Sieve of Erathosthenes prime numbers finding algorithm implemented in Go

sieve_of_eratosthenes.go

package main

import (
	"fmt"
	"log"
	"os"
	"strconv"
)

func Primes(n int) []int {
	numbers := make([]bool, n+1)

	for idx := range numbers {
		numbers[idx] = true
	}

	return Sieve(numbers)
}

func Sieve(s []bool) []int {
	l := len(s)
	s[0] = false
	s[1] = false

	// Filter out non-primes:
	for idx := 2; idx*idx < l; idx++ {
		if s[idx] {
			for i := 2 * idx; i < l; i += idx {
				s[i] = false
			}
		}
	}

	primes := make([]int, 0)

	// Gather remaining into their own slice:
	for idx, primep := range s {
		if primep {
			primes = append(primes, idx)
		}
	}

	return primes
}

func main() {
	var upTo int
	var err error

	if len(os.Args) == 1 {
		upTo = 100
	} else {
		upTo, err = strconv.Atoi(os.Args[1])
	}

	if err != nil {
		log.Fatal(err)
	}

	fmt.Printf("%v\n", Primes(upTo))
}

sieve_of_eratosthenes_test.go

package main

import (
	"reflect"
	"testing"
)

func TestPrimes(t *testing.T) {
	actual := Primes(23)
	expected := []int{2, 3, 5, 7, 11, 13, 17, 19, 23}

	if !reflect.DeepEqual(actual, expected) {
		t.Errorf("%v != %v", actual, expected)
	}
}