package primes_test
import (
	"primes"
	"testing"
	"github.com/matryer/is"
)
var primeCases = []struct {
	input    int
	expected []int
}{
	{0, []int{}},
	{1, []int{}},
	{2, []int{2}},
	{10, []int{2, 3, 5, 7}},
	{100, []int{2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97}},
}
var sieveCases = []struct {
	input    int
	expected []bool
}{
	// every non-prime is true, 0-th and 1-st is false, should be true
	{0, []bool{false}},
	{1, []bool{false, false}},
	{2, []bool{false, false, false}},
	{100, []bool{false, false, false, false, true, false, true, false, true, true, true, false, true, false, true, true, true, false, true, false, true, true, true, false, true, true, true, true, true, false, true, false, true, true, true, true, true, false, true, true, true, false, true, false, true, true, true, false, true, true, true, true, true, false, true, true, true, true, true, false, true, false, true, true, true, true, true, false, true, true, true, false, true, false, true, true, true, true, true, false, true, true, true, false, true, true, true, true, true, false, true, true, true, true, true, true, true, false, true, true, true}},
}
func TestPrimes(t *testing.T) {
	is := is.New(t)
	for _, tc := range primeCases {
		is.Equal(tc.expected, primes.Primes(tc.input))
	}
}
func TestSieve(t *testing.T) {
	is := is.New(t)
	for _, tc := range sieveCases {
		is.Equal(tc.expected, primes.JustSieve(tc.input))
	}
}
func BenchmarkPrimes(b *testing.B) {
	for i := 0; i < b.N; i++ {
		primes.Primes(1e6)
	}
}
func BenchmarkSieve(b *testing.B) {
	for i := 0; i < b.N; i++ {
		primes.JustSieve(1e6)
	}
}

// Package primes is for calculating primes using sieve of Eratosthenes.
package primes
import (
	"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
}
func JustSieve(limit int) []bool {
	sieve := make([]bool, limit+1)
	for p := 2; p <= limit; p++ {
		if !sieve[p] {
			for i := p * p; i <= limit; i += p {
				sieve[i] = true
			}
		}
	}
	return sieve
}