// Copyright (c) 2025-2026 Probo Inc <hello@probo.com>.
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

package slack

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"strconv"
	"time"
)

// VerifySignature implements the verification algorithm described in https://api.slack.com/authentication/verifying-requests-from-slack
func VerifySignature(signingSecret, timestamp, signature string, body []byte) error {
	// Check timestamp to prevent replay attacks
	ts, err := strconv.ParseInt(timestamp, 10, 64)
	if err != nil {
		return fmt.Errorf("invalid timestamp: %w", err)
	}

	now := time.Now().Unix()
	if abs(now-ts) > 60*5 { // 5 minutes
		return fmt.Errorf("request timestamp too old")
	}

	baseString := fmt.Sprintf("v0:%s:%s", timestamp, body)
	h := hmac.New(sha256.New, []byte(signingSecret))
	h.Write([]byte(baseString))
	expected := "v0=" + hex.EncodeToString(h.Sum(nil))

	if !hmac.Equal([]byte(expected), []byte(signature)) {
		return fmt.Errorf("signature mismatch")
	}

	return nil
}

func abs(n int64) int64 {
	if n < 0 {
		return -n
	}

	return n
}