lexer.js

// @flow strict

import { syntaxError } from '../error/syntaxError';

import type { Source } from './source';
import type { TokenKindEnum } from './tokenKind';
import { Token } from './ast';
import { TokenKind } from './tokenKind';
import { dedentBlockStringValue } from './blockString';

/**
 * Given a Source object, creates a Lexer for that source.
 * A Lexer is a stateful stream generator in that every time
 * it is advanced, it returns the next token in the Source. Assuming the
 * source lexes, the final Token emitted by the lexer will be of kind
 * EOF, after which the lexer will repeatedly return the same EOF token
 * whenever called.
 */
export class Lexer {
  source: Source;

  /**
   * The previously focused non-ignored token.
   */
  lastToken: Token;

  /**
   * The currently focused non-ignored token.
   */
  token: Token;

  /**
   * The (1-indexed) line containing the current token.
   */
  line: number;

  /**
   * The character offset at which the current line begins.
   */
  lineStart: number;

  constructor(source: Source) {
    const startOfFileToken = new Token(TokenKind.SOF, 0, 0, 0, 0, null);

    this.source = source;
    this.lastToken = startOfFileToken;
    this.token = startOfFileToken;
    this.line = 1;
    this.lineStart = 0;
  }

  /**
   * Advances the token stream to the next non-ignored token.
   */
  advance(): Token {
    this.lastToken = this.token;
    const token = (this.token = this.lookahead());
    return token;
  }

  /**
   * Looks ahead and returns the next non-ignored token, but does not change
   * the state of Lexer.
   */
  lookahead(): Token {
    let token = this.token;
    if (token.kind !== TokenKind.EOF) {
      do {
        // Note: next is only mutable during parsing, so we cast to allow this.
        token = token.next ?? ((token: any).next = readToken(this, token));
      } while (token.kind === TokenKind.COMMENT);
    }
    return token;
  }
}

/**
 * @internal
 */
export function isPunctuatorTokenKind(kind: TokenKindEnum): boolean %checks {
  return (
    kind === TokenKind.BANG ||
    kind === TokenKind.DOLLAR ||
    kind === TokenKind.AMP ||
    kind === TokenKind.PAREN_L ||
    kind === TokenKind.PAREN_R ||
    kind === TokenKind.SPREAD ||
    kind === TokenKind.COLON ||
    kind === TokenKind.EQUALS ||
    kind === TokenKind.AT ||
    kind === TokenKind.BRACKET_L ||
    kind === TokenKind.BRACKET_R ||
    kind === TokenKind.BRACE_L ||
    kind === TokenKind.PIPE ||
    kind === TokenKind.BRACE_R
  );
}

function printCharCode(code) {
  return (
    // NaN/undefined represents access beyond the end of the file.
    isNaN(code)
      ? TokenKind.EOF
      : // Trust JSON for ASCII.
      code < 0x007f
      ? JSON.stringify(String.fromCharCode(code))
      : // Otherwise print the escaped form.
        `"\\u${('00' + code.toString(16).toUpperCase()).slice(-4)}"`
  );
}

/**
 * Gets the next token from the source starting at the given position.
 *
 * This skips over whitespace until it finds the next lexable token, then lexes
 * punctuators immediately or calls the appropriate helper function for more
 * complicated tokens.
 */
function readToken(lexer: Lexer, prev: Token): Token {
  const source = lexer.source;
  const body = source.body;
  const bodyLength = body.length;

  const pos = positionAfterWhitespace(body, prev.end, lexer);
  const line = lexer.line;
  const col = 1 + pos - lexer.lineStart;

  if (pos >= bodyLength) {
    return new Token(TokenKind.EOF, bodyLength, bodyLength, line, col, prev);
  }

  const code = body.charCodeAt(pos);

  // SourceCharacter
  switch (code) {
    // !
    case 33:
      return new Token(TokenKind.BANG, pos, pos + 1, line, col, prev);
    // #
    case 35:
      return readComment(source, pos, line, col, prev);
    // $
    case 36:
      return new Token(TokenKind.DOLLAR, pos, pos + 1, line, col, prev);
    // &
    case 38:
      return new Token(TokenKind.AMP, pos, pos + 1, line, col, prev);
    // (
    case 40:
      return new Token(TokenKind.PAREN_L, pos, pos + 1, line, col, prev);
    // )
    case 41:
      return new Token(TokenKind.PAREN_R, pos, pos + 1, line, col, prev);
    // .
    case 46:
      if (body.charCodeAt(pos + 1) === 46 && body.charCodeAt(pos + 2) === 46) {
        return new Token(TokenKind.SPREAD, pos, pos + 3, line, col, prev);
      }
      break;
    // :
    case 58:
      return new Token(TokenKind.COLON, pos, pos + 1, line, col, prev);
    // =
    case 61:
      return new Token(TokenKind.EQUALS, pos, pos + 1, line, col, prev);
    // @
    case 64:
      return new Token(TokenKind.AT, pos, pos + 1, line, col, prev);
    // [
    case 91:
      return new Token(TokenKind.BRACKET_L, pos, pos + 1, line, col, prev);
    // ]
    case 93:
      return new Token(TokenKind.BRACKET_R, pos, pos + 1, line, col, prev);
    // {
    case 123:
      return new Token(TokenKind.BRACE_L, pos, pos + 1, line, col, prev);
    // |
    case 124:
      return new Token(TokenKind.PIPE, pos, pos + 1, line, col, prev);
    // }
    case 125:
      return new Token(TokenKind.BRACE_R, pos, pos + 1, line, col, prev);
    // A-Z _ a-z
    case 65:
    case 66:
    case 67:
    case 68:
    case 69:
    case 70:
    case 71:
    case 72:
    case 73:
    case 74:
    case 75:
    case 76:
    case 77:
    case 78:
    case 79:
    case 80:
    case 81:
    case 82:
    case 83:
    case 84:
    case 85:
    case 86:
    case 87:
    case 88:
    case 89:
    case 90:
    case 95:
    case 97:
    case 98:
    case 99:
    case 100:
    case 101:
    case 102:
    case 103:
    case 104:
    case 105:
    case 106:
    case 107:
    case 108:
    case 109:
    case 110:
    case 111:
    case 112:
    case 113:
    case 114:
    case 115:
    case 116:
    case 117:
    case 118:
    case 119:
    case 120:
    case 121:
    case 122:
      return readName(source, pos, line, col, prev);
    // - 0-9
    case 45:
    case 48:
    case 49:
    case 50:
    case 51:
    case 52:
    case 53:
    case 54:
    case 55:
    case 56:
    case 57:
      return readNumber(source, pos, code, line, col, prev);
    // "
    case 34:
      if (body.charCodeAt(pos + 1) === 34 && body.charCodeAt(pos + 2) === 34) {
        return readBlockString(source, pos, line, col, prev, lexer);
      }
      return readString(source, pos, line, col, prev);
  }

  throw syntaxError(source, pos, unexpectedCharacterMessage(code));
}

/**
 * Report a message that an unexpected character was encountered.
 */
function unexpectedCharacterMessage(code) {
  if (code < 0x0020 && code !== 0x0009 && code !== 0x000a && code !== 0x000d) {
    return `Cannot contain the invalid character ${printCharCode(code)}.`;
  }

  if (code === 39) {
    // '
    return 'Unexpected single quote character (\'), did you mean to use a double quote (")?';
  }

  return `Cannot parse the unexpected character ${printCharCode(code)}.`;
}

/**
 * Reads from body starting at startPosition until it finds a non-whitespace
 * character, then returns the position of that character for lexing.
 */
function positionAfterWhitespace(
  body: string,
  startPosition: number,
  lexer: Lexer,
): number {
  const bodyLength = body.length;
  let position = startPosition;
  while (position < bodyLength) {
    const code = body.charCodeAt(position);
    // tab | space | comma | BOM
    if (code === 9 || code === 32 || code === 44 || code === 0xfeff) {
      ++position;
    } else if (code === 10) {
      // new line
      ++position;
      ++lexer.line;
      lexer.lineStart = position;
    } else if (code === 13) {
      // carriage return
      if (body.charCodeAt(position + 1) === 10) {
        position += 2;
      } else {
        ++position;
      }
      ++lexer.line;
      lexer.lineStart = position;
    } else {
      break;
    }
  }
  return position;
}

/**
 * Reads a comment token from the source file.
 *
 * #[\u0009\u0020-\uFFFF]*
 */
function readComment(source, start, line, col, prev): Token {
  const body = source.body;
  let code;
  let position = start;

  do {
    code = body.charCodeAt(++position);
  } while (
    !isNaN(code) &&
    // SourceCharacter but not LineTerminator
    (code > 0x001f || code === 0x0009)
  );

  return new Token(
    TokenKind.COMMENT,
    start,
    position,
    line,
    col,
    prev,
    body.slice(start + 1, position),
  );
}

/**
 * Reads a number token from the source file, either a float
 * or an int depending on whether a decimal point appears.
 *
 * Int:   -?(0|[1-9][0-9]*)
 * Float: -?(0|[1-9][0-9]*)(\.[0-9]+)?((E|e)(+|-)?[0-9]+)?
 */
function readNumber(source, start, firstCode, line, col, prev): Token {
  const body = source.body;
  let code = firstCode;
  let position = start;
  let isFloat = false;

  if (code === 45) {
    // -
    code = body.charCodeAt(++position);
  }

  if (code === 48) {
    // 0
    code = body.charCodeAt(++position);
    if (code >= 48 && code <= 57) {
      throw syntaxError(
        source,
        position,
        `Invalid number, unexpected digit after 0: ${printCharCode(code)}.`,
      );
    }
  } else {
    position = readDigits(source, position, code);
    code = body.charCodeAt(position);
  }

  if (code === 46) {
    // .
    isFloat = true;

    code = body.charCodeAt(++position);
    position = readDigits(source, position, code);
    code = body.charCodeAt(position);
  }

  if (code === 69 || code === 101) {
    // E e
    isFloat = true;

    code = body.charCodeAt(++position);
    if (code === 43 || code === 45) {
      // + -
      code = body.charCodeAt(++position);
    }
    position = readDigits(source, position, code);
    code = body.charCodeAt(position);
  }

  // Numbers cannot be followed by . or NameStart
  if (code === 46 || isNameStart(code)) {
    throw syntaxError(
      source,
      position,
      `Invalid number, expected digit but got: ${printCharCode(code)}.`,
    );
  }

  return new Token(
    isFloat ? TokenKind.FLOAT : TokenKind.INT,
    start,
    position,
    line,
    col,
    prev,
    body.slice(start, position),
  );
}

/**
 * Returns the new position in the source after reading digits.
 */
function readDigits(source, start, firstCode) {
  const body = source.body;
  let position = start;
  let code = firstCode;
  if (code >= 48 && code <= 57) {
    // 0 - 9
    do {
      code = body.charCodeAt(++position);
    } while (code >= 48 && code <= 57); // 0 - 9
    return position;
  }
  throw syntaxError(
    source,
    position,
    `Invalid number, expected digit but got: ${printCharCode(code)}.`,
  );
}

/**
 * Reads a string token from the source file.
 *
 * "([^"\\\u000A\u000D]|(\\(u[0-9a-fA-F]{4}|["\\/bfnrt])))*"
 */
function readString(source, start, line, col, prev): Token {
  const body = source.body;
  let position = start + 1;
  let chunkStart = position;
  let code = 0;
  let value = '';

  while (
    position < body.length &&
    !isNaN((code = body.charCodeAt(position))) &&
    // not LineTerminator
    code !== 0x000a &&
    code !== 0x000d
  ) {
    // Closing Quote (")
    if (code === 34) {
      value += body.slice(chunkStart, position);
      return new Token(
        TokenKind.STRING,
        start,
        position + 1,
        line,
        col,
        prev,
        value,
      );
    }

    // SourceCharacter
    if (code < 0x0020 && code !== 0x0009) {
      throw syntaxError(
        source,
        position,
        `Invalid character within String: ${printCharCode(code)}.`,
      );
    }

    ++position;
    if (code === 92) {
      // \
      value += body.slice(chunkStart, position - 1);
      code = body.charCodeAt(position);
      switch (code) {
        case 34:
          value += '"';
          break;
        case 47:
          value += '/';
          break;
        case 92:
          value += '\\';
          break;
        case 98:
          value += '\b';
          break;
        case 102:
          value += '\f';
          break;
        case 110:
          value += '\n';
          break;
        case 114:
          value += '\r';
          break;
        case 116:
          value += '\t';
          break;
        case 117: {
          // uXXXX
          const charCode = uniCharCode(
            body.charCodeAt(position + 1),
            body.charCodeAt(position + 2),
            body.charCodeAt(position + 3),
            body.charCodeAt(position + 4),
          );
          if (charCode < 0) {
            const invalidSequence = body.slice(position + 1, position + 5);
            throw syntaxError(
              source,
              position,
              `Invalid character escape sequence: \\u${invalidSequence}.`,
            );
          }
          value += String.fromCharCode(charCode);
          position += 4;
          break;
        }
        default:
          throw syntaxError(
            source,
            position,
            `Invalid character escape sequence: \\${String.fromCharCode(
              code,
            )}.`,
          );
      }
      ++position;
      chunkStart = position;
    }
  }

  throw syntaxError(source, position, 'Unterminated string.');
}

/**
 * Reads a block string token from the source file.
 *
 * """("?"?(\\"""|\\(?!=""")|[^"\\]))*"""
 */
function readBlockString(source, start, line, col, prev, lexer): Token {
  const body = source.body;
  let position = start + 3;
  let chunkStart = position;
  let code = 0;
  let rawValue = '';

  while (position < body.length && !isNaN((code = body.charCodeAt(position)))) {
    // Closing Triple-Quote (""")
    if (
      code === 34 &&
      body.charCodeAt(position + 1) === 34 &&
      body.charCodeAt(position + 2) === 34
    ) {
      rawValue += body.slice(chunkStart, position);
      return new Token(
        TokenKind.BLOCK_STRING,
        start,
        position + 3,
        line,
        col,
        prev,
        dedentBlockStringValue(rawValue),
      );
    }

    // SourceCharacter
    if (
      code < 0x0020 &&
      code !== 0x0009 &&
      code !== 0x000a &&
      code !== 0x000d
    ) {
      throw syntaxError(
        source,
        position,
        `Invalid character within String: ${printCharCode(code)}.`,
      );
    }

    if (code === 10) {
      // new line
      ++position;
      ++lexer.line;
      lexer.lineStart = position;
    } else if (code === 13) {
      // carriage return
      if (body.charCodeAt(position + 1) === 10) {
        position += 2;
      } else {
        ++position;
      }
      ++lexer.line;
      lexer.lineStart = position;
    } else if (
      // Escape Triple-Quote (\""")
      code === 92 &&
      body.charCodeAt(position + 1) === 34 &&
      body.charCodeAt(position + 2) === 34 &&
      body.charCodeAt(position + 3) === 34
    ) {
      rawValue += body.slice(chunkStart, position) + '"""';
      position += 4;
      chunkStart = position;
    } else {
      ++position;
    }
  }

  throw syntaxError(source, position, 'Unterminated string.');
}

/**
 * Converts four hexadecimal chars to the integer that the
 * string represents. For example, uniCharCode('0','0','0','f')
 * will return 15, and uniCharCode('0','0','f','f') returns 255.
 *
 * Returns a negative number on error, if a char was invalid.
 *
 * This is implemented by noting that char2hex() returns -1 on error,
 * which means the result of ORing the char2hex() will also be negative.
 */
function uniCharCode(a, b, c, d) {
  return (
    (char2hex(a) << 12) | (char2hex(b) << 8) | (char2hex(c) << 4) | char2hex(d)
  );
}

/**
 * Converts a hex character to its integer value.
 * '0' becomes 0, '9' becomes 9
 * 'A' becomes 10, 'F' becomes 15
 * 'a' becomes 10, 'f' becomes 15
 *
 * Returns -1 on error.
 */
function char2hex(a) {
  return a >= 48 && a <= 57
    ? a - 48 // 0-9
    : a >= 65 && a <= 70
    ? a - 55 // A-F
    : a >= 97 && a <= 102
    ? a - 87 // a-f
    : -1;
}

/**
 * Reads an alphanumeric + underscore name from the source.
 *
 * [_A-Za-z][_0-9A-Za-z]*
 */
function readName(source, start, line, col, prev): Token {
  const body = source.body;
  const bodyLength = body.length;
  let position = start + 1;
  let code = 0;
  while (
    position !== bodyLength &&
    !isNaN((code = body.charCodeAt(position))) &&
    (code === 95 || // _
    (code >= 48 && code <= 57) || // 0-9
    (code >= 65 && code <= 90) || // A-Z
      (code >= 97 && code <= 122)) // a-z
  ) {
    ++position;
  }
  return new Token(
    TokenKind.NAME,
    start,
    position,
    line,
    col,
    prev,
    body.slice(start, position),
  );
}

// _ A-Z a-z
function isNameStart(code): boolean {
  return (
    code === 95 || (code >= 65 && code <= 90) || (code >= 97 && code <= 122)
  );
}