func TestPrimes(t *testing.T) {
	is := is.New(t)
	for _, tc := range primeCases {
		is.Equal(tc.expected, primes.Primes(tc.input))
	}
}

func BenchmarkPrimes(b *testing.B) {
	for i := 0; i < b.N; i++ {
		primes.Primes(1e6)
	}
}

// Package primes is for calculating primes using sieve of Eratosthenes.

// Package primes deals with prime numbers.

	"fmt"

const max = int(10e5)
// Sieve holds the factors for a given number.
type Sieve map[int][]int
var mysieve = make(Sieve, max)
func genSieve(n int) {
	for i := 1; i <= n; i++ {
		for j := i; j <= n; j += i {
			mysieve[j] = append(mysieve[j], i)
		}
	}
}
// ListPrimes returns the primes below n.
func ListPrimes(n int) ([]int, error) {
	if n > max && n < 1 {
		return nil, fmt.Errorf("%d is too high or low, max: %d, min: 1", n, max)
	}
	genSieve(n)
	var primes []int
	for i := 2; i < n; i++ {
		if len(mysieve[i]) == 2 {
			primes = append(primes, i)
		}
	}
	return primes, nil
}
func Primes(limit int) []int {
	sieve := make([]bool, limit+1)
	out := []int{}
	for p := 2; p <= limit; p++ {
		if !sieve[p] {
			out = append(out, p)
			for i := p * p; i <= limit; i += p {
				sieve[i] = true
			}
		}
	}
	return out
}

module primes
go 1.17
require github.com/matryer/is v1.4.0

// Package factors deals with factors.
package factors
import (
	"fmt"
)
const MAX = 100_000
// Sieve holds the factors for a given number.
type Sieve map[int][]int
var mysieve = make(Sieve, MAX)
func genSieve(n int) {
	for i := 1; i <= n; i++ {
		for j := i; j <= n; j += i {
			mysieve[j] = append(mysieve[j], i)
		}
	}
}
// ListPrimes returns the primes below n.
func ListPrimes(n int) ([]int, error) {
	if n > MAX && n < 1 {
		return nil, fmt.Errorf("%d is too high or low, max: %d, min: 1", n, MAX)
	}
	genSieve(n)
	var primes []int
	for i := 2; i < n; i++ {
		if len(mysieve[i]) == 2 {
			primes = append(primes, i)
		}
	}
	return primes, nil
}