cryptofun/prime/prime.go

package prime

import "math/rand"

const (
	// MaxPrime is to get a prime value below this number
	MaxPrime = 2000
	// MinPrime is to get a prime value above this number
	MinPrime = 500
)

// RandInt returns a random integer between two values
func RandInt(min int, max int) int {
	r := rand.Intn(max-min) + min
	return r
}

// RandPrime returns a random prime number between two values
func RandPrime(min int, max int) int {
	primes := SieveOfEratosthenes(max)
	randN := rand.Intn(len(primes)-0) + 0
	return primes[randN]
}

// SieveOfEratosthenes returns a list of primes less than N
func SieveOfEratosthenes(N int) (primes []int) {
	b := make([]bool, N)
	for i := 2; i < N; i++ {
		if b[i] == true {
			continue
		}
		primes = append(primes, i)
		for k := i * i; k < N; k += i {
			b[k] = true
		}
	}
	return
}

// Gcd returns the greatest common divisor
func Gcd(a, b int) int {
	var bgcd func(a, b, res int) int
	bgcd = func(a, b, res int) int {
		switch {
		case a == b:
			return res * a
		case a%2 == 0 && b%2 == 0:
			return bgcd(a/2, b/2, 2*res)
		case a%2 == 0:
			return bgcd(a/2, b, res)
		case b%2 == 0:
			return bgcd(a, b/2, res)
		case a > b:
			return bgcd(a-b, b, res)
		default:
			return bgcd(a, b-a, res)
		}
	}
	return bgcd(a, b, 1)
}