base62

Base62.java

public class Base62 {
    private final static String CHARS = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
    private final static int BASE = CHARS.length();

    public static String encode(long number) {
        var sb = new StringBuilder();
        while (number > 0) {
            var ch = CHARS.charAt((int) (number % BASE));
            number /= BASE;
            sb.insert(0, ch);
        }
        return sb.toString();
    }

    public static long decode(String str) {
        long number = 0;
        int power = 0;
        for (int i = str.length() - 1; i >= 0; i--) {
            var ch = str.charAt(i);
            var digit = CHARS.indexOf(ch);
            number += (long) (digit * Math.pow(BASE, power));
            power++;
        }
        return number;
    }

    public static void main(String[] args) {
        var num = 170916032679263329L;
        System.out.println(encode(num));
        System.out.println(decode(encode(num)));
    }
}