IEEE 754 規格による浮動小数点数の計算時に生じる誤差を補うため四則演算クラス

simple_decimal.js

/**
 * javascript Decimal クラス
 *
 * IEEE 754 規格による浮動小数点数の計算時に生じる誤差を補うために使用します
 *
 * example:
 *  - var a = new utils.Decimal(100).multi(0.0476).calc();
 *    print(a); // 4.760000000000001 => 4.76
 *
 * data case:
 *  - 0.0476 * 100 = 4.760000000000001 ==> 4.76
 *  - 0.000306 * 1000 = 3.0599999999999996 ==> 3.06
 *  - 0.05 * 3 = 0.15000000000000002 ==> 0.15
 *  - 0.01716 * 100000 = 1716.0000000000002 ==> 1716
 * 
 * @see IEEE 754 https://ja.wikipedia.org/wiki/IEEE_754
 * @see 元ネタ https://github.com/icoxfog417/simple_decimal
 */
var utils = {};

(function(){
  function sDec(value,asRaw){
    this.values = [];
    this.decimalDigitsMax = 0;
    this.add(value || 0);
  }

  sDec.prototype.valueOf = function(){
    return this.value / Math.pow(10, this.decimalDigits);
  }

  sDec.prototype.toString = function(){
    var val = this.valueOf().toFixed(this.decimalDigits).replace(/0+$/,"");
    return val;
  }

  sDec.prototype.val = function(){
     return this.value;
  }

  sDec.prototype.add = function(value, type){
    if(!(value === undefined || value == null || value == "")){
      if(!isNaN(value)){
        var i = String(value).indexOf('.');
        var decimalDigits = i >= 0 && String(value).length - i - 1 || 0;
        if (this.decimalDigitsMax < decimalDigits) {
          this.decimalDigitsMax = decimalDigits;
        }
        this.values.push({value: value, type: type});
      }
    }
    return this;
  }

  sDec.prototype.plus = function(value){
    return this.add(value, 'plus');
  }

  sDec.prototype.minus = function(value){
    return this.add(value, 'minus');
  }

  sDec.prototype.multi = function(value){
    return this.add(value, 'multi');
  }

  sDec.prototype.div = function(value){
    return this.add(value, 'div');
  }

  sDec.prototype.calc = function(){
    var decimalPart = Math.pow(10, this.decimalDigitsMax);
    var result = 0;
    this.values.forEach(function(v){
      var ret = result;
      switch(v.type) {
        case 'minus':
          result -= Math.round(parseFloat(v.value) * decimalPart);
          break;
        case 'multi':
          var tmp = Math.round(parseFloat(v.value) * decimalPart);
          result = result * tmp / decimalPart;
          break;
        case 'div':
          var tmp = Math.round(parseFloat(v.value) * decimalPart);
          result = result / tmp * decimalPart;
          break;
        default: // and 'plus'
          result += Math.round(parseFloat(v.value) * decimalPart);
          break;
      }
    });
    var i = String(result).indexOf('.');
    var decimalDigits = i >= 0 && String(result).length - i - 1 || 0;
    var _decimalPart = Math.pow(10, decimalDigits);
    return result * _decimalPart / (decimalPart * _decimalPart);
  }

  utils.Decimal = sDec;
})();