package pwhash

import (
	"crypto/pbkdf2"
	"crypto/rand"
	"crypto/sha512"
	"crypto/subtle"
	"errors"
)

const defaultIter = 12504615 // from tools/finditers

func HashIter(pw string, salt []byte, iter int) ([]byte, error) {
	return pbkdf2.Key(sha512.New, pw, salt, iter, 32)
}

func Hash(pw string) ([]byte, error) {
	buf := make([]byte, 40)
	salt := buf[32:]
	rand.Read(salt)
	hash, err := HashIter(pw, salt, defaultIter)
	if err != nil {
		return nil, err
	}
	copy(buf, hash)
	return buf, nil
}

var errBadPassword = errors.New("bad password")

func Check(pwHash []byte, pw string) error {
	if len(pwHash) < 32 {
		return errBadPassword
	}
	wantHash, salt := pwHash[:32], pwHash[32:]
	gotHash, err := HashIter(pw, salt, defaultIter)
	if err != nil {
		return err
	}
	if subtle.ConstantTimeCompare(gotHash, wantHash) == 0 {
		return errBadPassword
	}
	return nil
}