java - 我的概率/机会系统有什么错误？

Question

我目前正在使用机会系统来计算元素。

LootData 是一种对象类型，它存储一个被称为项目的值以及获得该“项目”的机会。

一个项目将包含一个 ItemStack(即项目)以及一个 double(即机会)。

似乎我的数学或我的代码已经关闭，因为一些机会较低的“项目”被更频繁地给出。

// Calculate total chance
double totalChance = 0;
for (LootData item: items)
    totalChance += item.getChance();

for (int i = 0; i < numberOfItems; i++) {
    // Select a 'random chance' from 0 to total chance
    double randomChance = Math.random() * totalChance;

    // Check which item the 'random chance' landed on
    for (LootData item: items) {
        randomChance -= item.getChance();

        if (randomChance < 0) {

            ItemStack stack = item.getItem().clone();
            stack.setAmount((int)(Math.random() * stack.getAmount() + 1));
            itemsToGive.add(stack);
            break;
        }

Answer 1

代码看起来不错，结果也合理。

也许存在转换问题？在不同数字类型之间切换可能会在意外位置截断值。

这是一个示例输出:

Item [  0 ] Freq [   0 ] Odds [ 0.012 ] Proportion [ 0.000 ]
Item [  1 ] Freq [   2 ] Odds [ 0.012 ] Proportion [ 0.020 ]
Item [  2 ] Freq [  14 ] Odds [ 0.122 ] Proportion [ 0.140 ]
Item [  3 ] Freq [  12 ] Odds [ 0.122 ] Proportion [ 0.120 ]
Item [  4 ] Freq [  36 ] Odds [ 0.366 ] Proportion [ 0.360 ]
Item [  5 ] Freq [  36 ] Odds [ 0.366 ] Proportion [ 0.360 ]

这是我写的算法和测试代码:

public class RandomTest {

    public static final int[] lootChance = new int[] { 1, 1, 10, 10, 30, 30 };
    public static final int totalLootChance;
    static {
        int total = 0;
        for ( int chance : lootChance ) {
            total += chance;
        }
        totalLootChance = total;
    }

    public static int lookupItem(double selection) {
        for ( int lootNo = 0; lootNo < lootChance.length; lootNo++ ) {
            selection -= lootChance[lootNo];
            if ( selection < 0 ) {
                return lootNo;
            }
        }
        throw new IllegalArgumentException("Selection [ " + selection + " ] out of range [ " + totalLootChance + " ]");
    }

    public static final int lootRuns = 100;

    public static void main(String[] args) {
        System.out.printf("Items [ %2d ] Weighed Total [ %3d ]\n", lootChance.length, totalLootChance);
        for ( int itemNo = 0; itemNo < lootChance.length; itemNo++) {
            System.out.printf("  [ %2d ] [ %3d ]\n", itemNo, lootChance[itemNo]);
        }

        int[] distribution = new int[ lootChance.length ];

        for ( int runNo = 0; runNo < lootRuns; runNo++ ) {
            double selection = Math.random() * totalLootChance;
            int itemSelection = lookupItem(selection);
            distribution[itemSelection]++;

            System.out.printf(
                "Run [ %3s ] Roll [ %04.2f ] Item [ %2d ]\n",
                runNo, selection, itemSelection);
        }

        for ( int lootNo = 0; lootNo < lootChance.length; lootNo++ ) {
            System.out.printf(
                "Item [ %2d ] Freq [ %3d ] Odds [ %02.3f ] Proportion [ %02.3f ]\n",
                lootNo,
                distribution[lootNo],
                (((double) lootChance[lootNo]) / totalLootChance),
                ((double) distribution[lootNo]) / lootRuns);
        }
    }
}