電卓パーサ

calculator.js

(function(expose){
expose.execute = function (source) {
  var tree = parse(source);
  return execTree(tree);
};

var execTree = function (tree) {
  if (typeof tree === 'number') {
    return tree;
  }

  if (tree instanceof Array && tree.length === 3) {
    var op = tree[0]
      , a = execTree(tree[1])
      , b = execTree(tree[2]);
    
    if (op === '+') {
      return a + b;
    }

    if (op === '-') {
      return a - b;
    }

    if (op === '*') {
      return a * b;
    }

    if (op === '/') {
      return a / b;
    }
  }

  throw new Error('bad tree');
};

var parse = expose.parse = function () {
  var text
    , ch
    , at
    , result;

  function tuple(op, a, b) {
    if (typeof op !== 'string') {
      throw new Error('bad tuple');
    }

    return [op, a, b];
  }

  function expression() {
    var op, a, b;

    white();
    a = term();
    white();

    if (ch === '+' || ch === '-') {
      op = ch;
      next();
      b = expression();
    }

    return op ? tuple(op, a, b) : a;
  }

  function term() {
    var op, a, b;

    a = factor();
    white();

    if (ch === '*' || ch === '/') {
      op = ch;
      next();
      white();
      b = term();
    }

    return op ? tuple(op, a, b) : a;
  }

  function factor() {
    if (ch === '(') {
      next();
      var value = expression();
      white();

      if (ch !== ')') throw new Error('expected ) but' + ch);

      next();
      return value;
    } else {
      return number();
    }
  }

  function number() {
    var match = (/[\+\-]?([1-9][0-9]+|[0-9])/).exec(text.slice(at-1))
      , str = match[0]
      , num = +str;
    
    for (var i = 0; i < str.length; i++) {
      next();
    }

    return num;
  }

  function white() {
    while (ch && ch <= ' ') {
      next();
    }
  }

  function next() {
    ch = text.charAt(at);
    at += 1;
    return ch;
  }

  return function (source) {
    text = source;
    ch = " ";
    at = 0;
    result = expression();
    white();

    return result;
  };
}();


}(typeof exports === 'undefined' ? window.calc = {} : exports));