[Java] Millar-Rabin primality test

https://casterian.net/algo/miller-rabin.html

The Casterian

평범한 소수 판별은 n^(1/2)인 반면

n^(1/4)까지 줄일 수 있다.

 

import java.util.*;
import java.io.*;

public class millar_rabin {
    static FastScanner fs = new FastScanner();
    static PrintWriter pw = new PrintWriter(System.out);
    static int[] primes = new int[]{2, 3, 5, 7, 11, 13, 17, 19, 23, 27};

    public static void main(String[] args) {
        int t = fs.nextInt();
        while (t-- > 0){
            solve(fs.nextInt());
        }

        pw.close();
    }

    static void solve(int n){
        boolean flag = true;
        if (n < 1000){
            flag = isPrime(n);
        }
        else{
            for (int i : primes){
                if (!mr(n, i)){
                    flag = false;
                    break;
                }
            }
        }

        if (flag) pw.println(n);
    }

    static boolean isPrime(int n){
        if (n == 1) return false;
        for (int i=2;i*i<=n;i++){
            if (n % i == 0) return false;
        }
        return true;
    }

    static int powmod(int x, int y, int m){
        x %= m;
        int r = 1;
        while (y > 0){
            if ((y & 1) == 1){
                r = (r * x) % m;
            }
            x = (x * x) % m;
            y /= 2;
        }
        return r;
    }

    static boolean mr(int n, int a){
        int d = n - 1;
        while ((d & 1) == 0){
            if (powmod(a, d, n) == n - 1){
                return true;
            }
            d /= 2;
        }

        int temp = powmod(a, d, n);
        return (temp == 1 || temp == n - 1);
    }

    // ----------input function----------

    static void sort(int[] a) {
        ArrayList<Integer> L = new ArrayList<>();
        for (int i : a)
            L.add(i);
        Collections.sort(L);
        for (int i = 0; i < a.length; i++)
            a[i] = L.get(i);
    }

    static class FastScanner {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        StringTokenizer st = new StringTokenizer("");

        String next() {
            while (!st.hasMoreTokens()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }

        int nextInt() {
            return Integer.parseInt(next());
        }

        double nextDouble() {
            return Double.parseDouble(next());
        }

        int[] readArray(int n) {
            int[] a = new int[n];
            for (int i = 0; i < n; i++)
                a[i] = nextInt();
            return a;
        }

        long nextLong() {
            return Long.parseLong(next());
        }
    }
}