// Package webhookreceiver demonstrates strict raw-body verification. Receivers
// must durably deduplicate event IDs together with their business effect before
// returning success. Delivery IDs change on replay; event IDs do not.
package webhookreceiver

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"errors"
	"net/http"
	"strconv"
	"strings"
	"time"
)

var ErrSignature = errors.New("invalid webhook signature")

type Key struct {
	Secret     []byte
	ValidUntil time.Time
}
type Identity struct {
	Delivery, Event, Attempt, Key string
	Number                        int
}

// Verify accepts current keys and explicitly time-limited previous keys. The
// receiver sets a previous key's ValidUntil no later than 24h after rotation.
func Verify(headers http.Header, raw []byte, now time.Time, keys map[string]Key) (Identity, error) {
	var identity Identity
	if len(raw) > 65536 {
		return identity, ErrSignature
	}
	fields := []string{}
	for _, name := range []string{"Agent-Hotel-Timestamp", "Agent-Hotel-Delivery-Id", "Agent-Hotel-Event-Id", "Agent-Hotel-Attempt-Id", "Agent-Hotel-Attempt", "Agent-Hotel-Key-Id", "Agent-Hotel-Signature"} {
		values := headers.Values(name)
		if len(values) != 1 || len(values[0]) == 0 || len(values[0]) > 128 || strings.ContainsAny(values[0], "\r\n\x00") {
			return identity, ErrSignature
		}
		fields = append(fields, values[0])
	}
	stamp, e := strconv.ParseInt(fields[0], 10, 64)
	if e != nil || strconv.FormatInt(stamp, 10) != fields[0] {
		return identity, ErrSignature
	}
	timestamp := time.Unix(stamp, 0)
	if timestamp.Before(now.Add(-5*time.Minute)) || timestamp.After(now.Add(5*time.Minute)) {
		return identity, ErrSignature
	}
	number, e := strconv.Atoi(fields[4])
	if e != nil || number < 1 || number > 11 || strconv.Itoa(number) != fields[4] {
		return identity, ErrSignature
	}
	key, ok := keys[fields[5]]
	if !ok || len(key.Secret) == 0 || (!key.ValidUntil.IsZero() && !now.Before(key.ValidUntil)) {
		return identity, ErrSignature
	}
	if len(fields[6]) != 67 || !strings.HasPrefix(fields[6], "v1=") || strings.ToLower(fields[6]) != fields[6] {
		return identity, ErrSignature
	}
	provided, e := hex.DecodeString(fields[6][3:])
	if e != nil {
		return identity, ErrSignature
	}
	h := hmac.New(sha256.New, key.Secret)
	h.Write([]byte("agent-hotel-webhook-v1\n" + strings.Join(fields[:5], "\n") + "\n"))
	h.Write(raw)
	if !hmac.Equal(provided, h.Sum(nil)) {
		return identity, ErrSignature
	}
	return Identity{Delivery: fields[1], Event: fields[2], Attempt: fields[3], Key: fields[5], Number: number}, nil
}