Dart SimpleJsonParser

main.dart

class SimpleJsonParser {
  int _index = 0;
  final String _json;

  SimpleJsonParser(this._json);

  dynamic parse() {
    final value = _parseValue();
    while (_index < _json.length && _json[_index].trim().isEmpty) {
      _index++;
    }
    if (_index < _json.length) {
      throw Exception('Unexpected character at index $_index');
    }
    return value;
  }

  dynamic _parseValue() {
    _skipWhitespace();
    if (_index >= _json.length) throw Exception('Unexpected end of input');
    final char = _json[_index];
    if (char == '{') return _parseObject();
    if (char == '[') return _parseArray();
    if (char == '"') return _parseString();
    if ('0123456789-'.contains(char)) return _parseNumber();
    throw Exception('Unexpected character at index $_index');
  }

  void _skipWhitespace() {
    while (_index < _json.length && _json[_index].trim().isEmpty) {
      _index++;
    }
  }

  Map<String, dynamic> _parseObject() {
    _index++; // {
    final map = <String, dynamic>{};
    while (_json[_index] != '}') {
      final key = _parseString();
      _skipWhitespace();
      if (_json[_index] != ':') throw Exception('Expected : at index $_index');
      _index++; // :
      final value = _parseValue();
      map[key] = value;
      _skipWhitespace();
      if (_json[_index] == ',') _index++;
      _skipWhitespace();
    }
    _index++; // }
    return map;
  }

  List<dynamic> _parseArray() {
    _index++; // [
    final list = <dynamic>[];
    while (_json[_index] != ']') {
      list.add(_parseValue());
      _skipWhitespace();
      if (_json[_index] == ',') _index++;
      _skipWhitespace();
    }
    _index++; // ]
    return list;
  }

  String _parseString() {
    _index++; // "
    final start = _index;
    while (_json[_index] != '"') {
      _index++;
    }
    final string = _json.substring(start, _index);
    _index++; // "
    return string;
  }

  num _parseNumber() {
    final start = _index;
    while (_index < _json.length && '0123456789.-eE'.contains(_json[_index])) {
      _index++;
    }
    final number = num.tryParse(_json.substring(start, _index));
    if (number == null) throw Exception('Invalid number at index $start');
    return number;
  }
}

void main() {
  final parser = SimpleJsonParser('{"key": "value", "key2": [1, 2, 3]}');
  print(parser.parse()); // {key: value, key2: [1, 2, 3]}
}