計算2的100萬次冪

計算2的100萬次冪

import java.math.BigDecimal;
import java.util.HashMap;
import java.util.concurrent.TimeUnit;

public class Client {
    public static void main(String[] args) {
        //計算2^100,0000

        long start = System.currentTimeMillis();
        int pow = 1000000;
        BigDecimal res = calPow(2, pow, new HashMap<>());
        
        System.out.println(System.currentTimeMillis() - start);
        System.out.println(res);
    }

    private static BigDecimal calPow(int n, int pow, HashMap<Integer, BigDecimal> countMap) {
        if (countMap.containsKey(pow)) {
            return countMap.get(pow);
        }
        //這裏的大數和小數是有關係的,比如64 = 2 * (33 -1)
        if (pow > 64) {
            int a = pow / 2;
            BigDecimal x = calPow(n, a, countMap).multiply(calPow(n, pow - a, countMap));
            countMap.put(pow, x);
            return x;
        } else if (pow < 33) {
            BigDecimal x = BigDecimal.valueOf(Math.pow(n, pow));
            countMap.put(pow, x);
            return x;
        }
        BigDecimal c = BigDecimal.valueOf(1);

        int mask = 1;
        for (int i = 0; i < 32; i++) {
            if ((pow & mask) != 0) {
                BigDecimal newPow = calPow(n, i, countMap);
                c = c.multiply(calPow(n, newPow.intValue(), countMap));
            }
            mask <<= 1;
        }

        countMap.put(pow, c);
        return c;
    }
}