go prime

main.go

package main

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

var one = big.NewInt(1)
var four = big.NewInt(4)
var two = big.NewInt(2)
var zero = big.NewInt(0)

func checkPrime(given *big.Int) bool {
	if given.Cmp(one) == 0 || given.Cmp(four) < 0 || new(big.Int).Mod(given, two).Cmp(zero) == 0 {
		return false
	}
	maxN := new(big.Int).Sqrt(given).Int64()

	for i := int64(2); i < maxN; i += 2 {
		if new(big.Int).Mod(given, big.NewInt(i+1)).Cmp(zero) == 0 {
			return false
		}
	}
	return true
}

func main() {
	primesBeforeList := []*big.Int{
		big.NewInt(60),
		big.NewInt(6000000000),
		big.NewInt(60000000000),
		new(big.Int).Exp(big.NewInt(2), big.NewInt(32), nil),
		new(big.Int).Exp(big.NewInt(2), big.NewInt(40), nil),
		new(big.Int).Exp(big.NewInt(2), big.NewInt(44), nil),
		new(big.Int).Exp(big.NewInt(2), big.NewInt(48), nil),
		new(big.Int).Exp(big.NewInt(2), big.NewInt(56), nil),
		new(big.Int).Exp(big.NewInt(2), big.NewInt(64), nil),
	}

	for _, primeBefore := range primesBeforeList {
		checkingFor := new(big.Int).Set(primeBefore)
		start := time.Now()
		if new(big.Int).Mod(primeBefore, big.NewInt(2)).Cmp(big.NewInt(0)) == 0 {
			primeBefore.Sub(primeBefore, big.NewInt(1))
		} else {
			primeBefore.Sub(primeBefore, big.NewInt(2))
		}

		for primeBefore.Cmp(big.NewInt(0)) > 0 && !checkPrime(primeBefore) {
			primeBefore.Sub(primeBefore, big.NewInt(2))
		}

		elapsed := time.Since(start)
		if primeBefore.Cmp(big.NewInt(0)) > 0 {
			fmt.Printf("First prime before %v is %v in %v\n", checkingFor, primeBefore, elapsed)
		} else {
			fmt.Printf("No previous prime found in %v\n", elapsed)
		}
	}
}