parser.js

module.exports = function (program) {
  const tokens = [];

  const length = program.length;

  let line = 1;
  let character = 1;

  const locations = [];

  for (let i = 0, starts = 0, ends = 0; i < length; i++) {
    const c = program[i];

    if (c === '\n') {
      ++line;
      character = 1;

      continue;
    }

    if ([ '+', '-', '<', '>', ',', '.', '[', ']', ].indexOf(c) > -1) {
      const token = {
        character,
        line,
        type: c,
      };

      if (c === '[') {
        ++starts;

        locations.push({
          character,
          line,
          tokenId: tokens.length,
        });
      }

      if (c === ']') {
        ++ends;

        if (starts < ends) {
          throw new SyntaxError(
            `An unmatched ] command was found at line ${line}, character ${character}.
            You can't close a loop without opening one!`
          );
        }

        const { tokenId, } = locations.pop();

        token.start = tokenId;
        tokens[tokenId].end = tokens.length;
      }

      tokens.push(token);
    }

    ++character;
  }

  if (locations.length) {
    const { line, character, } = locations.shift();

    throw new SyntaxError(
      `An unmatched [ was found at line ${line}, character ${character}.
      Don't forget to close your loops!`
    );
  }

  return tokens;
};