从一个无序，不相等的数组中，选取N个数，使其和为M实现算法

function getCombBySum(array,sum,tolerance,targetCount){
  var util = {
    /*
      get combination from array
      arr: target array
      num: combination item length
      return: one array that contain combination arrays
    */
    /*获取所有的可能组合
    如果是[1,2,3,4,5]取出3个 
    那么可能性就有10种 C(5,3)= C(5,2) 
    不用翻书了 给个公式 
    全排列  P(n,m)=n!/(n-m)!
    组合排列 C(5,2)=5!/2!*3!=5*4*3*2*1/[(2*1)*(3*2*1)]=10
    这是使用了循环加递归做出了组合排序
    */
    getCombination: function(arr, num) {
      var r=[];
      (function f(t, a, n){
        if (n==0) {
          return r.push(t);
        }

        for (var i=0,l=a.length; i<=l-n; i++){
          f(t.concat(a[i]), a.slice(i+1), n-1);
        }
      })([], arr, num);

      return r;
    },

    // take array index to a array
    // 获取数组的索引
    getArrayIndex: function(array) {
      var i = 0,
        r = [];
      for(i = 0;i<array.length;i++){
        r.push(i);
      }
      
      return r;
    }
  }

  var logic = {
    // sort the array,then get what's we need
    // 获取数组中比sum小的数
    init: function(array, sum) {
      // clone array
      var _array = array.concat(),
      r = [],
      i = 0;
      // sort by asc
      _array.sort(function(a,b){
        return a - b;
      });
      // get all number when it's less than or equal sum
      for(i = 0;i<_array.length;i++){
        if(_array[i]<=sum){
          r.push(_array[i]);
        }else{
          break;
        }
      }
      
      return r;
    },
    // important function
    core: function(array, sum, arrayIndex, count, r){
      console.log('count..', count)
      var i = 0,
        k = 0,
        combArray = [],
        _sum = 0,
        _cca = [],
        _cache = [];
      
      // get current count combination
      // 这里排序的不是原来的数组,而是求的索引后的数组
      combArray = util.getCombination(arrayIndex, count);
      console.log('combArray...', combArray);
      for(i = 0;i<combArray.length;i++){
        _cca = combArray[i];
        _sum = 0;
        _cache = [];
        // calculate the sum from combination
        for(k = 0;k<_cca.length;k++){
          _sum += array[_cca[k]];
          _cache.push(array[_cca[k]]);
        }
        if(Math.abs(_sum-sum) <= _tolerance){
          r.push(_cache);
        }      
      }
      
    }
    
  }

  var r = [],
    _array = [],
    _targetCount = 0,
    _tolerance = 0,
    _returnMark = 0;
  
  // check data
  _targetCount = targetCount || _targetCount;
  _tolerance = tolerance || _tolerance;
  
  _array = logic.init(array,sum);
  
  logic.core(_array, sum, util.getArrayIndex(_array), (_targetCount || _array.length), r);
  
  return r;
}
console.log('结果。。',getCombBySum([1, 3, 2, 4, 5, 8], 7, 0, 2))