package password

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

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

type params struct {
	memory      uint32 // en KiB
	iterations  uint32
	parallelism uint8
	saltLength  uint32
	keyLength   uint32
}

var defaultParams = &params{
	memory:      64 * 1024, // 64 MB
	iterations:  3,
	parallelism: 2,
	saltLength:  16,
	keyLength:   32,
}

// generateSalt crée un salt aléatoire cryptographiquement sûr
func generateSalt(length uint32) ([]byte, error) {
	salt := make([]byte, length)
	if _, err := rand.Read(salt); err != nil {
		return nil, err
	}
	return salt, nil
}

// HashPassword génère un hash Argon2id encodé (salt + hash + paramètres inclus)
func HashPassword(password string) (string, error) {
	salt, err := generateSalt(defaultParams.saltLength)
	if err != nil {
		return "", err
	}

	hash := argon2.IDKey(
		[]byte(password),
		salt,
		defaultParams.iterations,
		defaultParams.memory,
		defaultParams.parallelism,
		defaultParams.keyLength,
	)

	// Format standard: $argon2id$v=19$m=65536,t=3,p=2$salt$hash
	b64Salt := base64.RawStdEncoding.EncodeToString(salt)
	b64Hash := base64.RawStdEncoding.EncodeToString(hash)

	encoded := fmt.Sprintf(
		"$argon2id$v=%d$m=%d,t=%d,p=%d$%s$%s",
		argon2.Version,
		defaultParams.memory,
		defaultParams.iterations,
		defaultParams.parallelism,
		b64Salt,
		b64Hash,
	)

	return encoded, nil
}

// VerifyPassword compare un mot de passe en clair avec un hash stocké
func VerifyPassword(password, encodedHash string) (bool, error) {
	p, salt, hash, err := decodeHash(encodedHash)
	if err != nil {
		return false, err
	}

	otherHash := argon2.IDKey(
		[]byte(password),
		salt,
		p.iterations,
		p.memory,
		p.parallelism,
		p.keyLength,
	)

	// Comparaison en temps constant pour éviter les timing attacks
	if subtle.ConstantTimeCompare(hash, otherHash) == 1 {
		return true, nil
	}
	return false, nil
}

// decodeHash parse la chaîne encodée pour en extraire params, salt et hash
func decodeHash(encodedHash string) (*params, []byte, []byte, error) {
	parts := strings.Split(encodedHash, "$")
	if len(parts) != 6 {
		return nil, nil, nil, errors.New("format de hash invalide")
	}

	var version int
	_, err := fmt.Sscanf(parts[2], "v=%d", &version)
	if err != nil {
		return nil, nil, nil, err
	}
	if version != argon2.Version {
		return nil, nil, nil, errors.New("version argon2 incompatible")
	}

	p := &params{}
	_, err = fmt.Sscanf(parts[3], "m=%d,t=%d,p=%d", &p.memory, &p.iterations, &p.parallelism)
	if err != nil {
		return nil, nil, nil, err
	}

	salt, err := base64.RawStdEncoding.DecodeString(parts[4])
	if err != nil {
		return nil, nil, nil, err
	}
	p.saltLength = uint32(len(salt))

	hash, err := base64.RawStdEncoding.DecodeString(parts[5])
	if err != nil {
		return nil, nil, nil, err
	}
	p.keyLength = uint32(len(hash))

	return p, salt, hash, nil
}