package events

import (
	"crypto/hmac"
	"crypto/sha256"
	"crypto/subtle"
	"encoding/hex"
	"errors"
	"fmt"
)

// giteaSignatureHeader carries the hex HMAC-SHA256 of the raw body.
const giteaSignatureHeader = "X-Gitea-Signature"

// Default shared-secret header names (overridable per source in config;
// gitea is always HMAC and ignores the override).
const (
	DefaultRedmineSecretHeader   = "X-Redmine-Webhook-Secret"
	DefaultDiscourseSecretHeader = "X-Discourse-Webhook-Secret"
)

// ErrUnverified marks every verification failure so callers reject with 401
// without distinguishing WHY (no oracle for attackers poking the receiver).
var ErrUnverified = errors.New("webhook not verified")

// VerifyGiteaHMAC checks the X-Gitea-Signature header (hex HMAC-SHA256 of
// the raw request body) against the shared secret. Comparison is
// constant-time; error text never echoes header values.
func VerifyGiteaHMAC(body []byte, headerValue, secret string) error {
	if headerValue == "" {
		return fmt.Errorf("%w: missing %s header", ErrUnverified, giteaSignatureHeader)
	}
	got, err := hex.DecodeString(headerValue)
	if err != nil {
		return fmt.Errorf("%w: %s header is not valid hex", ErrUnverified, giteaSignatureHeader)
	}
	mac := hmac.New(sha256.New, []byte(secret))
	mac.Write(body)
	if !hmac.Equal(got, mac.Sum(nil)) {
		return fmt.Errorf("%w: %s does not match body", ErrUnverified, giteaSignatureHeader)
	}
	return nil
}

// VerifySharedSecret constant-time compares the value of the provider's
// secret header against the configured secret.
func VerifySharedSecret(headerName, headerValue, secret string) error {
	if headerValue == "" {
		return fmt.Errorf("%w: missing %s header", ErrUnverified, headerName)
	}
	if subtle.ConstantTimeCompare([]byte(headerValue), []byte(secret)) != 1 {
		return fmt.Errorf("%w: %s does not verify", ErrUnverified, headerName)
	}
	return nil
}

// GiteaSignatureHeader exposes the fixed HMAC header name (config/docs).
func GiteaSignatureHeader() string { return giteaSignatureHeader }

// SignGitea computes the expected HMAC for a body+secret (tests, smoke).
func SignGitea(body []byte, secret string) string {
	mac := hmac.New(sha256.New, []byte(secret))
	mac.Write(body)
	return hex.EncodeToString(mac.Sum(nil))
}