package auth

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

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

const (
	argon2Time    = 3
	argon2Memory  = 64 * 1024
	argon2Threads = 4
	argon2KeyLen  = 32
	argon2SaltLen = 16

	minPasswordLength = 14
	minPassphraseWords = 5
	maxPasswordLength  = 256
)

var (
	ErrPasswordTooLong     = errors.New("password must be at most 256 characters")
	ErrPasswordTooWeak     = errors.New("password must be at least 14 characters or at least 5 words")
	ErrPasswordMatchesUser = errors.New("password cannot match the login name")
	ErrPasswordCommon      = errors.New("password is too common")
	ErrPasswordNotPrintable = errors.New("password may only contain printable characters")
)

// ValidatePassword checks password policy rules.
func ValidatePassword(password string, username string) error {
	if len(password) > maxPasswordLength {
		return ErrPasswordTooLong
	}
	for _, runeValue := range password {
		if !unicode.IsPrint(runeValue) {
			return ErrPasswordNotPrintable
		}
	}
	if strings.EqualFold(strings.TrimSpace(password), strings.TrimSpace(username)) {
		return ErrPasswordMatchesUser
	}
	if isCommonPassword(password) {
		return ErrPasswordCommon
	}

	wordCount := countWords(password)
	if len(password) >= minPasswordLength || wordCount >= minPassphraseWords {
		return nil
	}
	return ErrPasswordTooWeak
}

func countWords(password string) int {
	fields := strings.Fields(password)
	return len(fields)
}

// HashPassword returns a PHC-formatted argon2id hash string.
func HashPassword(password string) (string, error) {
	salt := make([]byte, argon2SaltLen)
	if _, err := rand.Read(salt); err != nil {
		return "", fmt.Errorf("generate salt: %w", err)
	}

	hash := argon2.IDKey(
		[]byte(password),
		salt,
		argon2Time,
		argon2Memory,
		argon2Threads,
		argon2KeyLen,
	)

	return fmt.Sprintf(
		"$argon2id$v=%d$m=%d,t=%d,p=%d$%s$%s",
		argon2.Version,
		argon2Memory,
		argon2Time,
		argon2Threads,
		base64.RawStdEncoding.EncodeToString(salt),
		base64.RawStdEncoding.EncodeToString(hash),
	), nil
}

// VerifyPassword compares password against a PHC argon2id hash.
func VerifyPassword(password string, encodedHash string) (bool, error) {
	parts := strings.Split(encodedHash, "$")
	if len(parts) != 6 || parts[1] != "argon2id" {
		return false, errors.New("unsupported password hash format")
	}

	var version int
	if _, err := fmt.Sscanf(parts[2], "v=%d", &version); err != nil {
		return false, errors.New("invalid hash version")
	}

	var memory uint32
	var timeCost uint32
	var threads uint8
	if _, err := fmt.Sscanf(parts[3], "m=%d,t=%d,p=%d", &memory, &timeCost, &threads); err != nil {
		return false, errors.New("invalid hash parameters")
	}

	salt, err := base64.RawStdEncoding.DecodeString(parts[4])
	if err != nil {
		return false, errors.New("invalid hash salt")
	}
	expectedHash, err := base64.RawStdEncoding.DecodeString(parts[5])
	if err != nil {
		return false, errors.New("invalid hash digest")
	}

	computed := argon2.IDKey(
		[]byte(password),
		salt,
		timeCost,
		memory,
		threads,
		uint32(len(expectedHash)),
	)
	return subtle.ConstantTimeCompare(computed, expectedHash) == 1, nil
}