// Package auth holds the gateway's credential validators. The only non-trivial
// one is the Telegram Web App initData HMAC check; guest and email logins carry
// no gateway-side secret and are validated by the backend. The validator is an
// interface so handlers test against fixtures without a bot token.
package auth

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"errors"
	"net/url"
	"sort"
	"strconv"
	"strings"
	"time"
)

// ErrInvalidInitData is returned when initData fails HMAC validation, is missing
// the hash, is malformed, or is older than the freshness window.
var ErrInvalidInitData = errors.New("auth: invalid telegram init data")

// defaultMaxAge bounds how old a validated initData payload may be.
const defaultMaxAge = 24 * time.Hour

// TelegramUser is the identity extracted from a validated initData payload. ID
// is the platform user id used as the identity's external_id.
type TelegramUser struct {
	ID        string
	Username  string
	FirstName string
}

// TelegramValidator validates Telegram Web App launch data and returns the
// authenticated user.
type TelegramValidator interface {
	Validate(initData string) (TelegramUser, error)
}

// HMACValidator validates initData against a bot token per Telegram's documented
// algorithm: the data-check string is HMAC-SHA256'd under a secret derived from
// the bot token, and the result is compared with the supplied hash.
type HMACValidator struct {
	botToken string
	maxAge   time.Duration
	now      func() time.Time
}

// NewHMACValidator constructs a validator for botToken.
func NewHMACValidator(botToken string) *HMACValidator {
	return &HMACValidator{botToken: botToken, maxAge: defaultMaxAge, now: time.Now}
}

// Validate parses and verifies initData, returning the authenticated user.
func (v *HMACValidator) Validate(initData string) (TelegramUser, error) {
	values, err := url.ParseQuery(initData)
	if err != nil {
		return TelegramUser{}, ErrInvalidInitData
	}
	hash := values.Get("hash")
	if hash == "" {
		return TelegramUser{}, ErrInvalidInitData
	}
	values.Del("hash")

	if !v.checkSignature(values, hash) {
		return TelegramUser{}, ErrInvalidInitData
	}
	if err := v.checkFreshness(values.Get("auth_date")); err != nil {
		return TelegramUser{}, err
	}
	return parseUser(values.Get("user"))
}

// checkSignature recomputes the HMAC over the sorted data-check string and
// compares it with hash in constant time.
func (v *HMACValidator) checkSignature(values url.Values, hash string) bool {
	keys := make([]string, 0, len(values))
	for k := range values {
		keys = append(keys, k)
	}
	sort.Strings(keys)
	lines := make([]string, 0, len(keys))
	for _, k := range keys {
		lines = append(lines, k+"="+values.Get(k))
	}
	dataCheck := strings.Join(lines, "\n")

	secret := hmacSHA256([]byte("WebAppData"), []byte(v.botToken))
	want := hmacSHA256(secret, []byte(dataCheck))
	got, err := hex.DecodeString(hash)
	if err != nil {
		return false
	}
	return hmac.Equal(want, got)
}

// checkFreshness rejects an auth_date older than the validator's window.
func (v *HMACValidator) checkFreshness(authDate string) error {
	if authDate == "" {
		return ErrInvalidInitData
	}
	secs, err := strconv.ParseInt(authDate, 10, 64)
	if err != nil {
		return ErrInvalidInitData
	}
	if v.now().Sub(time.Unix(secs, 0)) > v.maxAge {
		return ErrInvalidInitData
	}
	return nil
}

// parseUser extracts the user id and names from the user JSON field.
func parseUser(userJSON string) (TelegramUser, error) {
	if userJSON == "" {
		return TelegramUser{}, ErrInvalidInitData
	}
	var u struct {
		ID        int64  `json:"id"`
		Username  string `json:"username"`
		FirstName string `json:"first_name"`
	}
	if err := json.Unmarshal([]byte(userJSON), &u); err != nil || u.ID == 0 {
		return TelegramUser{}, ErrInvalidInitData
	}
	return TelegramUser{
		ID:        strconv.FormatInt(u.ID, 10),
		Username:  u.Username,
		FirstName: u.FirstName,
	}, nil
}

// hmacSHA256 returns HMAC-SHA256(message) under key.
func hmacSHA256(key, message []byte) []byte {
	h := hmac.New(sha256.New, key)
	h.Write(message)
	return h.Sum(nil)
}