PEG math parser

.pegjs

{
  function filledArray(count, value) {
    var result = new Array(count), i;
    for (i = 0; i < count; i++) {
      result[i] = value;
    }
    return result;
  }

  function extractOptional(optional, index) {
    return optional ? optional[index] : null;
  }

  function extractList(list, index) {
    var result = new Array(list.length), i;
    for (i = 0; i < list.length; i++) {
      result[i] = list[i][index];
    }
    return result;
  }

  function buildList(head, tail, index) {
    return [head].concat(extractList(tail, index));
  }

  function buildTree(head, tail, builder) {
    var result = head, i;
    for (i = 0; i < tail.length; i++) {
      result = builder(result, tail[i]);
    }
    return result;
  }

  function optionalList(value) {
    return value !== null ? value : [];
  }
}

Expression
  = head:Term tail:(_ ("+" / "-") _ Term)* {
      var result = head, i;

      for (i = 0; i < tail.length; i++) {
        if (tail[i][1] === "+") { result += tail[i][3]; }
        if (tail[i][1] === "-") { result -= tail[i][3]; }
      }

      return result;
    }

Term
  = head:Factor tail:(_ ("*" / "/") _ Factor)* {
      var result = head, i;

      for (i = 0; i < tail.length; i++) {
        if (tail[i][1] === "*") { result *= tail[i][3]; }
        if (tail[i][1] === "/") { result /= tail[i][3]; }
      }

      return result;
    }

/* 
 * (2)関数 functionを先頭に追加
 */ 
Factor
  = function
  /  "(" _ expr:Expression _ ")" { return expr; }
  / Num


Num
    = _ head:[0-9]+ tail:('.' [0-9]+)? _ {
        var result = head.join('');
        if (tail && tail[1]) {
            result += '.' + tail[1].join('');
        }
        return parseFloat(result);
    }

_ "whitespace"
  = [ \t\n\r]*

/*
 * (3)識別子
 */
identifier
   = [A-Za-z_][A-Za-z0-9_]+ { return text(); }
   
const
   = identifier 
   / "PI"+ { return 3; }
   / Num

/*
 * (4)パラメータリスト
 */
ParameterList
 = head:Expression tail:( _ "," _ Expression )* {
      var ret = buildList(head,tail,3)
      return ret;
   }

/*
 * (5)関数追加(複数パラメータ対応)
 */
function 
  = func:identifier _ "(" _ params:ParameterList? _ ")" { 
    const funcName = func.toLowerCase();
    // 関数実行処理
    switch(funcName) {
      case "sum" : // sum関数を実行
          if (params == null) {
             alert("パラメータがありません:sum")
          }
          return params.reduce(function(l,r) { return l + r })
      default :
          return 0;
    }
}