package database

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

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

// Argon2 parameters - these are up-to-date secure defaults
const (
	argon2Time    = 1
	argon2Memory  = 64 * 1024 // 64 MB
	argon2Threads = 4
	argon2KeyLen  = 32
	argon2SaltLen = 16
)

// hashParts is the expected number of $-separated segments
// in an encoded Argon2id hash string.
const hashParts = 6

// minPasswordComplexityLen is the minimum password length that
// triggers per-character-class complexity enforcement.
const minPasswordComplexityLen = 4

// Sentinel errors returned by decodeHash.
var (
	errInvalidHashFormat    = errors.New("invalid hash format")
	errInvalidAlgorithm     = errors.New("invalid algorithm")
	errIncompatibleVersion  = errors.New("incompatible argon2 version")
	errSaltLengthOutOfRange = errors.New("salt length out of range")
	errHashLengthOutOfRange = errors.New("hash length out of range")
)

// PasswordConfig holds Argon2 configuration
type PasswordConfig struct {
	Time    uint32
	Memory  uint32
	Threads uint8
	KeyLen  uint32
	SaltLen uint32
}

// DefaultPasswordConfig returns secure default Argon2 parameters
func DefaultPasswordConfig() *PasswordConfig {
	return &PasswordConfig{
		Time:    argon2Time,
		Memory:  argon2Memory,
		Threads: argon2Threads,
		KeyLen:  argon2KeyLen,
		SaltLen: argon2SaltLen,
	}
}

// HashPassword generates an Argon2id hash of the password
func HashPassword(password string) (string, error) {
	config := DefaultPasswordConfig()

	// Generate a salt
	salt := make([]byte, config.SaltLen)

	_, err := rand.Read(salt)
	if err != nil {
		return "", err
	}

	// Generate the hash
	hash := argon2.IDKey(
		[]byte(password),
		salt,
		config.Time,
		config.Memory,
		config.Threads,
		config.KeyLen,
	)

	// Encode the hash and parameters
	b64Salt := base64.RawStdEncoding.EncodeToString(salt)
	b64Hash := base64.RawStdEncoding.EncodeToString(hash)

	// Format: $argon2id$v=19$m=65536,t=1,p=4$salt$hash
	encoded := fmt.Sprintf(
		"$argon2id$v=%d$m=%d,t=%d,p=%d$%s$%s",
		argon2.Version,
		config.Memory,
		config.Time,
		config.Threads,
		b64Salt,
		b64Hash,
	)

	return encoded, nil
}

// VerifyPassword checks if the provided password matches the hash
func VerifyPassword(
	password, encodedHash string,
) (bool, error) {
	// Extract parameters and hash from encoded string
	config, salt, hash, err := decodeHash(encodedHash)
	if err != nil {
		return false, err
	}

	// Generate hash of the provided password
	otherHash := argon2.IDKey(
		[]byte(password),
		salt,
		config.Time,
		config.Memory,
		config.Threads,
		config.KeyLen,
	)

	// Compare hashes using constant time comparison
	return subtle.ConstantTimeCompare(hash, otherHash) == 1, nil
}

// decodeHash extracts parameters, salt, and hash from an
// encoded hash string.
func decodeHash(
	encodedHash string,
) (*PasswordConfig, []byte, []byte, error) {
	parts := strings.Split(encodedHash, "$")
	if len(parts) != hashParts {
		return nil, nil, nil, errInvalidHashFormat
	}

	if parts[1] != "argon2id" {
		return nil, nil, nil, errInvalidAlgorithm
	}

	version, err := parseVersion(parts[2])
	if err != nil {
		return nil, nil, nil, err
	}

	if version != argon2.Version {
		return nil, nil, nil, errIncompatibleVersion
	}

	config, err := parseParams(parts[3])
	if err != nil {
		return nil, nil, nil, err
	}

	salt, err := decodeSalt(parts[4])
	if err != nil {
		return nil, nil, nil, err
	}

	config.SaltLen = uint32(len(salt)) //nolint:gosec // validated in decodeSalt

	hash, err := decodeHashBytes(parts[5])
	if err != nil {
		return nil, nil, nil, err
	}

	config.KeyLen = uint32(len(hash)) //nolint:gosec // validated in decodeHashBytes

	return config, salt, hash, nil
}

func parseVersion(s string) (int, error) {
	var version int

	_, err := fmt.Sscanf(s, "v=%d", &version)
	if err != nil {
		return 0, fmt.Errorf("parsing version: %w", err)
	}

	return version, nil
}

func parseParams(s string) (*PasswordConfig, error) {
	config := &PasswordConfig{}

	_, err := fmt.Sscanf(
		s, "m=%d,t=%d,p=%d",
		&config.Memory, &config.Time, &config.Threads,
	)
	if err != nil {
		return nil, fmt.Errorf("parsing params: %w", err)
	}

	return config, nil
}

func decodeSalt(s string) ([]byte, error) {
	salt, err := base64.RawStdEncoding.DecodeString(s)
	if err != nil {
		return nil, fmt.Errorf("decoding salt: %w", err)
	}

	saltLen := len(salt)
	if saltLen < 0 || saltLen > int(^uint32(0)) {
		return nil, errSaltLengthOutOfRange
	}

	return salt, nil
}

func decodeHashBytes(s string) ([]byte, error) {
	hash, err := base64.RawStdEncoding.DecodeString(s)
	if err != nil {
		return nil, fmt.Errorf("decoding hash: %w", err)
	}

	hashLen := len(hash)
	if hashLen < 0 || hashLen > int(^uint32(0)) {
		return nil, errHashLengthOutOfRange
	}

	return hash, nil
}

// GenerateRandomPassword generates a cryptographically secure
// random password.
func GenerateRandomPassword(length int) (string, error) {
	const (
		uppercase = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
		lowercase = "abcdefghijklmnopqrstuvwxyz"
		digits    = "0123456789"
		special   = "!@#$%^&*()_+-=[]{}|;:,.<>?"
	)

	// Combine all character sets
	allChars := uppercase + lowercase + digits + special

	// Create password slice
	password := make([]byte, length)

	// Ensure at least one character from each set
	if length >= minPasswordComplexityLen {
		password[0] = uppercase[cryptoRandInt(len(uppercase))]
		password[1] = lowercase[cryptoRandInt(len(lowercase))]
		password[2] = digits[cryptoRandInt(len(digits))]
		password[3] = special[cryptoRandInt(len(special))]

		// Fill the rest randomly from all characters
		for i := minPasswordComplexityLen; i < length; i++ {
			password[i] = allChars[cryptoRandInt(len(allChars))]
		}

		// Shuffle the password to avoid predictable pattern
		for i := range len(password) - 1 {
			j := cryptoRandInt(len(password) - i)
			idx := len(password) - 1 - i
			password[idx], password[j] = password[j], password[idx]
		}
	} else {
		// For very short passwords, just use all characters
		for i := range length {
			password[i] = allChars[cryptoRandInt(len(allChars))]
		}
	}

	return string(password), nil
}

// cryptoRandInt generates a cryptographically secure random
// integer in [0, upperBound).
func cryptoRandInt(upperBound int) int {
	if upperBound <= 0 {
		panic("upperBound must be positive")
	}

	nBig, err := rand.Int(
		rand.Reader,
		big.NewInt(int64(upperBound)),
	)
	if err != nil {
		panic(fmt.Sprintf("crypto/rand error: %v", err))
	}

	return int(nBig.Int64())
}