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package class016;
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public class Code01_RandToRand {
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// 条件，且代码不可修改，1~5
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public static int f() {
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return (int) (Math.random() * 5) + 1;
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}
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// 等概率返回 0~4
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public static int a() {
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return f() - 1;
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}
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// 等概率返回 0 ~ 24
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public static int b() {
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return a() * 5 + a();
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}
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// 等概率返回 1~7
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public static int g1() {
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int t = 0;
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do {
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t = b();
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} while (t > 20);
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// t 0 ~ 20
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return (t % 7) + 1;
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}
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// 利用f函数，请等概率生成0和1
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public static int c() {
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int t = 0;
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do {
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t = f();
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} while (t == 3);
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return t < 3 ? 0 : 1;
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}
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// 等概率返回 1~7
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public static int g2() {
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int t = 0;
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do {
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t = (c() << 2) + (c() << 1) + c();
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} while (t == 7);
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return t + 1;
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}
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// 这个结构是唯一的随机机制
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// 你只能初始化并使用，不可修改
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public static class RandomBox {
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private final int min;
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private final int max;
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// 初始化时请一定不要让mi==ma
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public RandomBox(int mi, int ma) {
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min = mi;
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max = ma;
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}
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public int random() {
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return min + (int) (Math.random() * (max - min + 1));
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}
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public int min() {
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return min;
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}
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public int max() {
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return max;
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}
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}
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// 利用条件RandomBox，如何等概率返回0和1
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public static int rand01(RandomBox randomBox) {
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int min = randomBox.min();
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int max = randomBox.max();
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// min ~ max
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int size = max - min + 1;
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// size是不是奇数，odd 奇数
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boolean odd = (size & 1) != 0;
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int mid = size / 2;
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int ans = 0;
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do {
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ans = randomBox.random() - min;
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} while (odd && ans == mid);
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return ans < mid ? 0 : 1;
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}
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// 给你一个RandomBox，这是唯一能借助的随机机制
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// 等概率返回from~to范围上任何一个数
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// 要求from<=to
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public static int random(RandomBox randomBox, int from, int to) {
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if (from == to) {
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return from;
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}
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// 3 ~ 9
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// 0 ~ 6
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// 0 ~ range
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int range = to - from;
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int num = 1;
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// 求0～range需要几个2进制位
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while ((1 << num) - 1 < range) {
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num++;
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}
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// 我们一共需要num位
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// 最终的累加和，首先+0位上是1还是0，1位上是1还是0，2位上是1还是0...
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int ans = 0;
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do {
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ans = 0;
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for (int i = 0; i < num; i++) {
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ans |= (rand01(randomBox) << i);
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}
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} while (ans > range);
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return ans + from;
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}
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public static void main(String[] args) {
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int hasFrom = 1;
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int hasTo = 19;
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int from = 7;
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int to = 29;
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RandomBox randomBox = new RandomBox(hasFrom, hasTo);
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int[] ans = new int[to + 1];
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int testTime1 = 1000000;
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for (int i = 0; i < testTime1; i++) {
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ans[random(randomBox, from, to)]++;
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}
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for (int i = 0; i < ans.length; i++) {
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System.out.println(i + " 出现了 ： " + ans[i]);
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}
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System.out.println("==========");
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}
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}
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