JavaScript 中的背包变体

Posted 2023-02-16

【中文标题】JavaScript 中的背包变体

【英文标题】：Knapsack variant in JavaScript

【发布时间】：2015-08-20 20:47:38

【问题描述】：

我尝试在 javascript 中实现这个knapsack problem solution algorithm，但我得到的解决方案 s_opt 的总权重大于 L_max。

我做错了什么？

我怀疑这可能与 Closures in recursion 有关。

/*
GENERAL:
   Assume we have a knapsack and we want to bring as much stuff as possible.
   Of each thing we have several variants to choose from. Each of these variants have
   different value and takes different amount of space.

DEFINITIONS:
    L_max = integer, size of the knapsack for the entire problem having N items
    l = matrix, having the elements l[i-1][j-1] representing the space taken
            by variant j of item i (-1 since indexing the matrices has index starting on zero, i.e. item i is stored at position i-1)
    p = matrix, having the elements p[i-1][j-1] representing the value given by
            by variant j of item i
    n = total number of items (used in a sub-problem)
    N = total number of items (used in the full problem, N >= n)
    s_opt = vector having the optimal combination of variant selections s_i, i.e. s_opt = arg max p_sum

*/
function knapsack(L_max,l,p) 

    // constructing (initializing) - they are private members
    var self = this; // in order for private functions to be able read variables
    this.N = l.length;
    var DCached = []; // this is only used by a private function so it doesnt need to made public using this.*
    this.s_opt = [];
    this.p_mean = null;
    this.L_max = L_max;

    // define public optimization function for the entire problem
    // when this is completed the user can read
    // s_opt to get the solution and
    // p_mean to know the quality of the solution
    this.optimize = function() 
        self.p_mean = D(self.N,self.L_max) / Math.max(1,self.N);
    

    // define private sub-problem optimization function
    var D = function(n,r) 
        if (r<0)
         return -Infinity;
        if (n==0)
         return 0;
        if(DCached[n-1] != null) 
         if(DCached[n-1][r-1] != null) 
            return DCached[n-1][r-1];
         
        
        var p_max = -Infinity;
        var p_sum;
        var J = l[n-1].length;
        for(var j = 0; j < J; j++) 
         p_sum = p[n-1][j] + D( n-1 , r - l[n-1][j] );
         if(p_sum>p_max) 
             p_max = p_sum;
             self.s_opt[n-1] = j;
         
        
        DCached[n-1] = [];
        DCached[n-1][r-1] = p_max;
        return p_max;
    

使用此背包求解器的客户端执行以下操作：

var knapsackSolution = new knapsack(5,l,p);
knapsackSolution.optimize();
// now the client can access knapsackSolution.s_opt containing the solution.

【问题讨论】：

l 和 p 包含什么？ Infinity 是什么？

@NinaScholz: l 是一个矩阵，其中的元素 l[i-1][j-1] 表示项目 i 的变体 j 占用的空间。 p 是一个矩阵，其元素 p[i-1][j-1] 表示由项目 i 的变量 j 给出的值。关于Infinity，请看w3schools.com/jsref/jsref_infinity.asp

【参考方案1】：

我找到了解决方案。在解决子问题D(n,r) 时，问题中的代码返回了优化值，但它并没有真正以正确的方式管理数组s_opt。在下面粘贴的修改后的解决方案中，我修复了这个问题。不仅返回背包的优化值，还返回一组选择的变体（例如最大值的 arg）。缓存也被修改以管理解决方案的这两个部分（最大值和 arg 最大值）。

下面的代码还包含一个附加功能。用户现在还可以传递一个值maxComputingComplexity，以某种启发式方式控制问题的计算量。

    /*
GENERAL:
    Assume we have a knapsack and we want to bring as much stuff as possible.
    Of each thing we have several variants to choose from. Each of these variants have
    different value and takes different amount of space.

    The quantity of each variant is one.

DEFINITIONS:
    L_max = integer, size of the knapsack, e.g. max number of letters, for the entire problem having N items
    l = matrix, having the elements l[i-1][j-1] representing the space taken
            by variant j of item i (-1 since indexing the matrices has index starting on zero, i.e. item i is stored at position i-1)
    p = matrix, having the elements p[i-1][j-1] representing the value given by
            by variant j of item i
    maxComputingComplexity = value limiting the product L_max*self.N*M_max in order to make the optimization
            complete in limited amount of time. It has a serious implication, since it may cut the list of alternatives
            so that only the first alternatives are used in the computation, meaning that the input should be well
            ordered
    n = total number of items (used in a sub-problem)
    N = total number of items (used in the full problem, N >= n)
    M_i = number of variants of item i
    s_i = which variant is chosen to pack of item i
    s = vector of elements s_i representing a possible solution
    r = maximum total space in the knapsack, i.e. sum(l[i][s_i]) <= r
    p_sum = sum of the values of the selected variants, i.e. sum(p[i][s_i]
    s_opt = vector having the optimal combination of variant selections s_i, i.e. s_opt = arg max p_sum

    In order to solve this, let us see p_sum as a function
    D(n,r) = p_sum (just seeing it as a function of the sub-problem n combined with the maximum total space r)
RESULT:
*/

function knapsack(L_max,l,p,maxComputingComplexity) 


    // constructing (initializing) - they are private members
    var self = this; // in order for private functions to be able read variables
    this.N = l.length;
    var DCached = []; // this is only used by a private function so it doesnt need to made public using this.*
    //this.s_opt = [];
    //this.p_mean = null;
    this.L_max = L_max;
    this.maxComputingComplexity = maxComputingComplexity;
    //console.log("knapsack: Creating knapsack. N=" + N + ". L_max=" + L_max + ".");

    // object to store the solution (both big problem and sub-problems)
    function result(p_max,s_opt) 
        this.p_max = p_max; //max value
        this.s_opt = s_opt; //arg max value
    

    // define public optimization function for the entire problem
    // when this is completed the user can read
    // s_opt to get the solution and
    // p_mean to know the quality of the solution
    // computing complexity O(L_max*self.N*M_max),
    // think O=L_max*N*M_max => M_max=O/L_max/N => 3=x/140/20 => x=3*140*20 => x=8400
    this.optimize = function() 
        var M_max = Math.max(maxComputingComplexity / (L_max*self.N),2); //totally useless if not at least two
        console.log("optimize: Setting M_max =" + M_max);
        return D(self.N,self.L_max,M_max);
        //self.p_mean = mainResult.D / Math.max(1,self.N);
        // console.log...
    

    // Define private sub-problem optimization function.
    // The function reads to "global" variables, p and l
    // and as arguments it takes
    //      n delimiting the which sub-set of items to be able to include (from p and l)
    //      r setting the max space that this sub-set of items may take
    // Based on these arguments the function optimizes D
    // and returns
    //      D the max value that can be obtained by combining the things
    //      s_opt the selection (array of length n) of things optimizing D
    var D = function(n,r,M_max) 
        // Start by checking whether the value is already cached...
        if(DCached[n-1] != null) 
         if(DCached[n-1][r-1] != null) 
            //console.log("knapsack.D: n=" + n + " r=" + r + " returning from cache.");
            return DCached[n-1][r-1];
         

        
        var D_result = new result(-Infinity, []); // here we will manage the result
        //D_result.s_opt[n-1] = 0; // just put something there to start with
        if (r<0) 
            //D_result.p_max = -Infinity;
            return D_result;
        
        if (n==0) 
            D_result.p_max = 0;
            return D_result;
        
        var p_sum;
        //self.s_opt[n] = 0; not needed
        var J = Math.min(l[n-1].length,M_max);
        var D_minusOneResult; //storing the result when optimizing all previous items given a max length
        for(var j = 0; j < J; j++) 
            D_minusOneResult = D( n-1 , r - l[n-1][j] , M_max)
            p_sum = p[n-1][j] + D_minusOneResult.p_max;
            if(p_sum > D_result.p_max) 
                D_result.p_max = p_sum;
                D_result.s_opt = D_minusOneResult.s_opt;
                D_result.s_opt[n-1] = j;
            

        
        DCached[n-1] = [];
        DCached[n-1][r-1] = D_result;
        //console.log("knapsack.D: n=" + n + " r=" + r + " p_max= "+ p_max);
        return D_result;