Task: Montgomery reduction
Go: package main

import (
    "fmt"
    "math/big"
    "math/rand"
    "time"
)


type mont struct {
    n  uint     
    m  *big.Int 
    r2 *big.Int 
}


func newMont(m *big.Int) *mont {
    if m.Bit(0) != 1 {
        return nil
    }
    n := uint(m.BitLen())
    x := big.NewInt(1)
    x.Sub(x.Lsh(x, n), m)
    return &mont{n, new(big.Int).Set(m), x.Mod(x.Mul(x, x), m)}
}


func (m mont) reduce(t *big.Int) *big.Int {
    a := new(big.Int).Set(t)
    for i := uint(0); i < m.n; i++ {
        if a.Bit(0) == 1 {
            a.Add(a, m.m)
        }
        a.Rsh(a, 1)
    }
    if a.Cmp(m.m) >= 0 {
        a.Sub(a, m.m)
    }
    return a
}


func main() {
    const n = 100 

    
    rnd := rand.New(rand.NewSource(time.Now().UnixNano()))
    one := big.NewInt(1)
    r1 := new(big.Int).Lsh(one, n-1)
    r2 := new(big.Int).Lsh(one, n-2)
    m := new(big.Int)
    m.Or(r1, m.Or(m.Lsh(m.Rand(rnd, r2), 1), one))

    
    mr := newMont(m)

    
    
    x1 := new(big.Int).Rand(rnd, m)
    x2 := new(big.Int).Rand(rnd, m)

    
    
    t1 := new(big.Int).Mul(x1, mr.r2)
    t2 := new(big.Int).Mul(x2, mr.r2)

    
    r1 = mr.reduce(t1)
    r2 = mr.reduce(t2)

    
    fmt.Println("b:  2")
    fmt.Println("n: ", mr.n)
    fmt.Println("r: ", new(big.Int).Lsh(one, mr.n))
    fmt.Println("m: ", mr.m)
    fmt.Println("t1:", t1)
    fmt.Println("t2:", t2)
    fmt.Println("r1:", r1)
    fmt.Println("r2:", r2)

    
    fmt.Println()
    fmt.Println("Original x1:       ", x1)
    fmt.Println("Recovererd from r1:", mr.reduce(r1))
    fmt.Println("Original x2:       ", x2)
    fmt.Println("Recovererd from r2:", mr.reduce(r2))

    
    fmt.Println("\nMontgomery computation of x1 ^ x2 mod m:")
    
    
    prod := mr.reduce(mr.r2)             
    base := mr.reduce(t1.Mul(x1, mr.r2)) 
    exp := new(big.Int).Set(x2)          
    for exp.BitLen() > 0 {
        if exp.Bit(0) == 1 {
            prod = mr.reduce(prod.Mul(prod, base))
        }
        exp.Rsh(exp, 1)
        base = mr.reduce(base.Mul(base, base))
    }
    fmt.Println(mr.reduce(prod))

    
    fmt.Println("\nLibrary-based computation of x1 ^ x2 mod m:")
    fmt.Println(new(big.Int).Exp(x1, x2, m))
}

Java: import java.math.BigInteger;

public class MontgomeryReduction {
    private static final BigInteger ZERO = BigInteger.ZERO;
    private static final BigInteger ONE = BigInteger.ONE;
    private static final BigInteger TWO = BigInteger.valueOf(2);

    public static class Montgomery {
        public static final int BASE = 2;

        BigInteger m;
        BigInteger rrm;
        int n;

        public Montgomery(BigInteger m) {
            if (m.compareTo(BigInteger.ZERO) <= 0 || !m.testBit(0)) {
                throw new IllegalArgumentException();
            }
            this.m = m;
            this.n = m.bitLength();
            this.rrm = ONE.shiftLeft(n * 2).mod(m);
        }

        public BigInteger reduce(BigInteger t) {
            BigInteger a = t;
            for (int i = 0; i < n; i++) {
                if (a.testBit(0)) a = a.add(this.m);
                a = a.shiftRight(1);
            }
            if (a.compareTo(m) >= 0) a = a.subtract(this.m);
            return a;
        }
    }

    public static void main(String[] args) {
        BigInteger m  = new BigInteger("750791094644726559640638407699");
        BigInteger x1 = new BigInteger("540019781128412936473322405310");
        BigInteger x2 = new BigInteger("515692107665463680305819378593");

        Montgomery mont = new Montgomery(m);
        BigInteger t1 = x1.multiply(mont.rrm);
        BigInteger t2 = x2.multiply(mont.rrm);

        BigInteger r1 = mont.reduce(t1);
        BigInteger r2 = mont.reduce(t2);
        BigInteger r = ONE.shiftLeft(mont.n);

        System.out.printf("b :  %s\n", Montgomery.BASE);
        System.out.printf("n :  %s\n", mont.n);
        System.out.printf("r :  %s\n", r);
        System.out.printf("m :  %s\n", mont.m);
        System.out.printf("t1:  %s\n", t1);
        System.out.printf("t2:  %s\n", t2);
        System.out.printf("r1:  %s\n", r1);
        System.out.printf("r2:  %s\n", r2);
        System.out.println();
        System.out.printf("Original x1       :  %s\n", x1);
        System.out.printf("Recovered from r1 :  %s\n", mont.reduce(r1));
        System.out.printf("Original x2       :  %s\n", x2);
        System.out.printf("Recovered from r2 :  %s\n", mont.reduce(r2));

        System.out.println();
        System.out.println("Montgomery computation of x1 ^ x2 mod m :");
        BigInteger prod = mont.reduce(mont.rrm);
        BigInteger base = mont.reduce(x1.multiply(mont.rrm));
        BigInteger exp = x2;
        while (exp.bitLength()>0) {
            if (exp.testBit(0)) prod=mont.reduce(prod.multiply(base));
            exp = exp.shiftRight(1);
            base = mont.reduce(base.multiply(base));
        }
        System.out.println(mont.reduce(prod));

        System.out.println();
        System.out.println("Library-based computation of x1 ^ x2 mod m :");
        System.out.println(x1.modPow(x2, m));
    }
}