package webhook

import (
	"bytes"
	"context"
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"net"
	"net/http"
	"syscall"
	"time"
)

// Attempts is how many times a delivery is tried. Chapter 23.4.
const Attempts = 3

// Client refuses a denied address, whatever a name resolved to or a redirect asks for.
func Client() *http.Client {
	// Checked in the dialer, after resolving and before connecting, closing the window between.
	dialer := &net.Dialer{
		Timeout: 10 * time.Second,
		Control: func(network, address string, c syscall.RawConn) error {
			return AllowedAddr(address)
		},
	}
	return &http.Client{
		Timeout: 30 * time.Second,
		CheckRedirect: func(req *http.Request, via []*http.Request) error {
			// Re-check every redirect, because a pass that then hops to 169.254.169.254 is the attack.
			if len(via) >= 5 {
				return fmt.Errorf("too many redirects")
			}
			return Allowed(req.Context(), req.URL.String())
		},
		Transport: &http.Transport{
			DialContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
				if err := AllowedAddr(addr); err != nil {
					return nil, err
				}
				return dialer.DialContext(ctx, network, addr)
			},
		},
	}
}

// Sign is HMAC-SHA256, with the config naming a secret whose value lives on the server.
func Sign(secret string, body []byte) string {
	mac := hmac.New(sha256.New, []byte(secret))
	mac.Write(body)
	return "sha256=" + hex.EncodeToString(mac.Sum(nil))
}

// Deliver posts one event, retrying with backoff up to attempts times.
func Deliver(ctx context.Context, url, secret string, payload any, attempts int) error {
	body, err := json.Marshal(payload)
	if err != nil {
		return err
	}
	if err := Allowed(ctx, url); err != nil {
		return err
	}
	client := Client()

	var last error
	for attempt := range max(attempts, 1) {
		if attempt > 0 {
			select {
			case <-ctx.Done():
				return ctx.Err()
			case <-time.After(time.Duration(1<<attempt) * time.Second):
			}
		}
		req, err := http.NewRequestWithContext(ctx, http.MethodPost, url, bytes.NewReader(body))
		if err != nil {
			return err
		}
		req.Header.Set("Content-Type", "application/json")
		req.Header.Set("User-Agent", "barerepo")
		if secret != "" {
			req.Header.Set("Barerepo-Signature", Sign(secret, body))
		}
		resp, err := client.Do(req)
		if err != nil {
			last = err
			continue
		}
		resp.Body.Close()
		if resp.StatusCode < 300 {
			return nil
		}
		last = fmt.Errorf("%s", resp.Status)
	}
	return last
}