// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import "dart:_boxed_int";
import "dart:_compact_hash" show createMapFromStringKeyValueListUnsafe;
import "dart:_error_utils";
import "dart:_internal"
    show patch, POWERS_OF_TEN, unsafeCast, pushWasmArray, popWasmArray;
import "dart:_js_string_convert";
import "dart:_js_types";
import "dart:_js_helper" show JS, jsStringFromDartString, JSExternWrapperExt;
import "dart:_list"
    show GrowableList, WasmListBaseUnsafeExtensions, WasmListBase;
import "dart:_string";
import "dart:_string_helper";
import "dart:_object_helper";
import "dart:_typed_data";
import "dart:_object_helper";
import "dart:_wasm";
import "dart:typed_data" show Uint8List;

@patch
dynamic _parseJson(
  String source,
  Object? Function(Object? key, Object? value)? reviver,
) {
  final listener = _JsonListener(reviver);
  final parser = _JSStringImplParser(listener);
  parser.setNewChunk(unsafeCast<JSStringImpl>(source), source.length);
  parser.parse(0);
  parser.close();
  return listener.result;
}

@patch
class Utf8Decoder {
  @patch
  Converter<List<int>, T> fuse<T>(Converter<String, T> next) {
    if (next is JsonDecoder) {
      return _JsonUtf8Decoder(
            (next as JsonDecoder)._reviver,
            this._allowMalformed,
          )
          as dynamic /*=Converter<List<int>, T>*/;
    }
    return super.fuse<T>(next);
  }
}

class _JsonUtf8Decoder extends Converter<List<int>, Object?> {
  final Object? Function(Object? key, Object? value)? _reviver;
  final bool _allowMalformed;

  _JsonUtf8Decoder(this._reviver, this._allowMalformed);

  Object? convert(List<int> input) {
    var parser = _JsonUtf8DecoderSink._createParser(_reviver, _allowMalformed);
    parser.parseChunk(input, 0, input.length);
    parser.close();
    return parser.result;
  }

  ByteConversionSink startChunkedConversion(Sink<Object?> sink) =>
      _JsonUtf8DecoderSink(_reviver, sink, _allowMalformed);
}

//// Implementation ///////////////////////////////////////////////////////////

// Simple API for JSON parsing.

/**
 * A [_JsonListener] builds data objects from the parser events.
 *
 * This is a simple stack-based object builder. It keeps the most recently
 * seen value in a variable, and uses it depending on the following event.
 */
class _JsonListener {
  _JsonListener(this.reviver);

  final Object? Function(Object? key, Object? value)? reviver;

  /**
   * Stack used to handle nested containers.
   *
   * The current container is pushed on the stack when a new one is started.
   */
  WasmArray<GrowableList?> stack = WasmArray<GrowableList?>(0);
  int stackLength = 0;

  void stackPush(WasmArray<Object?>? value, int valueLength) {
    final GrowableList<Object?>? valueAsList = value == null
        ? null
        : GrowableList.withDataAndLength(value, valueLength);

    // `GrowableList._nextCapacity` is copied here as the next capacity. We
    // can't use `GrowableList._nextCapacity` as tear-off as it's difficult to
    // inline tear-offs manually in the `pushWasmArray` compiler.
    pushWasmArray<GrowableList<Object?>?>(
      this.stack,
      this.stackLength,
      valueAsList,
      (stackLength * 2) | 3,
    );
  }

  GrowableList<dynamic>? stackPop() {
    assert(stackLength != 0);
    return popWasmArray<GrowableList<dynamic>?>(stack, stackLength);
  }

  /** Contents of the current container being built, or null if not building a
   * container.
   *
   * When building a [Map] this will contain array of key-value pairs.
   */
  WasmArray<Object?>? currentContainer = null;
  int currentContainerLength = 0;

  void currentContainerPush(Object? value) {
    WasmArray<Object?> currentContainerNonNull = unsafeCast<WasmArray<Object?>>(
      this.currentContainer,
    );
    // Per `_HashBase` invariant capacity needs to be a power of two.
    pushWasmArray<Object?>(
      currentContainerNonNull,
      this.currentContainerLength,
      value,
      currentContainerLength == 0 ? 8 : (currentContainerLength * 2),
    );
    currentContainer = currentContainerNonNull;
  }

  /** The most recently read value. */
  Object? value;

  /** Pushes the currently active container. */
  void beginContainer() {
    stackPush(currentContainer, currentContainerLength);
    currentContainer = const WasmArray<Object?>.literal([]);
    currentContainerLength = 0;
  }

  /** Pops the top container from the [stack]. */
  void popContainer() {
    final GrowableList<dynamic>? currentContainerList = stackPop();
    if (currentContainerList == null) {
      currentContainer = null;
      currentContainerLength = 0;
    } else {
      currentContainer = currentContainerList.data;
      currentContainerLength = currentContainerList.length;
    }
  }

  void handleString(String value) {
    this.value = value;
  }

  @pragma('wasm:prefer-inline')
  void handleIntegerNumber(int value) {
    if (value.toWasmI64().leU(0xff.toWasmI64())) {
      handleNumber(_intBoxes256[value]);
      return;
    }
    handleNumber(value);
  }

  void handleNumber(num value) {
    this.value = value;
  }

  void handleBool(bool value) {
    this.value = value;
  }

  void handleNull() {
    this.value = null;
  }

  void beginObject() {
    beginContainer();
  }

  void propertyName() {
    currentContainerPush(value);
    value = null;
  }

  void propertyValue() {
    if (reviver case final reviver?) {
      final keyValuePairs = unsafeCast<WasmArray<Object?>>(
        currentContainer,
      ); // null deref
      final key = keyValuePairs[currentContainerLength - 1];
      currentContainerPush(reviver(key, value));
    } else {
      currentContainerPush(value);
    }
    value = null;
  }

  void endObject() {
    value = createMapFromStringKeyValueListUnsafe<String, dynamic>(
      unsafeCast<WasmArray<Object?>>(currentContainer),
      currentContainerLength,
    );
    popContainer();
  }

  void beginArray() {
    beginContainer();
  }

  void arrayElement() {
    var reviver = this.reviver;
    if (reviver != null) {
      value = reviver(currentContainerLength, value);
    }
    currentContainerPush(value);
    value = null;
  }

  void endArray() {
    value = GrowableList.withDataAndLength(
      unsafeCast<WasmArray<Object?>>(currentContainer),
      currentContainerLength,
    );
    popContainer();
  }

  /**
   * Read out the final result of parsing a JSON string.
   *
   * Must only be called when the entire input has been parsed.
   */
  dynamic get result {
    assert(currentContainer == null);
    var reviver = this.reviver;
    if (reviver != null) {
      return reviver(null, value);
    } else {
      return value;
    }
  }
}

/**
 * Buffer holding parts of a numeral.
 *
 * The buffer contains the characters of a JSON number.
 * These are all ASCII, so an `array i8` is used as backing store.
 *
 * This buffer is used when a JSON number is split between separate chunks.
 */
class _NumberBuffer {
  static const int minCapacity = 16;
  static const int defaultOverhead = 5;
  WasmArray<WasmI8> array;
  int length = 0;
  _NumberBuffer(int initialCapacity)
    : array = WasmArray<WasmI8>(_initialCapacity(initialCapacity));

  int get capacity => array.length;

  void clear() {
    length = 0;
  }

  // Pick an initial capacity greater than the first part's size.
  // The typical use case has two parts, this is the attempt at
  // guessing the size of the second part without overdoing it.
  // The default estimate of the second part is [defaultOverhead],
  // then round to multiplum of four, and return the result,
  // or [minCapacity] if that is greater.
  static int _initialCapacity(int minCapacity) {
    minCapacity += defaultOverhead;
    if (minCapacity < _NumberBuffer.minCapacity) {
      return _NumberBuffer.minCapacity;
    }
    minCapacity = (minCapacity + 3) & ~3; // Round to multiple of four.
    return minCapacity;
  }

  // Grows to the exact size asked for.
  void ensureCapacity(int newCapacity) {
    WasmArray<WasmI8> array = this.array;
    if (newCapacity <= array.length) return;
    WasmArray<WasmI8> newArray = WasmArray<WasmI8>(newCapacity);
    newArray.copy(0, array, 0, array.length);
    this.array = newArray;
  }

  String getString() => JSStringImpl.fromAsciiBytes(array, 0, length);

  // TODO(lrn): See if parsing of numbers can be abstracted to something
  // not only working on strings, but also on char-code lists, without losing
  // performance.
  num parseNum() => num.parse(getString());
  double parseDouble() => _jsParseValidFloat(getString());
}

/**
 * Base class for the common monomorphic parts of all chunked JSON parsers.
 *
 * The members in this class should not be overridden and should not have
 * polymorphism to make sure the generated code for the subclasses is optimal.
 */
abstract class _ChunkedJsonParserState {
  final _JsonListener listener;

  _ChunkedJsonParserState(this.listener);

  // The current parsing state.
  int state = _ChunkedJsonParser.STATE_INITIAL;

  WasmArray<WasmI64> states = WasmArray<WasmI64>(0);
  int statesLength = 0;

  /**
   * Stores tokenizer state between chunks.
   *
   * This state is stored when a chunk stops in the middle of a
   * token (string, numeral, boolean or null).
   *
   * The partial state is used to continue parsing on the next chunk.
   * The previous chunk is not retained, any data needed are stored in
   * this integer, or in the [buffer] field as a string-building buffer
   * or a [_NumberBuffer].
   *
   * Prefix state stored in [prefixState] as bits.
   *
   *            ..00 : No partial value (NO_PARTIAL).
   *
   *         ..00001 : Partial string, not inside escape.
   *         ..00101 : Partial string, after '\'.
   *     ..vvvv1dd01 : Partial \u escape.
   *                   The 'dd' bits (2-3) encode the number of hex digits seen.
   *                   Bits 5-16 encode the value of the hex digits seen so far.
   *
   *        ..0ddd10 : Partial numeral.
   *                   The `ddd` bits store the parts of in the numeral seen so
   *                   far, as the constants `NUM_*` defined above.
   *                   The characters of the numeral are stored in [buffer]
   *                   as a [_NumberBuffer].
   *
   *      ..0ddd0011 : Partial 'null' keyword.
   *      ..0ddd0111 : Partial 'true' keyword.
   *      ..0ddd1011 : Partial 'false' keyword.
   *      ..0ddd1111 : Partial UTF-8 BOM byte sequence ("\xEF\xBB\xBF").
   *                   For all keywords, the `ddd` bits encode the number
   *                   of letters seen.
   *                   The BOM byte sequence is only used by [_JsonUtf8Parser],
   *                   and only at the very beginning of input.
   */
  int partialState = _ChunkedJsonParser.NO_PARTIAL;

  /**
   * String parts stored while parsing a string.
   */
  StringBuffer? _stringBuffer;

  @pragma('wasm:prefer-inline')
  StringBuffer get stringBuffer => _stringBuffer ??= StringBuffer();

  /**
   * Number parts stored while parsing a number.
   */
  _NumberBuffer? _numberBuffer;

  @pragma('wasm:prefer-inline')
  _NumberBuffer get numberBuffer =>
      _numberBuffer ??= _NumberBuffer(_NumberBuffer.minCapacity);

  /**
   * Push the current parse [state] on a stack.
   *
   * State is pushed when a new array or object literal starts,
   * so the parser can go back to the correct value when the literal ends.
   */
  void saveState(int state) {
    pushWasmArray<WasmI64>(
      states,
      statesLength,
      state.toWasmI64(),
      (statesLength * 2) | 3,
    );
  }

  /**
   * Restore a state pushed with [saveState].
   */
  int restoreState() {
    assert(statesLength > 0);
    return popWasmArray<WasmI64>(states, statesLength).toInt();
  }

  /**
   * Read out the result after successfully closing the parser.
   *
   * The parser is closed by calling [close] or calling [addSourceChunk] with
   * `true` as second (`isLast`) argument.
   */
  dynamic get result => listener.result;
}

/**
 * Chunked JSON parser implementation.
 *
 * This is a mixin instead of a base class to allow compilers to specialize
 * applications otherwise accessing chunk characters becomes polymorphic.
 */
mixin _ChunkedJsonParser<T> on _ChunkedJsonParserState {
  // A simple non-recursive state-based parser for JSON.
  //
  // Literal values accepted in states ARRAY_EMPTY, ARRAY_COMMA, OBJECT_COLON
  // and strings also in OBJECT_EMPTY, OBJECT_COMMA.
  //               VALUE  STRING  :  ,  }  ]        Transitions to
  // EMPTY            X      X                   -> END
  // ARRAY_EMPTY      X      X             @     -> ARRAY_VALUE / pop
  // ARRAY_VALUE                     @     @     -> ARRAY_COMMA / pop
  // ARRAY_COMMA      X      X                   -> ARRAY_VALUE
  // OBJECT_EMPTY            X          @        -> OBJECT_KEY / pop
  // OBJECT_KEY                   @              -> OBJECT_COLON
  // OBJECT_COLON     X      X                   -> OBJECT_VALUE
  // OBJECT_VALUE                    @  @        -> OBJECT_COMMA / pop
  // OBJECT_COMMA            X                   -> OBJECT_KEY
  // END
  // Starting a new array or object will push the current state. The "pop"
  // above means restoring this state and then marking it as an ended value.
  // X means generic handling, @ means special handling for just that
  // state - that is, values are handled generically, only punctuation
  // cares about the current state.
  // Values for states are chosen so bits 0 and 1 tell whether
  // a string/value is allowed, and setting bits 0 through 2 after a value
  // gets to the next state (not empty, doesn't allow a value).

  // State building-block constants.
  static const int TOP_LEVEL = 0;
  static const int INSIDE_ARRAY = 1;
  static const int INSIDE_OBJECT = 2;
  static const int AFTER_COLON = 3; // Always inside object.

  static const int ALLOW_STRING_MASK = 8; // Allowed if zero.
  static const int ALLOW_VALUE_MASK = 4; // Allowed if zero.
  static const int ALLOW_VALUE = 0;
  static const int STRING_ONLY = 4;
  static const int NO_VALUES = 12;

  // Objects and arrays are "empty" until their first property/element.
  // At this position, they may either have an entry or a close-bracket.
  static const int EMPTY = 0;
  static const int NON_EMPTY = 16;
  static const int EMPTY_MASK = 16; // Empty if zero.

  // Actual states               : Context | Is empty? | Next?
  static const int STATE_INITIAL = TOP_LEVEL | EMPTY | ALLOW_VALUE;
  static const int STATE_END = TOP_LEVEL | NON_EMPTY | NO_VALUES;

  static const int STATE_ARRAY_EMPTY = INSIDE_ARRAY | EMPTY | ALLOW_VALUE;
  static const int STATE_ARRAY_VALUE = INSIDE_ARRAY | NON_EMPTY | NO_VALUES;
  static const int STATE_ARRAY_COMMA = INSIDE_ARRAY | NON_EMPTY | ALLOW_VALUE;

  static const int STATE_OBJECT_EMPTY = INSIDE_OBJECT | EMPTY | STRING_ONLY;
  static const int STATE_OBJECT_KEY = INSIDE_OBJECT | NON_EMPTY | NO_VALUES;
  static const int STATE_OBJECT_COLON = AFTER_COLON | NON_EMPTY | ALLOW_VALUE;
  static const int STATE_OBJECT_VALUE = AFTER_COLON | NON_EMPTY | NO_VALUES;
  static const int STATE_OBJECT_COMMA = INSIDE_OBJECT | NON_EMPTY | STRING_ONLY;

  // Bits set in state after successfully reading a value.
  // This transitions the state to expect the next punctuation.
  static const int VALUE_READ_BITS = NON_EMPTY | NO_VALUES;

  // Character code constants.
  static const int BACKSPACE = 0x08;
  static const int TAB = 0x09;
  static const int NEWLINE = 0x0a;
  static const int CARRIAGE_RETURN = 0x0d;
  static const int FORM_FEED = 0x0c;
  static const int SPACE = 0x20;
  static const int QUOTE = 0x22;
  static const int PLUS = 0x2b;
  static const int COMMA = 0x2c;
  static const int MINUS = 0x2d;
  static const int DECIMALPOINT = 0x2e;
  static const int SLASH = 0x2f;
  static const int CHAR_0 = 0x30;
  static const int CHAR_9 = 0x39;
  static const int COLON = 0x3a;
  static const int CHAR_E = 0x45;
  static const int LBRACKET = 0x5b;
  static const int BACKSLASH = 0x5c;
  static const int RBRACKET = 0x5d;
  static const int CHAR_a = 0x61;
  static const int CHAR_b = 0x62;
  static const int CHAR_e = 0x65;
  static const int CHAR_f = 0x66;
  static const int CHAR_l = 0x6c;
  static const int CHAR_n = 0x6e;
  static const int CHAR_r = 0x72;
  static const int CHAR_s = 0x73;
  static const int CHAR_t = 0x74;
  static const int CHAR_u = 0x75;
  static const int LBRACE = 0x7b;
  static const int RBRACE = 0x7d;

  // State of partial value at chunk split.
  static const int NO_PARTIAL = 0;
  static const int PARTIAL_STRING = 1;
  static const int PARTIAL_NUMERAL = 2;
  static const int PARTIAL_KEYWORD = 3;
  static const int MASK_PARTIAL = 3;

  // Partial states for numerals. Values can be |'ed with PARTIAL_NUMERAL.
  static const int NUM_SIGN = 0; // After initial '-'.
  static const int NUM_ZERO = 4; // After '0' as first digit.
  static const int NUM_DIGIT = 8; // After digit, no '.' or 'e' seen.
  static const int NUM_DOT = 12; // After '.'.
  static const int NUM_DOT_DIGIT = 16; // After a decimal digit (after '.').
  static const int NUM_E = 20; // After 'e' or 'E'.
  static const int NUM_E_SIGN = 24; // After '-' or '+' after 'e' or 'E'.
  static const int NUM_E_DIGIT = 28; // After exponent digit.
  static const int NUM_SUCCESS = 32; // Never stored as partial state.

  // Partial states for strings.
  static const int STR_PLAIN = 0; // Inside string, but not escape.
  static const int STR_ESCAPE = 4; // After '\'.
  static const int STR_U = 16; // After '\u' and 0-3 hex digits.
  static const int STR_U_COUNT_SHIFT = 2; // Hex digit count in bits 2-3.
  static const int STR_U_VALUE_SHIFT = 5; // Hex digit value in bits 5+.

  // Partial states for keywords.
  static const int KWD_TYPE_MASK = 12;
  static const int KWD_TYPE_SHIFT = 2;
  static const int KWD_NULL = 0; // Prefix of "null" seen.
  static const int KWD_TRUE = 4; // Prefix of "true" seen.
  static const int KWD_FALSE = 8; // Prefix of "false" seen.
  static const int KWD_BOM = 12; // Prefix of BOM seen.
  static const int KWD_COUNT_SHIFT = 4; // Prefix length in bits 4+.

  // Mask used to mask off two lower bits.
  static const int TWO_BIT_MASK = 3;

  /**
   * Finalizes the parsing.
   *
   * Throws if the source read so far doesn't end up with a complete
   * parsed value. That means it must not be inside a list or object
   * literal, and any partial value read should also be a valid complete
   * value.
   *
   * The only valid partial state is a number that ends in a digit, and
   * only if the number is the entire JSON value being parsed
   * (otherwise it would be inside a list or object).
   * Such a number will be completed. Any other partial state is an error.
   */
  void close() {
    if (partialState != NO_PARTIAL) {
      int partialType = partialState & MASK_PARTIAL;
      if (partialType == PARTIAL_NUMERAL) {
        int numState = partialState & ~MASK_PARTIAL;
        // A partial number might be a valid number if we know it's done.
        // There is an unnecessary overhead if input is a single number,
        // but this is assumed to be rare.
        finishChunkNumber(numState, 0, 0);
      } else if (partialType == PARTIAL_STRING) {
        fail(chunkEnd, "Unterminated string");
      } else {
        assert(partialType == PARTIAL_KEYWORD);
        fail(chunkEnd); // Incomplete literal.
      }
    }
    if (state != STATE_END) {
      fail(chunkEnd);
    }
  }

  /**
   * Length of current chunk.
   *
   * The valid arguments to [getChar] are 0 .. `chunkEnd - 1`.
   */
  int get chunkEnd;

  /**
   * Returns the chunk itself.
   *
   * Only used by [fail] to include the chunk in the thrown [FormatException].
   */
  T get chunk;

  /**
   * Get character/code unit of current chunk.
   *
   * The [index] must be non-negative and less than `chunkEnd`.
   * In practice, [index] will be no smaller than the `start` argument passed
   * to [parse].
   */
  int getChar(int index);

  /**
   * Returns [true] if [getChar] is returning UTF16 code units.
   *
   * Otherwise it is expected that [getChar] is returning UTF8 bytes.
   */
  bool get isUtf16Input;

  /**
   * Copy ASCII characters from start to end of chunk into a list.
   *
   * Used for number buffer (always copies ASCII, so encoding is not important).
   */
  void copyCharsToList(
    int start,
    int end,
    WasmArray<WasmI8> target,
    int offset,
  );

  /**
   * Build a string using input code units.
   *
   * Creates a string buffer and enables adding characters and slices
   * to that buffer.
   * The buffer is stored in the [buffer] field. If the string is unterminated,
   * the same buffer is used to continue parsing in the next chunk.
   */
  void beginString();
  /**
   * Add single character code to string being built.
   *
   * Used for unparsed escape sequences.
   */
  void addCharToString(int charCode);

  /**
   * Adds slice of current chunk to string being built.
   *
   * The [start] positions is inclusive, [end] is exclusive.
   * The [bits] is a bit-wise OR of the range of bytes/charcodes.
   * The [codeUnitsCache] is pre-populated from the range of bytes/charcodes.
   *
   * The [codeUnitsCache] may be shorter than the range of bytes/charcodes in
   * which case it cannot be used, the implementation has to populate a separate
   * wasm array for the range.
   *
   * The [codeUnitsCache] should only be used if the [bits] indicate the string
   * is ASCII as in that case the bytes in `[start..end[` we stored in the array
   * are also code units.
   */
  void addSliceToString(
    int start,
    int end,
    int bits,
    WasmArray<WasmI16> codeUnitsCache,
  );

  /**
   * Adds a string slice to the string being built.
   */
  void addStringSliceToString(String string);

  /** Finalizes the string being built and returns it as a String. */
  String endString();

  /**
   * Extracts a literal string from a slice of the current chunk.
   *
   * No interpretation of the content is performed, except for converting
   * the source format to string.
   * This can be implemented more or less efficiently depending on the
   * underlying source.
   *
   * This is used for string literals that contain no escapes.
   */
  String getString(
    int start,
    int end,
    int bits,
    WasmArray<WasmI16> codeUnitsCache,
  );

  /**
   * Same as [getString] but with [hash] containing the already computed string
   * hash.
   *
   * Used to create string objects for keys in json objects.
   */
  String getJsonObjectKeyString(int start, int end, int bits, int hash);

  /**
   * Parse a slice of the current chunk as a double.
   *
   * The format is expected to be correct.
   * This is used by [parseNumber] when the double value cannot be
   * built exactly during parsing.
   */
  double parseDouble(int start, int end) {
    return _jsParseValidFloat(
      getString(start, end, 0x7f, _emptyCodeUnitsCache),
    );
  }

  /**
   * Continues parsing a partial value.
   */
  int parsePartial(int position) {
    if (position == chunkEnd) return position;
    int partialState = this.partialState;
    assert(partialState != NO_PARTIAL);
    int partialType = partialState & MASK_PARTIAL;
    this.partialState = NO_PARTIAL;
    partialState = partialState & ~MASK_PARTIAL;
    assert(partialType != 0);
    if (partialType == PARTIAL_STRING) {
      position = parsePartialString(position, partialState);
    } else if (partialType == PARTIAL_NUMERAL) {
      position = parsePartialNumber(position, partialState);
    } else if (partialType == PARTIAL_KEYWORD) {
      position = parsePartialKeyword(position, partialState);
    }
    return position;
  }

  /**
   * Parses the remainder of a number into the number buffer.
   *
   * Syntax is checked while pasing.
   * Starts at position, which is expected to be the start of the chunk,
   * and returns the index of the first non-number-literal character found,
   * or chunkEnd if the entire chunk is a valid number continuation.
   * Throws if a syntax error is detected.
   */
  int parsePartialNumber(int position, int state) {
    int start = position;
    // Primitive implementation, can be optimized.
    int end = chunkEnd;
    toBailout:
    {
      if (position == end) break toBailout;
      int char = getChar(position);
      int digit = char ^ CHAR_0;
      if (state == NUM_SIGN) {
        if (digit <= 9) {
          if (digit == 0) {
            state = NUM_ZERO;
          } else {
            state = NUM_DIGIT;
          }
          position++;
          if (position == end) break toBailout;
          char = getChar(position);
          digit = char ^ CHAR_0;
        } else {
          fail(position);
        }
      }
      if (state == NUM_ZERO) {
        // JSON does not allow insignificant leading zeros (e.g., "09").
        if (digit <= 9) fail(position);
        state = NUM_DIGIT;
      }
      while (state == NUM_DIGIT) {
        if (digit > 9) {
          if (char == DECIMALPOINT) {
            state = NUM_DOT;
          } else if ((char | 0x20) == CHAR_e) {
            state = NUM_E;
          } else {
            finishChunkNumber(state, start, position);
            return position;
          }
        }
        position++;
        if (position == end) break toBailout;
        char = getChar(position);
        digit = char ^ CHAR_0;
      }
      if (state == NUM_DOT) {
        if (digit > 9) fail(position);
        state = NUM_DOT_DIGIT;
      }
      while (state == NUM_DOT_DIGIT) {
        if (digit > 9) {
          if ((char | 0x20) == CHAR_e) {
            state = NUM_E;
          } else {
            finishChunkNumber(state, start, position);
            return position;
          }
        }
        position++;
        if (position == end) break toBailout;
        char = getChar(position);
        digit = char ^ CHAR_0;
      }
      if (state == NUM_E) {
        if (char == PLUS || char == MINUS) {
          state = NUM_E_SIGN;
          position++;
          if (position == end) break toBailout;
          char = getChar(position);
          digit = char ^ CHAR_0;
        }
      }
      assert(state >= NUM_E);
      while (digit <= 9) {
        state = NUM_E_DIGIT;
        position++;
        if (position == end) break toBailout;
        char = getChar(position);
        digit = char ^ CHAR_0;
      }
      finishChunkNumber(state, start, position);
      return position;
    }
    // Bailout code in case the current chunk ends while parsing the numeral.
    assert(position == end);
    continueChunkNumber(state, start);
    return chunkEnd;
  }

  /**
   * Continues parsing a partial string literal.
   *
   * Handles partial escapes and then hands the parsing off to
   * [parseStringToBuffer].
   */
  int parsePartialString(int position, int partialState) {
    if (partialState == STR_PLAIN) {
      return parseStringToBuffer(position);
    }
    if (partialState == STR_ESCAPE) {
      position = parseStringEscape(position);
      // parseStringEscape sets partialState if it sees the end.
      if (position == chunkEnd) return position;
      return parseStringToBuffer(position);
    }
    assert((partialState & STR_U) != 0);
    int value = partialState >> STR_U_VALUE_SHIFT;
    int count = (partialState >> STR_U_COUNT_SHIFT) & TWO_BIT_MASK;
    for (int i = count; i < 4; i++, position++) {
      if (position == chunkEnd) return chunkStringEscapeU(i, value);
      int char = getChar(position);
      int digit = parseHexDigit(char);
      if (digit < 0) fail(position, "Invalid hex digit");
      value = 16 * value + digit;
    }
    addCharToString(value);
    return parseStringToBuffer(position);
  }

  /**
   * Continues parsing a partial keyword.
   */
  int parsePartialKeyword(int position, int partialState) {
    int keywordType = partialState & KWD_TYPE_MASK;
    int count = partialState >> KWD_COUNT_SHIFT;
    int keywordTypeIndex = keywordType >> KWD_TYPE_SHIFT;
    String keyword = const [
      "null",
      "true",
      "false",
      "\xEF\xBB\xBF",
    ][keywordTypeIndex];
    assert(count < keyword.length);
    do {
      if (position == chunkEnd) {
        this.partialState =
            PARTIAL_KEYWORD | keywordType | (count << KWD_COUNT_SHIFT);
        return chunkEnd;
      }
      int expectedChar = keyword.codeUnitAtUnchecked(count);
      if (getChar(position) != expectedChar) {
        if (count == 0) {
          assert(keywordType == KWD_BOM);
          return position;
        }
        fail(position);
      }
      position++;
      count++;
    } while (count < keyword.length);
    if (keywordType == KWD_NULL) {
      listener.handleNull();
    } else if (keywordType != KWD_BOM) {
      listener.handleBool(keywordType == KWD_TRUE);
    }
    return position;
  }

  /** Convert hex-digit to its value. Returns -1 if char is not a hex digit. */
  int parseHexDigit(int char) {
    int digit = char ^ 0x30;
    if (digit <= 9) return digit;
    int letter = (char | 0x20) ^ 0x60;
    // values 1 .. 6 are 'a' through 'f'
    if (letter <= 6 && letter > 0) return letter + 9;
    return -1;
  }

  /**
   * Parses the current chunk as a chunk of JSON.
   *
   * Starts parsing at [position] and continues until [chunkEnd].
   * Continues parsing where the previous chunk (if any) ended.
   */
  void parse(int position) {
    int length = chunkEnd;
    if (partialState != NO_PARTIAL) {
      position = parsePartial(position);
      if (position == length) return;
    }

    int state = this.state;
    outer:
    while (position < length) {
      int char = 0;
      do {
        char = getChar(position);
        if (isUtf16Input && char > 0xFF) {
          break;
        }
        if ((_characterAttributes.readUnsigned(char) & CHAR_WHITESPACE) == 0) {
          break;
        }
        position++;
        if (position >= length) {
          break outer;
        }
      } while (true);

      switch (char) {
        case QUOTE:
          if ((state & ALLOW_STRING_MASK) != 0) fail(position);
          final calculateHash =
              state == STATE_OBJECT_EMPTY || state == STATE_OBJECT_COMMA;
          state |= VALUE_READ_BITS;
          if (calculateHash) {
            position = parseStringUsedAsKey(position + 1);
          } else {
            position = parseString(position + 1);
          }
          break;
        case LBRACKET:
          if ((state & ALLOW_VALUE_MASK) != 0) fail(position);
          listener.beginArray();
          saveState(state);
          state = STATE_ARRAY_EMPTY;
          position++;
          break;
        case LBRACE:
          if ((state & ALLOW_VALUE_MASK) != 0) fail(position);
          listener.beginObject();
          saveState(state);
          state = STATE_OBJECT_EMPTY;
          position++;
          break;
        case CHAR_n:
          if ((state & ALLOW_VALUE_MASK) != 0) fail(position);
          state |= VALUE_READ_BITS;
          position = parseNull(position);
          break;
        case CHAR_f:
          if ((state & ALLOW_VALUE_MASK) != 0) fail(position);
          state |= VALUE_READ_BITS;
          position = parseFalse(position);
          break;
        case CHAR_t:
          if ((state & ALLOW_VALUE_MASK) != 0) fail(position);
          state |= VALUE_READ_BITS;
          position = parseTrue(position);
          break;
        case COLON:
          if (state != STATE_OBJECT_KEY) fail(position);
          listener.propertyName();
          state = STATE_OBJECT_COLON;
          position++;
          break;
        case COMMA:
          if (state == STATE_OBJECT_VALUE) {
            listener.propertyValue();
            state = STATE_OBJECT_COMMA;
            position++;
          } else if (state == STATE_ARRAY_VALUE) {
            listener.arrayElement();
            state = STATE_ARRAY_COMMA;
            position++;
          } else {
            fail(position);
          }
          break;
        case RBRACKET:
          if (state == STATE_ARRAY_EMPTY) {
            listener.endArray();
          } else if (state == STATE_ARRAY_VALUE) {
            listener.arrayElement();
            listener.endArray();
          } else {
            fail(position);
          }
          state = restoreState() | VALUE_READ_BITS;
          position++;
          break;
        case RBRACE:
          if (state == STATE_OBJECT_EMPTY) {
            listener.endObject();
          } else if (state == STATE_OBJECT_VALUE) {
            listener.propertyValue();
            listener.endObject();
          } else {
            fail(position);
          }
          state = restoreState() | VALUE_READ_BITS;
          position++;
          break;
        default:
          if ((state & ALLOW_VALUE_MASK) != 0) fail(position);
          state |= VALUE_READ_BITS;
          position = parseNumber(char, position);
          break;
      }
    }
    this.state = state;
  }

  /**
   * Parses a "true" literal starting at [position].
   *
   * The character `source[position]` must be "t".
   */
  int parseTrue(int position) {
    assert(getChar(position) == CHAR_t);
    if (chunkEnd < position + 4) {
      return parseKeywordPrefix(position, "true", KWD_TRUE);
    }
    if (getChar(position + 1) != CHAR_r ||
        getChar(position + 2) != CHAR_u ||
        getChar(position + 3) != CHAR_e) {
      fail(position);
    }
    listener.handleBool(true);
    return position + 4;
  }

  /**
   * Parses a "false" literal starting at [position].
   *
   * The character `source[position]` must be "f".
   */
  int parseFalse(int position) {
    assert(getChar(position) == CHAR_f);
    if (chunkEnd < position + 5) {
      return parseKeywordPrefix(position, "false", KWD_FALSE);
    }
    if (getChar(position + 1) != CHAR_a ||
        getChar(position + 2) != CHAR_l ||
        getChar(position + 3) != CHAR_s ||
        getChar(position + 4) != CHAR_e) {
      fail(position);
    }
    listener.handleBool(false);
    return position + 5;
  }

  /**
   * Parses a "null" literal starting at [position].
   *
   * The character `source[position]` must be "n".
   */
  int parseNull(int position) {
    assert(getChar(position) == CHAR_n);
    if (chunkEnd < position + 4) {
      return parseKeywordPrefix(position, "null", KWD_NULL);
    }
    if (getChar(position + 1) != CHAR_u ||
        getChar(position + 2) != CHAR_l ||
        getChar(position + 3) != CHAR_l) {
      fail(position);
    }
    listener.handleNull();
    return position + 4;
  }

  int parseKeywordPrefix(int position, String chars, int type) {
    assert(getChar(position) == chars.codeUnitAt(0));
    int length = chunkEnd;
    int start = position;
    int count = 1;
    while (++position < length) {
      int char = getChar(position);
      if (char != chars.codeUnitAtUnchecked(count)) fail(start);
      count++;
    }
    this.partialState = PARTIAL_KEYWORD | type | (count << KWD_COUNT_SHIFT);
    return length;
  }

  static const int CHAR_SIMPLE_STRING_END = 1;
  static const int CHAR_WHITESPACE = 2;

  /**
   * [_characterAttributes] string was generated using the following code:
   *
   * ```
   * int $(String ch) => ch.codeUnitAt(0);
   * final list = Uint8List(256);
   * for (var i = 0; i < $(' '); i++) {
   *   list[i] |= CHAR_SIMPLE_STRING_END;
   * }
   * list[$('"')] |= CHAR_SIMPLE_STRING_END;
   * list[$('\\')] |= CHAR_SIMPLE_STRING_END;
   * list[$(' ')] |= CHAR_WHITESPACE;
   * list[$('\r')] |= CHAR_WHITESPACE;
   * list[$('\n')] |= CHAR_WHITESPACE;
   * list[$('\t')] |= CHAR_WHITESPACE;
   * for (var i = 0; i < 256; i += 64) {
   *   for (var v in list.skip(i).take(64)) {
   *     print('${v + $(' ')},');
   *   }
   * }
   * ```
   */
  static const _characterAttributes = ImmutableWasmArray<WasmI8>.literal([
    33, 33, 33, 33, 33, 33, 33, 33, 33, 35, 35, 33, 33, 35, 33, 33, 33, 33, //
    33, 33, 33, 33, 33, 33, 33, 33, 33, 33, 33, 33, 33, 33, 34, 32, 33, 32, //
    32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, //
    32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, //
    32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, //
    32, 32, 33, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, //
    32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, //
    32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, //
    32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, //
    32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, //
    32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, //
    32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, //
    32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, //
    32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, 32, //
    32, 32, 32, 32,
  ]);

  /**
   * Parses a string value.
   *
   * Initial [position] is right after the initial quote.
   * Returned position right after the final quote.
   */
  int parseString(int position) {
    return _parseStringInternal(position, false);
  }

  /**
   * Same as [parseString] but the caller knows this string will be used as a
   * key in a json object.
   */
  int parseStringUsedAsKey(int position) {
    return _parseStringInternal(position, true);
  }

  @pragma('wasm:prefer-inline')
  int _parseStringInternal(int position, bool isJsonObjectKey) {
    // Format: '"'([^\x00-\x1f\\\"]|'\\'[bfnrt/\\"])*'"'
    // Initial position is right after first '"'.
    int start = position;
    int end = chunkEnd;
    int char = 0;
    int bits = 0;
    int hash = 0;

    // If our input is a JSString then we can obtain substrings using the
    // `js-string:substring` function for it. That means we never have to go
    // via an intermediary `WasmArray<WasmI16>` to create the JS string.
    //
    // If instead our intput is bytes (i.e., fused json+utf8 decoder) then we
    // create JS strings using the `js-string:fromCharCodeArray` API. That
    // requires copying bytes to an intermediary `WasmArray<WasmI16>`. To avoid
    // doing two O(n) traversals over the input (first here in this function and
    // later on when creating the array) - already when we consume the bytes
    // here we opportunistically (i.e., assuming it's a shorter string,
    // assuming it's ASCII) store the bytes into a pre-allocated
    // `WasmArray<WasmI16>`. In the common case this will then avoid the
    // additional O(n) pass.
    //
    // The `!isUtf16Input` condition below checks whether input is bytes (and
    // will be compile-time evaluted to a constant due each parser returning a
    // constant and the compiler copying the mixin for each implementation).
    //
    // Though if the string we currently parse is used as a key in a json
    // object we'll with high likelyhood not need to allocate a JS String. This
    // is due to the fact that we first try to look whether we have a matching
    // string in the string intern cache. We therefore do not need (in the
    // common case) go via an intermediary `WasmArray<WasmI16>`,  so we'll
    // not popualte it for json keys.
    //
    // The `!isJsonObjectKey` condition below checks for that case.
    final bool useCharCodeCache = !isUtf16Input && !isJsonObjectKey;
    final WasmArray<WasmI16> codeUnitsCache;
    final int codeUnitsCacheSize;
    if (useCharCodeCache) {
      codeUnitsCache = _codeUnitsCache;
      codeUnitsCacheSize = _codeUnitsCacheSize;
    } else {
      codeUnitsCache = _emptyCodeUnitsCache;
      codeUnitsCacheSize = _emptyCodeUnitsCacheSize;
    }
    int cacheIndex = 0;

    if (position < end) {
      do {
        // Caveat: do not combine the following two lines together. It helps
        // compiler to generate better code (it currently can't reorder operations
        // to reduce register pressure).
        char = getChar(position);
        bits |= char; // Includes final '"', but that never matters.
        position++;
        if (!isUtf16Input || char <= 0xFF) {
          if ((_characterAttributes.readUnsigned(char) &
                  CHAR_SIMPLE_STRING_END) !=
              0) {
            break;
          }
        }
        if (isJsonObjectKey) {
          hash = stringCombineHashes(hash, char);
        }
        if (codeUnitsCacheSize > 0 && cacheIndex < codeUnitsCacheSize) {
          codeUnitsCache.write(cacheIndex++, char);
        }
      } while (position < end);
      if (char == QUOTE) {
        int sliceEnd = position - 1;
        listener.handleString(
          isJsonObjectKey
              ? getJsonObjectKeyString(
                  start,
                  sliceEnd,
                  bits,
                  stringFinalizeHash(hash),
                )
              : getString(start, sliceEnd, bits, codeUnitsCache),
        );
        return sliceEnd + 1;
      }
      if (char == BACKSLASH) {
        int sliceEnd = position - 1;
        if (start < sliceEnd) {
          final s = getString(start, sliceEnd, bits, codeUnitsCache);
          beginString();
          addStringSliceToString(s);
        } else {
          beginString();
        }
        return parseStringToBuffer(sliceEnd);
      }
      if (char < SPACE) {
        fail(position - 1, "Control character in string");
      }
    }
    beginString();
    if (start < end) {
      addSliceToString(start, end, bits, codeUnitsCache);
    }
    return chunkString(STR_PLAIN);
  }

  /**
   * Sets up a partial string state.
   *
   * The state is either not inside an escape, or right after a backslash.
   * For partial strings ending inside a Unicode escape, use
   * [chunkStringEscapeU].
   */
  int chunkString(int stringState) {
    partialState = PARTIAL_STRING | stringState;
    return chunkEnd;
  }

  /**
   * Sets up a partial string state for a partially parsed Unicode escape.
   *
   * The partial string state includes the current [buffer] and the
   * number of hex digits of the Unicode seen so far (e.g., for `"\u30')
   * the state knows that two digits have been seen, and what their value is.
   *
   * Returns [chunkEnd] so it can be used as part of a return statement.
   */
  int chunkStringEscapeU(int count, int value) {
    partialState =
        PARTIAL_STRING |
        STR_U |
        (count << STR_U_COUNT_SHIFT) |
        (value << STR_U_VALUE_SHIFT);
    return chunkEnd;
  }

  /**
   * Parses the remainder of a string literal into a buffer.
   *
   * The buffer is stored in [buffer] and its underlying format depends on
   * the input chunk type. For example UTF-8 decoding happens in the
   * buffer, not in the parser, since all significant JSON characters are ASCII.
   *
   * This function scans through the string literal for escapes, and copies
   * slices of non-escape characters using [addSliceToString].
   */
  int parseStringToBuffer(int position) {
    int end = chunkEnd;
    int start = position;
    int bits = 0;

    final bool useCharCodeCache = !isUtf16Input;

    final WasmArray<WasmI16> codeUnitsCache;
    final int codeUnitsCacheSize;
    if (useCharCodeCache) {
      codeUnitsCache = _codeUnitsCache;
      codeUnitsCacheSize = _codeUnitsCacheSize;
    } else {
      codeUnitsCache = _emptyCodeUnitsCache;
      codeUnitsCacheSize = _emptyCodeUnitsCacheSize;
    }

    int cacheIndex = 0;

    while (true) {
      if (position == end) {
        if (position > start) {
          addSliceToString(start, position, bits, codeUnitsCache);
        }
        return chunkString(STR_PLAIN);
      }

      int char = 0;
      do {
        char = getChar(position);
        bits |= char; // Includes final '"', but that never matters.
        position++;
        if (!isUtf16Input || char <= 0xFF) {
          if ((_characterAttributes.readUnsigned(char) &
                  CHAR_SIMPLE_STRING_END) !=
              0) {
            break;
          }
        }

        if (cacheIndex < codeUnitsCacheSize) {
          codeUnitsCache.write(cacheIndex++, char);
        }
      } while (position < end);

      if (char < SPACE) {
        fail(position - 1); // Control character in string.
      }

      if (char == QUOTE) {
        int quotePosition = position - 1;
        if (quotePosition > start) {
          addSliceToString(start, quotePosition, bits, codeUnitsCache);
        }
        listener.handleString(endString());
        return position;
      }

      if (char != BACKSLASH) {
        continue;
      }

      // Handle escape.
      if (position - 1 > start) {
        addSliceToString(start, position - 1, bits, codeUnitsCache);
      }

      if (position == end) return chunkString(STR_ESCAPE);
      position = parseStringEscape(position);
      if (position == end) return position;
      start = position;
      cacheIndex = 0;
      bits = 0;
    }
  }

  /**
   * Parse a string escape.
   *
   * Position is right after the initial backslash.
   * The following escape is parsed into a character code which is added to
   * the current string buffer using [addCharToString].
   *
   * Returns position after the last character of the escape.
   */
  int parseStringEscape(int position) {
    int char = getChar(position++);
    int length = chunkEnd;
    switch (char) {
      case CHAR_b:
        char = BACKSPACE;
        break;
      case CHAR_f:
        char = FORM_FEED;
        break;
      case CHAR_n:
        char = NEWLINE;
        break;
      case CHAR_r:
        char = CARRIAGE_RETURN;
        break;
      case CHAR_t:
        char = TAB;
        break;
      case SLASH:
      case BACKSLASH:
      case QUOTE:
        break;
      case CHAR_u:
        int hexStart = position - 1;
        int value = 0;
        for (int i = 0; i < 4; i++) {
          if (position == length) return chunkStringEscapeU(i, value);
          char = getChar(position++);
          int digit = char ^ 0x30;
          value *= 16;
          if (digit <= 9) {
            value += digit;
          } else {
            digit = (char | 0x20) - CHAR_a;
            // digit < 0 || digit > 5
            if (digit.gtU(5)) {
              fail(hexStart, "Invalid unicode escape");
            }
            value += digit + 10;
          }
        }
        char = value;
        break;
      default:
        if (char < SPACE) fail(position, "Control character in string");
        fail(position, "Unrecognized string escape");
    }
    addCharToString(char);
    if (position == length) return chunkString(STR_PLAIN);
    return position;
  }

  /// Sets up a partial numeral state.
  /// Returns chunkEnd to allow easy one-line bailout tests.
  int beginChunkNumber(int state, int start) {
    int end = chunkEnd;
    int length = end - start;
    numberBuffer.ensureCapacity(length);
    copyCharsToList(start, end, numberBuffer.array, 0);
    numberBuffer.length = length;
    this.partialState = PARTIAL_NUMERAL | state;
    return end;
  }

  void addNumberChunk(int start, int end, int overhead) {
    int length = end - start;
    int count = numberBuffer.length;
    int newCount = count + length;
    int newCapacity = newCount + overhead;
    numberBuffer.ensureCapacity(newCapacity);
    copyCharsToList(start, end, numberBuffer.array, count);
    numberBuffer.length = newCount;
  }

  // Continues an already chunked number across an entire chunk.
  int continueChunkNumber(int state, int start) {
    int end = chunkEnd;
    addNumberChunk(start, end, _NumberBuffer.defaultOverhead);
    this.partialState = PARTIAL_NUMERAL | state;
    return end;
  }

  void finishChunkNumber(int state, int start, int end) {
    if (state == NUM_ZERO) {
      listener.handleIntegerNumber(0);
      numberBuffer.clear();
      return;
    }
    if (end > start) {
      addNumberChunk(start, end, 0);
    }
    if (state == NUM_DIGIT) {
      num value = numberBuffer.parseNum();
      listener.handleNumber(value);
    } else if (state == NUM_DOT_DIGIT || state == NUM_E_DIGIT) {
      listener.handleNumber(numberBuffer.parseDouble());
    } else {
      fail(chunkEnd, "Unterminated number literal");
    }
    numberBuffer.clear();
  }

  int parseNumber(int char, int position) {
    // Also called on any unexpected character.
    // Format:
    //  '-'?('0'|[1-9][0-9]*)('.'[0-9]+)?([eE][+-]?[0-9]+)?
    int start = position;
    int length = chunkEnd;
    // Collects an int value while parsing. Used for both an integer literal,
    // and the exponent part of a double literal.
    // Stored as negative to ensure we can represent -2^63.
    int intValue = 0;
    double doubleValue = 0.0; // Collect double value while parsing.
    // 1 if there is no leading -, -1 if there is.
    int sign = 1;
    bool isDouble = false;
    // Break this block when the end of the number literal is reached.
    // At that time, position points to the next character, and isDouble
    // is set if the literal contains a decimal point or an exponential.
    if (char == MINUS) {
      sign = -1;
      position++;
      if (position == length) return beginChunkNumber(NUM_SIGN, start);
      char = getChar(position);
    }
    int digit = char ^ CHAR_0;
    if (digit > 9) {
      if (sign < 0) {
        fail(position, "Missing expected digit");
      } else {
        // If it doesn't even start out as a numeral.
        fail(position);
      }
    }
    if (digit == 0) {
      position++;
      if (position == length) return beginChunkNumber(NUM_ZERO, start);
      char = getChar(position);
      digit = char ^ CHAR_0;
      // If starting with zero, next character must not be digit.
      if (digit <= 9) fail(position);
    } else {
      int digitCount = 0;
      do {
        if (digitCount >= 18) {
          // Check for overflow.
          // Is 1 if digit is 8 or 9 and sign == 0, or digit is 9 and sign < 0;
          int highDigit = digit >> 3;
          if (sign < 0) highDigit &= digit;
          if (digitCount == 19 || intValue - highDigit < -922337203685477580) {
            isDouble = true;
            // Big value that we know is not trusted to be exact later,
            // forcing reparsing using `double.parse`.
            doubleValue = 9223372036854775808.0;
          }
        }
        intValue = 10 * intValue - digit;
        digitCount++;
        position++;
        if (position == length) return beginChunkNumber(NUM_DIGIT, start);
        char = getChar(position);
        digit = char ^ CHAR_0;
      } while (digit <= 9);
    }
    if (char == DECIMALPOINT) {
      if (!isDouble) {
        isDouble = true;
        doubleValue = (intValue == 0) ? 0.0 : -intValue.toDouble();
      }
      intValue = 0;
      position++;
      if (position == length) return beginChunkNumber(NUM_DOT, start);
      char = getChar(position);
      digit = char ^ CHAR_0;
      if (digit > 9) fail(position);
      do {
        doubleValue = 10.0 * doubleValue + digit;
        intValue -= 1;
        position++;
        if (position == length) return beginChunkNumber(NUM_DOT_DIGIT, start);
        char = getChar(position);
        digit = char ^ CHAR_0;
      } while (digit <= 9);
    }
    if ((char | 0x20) == CHAR_e) {
      if (!isDouble) {
        isDouble = true;
        doubleValue = (intValue == 0) ? 0.0 : -intValue.toDouble();
        intValue = 0;
      }
      position++;
      if (position == length) return beginChunkNumber(NUM_E, start);
      char = getChar(position);
      int expSign = 1;
      int exponent = 0;
      if (((char + 1) | 2) == 0x2e /*+ or -*/ ) {
        expSign = 0x2C - char; // -1 for MINUS, +1 for PLUS
        position++;
        if (position == length) return beginChunkNumber(NUM_E_SIGN, start);
        char = getChar(position);
      }
      digit = char ^ CHAR_0;
      if (digit > 9) {
        fail(position, "Missing expected digit");
      }
      bool exponentOverflow = false;
      do {
        exponent = 10 * exponent + digit;
        if (exponent > 400) exponentOverflow = true;
        position++;
        if (position == length) return beginChunkNumber(NUM_E_DIGIT, start);
        char = getChar(position);
        digit = char ^ CHAR_0;
      } while (digit <= 9);
      if (exponentOverflow) {
        if (doubleValue == 0.0 || expSign < 0) {
          listener.handleNumber(sign < 0 ? -0.0 : 0.0);
        } else {
          listener.handleNumber(
            sign < 0 ? double.negativeInfinity : double.infinity,
          );
        }
        return position;
      }
      intValue += expSign * exponent;
    }
    if (!isDouble) {
      int bitFlag = -(sign + 1) >> 1; // 0 if sign == -1, -1 if sign == 1
      // Negate if bitFlag is -1 by doing ~intValue + 1
      listener.handleIntegerNumber((intValue ^ bitFlag) - bitFlag);
      return position;
    }
    // Double values at or above this value (2 ** 53) may have lost precision.
    // Only trust results that are below this value.
    const double maxExactDouble = 9007199254740992.0;
    if (doubleValue < maxExactDouble) {
      int exponent = intValue;
      double signedMantissa = doubleValue * sign;
      if (exponent >= -22) {
        if (exponent < 0) {
          listener.handleNumber(signedMantissa / POWERS_OF_TEN[-exponent]);
          return position;
        }
        if (exponent == 0) {
          listener.handleNumber(signedMantissa);
          return position;
        }
        if (exponent <= 22) {
          listener.handleNumber(signedMantissa * POWERS_OF_TEN[exponent]);
          return position;
        }
      }
    }
    // If the value is outside the range +/-maxExactDouble or
    // exponent is outside the range +/-22, then we can't trust simple double
    // arithmetic to get the exact result, so we use the system double parsing.
    listener.handleNumber(parseDouble(start, position));
    return position;
  }

  Never fail(int position, [String? message]) {
    if (message == null) {
      message = "Unexpected character";
      if (position == chunkEnd) message = "Unexpected end of input";
    }
    throw FormatException(message, chunk, position);
  }
}

/**
 * Chunked JSON parser that parses [JSStringImpl] chunks.
 */
class _JSStringImplParser extends _ChunkedJsonParserState
    with _ChunkedJsonParser<JSStringImpl> {
  @pragma('wasm:initialize-at-startup')
  static WasmArray<WasmI16> _emptyChunk = WasmArray<WasmI16>(0);

  JSStringImpl _chunkAsString = unsafeCast<JSStringImpl>('');
  WasmArray<WasmI16> _chunkAsArray = _emptyChunk;
  int chunkEnd = 0;

  _JSStringImplParser(_JsonListener listener) : super(listener);

  void setNewChunk(JSStringImpl string, int end) {
    final externRef = string.wrappedExternRef;
    final array = WasmArray<WasmI16>(end);
    if (string.length == end) {
      jsStringIntoCharCodeArray(externRef, array, 0.toWasmI32());
    } else {
      for (int i = 0; i < end; ++i) array.write(i, jsCharCodeAt(externRef, i));
    }
    _chunkAsString = string;
    _chunkAsArray = array;
    chunkEnd = end;
  }

  @override
  JSStringImpl get chunk => _chunkAsString;

  @pragma('wasm:prefer-inline')
  bool get isUtf16Input => true;

  @pragma('wasm:prefer-inline')
  int getChar(int position) => _chunkAsArray.readUnsigned(position);

  @pragma('wasm:prefer-inline')
  String getString(
    int start,
    int end,
    int bits,
    WasmArray<WasmI16> codeUnitsCache,
  ) {
    // NOTE: We don't use [codeUnitsCache]. TFA's signature tree shaker should
    // remove its usage and the caller shouldn't even populate anything (due
    // to using [_emptyCodeUnitsCache]).
    return _chunkAsString.substringUnchecked(start, end);
  }

  String getJsonObjectKeyString(int start, int end, int bits, int stringHash) {
    return _internJSString(
      _chunkAsString,
      _chunkAsArray,
      stringHash,
      start,
      end,
    );
  }

  void beginString() {
    assert(stringBuffer.isEmpty);
  }

  void addSliceToString(
    int start,
    int end,
    int bits,
    WasmArray<WasmI16> codeUnitsCache,
  ) {
    addStringSliceToString(_chunkAsString.substringUnchecked(start, end));
  }

  void addStringSliceToString(String string) {
    stringBuffer.write(string);
  }

  void addCharToString(int charCode) {
    stringBuffer.writeCharCode(charCode);
  }

  String endString() {
    final string = stringBuffer.toString();
    stringBuffer.clear();
    return string;
  }

  void copyCharsToList(
    int start,
    int end,
    WasmArray<WasmI8> target,
    int offset,
  ) {
    int length = end - start;
    for (int i = 0; i < length; i++) {
      target.write(offset + i, _chunkAsArray.readUnsigned(start + i));
    }
  }

  double parseDouble(int start, int end) {
    return _jsParseValidFloat(
      getString(start, end, 0x7f, _emptyCodeUnitsCache),
    );
  }

  void close() {
    super.close();
    _jsStringInternCache.fill(0, null, _jsStringInternCacheSize);
  }
}

const _emptyCodeUnitsCacheSize = 0;
@pragma('wasm:initialize-at-startup')
final _emptyCodeUnitsCache = WasmArray<WasmI16>(_emptyCodeUnitsCacheSize);

const int _codeUnitsCacheSize = 512;
@pragma("wasm:initialize-at-startup")
final _codeUnitsCache = WasmArray<WasmI16>(_codeUnitsCacheSize);

const int _jsStringInternCacheSize = 512;
@pragma("wasm:initialize-at-startup")
final WasmArray<JSStringImpl?> _jsStringInternCache = WasmArray<JSStringImpl?>(
  _jsStringInternCacheSize,
);
JSStringImpl _internJSString(
  JSStringImpl source,
  WasmArray<WasmI16> sourceAsArray,
  int stringHash,
  int start,
  int end,
) {
  final length = end - start;
  final int index = stringHash & (_jsStringInternCacheSize - 1);
  final existing = _jsStringInternCache[index];

  insert:
  {
    if (existing != null) {
      if (existing.length == length) {
        if (getIdentityHashField(existing).toWasmI32() ==
            stringHash.toWasmI32()) {
          for (int j = start, i = 0; i < length; ++i, ++j) {
            if (existing.codeUnitAtUnchecked(i) !=
                sourceAsArray.readUnsigned(j)) {
              break insert;
            }
          }
          return existing;
        }
      }
    }
  }

  final result = unsafeCast<JSStringImpl>(
    source.substringUnchecked(start, end),
  );
  setIdentityHashField(result, stringHash);
  _jsStringInternCache[index] = result;
  return result;
}

JSStringImpl _internJSStringFromAsciiSlice(
  U8List source,
  int stringHash,
  int start,
  int end,
) {
  final length = end - start;
  final int index = stringHash & (_jsStringInternCacheSize - 1);
  final existing = _jsStringInternCache[index];

  insert:
  {
    if (existing != null) {
      if (getIdentityHashField(existing).toWasmI32() ==
          stringHash.toWasmI32()) {
        if (existing.length == length) {
          for (int j = start, i = 0; i < length; ++i, ++j) {
            if (existing.codeUnitAtUnchecked(i) != source[j]) {
              break insert;
            }
          }
          return existing;
        }
      }
    }
  }

  final result = JSStringImpl.fromAsciiBytes(
    source.data,
    source.offsetInElements + start,
    source.offsetInElements + end,
  );
  setIdentityHashField(result, stringHash);
  _jsStringInternCache[index] = result;
  return result;
}

@patch
class JsonDecoder {
  @patch
  StringConversionSink startChunkedConversion(Sink<Object?> sink) =>
      _JsonStringDecoderSink(this._reviver, sink);
}

/**
 * Implements the chunked conversion from a JSON string to its corresponding
 * object.
 *
 * The sink only creates one object, but its input can be chunked.
 */
class _JsonStringDecoderSink extends StringConversionSinkBase {
  final _JsonListener _listener;

  late final _JSStringImplParser _stringParser;

  final Object? Function(Object? key, Object? value)? _reviver;

  final Sink<Object?> _sink;

  _JsonStringDecoderSink(this._reviver, this._sink)
    : _listener = _JsonListener(_reviver) {
    _stringParser = _JSStringImplParser(_listener);
  }

  void addSlice(String chunk, int start, int end, bool isLast) {
    final parser = _stringParser;
    parser.setNewChunk(unsafeCast<JSStringImpl>(chunk), end);
    parser.parse(start);
    if (isLast) parser.close();
  }

  void add(String chunk) {
    addSlice(chunk, 0, chunk.length, false);
  }

  void close() {
    _stringParser.close();
    _sink.add(_listener.result);
    _sink.close();
  }

  ByteConversionSink asUtf8Sink(bool allowMalformed) =>
      _JsonUtf8DecoderSink(_reviver, _sink, allowMalformed);
}

/**
 * Chunked JSON parser that parses UTF-8 chunks.
 */
class _JsonUtf8Parser extends _ChunkedJsonParserState
    with _ChunkedJsonParser<U8List> {
  static final U8List emptyChunk = U8List(0);

  final _Utf8Decoder decoder;
  U8List chunk = emptyChunk;
  int chunkEnd = 0;

  _JsonUtf8Parser(_JsonListener listener, bool allowMalformed)
    : decoder = _Utf8Decoder(allowMalformed),
      super(listener) {
    // Starts out checking for an optional BOM (KWD_BOM, count = 0).
    partialState =
        _ChunkedJsonParser.PARTIAL_KEYWORD | _ChunkedJsonParser.KWD_BOM;
  }

  void parseChunk(List<int> value, int start, int end) {
    if (value is U8List) {
      chunk = value;
    } else {
      final bytes = U8List(end - start);
      bytes.setRange(0, bytes.length, value, start);
      end = bytes.length;
      start = 0;
      chunk = bytes;
    }
    chunkEnd = end;
    parse(start);
  }

  @pragma('wasm:prefer-inline')
  bool get isUtf16Input => false;

  @pragma('wasm:prefer-inline')
  int getChar(int position) => chunk.getUnchecked(position);

  String getString(
    int start,
    int end,
    int bits,
    WasmArray<WasmI16> codeUnitsCache,
  ) {
    final int length = end - start;
    if (length == 0) return '';

    if (bits <= 0x7f) {
      return _stringFromBytesOrCache(start, end, codeUnitsCache);
    }
    beginString();
    addStringSliceToString(decoder.convertChunked(chunk, start, end));
    return endString();
  }

  String getJsonObjectKeyString(int start, int end, int bits, int stringHash) {
    // We can only rely on [stringHash] if [bits] tell us it was ASCII.
    if (bits <= 0x7f) {
      return _internJSStringFromAsciiSlice(chunk, stringHash, start, end);
    }
    beginString();
    addStringSliceToString(decoder.convertChunked(chunk, start, end));
    return endString();
  }

  void beginString() {
    assert(stringBuffer.isEmpty);
    decoder.reset();
  }

  void addSliceToString(
    int start,
    int end,
    int bits,
    WasmArray<WasmI16> codeUnitsCache,
  ) {
    if (!decoder.expectsContinuation && bits <= 0x7f) {
      addStringSliceToString(
        _stringFromBytesOrCache(start, end, codeUnitsCache),
      );
      return;
    }
    addStringSliceToString(decoder.convertChunked(chunk, start, end));
  }

  JSStringImpl _stringFromBytesOrCache(
    int start,
    int end,
    WasmArray<WasmI16> codeUnitsCache,
  ) {
    final length = end - start;
    if (length <= codeUnitsCache.length) {
      assert(
        _verifyCacheIsValid(
          codeUnitsCache,
          chunk.data,
          chunk.offsetInElements + start,
          chunk.offsetInElements + end,
        ),
      );
      return JSStringImpl.fromCharCodeArray(codeUnitsCache, 0, length);
    }
    return JSStringImpl.fromAsciiBytes(
      chunk.data,
      chunk.offsetInElements + start,
      chunk.offsetInElements + end,
    );
  }

  void addStringSliceToString(String string) {
    stringBuffer.write(string);
  }

  void addCharToString(int charCode) {
    decoder.flush(stringBuffer);
    stringBuffer.writeCharCode(charCode);
  }

  String endString() {
    decoder.flush(stringBuffer);
    final string = stringBuffer.toString();
    stringBuffer.clear();
    return string;
  }

  void copyCharsToList(
    int start,
    int end,
    WasmArray<WasmI8> target,
    int offset,
  ) {
    int length = end - start;
    target.copy(offset, chunk.data, start, length);
  }

  double parseDouble(int start, int end) {
    final result = JSStringImpl.fromAsciiBytes(
      chunk.data,
      chunk.offsetInElements + start,
      chunk.offsetInElements + end,
    );
    return _jsParseValidFloat(result);
  }
}

bool _verifyCacheIsValid(
  WasmArray<WasmI16> codeUnitsCache,
  WasmArray<WasmI8> source,
  int start,
  int end,
) {
  final length = end - start;
  assert(length <= codeUnitsCache.length);
  for (int i = 0; i < length && i < codeUnitsCache.length; ++i) {
    if (source.readUnsigned(start + i) != codeUnitsCache.readUnsigned(i)) {
      return false;
    }
  }
  return true;
}

/**
 * Implements the chunked conversion from a UTF-8 encoding of JSON
 * to its corresponding object.
 */
class _JsonUtf8DecoderSink extends ByteConversionSink {
  final _JsonUtf8Parser _parser;
  final Sink<Object?> _sink;

  _JsonUtf8DecoderSink(reviver, this._sink, bool allowMalformed)
    : _parser = _createParser(reviver, allowMalformed);

  static _JsonUtf8Parser _createParser(
    Object? Function(Object? key, Object? value)? reviver,
    bool allowMalformed,
  ) => _JsonUtf8Parser(_JsonListener(reviver), allowMalformed);

  void addSlice(List<int> chunk, int start, int end, bool isLast) {
    _addChunk(chunk, start, end);
    if (isLast) close();
  }

  void add(List<int> chunk) {
    _addChunk(chunk, 0, chunk.length);
  }

  void _addChunk(List<int> chunk, int start, int end) {
    _parser.parseChunk(chunk, start, end);
  }

  void close() {
    _parser.close();
    var decoded = _parser.result;
    _sink.add(decoded);
    _sink.close();
  }
}

@patch
class _Utf8Decoder {
  /// Flags indicating presence of the various kinds of bytes in the input.
  int _scanFlags = 0;

  /// How many bytes of the BOM have been read so far. Set to -1 when the BOM
  /// has been skipped (or was not present).
  int _bomIndex = 0;

  // Table for the scanning phase, which quickly scans through the input.
  //
  // Each input byte is looked up in the table, providing a size and some flags.
  // The sizes are summed, and the flags are or'ed together.
  //
  // The resulting size and flags indicate:
  // A) How many UTF-16 code units will be emitted by the decoding of this
  //    input. This can be used to allocate a string of the correct length up
  //    front.
  // B) Which decoder and resulting string representation is appropriate. There
  //    are three cases:
  //    1) Pure ASCII (flags == 0): The input can simply be put into a
  //       OneByteString without further decoding.
  //    2) Latin1 (flags == (flagLatin1 | flagExtension)): The result can be
  //       represented by a OneByteString, and the decoder can assume that only
  //       Latin1 characters are present.
  //    3) Arbitrary input (otherwise): Needs a full-featured decoder. Output
  //       can be represented by a TwoByteString.

  static const int sizeMask = 0x03;
  static const int flagsMask = 0x3C;

  static const int flagExtension = 1 << 2;
  static const int flagLatin1 = 1 << 3;
  static const int flagNonLatin1 = 1 << 4;
  static const int flagIllegal = 1 << 5;

  // ASCII     'A' = 64 + (1);
  // Extension 'D' = 64 + (0 | flagExtension);
  // Latin1    'I' = 64 + (1 | flagLatin1);
  // BMP       'Q' = 64 + (1 | flagNonLatin1);
  // Non-BMP   'R' = 64 + (2 | flagNonLatin1);
  // Illegal   'a' = 64 + (1 | flagIllegal);
  // Illegal   'b' = 64 + (2 | flagIllegal);
  static const scanTable = ImmutableWasmArray<WasmI8>.literal([
    // 00-1F
    65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65,
    65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65,

    // 20-3F
    65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65,
    65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65,

    // 40-5F
    65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65,
    65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65,

    // 60-7F
    65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65,
    65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65,

    // 80-9F
    68, 68, 68, 68, 68, 68, 68, 68, 68, 68, 68, 68, 68, 68, 68, 68, 68, 68, 68,
    68, 68, 68, 68, 68, 68, 68, 68, 68, 68, 68, 68, 68,

    // A0-BF
    68, 68, 68, 68, 68, 68, 68, 68, 68, 68, 68, 68, 68, 68, 68, 68, 68, 68, 68,
    68, 68, 68, 68, 68, 68, 68, 68, 68, 68, 68, 68, 68,

    // C0-DF
    97, 97, 73, 73, 81, 81, 81, 81, 81, 81, 81, 81, 81, 81, 81, 81, 81, 81, 81,
    81, 81, 81, 81, 81, 81, 81, 81, 81, 81, 81, 81, 81,

    // E0-FF
    81, 81, 81, 81, 81, 81, 81, 81, 81, 81, 81, 81, 81, 81, 81, 81, 82, 82, 82,
    82, 82, 98, 98, 98, 98, 98, 98, 98, 98, 98, 98, 98,
  ]);

  /// Reset the decoder to a state where it is ready to decode a new string but
  /// will not skip a leading BOM. Used by the fused UTF-8 / JSON decoder.
  void reset() {
    _state = initial;
    _bomIndex = -1;
  }

  int scan(WasmArray<WasmI8> bytes, int start, int end) {
    int size = 0;
    int flags = 0;
    int asciiCharacterIndex = 0;
    for (int i = start; i < end; ++i) {
      final byte = bytes.readUnsigned(i);
      final int t = scanTable.readUnsigned(byte);
      size += t & sizeMask;
      flags |= t;
      _characterArray.write(asciiCharacterIndex++, byte);
    }
    _scanFlags |= flags & flagsMask;
    return size;
  }

  // The VM decoder handles BOM explicitly instead of via the state machine.
  @patch
  _Utf8Decoder(this.allowMalformed) : _state = initial;

  @patch
  String convertSingle(List<int> codeUnits, int start, int? maybeEnd) {
    int end = RangeErrorUtils.checkValidRange(
      start,
      maybeEnd,
      codeUnits.length,
    );
    if (start == end) return "";

    if (codeUnits is JSUint8ArrayImpl) {
      JSStringImpl? decoded = decodeUtf8JS(
        codeUnits,
        start,
        end,
        allowMalformed,
      );
      if (decoded != null) return decoded;
    }

    final WasmArray<WasmI8> bytes;
    int errorOffset = 0;
    if (codeUnits is U8List) {
      final u8list = unsafeCast<U8List>(codeUnits);
      bytes = u8list.data;
      final offsetInBytes = u8list.offsetInBytes;
      errorOffset = -start - offsetInBytes;
      start += offsetInBytes;
      end += offsetInBytes;
    } else {
      // If we're passed a `List<int>` other than `U8List` or a JS typed array,
      // it means the performance is not too important. Convert the input to
      // `U8List` to avoid shipping another UTF-8 decoder for `List<int>`.
      final length = end - start;
      bytes = WasmArray(length);
      int bytesIdx = 0;
      for (int codeUnitsIdx = start; codeUnitsIdx < end; codeUnitsIdx += 1) {
        int byte = codeUnits[codeUnitsIdx];
        // byte < 0 || byte > 255
        if (byte.gtU(255)) {
          if (allowMalformed) {
            byte = 0xFF;
          } else {
            throw FormatException(
              'Invalid UTF-8 byte',
              codeUnits,
              codeUnitsIdx,
            );
          }
        }
        bytes.write(bytesIdx++, byte);
      }
      start = 0;
      end = length;
      errorOffset = 0;
    }

    // Skip initial BOM.
    start += skipBomSingle(bytes, start, end);

    final int length = end - start;
    if (length < _characterArraySize) {
      return _convertChunkInternal(
        bytes,
        start,
        end,
        codeUnits,
        errorOffset,
        false,
      );
    }

    String result = '';
    const maxBytes = _maxChunkSizeInBytes;
    final int chunks = length ~/ maxBytes;
    final int remaining = length % maxBytes;
    for (int i = 0; i < chunks; ++i) {
      result += _convertChunkInternal(
        bytes,
        start,
        start + maxBytes,
        codeUnits,
        errorOffset,
        true,
      );
      start += maxBytes;
    }
    if (remaining > 0) {
      assert(remaining < maxBytes);
      result += _convertChunkInternal(
        bytes,
        start,
        end,
        codeUnits,
        errorOffset,
        true,
      );
      start += remaining;
    }
    assert(start == end);
    return result;
  }

  @patch
  String convertChunked(List<int> codeUnits, int start, int? maybeEnd) {
    int end = RangeErrorUtils.checkValidRange(
      start,
      maybeEnd,
      codeUnits.length,
    );
    if (start == end) return "";

    final WasmArray<WasmI8> bytes;
    int errorOffset = 0;
    if (codeUnits is U8List) {
      final u8list = unsafeCast<U8List>(codeUnits);
      bytes = u8list.data;
      final offsetInBytes = u8list.offsetInBytes;
      errorOffset = -start - offsetInBytes;
      start += offsetInBytes;
      end += offsetInBytes;
    } else {
      bytes = _makeI8Array(codeUnits, start, end);
      errorOffset = -start;
      end -= start;
      start = 0;
    }

    // Skip initial BOM.
    start += skipBomChunked(bytes, start, end);

    final int length = end - start;
    if (length < _characterArraySize) {
      return _convertChunkInternal(
        bytes,
        start,
        end,
        codeUnits,
        errorOffset,
        true,
      );
    }

    String result = '';
    const maxBytes = _maxChunkSizeInBytes;
    final int chunks = length ~/ maxBytes;
    final int remaining = length % maxBytes;
    for (int i = 0; i < chunks; ++i) {
      result += _convertChunkInternal(
        bytes,
        start,
        start + maxBytes,
        codeUnits,
        errorOffset,
        true,
      );
      start += maxBytes;
    }
    if (remaining > 0) {
      assert(remaining < maxBytes);
      result += _convertChunkInternal(
        bytes,
        start,
        end,
        codeUnits,
        errorOffset,
        true,
      );
      start += remaining;
    }
    assert(start == end);
    return result;
  }

  @pragma('wasm:initialize-at-startup')
  static final _characterArray = WasmArray<WasmI16>(_characterArraySize);
  static const _characterArraySize = 1025;

  // We use the pre-allocated [_characterArray] to avoid temporary allocations
  // of `WasmArray<WasmI16>` objects when calling
  // `wasm:js-string.fromCharCodeArray` builtin.
  //
  // This constant defines the maximum size of a chunk of utf-8 encoded
  // bytes that will - when decoded to UTF-16 code units - fit into the
  // [_characterArray].
  //
  // The reason we subtract one from the [_characterArraySize] is the following:
  // When processing a chunk of N bytes we may have a carryover state from the
  // previous chunk that contains an uninished unicode point that requires
  // encoding it as a surrogate pair (i.e. 2 UTF-16 code units).
  //
  // So when e.g. processing 1024 bytes, the first byte may finish a unicode
  // point that requires 2 UTF-16 code units, the remaining 1023 bytes may be
  // ascii. So the 1024 byte chunk may turn into 1025 UTF-16 code units.
  //
  // => The [_characterArraySize] must be one larger than
  // [_maxChunkSizeInBytes].
  static const _maxChunkSizeInBytes = _characterArraySize - 1;

  @pragma('wasm:prefer-inline')
  String _convertChunkInternal(
    WasmArray<WasmI8> bytes,
    int start,
    int end,
    List<int> bytesForError,
    int errorOffset,
    bool isChunk,
  ) {
    assert(0 <= start && start <= end);
    assert(0 <= end && end <= bytes.length);

    if (start == end) return "";

    // Scan input to determine size and appropriate decoder.
    int size = scan(bytes, start, end);
    int flags = _scanFlags;

    final int carryOverState = _state;
    final int carryOverChar = _charOrIndex;
    if (isChunk) {
      // Adjust scan flags and size based on carry-over state.
      switch (_state) {
        case IA:
          break;
        case X1:
          flags |= _charOrIndex < (0x100 >> 6) ? flagLatin1 : flagNonLatin1;
          if (end - start >= 1) {
            size += _charOrIndex < (0x10000 >> 6) ? 1 : 2;
          }
          break;
        case X2:
          flags |= flagNonLatin1;
          if (end - start >= 2) {
            size += _charOrIndex < (0x10000 >> 12) ? 1 : 2;
          }
          break;
        case TO:
        case TS:
          flags |= flagNonLatin1;
          if (end - start >= 2) size += 1;
          break;
        case X3:
        case QO:
        case QR:
          flags |= flagNonLatin1;
          if (end - start >= 3) size += 2;
          break;
      }
    }

    if (flags == 0) {
      // Pure ASCII.
      assert(_state == accept);
      assert(size == end - start);
      return decodeAscii(bytes, start, end, size);
    }

    // Do not include any final, incomplete character in size.
    int extensionCount = 0;
    int i = end - 1;
    while (i >= start && (bytes.readUnsigned(i) & 0xC0) == 0x80) {
      extensionCount++;
      i--;
    }
    if (i >= start &&
        bytes.readUnsigned(i) >= ((~0x3F >> extensionCount) & 0xFF)) {
      size -= bytes.readUnsigned(i) >= 0xF0 ? 2 : 1;
    }

    String result;
    if (flags == (flagLatin1 | flagExtension)) {
      // Latin1.
      result = decode8(bytes, start, end, size);
    } else {
      // Arbitrary Unicode.
      result = decode16(bytes, start, end, size);
    }
    if (isChunk) {
      if (!isErrorState(_state)) {
        return result;
      }
      assert(_bomIndex == -1);

      if (!allowMalformed) {
        final String message = errorDescription(_state);
        _state = initial; // Ready for more input.
        throw FormatException(
          message,
          bytesForError,
          errorOffset + _charOrIndex,
        );
      }
    } else {
      if (_state == accept) {
        return result;
      }

      if (!allowMalformed) {
        if (!isErrorState(_state)) {
          // Unfinished sequence.
          _state = errorUnfinished;
          _charOrIndex = end;
        }
        final String message = errorDescription(_state);
        throw FormatException(
          message,
          bytesForError,
          errorOffset + _charOrIndex,
        );
      }
    }

    // Start over on slow path.
    _state = carryOverState;
    _charOrIndex = carryOverChar;
    result = _decodeGeneral(bytes, start, end, !isChunk);
    assert(!isErrorState(_state));
    return result;
  }

  @pragma("wasm:prefer-inline")
  int skipBomSingle(WasmArray<WasmI8> bytes, int start, int end) {
    if (end - start >= 3 &&
        bytes.readUnsigned(start) == 0xEF &&
        bytes.readUnsigned(start + 1) == 0xBB &&
        bytes.readUnsigned(start + 2) == 0xBF) {
      return 3;
    }
    return 0;
  }

  @pragma("wasm:prefer-inline")
  int skipBomChunked(WasmArray<WasmI8> bytes, int start, int end) {
    assert(start <= end);
    int bomIndex = _bomIndex;
    // Already skipped?
    if (bomIndex == -1) return 0;

    const bomValues = ImmutableWasmArray<WasmI8>.literal([0xEF, 0xBB, 0xBF]);
    int i = 0;
    while (bomIndex < 3) {
      if (start + i == end) {
        // Unfinished BOM.
        _bomIndex = bomIndex;
        return 0;
      }
      if (bytes.readUnsigned(start + i++) !=
          bomValues.readUnsigned(bomIndex++)) {
        // No BOM.
        _bomIndex = -1;
        return 0;
      }
    }
    // Complete BOM.
    _bomIndex = -1;
    _state = initial;
    return i;
  }

  String decodeAscii(WasmArray<WasmI8> bytes, int start, int end, int size) {
    assert(size == (end - start));
    assert(start < end);
    assert(size <= _characterArraySize);

    // In the ascii case the wasm array should be up-to-date already (populated
    // during [scan]).
    assert(
      (() {
        for (int i = 0; i < size; ++i) {
          if (_characterArray.readUnsigned(i) !=
              bytes.readUnsigned(start + i)) {
            return false;
          }
        }
        return true;
      })(),
    );
    return JSStringImpl.fromCharCodeArray(_characterArray, 0, size);
  }

  String decode8(WasmArray<WasmI8> bytes, int start, int end, int size) {
    assert(start < end);
    assert(size <= _characterArraySize);

    int i = start;
    int j = 0;
    if (_state == X1) {
      // Half-way though 2-byte sequence
      assert(_charOrIndex == 2 || _charOrIndex == 3);
      final int e = bytes.readUnsigned(i++) ^ 0x80;
      if (e >= 0x40) {
        _state = errorMissingExtension;
        _charOrIndex = i - 1;
        return "";
      }
      _characterArray.write(j++, ((_charOrIndex << 6) | e) & 0xFF);
      _state = accept;
    }
    assert(_state == accept);
    while (i < end) {
      int byte = bytes.readUnsigned(i++);
      if (byte >= 0x80) {
        if (byte < 0xC0) {
          _state = errorUnexpectedExtension;
          _charOrIndex = i - 1;
          return "";
        }
        assert(byte == 0xC2 || byte == 0xC3);
        if (i == end) {
          _state = X1;
          _charOrIndex = byte & 0x1F;
          break;
        }
        final int e = bytes.readUnsigned(i++) ^ 0x80;
        if (e >= 0x40) {
          _state = errorMissingExtension;
          _charOrIndex = i - 1;
          return "";
        }
        byte = (byte << 6) | e;
      }
      _characterArray.write(j++, byte & 0xFF);
    }
    assert(j == size);
    return JSStringImpl.fromCharCodeArray(_characterArray, 0, size);
  }

  // This table is the Wasm array version of `_Utf8Decoder.transitionTable`,
  // refer to the original type for documentation of the values.
  static const _transitionTable = ImmutableWasmArray<WasmI8>.literal([
    32, 0, 48, 58, 88, 69, 67, 67, 67, 67, 67, 78, 58, 108, 68, 98, 32, 0, //
    48, 58, 88, 69, 67, 67, 67, 67, 67, 78, 118, 108, 68, 98, 32, 0, 48, 58, //
    88, 69, 67, 67, 67, 67, 67, 78, 58, 108, 68, 98, 32, 65, 65, 65, 65, 65, //
    0, 0, 0, 0, 0, 65, 65, 65, 65, 65, 48, 48, 48, 48, 48, 65, 65, 65, 65, //
    65, 58, 58, 58, 58, 58, 65, 65, 65, 65, 65, 71, 71, 48, 48, 48, 65, 65, //
    65, 65, 65, 48, 48, 75, 75, 75, 65, 65, 65, 65, 65, 71, 58, 58, 58, 58, //
    65, 65, 65, 65, 65, 58, 73, 73, 73, 73, 65, 65, 65, 65, 65, 48, 48, 48, //
    128, 48, 65, 65, 65, 65, 65, 0, 0, 0, 0, 32, 65, 65, 65, 65, 65,
  ]);

  // This table is the Wasm array version of `_Utf8Decoder.typeTable`,
  // refer to the original type for documentation of the values.
  static const _typeTable = ImmutableWasmArray<WasmI8>.literal([
    65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, //
    65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, //
    65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, //
    65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, //
    65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, //
    65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, //
    65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, //
    65, 65, 70, 70, 70, 70, 70, 70, 70, 70, 70, 70, 70, 70, 70, 70, 70, 70, //
    71, 71, 71, 71, 71, 71, 71, 71, 71, 71, 71, 71, 71, 71, 71, 71, 72, 72, //
    72, 72, 72, 72, 72, 72, 72, 72, 72, 72, 72, 72, 72, 72, 72, 72, 72, 72, //
    72, 72, 72, 72, 72, 72, 72, 73, 72, 72, 72, 74, 69, 69, 66, 66, 66, 66, //
    66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, //
    66, 66, 66, 66, 66, 66, 66, 66, 75, 67, 67, 67, 67, 67, 67, 67, 67, 67, //
    67, 67, 67, 68, 67, 76, 79, 78, 78, 78, 77, 69, 69, 69, 69, 69, 69, 69, //
    69, 69, 69, 69,
  ]);

  String decode16(WasmArray<WasmI8> bytes, int start, int end, int size) {
    assert(start < end);
    assert(size <= _characterArraySize);

    int i = start;
    int j = 0;
    int state = _state;
    int char;

    // First byte
    assert(!isErrorState(state));
    final int byte = bytes.readUnsigned(i++);
    final int type = _typeTable.readUnsigned(byte) & typeMask;
    if (state == accept) {
      char = byte & (shiftedByteMask >> type);
      state = _transitionTable.readUnsigned(type);
    } else {
      char = (byte & 0x3F) | (_charOrIndex << 6);
      state = _transitionTable.readUnsigned(state + type);
    }

    while (i < end) {
      final int byte = bytes.readUnsigned(i++);
      final int type = _typeTable.readUnsigned(byte) & typeMask;
      if (state == accept) {
        if (char >= 0x10000) {
          assert(char < 0x110000);
          _characterArray.write(j++, 0xD7C0 + (char >> 10));
          _characterArray.write(j++, 0xDC00 + (char & 0x3FF));
        } else {
          _characterArray.write(j++, char);
        }
        char = byte & (shiftedByteMask >> type);
        state = _transitionTable.readUnsigned(type);
      } else if (isErrorState(state)) {
        _state = state;
        _charOrIndex = i - 2;
        return "";
      } else {
        char = (byte & 0x3F) | (char << 6);
        state = _transitionTable.readUnsigned(state + type);
      }
    }

    // Final write?
    if (state == accept) {
      if (char >= 0x10000) {
        assert(char < 0x110000);
        _characterArray.write(j++, 0xD7C0 + (char >> 10));
        _characterArray.write(j++, 0xDC00 + (char & 0x3FF));
      } else {
        _characterArray.write(j++, char);
      }
    } else if (isErrorState(state)) {
      _state = state;
      _charOrIndex = end - 1;
      return "";
    }

    _state = state;
    _charOrIndex = char;

    assert(j == size);
    return JSStringImpl.fromCharCodeArray(_characterArray, 0, size);
  }

  String _decodeGeneral(
    WasmArray<WasmI8> bytes,
    int start,
    int end,
    bool single,
  ) {
    int state = _state;
    int char = _charOrIndex;
    final StringBuffer buffer = StringBuffer();
    int i = start;
    int byte = bytes.readUnsigned(i++);
    loop:
    while (true) {
      multibyte:
      while (true) {
        int type = _typeTable.readUnsigned(byte) & typeMask;
        char = (state <= afterBom)
            ? byte & (shiftedByteMask >> type)
            : (byte & 0x3F) | (char << 6);
        state = _transitionTable.readUnsigned(state + type);
        if (state == accept) {
          buffer.writeCharCode(char);
          if (i == end) break loop;
          break multibyte;
        } else if (isErrorState(state)) {
          if (allowMalformed) {
            switch (state) {
              case errorInvalid:
              case errorUnexpectedExtension:
                // A single byte that can't start a sequence.
                buffer.writeCharCode(unicodeReplacementCharacterRune);
                break;
              case errorMissingExtension:
                // Unfinished sequence followed by a byte that can start a
                // sequence.
                buffer.writeCharCode(unicodeReplacementCharacterRune);
                // Re-parse offending byte.
                i -= 1;
                break;
              default:
                // Unfinished sequence followed by a byte that can't start a
                // sequence.
                buffer.writeCharCode(unicodeReplacementCharacterRune);
                buffer.writeCharCode(unicodeReplacementCharacterRune);
                break;
            }
            state = initial;
          } else {
            _state = state;
            _charOrIndex = i - 1;
            return "";
          }
        }
        if (i == end) break loop;
        byte = bytes.readUnsigned(i++);
      }

      final int markStart = i;
      byte = bytes.readUnsigned(i++);
      if (byte < 128) {
        int markEnd = end;
        while (i < end) {
          byte = bytes.readUnsigned(i++);
          if (byte >= 128) {
            markEnd = i - 1;
            break;
          }
        }
        assert(markStart < markEnd);
        for (int m = markStart; m < markEnd; m++) {
          buffer.writeCharCode(bytes.readUnsigned(m));
        }
        if (markEnd == end) break loop;
      }
    }

    if (single && state > afterBom) {
      // Unfinished sequence.
      if (allowMalformed) {
        buffer.writeCharCode(unicodeReplacementCharacterRune);
      } else {
        _state = errorUnfinished;
        _charOrIndex = end;
        return "";
      }
    }
    _state = state;
    _charOrIndex = char;
    return buffer.toString();
  }
}

WasmArray<WasmI8> _makeI8Array(List<int> codeUnits, int start, int end) {
  if (codeUnits is WasmListBase) {
    return _makeI8ArrayFromWasmListBase(
      unsafeCast<WasmListBase<int>>(codeUnits),
      start,
      end,
    );
  }

  if (codeUnits is WasmI8ArrayBase) {
    return _makeI8ArrayFromWasmI8ArrayBase(
      unsafeCast<WasmI8ArrayBase>(codeUnits),
      start,
      end,
    );
  }

  final int length = end - start;
  final WasmArray<WasmI8> bytes = WasmArray(length);
  for (int i = 0; i < length; i++) {
    int b = codeUnits[start + i];
    if ((b & ~0xFF) != 0) {
      // Replace invalid byte values by FF, which is also invalid.
      b = 0xFF;
    }
    bytes.write(i, b);
  }
  return bytes;
}

WasmArray<WasmI8> _makeI8ArrayFromWasmListBase(
  WasmListBase<int> codeUnits,
  int start,
  int end,
) {
  final int length = end - start;
  final WasmArray<WasmI8> bytes = WasmArray(length);
  final WasmArray<Object?> listData = codeUnits.data;
  for (int i = 0; i < length; i++) {
    int b = unsafeCast<int>(listData[start + i]);
    if ((b & ~0xFF) != 0) {
      // Replace invalid byte values by FF, which is also invalid.
      b = 0xFF;
    }
    bytes.write(i, b);
  }
  return bytes;
}

WasmArray<WasmI8> _makeI8ArrayFromWasmI8ArrayBase(
  WasmI8ArrayBase codeUnits,
  int start,
  int end,
) {
  final int length = end - start;
  final WasmArray<WasmI8> bytes = WasmArray(length);
  final WasmArray<WasmI8> listData = codeUnits.data;
  final listDataOffset = codeUnits.offsetInBytes;
  for (int i = 0; i < length; i++) {
    int b = listData.readSigned(listDataOffset + start + i);
    if ((b & ~0xFF) != 0) {
      // Replace invalid byte values by FF, which is also invalid.
      b = 0xFF;
    }
    bytes.write(i, b);
  }
  return bytes;
}

// Assumes the given [string] is a valid float, so it can rely on the implicit
// string to number conversion in JS using `+<string-of-number>`.
double _jsParseValidFloat(String string) =>
    JS<double>('(s) => +s', jsStringFromDartString(string).wrappedExternRef);

const ImmutableWasmArray<BoxedInt> _intBoxes256 = ImmutableWasmArray.literal([
  0, 1, 2, 3, 4, 5, 6, 7, //
  8, 9, 10, 11, 12, 13, 14, 15, //
  16, 17, 18, 19, 20, 21, 22, 23, //
  24, 25, 26, 27, 28, 29, 30, 31, //
  32, 33, 34, 35, 36, 37, 38, 39, //
  40, 41, 42, 43, 44, 45, 46, 47, //
  48, 49, 50, 51, 52, 53, 54, 55, //
  56, 57, 58, 59, 60, 61, 62, 63, //
  64, 65, 66, 67, 68, 69, 70, 71, //
  72, 73, 74, 75, 76, 77, 78, 79, //
  80, 81, 82, 83, 84, 85, 86, 87, //
  88, 89, 90, 91, 92, 93, 94, 95, //
  96, 97, 98, 99, 100, 101, 102, 103, //
  104, 105, 106, 107, 108, 109, 110, 111, //
  112, 113, 114, 115, 116, 117, 118, 119, //
  120, 121, 122, 123, 124, 125, 126, 127, //
  128, 129, 130, 131, 132, 133, 134, 135, //
  136, 137, 138, 139, 140, 141, 142, 143, //
  144, 145, 146, 147, 148, 149, 150, 151, //
  152, 153, 154, 155, 156, 157, 158, 159, //
  160, 161, 162, 163, 164, 165, 166, 167, //
  168, 169, 170, 171, 172, 173, 174, 175, //
  176, 177, 178, 179, 180, 181, 182, 183, //
  184, 185, 186, 187, 188, 189, 190, 191, //
  192, 193, 194, 195, 196, 197, 198, 199, //
  200, 201, 202, 203, 204, 205, 206, 207, //
  208, 209, 210, 211, 212, 213, 214, 215, //
  216, 217, 218, 219, 220, 221, 222, 223, //
  224, 225, 226, 227, 228, 229, 230, 231, //
  232, 233, 234, 235, 236, 237, 238, 239, //
  240, 241, 242, 243, 244, 245, 246, 247, //
  248, 249, 250, 251, 252, 253, 254, 255, //
]);
