java 根据ID生成6位邀请码

package com.digital.utils;

import java.util.Arrays;

/**
 * @author ye
 * @date 2022/3/16
 */
public class InvitationCodeUtil {
    /**
     * 随机字符串
     */
    private static final char[] CHARS = new char[] {'f', 'l', 'g', 'w', '5', 'x', 'c', '3',
            '9', 'z', 'm', '6', '7', 'y', 'r', 't', '2', 'h', 's', '8', 'd', 'v', 'e', 'j', '4', 'k',
            'q', 'p', 'u', 'a', 'n', 'b'};

    private final static int CHARS_LENGTH = 32;
    /**
     * 邀请码长度
     */
    private final static int CODE_LENGTH = 6;

    /**
     * 随机数据
     */
    private final static long SLAT = 1234561L;

    /**
     * PRIME1 与 CHARS 的长度 L互质，可保证 ( id * PRIME1) % L 在 [0,L)上均匀分布
     */
    private final static int PRIME1 = 3;

    /**
     * PRIME2 与 CODE_LENGTH 互质，可保证 ( index * PRIME2) % CODE_LENGTH  在 [0，CODE_LENGTH）上均匀分布
     */
    private final static int PRIME2 = 11;

    /**
     * 生成邀请码, 默认为 8位邀请码
     *
     * @param id 唯一的id. 支持的最大值为: (32^7 - {@link #SLAT})/{@link #PRIME1} = 11452834602
     * @return
     */
    public static String gen(Long id) {
        return gen(id, CODE_LENGTH);
    }

    /**
     * 生成邀请码
     *
     * @param id 唯一的id主键. 支持的最大值为: (32^7 - {@link #SLAT})/{@link #PRIME1}
     * @return code
     */
    public static String gen(Long id, int length) {

        // 补位
        id = id * PRIME1 + SLAT;
        //将 id 转换成32进制的值
        long[] b = new long[CODE_LENGTH];
        // 32进制数
        b[0] = id;
        for (int i = 0; i < CODE_LENGTH - 1; i++) {
            b[i + 1] = b[i] / CHARS_LENGTH;
            // 按位扩散
            b[i] = (b[i] + i * b[0]) % CHARS_LENGTH;
        }

        long tmp = 0;
        for (int i = 0; i < length - 2; i++) {
            tmp += b[i];
        }
        b[length - 1] = tmp * PRIME1 % CHARS_LENGTH;

        // 进行混淆
        long[] codeIndexArray = new long[CODE_LENGTH];
        for (int i = 0; i < CODE_LENGTH; i++) {
            codeIndexArray[i] = b[i * PRIME2 % CODE_LENGTH];
        }

        StringBuilder buffer = new StringBuilder();
        Arrays.stream(codeIndexArray).boxed().map(Long::intValue).map(t -> CHARS[t]).forEach(buffer::append);
        return buffer.toString();
    }

    /**
     * 将邀请码解密成原来的id
     *
     * @param code 邀请码
     * @return id
     */
    public static Long decode(String code) {
        if (code.length() != CODE_LENGTH) {
            return null;
        }
        // 将字符还原成对应数字
        long[] a = new long[CODE_LENGTH];
        for (int i = 0; i < CODE_LENGTH; i++) {
            char c = code.charAt(i);
            int index = findIndex(c);
            if (index == -1) {
                // 异常字符串
                return null;
            }
            a[i * PRIME2 % CODE_LENGTH] = index;
        }

        long[] b = new long[CODE_LENGTH];
        for (int i = CODE_LENGTH - 2; i >= 0; i--) {
            b[i] = (a[i] - a[0]*i + CHARS_LENGTH * i) % CHARS_LENGTH;
        }

        long res = 0;
        for (int i = CODE_LENGTH - 2; i >= 0; i--) {
            res += b[i];
            res *= (i > 0 ? CHARS_LENGTH : 1);
        }
        return (res - SLAT) / PRIME1;
    }

    /**
     * 查找对应字符的index
     *
     * @param c 字符
     * @return index
     */
    public static int findIndex(char c) {
        for (int i = 0; i < CHARS_LENGTH; i++) {
            if (CHARS[i] == c) {
                return i;
            }
        }
        return -1;
    }


    public static void main(String[] args) {
        for (int i = 1; i < 1000; i++) {
            String s = gen(Long.valueOf(i));
            System.out.println(s + "-----" + decode(s));
        }
    }

}