package authn

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

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

const (
	argonTime    uint32 = 2
	argonMemory  uint32 = 32 * 1024
	argonThreads uint8  = 1
	argonKeyLen  uint32 = 32
	saltLen             = 16
)

func HashPassword(password string) (string, error) {
	if len(password) < 12 {
		return "", errors.New("password must contain at least 12 characters")
	}
	if len(password) > 1024 {
		return "", errors.New("password must not exceed 1024 bytes")
	}
	salt := make([]byte, saltLen)
	if _, err := rand.Read(salt); err != nil {
		return "", err
	}
	hash := argon2.IDKey([]byte(password), salt, argonTime, argonMemory, argonThreads, argonKeyLen)
	return fmt.Sprintf("$argon2id$v=19$m=%d,t=%d,p=%d$%s$%s",
		argonMemory,
		argonTime,
		argonThreads,
		base64.RawStdEncoding.EncodeToString(salt),
		base64.RawStdEncoding.EncodeToString(hash),
	), nil
}

func VerifyPassword(encoded, password string) bool {
	if len(password) > 1024 {
		return false
	}
	parts := strings.Split(encoded, "$")
	if len(parts) != 6 || parts[1] != "argon2id" || parts[2] != "v=19" {
		return false
	}
	params := strings.Split(parts[3], ",")
	if len(params) != 3 {
		return false
	}
	memory, ok := parseParam(params[0], "m=")
	if !ok || memory < 8*1024 || memory > 256*1024 {
		return false
	}
	timeCost, ok := parseParam(params[1], "t=")
	if !ok || timeCost < 1 || timeCost > 10 {
		return false
	}
	threads, ok := parseParam(params[2], "p=")
	if !ok || threads < 1 || threads > 16 {
		return false
	}
	salt, err := base64.RawStdEncoding.DecodeString(parts[4])
	if err != nil || len(salt) < 16 || len(salt) > 64 {
		return false
	}
	expected, err := base64.RawStdEncoding.DecodeString(parts[5])
	if err != nil || len(expected) < 16 || len(expected) > 64 {
		return false
	}
	actual := argon2.IDKey([]byte(password), salt, uint32(timeCost), uint32(memory), uint8(threads), uint32(len(expected)))
	return subtle.ConstantTimeCompare(actual, expected) == 1
}

func parseParam(value, prefix string) (uint64, bool) {
	raw, ok := strings.CutPrefix(value, prefix)
	if !ok {
		return 0, false
	}
	n, err := strconv.ParseUint(raw, 10, 32)
	return n, err == nil
}