package auth

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

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

type Argon2Params struct {
	Memory  uint32
	Time    uint32
	Threads uint8
}

var DefaultParams = Argon2Params{
	Memory:  65536,
	Time:    3,
	Threads: 4,
}

const saltLen = 16
const keyLen = 32

// HashPassword generates an Argon2id hash string.
func HashPassword(password string, params Argon2Params) (string, error) {
	salt := make([]byte, saltLen)
	if _, err := rand.Read(salt); err != nil {
		return "", fmt.Errorf("generate salt: %w", err)
	}

	key := argon2.IDKey([]byte(password), salt, params.Time, params.Memory, params.Threads, keyLen)

	return fmt.Sprintf("$argon2id$v=%d$m=%d,t=%d,p=%d$%s$%s",
		argon2.Version,
		params.Memory, params.Time, params.Threads,
		base64.RawStdEncoding.EncodeToString(salt),
		base64.RawStdEncoding.EncodeToString(key),
	), nil
}

// VerifyPassword checks a password against an Argon2id hash string.
func VerifyPassword(password, hash string) (bool, error) {
	parts := strings.Split(hash, "$")
	if len(parts) != 6 || parts[1] != "argon2id" {
		return false, fmt.Errorf("invalid hash format")
	}

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

	var memory uint32
	var time uint32
	var threads uint8
	if _, err := fmt.Sscanf(parts[3], "m=%d,t=%d,p=%d", &memory, &time, &threads); err != nil {
		return false, fmt.Errorf("parse params: %w", err)
	}

	salt, err := base64.RawStdEncoding.DecodeString(parts[4])
	if err != nil {
		return false, fmt.Errorf("decode salt: %w", err)
	}

	expectedKey, err := base64.RawStdEncoding.DecodeString(parts[5])
	if err != nil {
		return false, fmt.Errorf("decode key: %w", err)
	}

	key := argon2.IDKey([]byte(password), salt, time, memory, threads, uint32(len(expectedKey)))

	return subtle.ConstantTimeCompare(key, expectedKey) == 1, nil
}