package password

import (
	"crypto/rand"
	"crypto/subtle"
	"encoding/base64"
	"errors"
	"fmt"
	"strings"

	"golang.org/x/crypto/argon2"
)

const (
	argonTime    = 3
	argonMemory  = 64 * 1024
	argonThreads = 2
	argonKeyLen  = 32
	argonSaltLen = 16
)

// Hash hashes a plaintext password with argon2id.
func Hash(plain string) (string, error) {
	salt := make([]byte, argonSaltLen)
	if _, err := rand.Read(salt); err != nil {
		return "", err
	}
	key := argon2.IDKey([]byte(plain), salt, argonTime, argonMemory, argonThreads, argonKeyLen)
	enc := base64.RawStdEncoding
	return fmt.Sprintf("$argon2id$v=%d$m=%d,t=%d,p=%d$%s$%s",
		argon2.Version, argonMemory, argonTime, argonThreads,
		enc.EncodeToString(salt), enc.EncodeToString(key)), nil
}

// Verify checks a plaintext password against an argon2id hash string.
func Verify(plain, encoded string) (bool, error) {
	parts := strings.Split(encoded, "$")
	if len(parts) != 6 || parts[1] != "argon2id" {
		return false, errors.New("malformed password hash")
	}
	var version int
	var memory uint32
	var time_ uint32
	var threads uint8
	if _, err := fmt.Sscanf(parts[2], "v=%d", &version); err != nil {
		return false, err
	}
	if _, err := fmt.Sscanf(parts[3], "m=%d,t=%d,p=%d", &memory, &time_, &threads); err != nil {
		return false, err
	}
	enc := base64.RawStdEncoding
	salt, err := enc.DecodeString(parts[4])
	if err != nil {
		return false, err
	}
	want, err := enc.DecodeString(parts[5])
	if err != nil {
		return false, err
	}
	got := argon2.IDKey([]byte(plain), salt, time_, memory, threads, uint32(len(want)))
	return subtle.ConstantTimeCompare(got, want) == 1, nil
}