package main

import (
	"crypto/hmac"
	"crypto/sha256"
	"crypto/subtle"
	"encoding/base64"
	"net/http"
	"strconv"
	"strings"
	"time"
)

const (
	sessionCookieName = "mangabm_session"
	// 60 days: long enough that a phone stays logged in between reading spells.
	sessionTTL = 60 * 24 * time.Hour
	// Domain separation, so the session key can never collide with any other
	// use of API_TOKEN. Changing this string logs everyone out.
	sessionKeyPurpose = "mangabm-web-session-v1"
)

// sessionKey derives the cookie-signing key from the API token. Sessions are
// stateless — there is no session table — so rotating API_TOKEN invalidates
// every outstanding cookie at once.
func sessionKey(apiToken string) []byte {
	sum := sha256.Sum256([]byte(apiToken + sessionKeyPurpose))
	return sum[:]
}

// signSession encodes "<expiryMs>.<base64url HMAC(expiryMs)>".
func signSession(key []byte, expiryMs int64) string {
	payload := strconv.FormatInt(expiryMs, 10)
	return payload + "." + sessionMAC(key, payload)
}

func sessionMAC(key []byte, payload string) string {
	mac := hmac.New(sha256.New, key)
	mac.Write([]byte(payload))
	return base64.RawURLEncoding.EncodeToString(mac.Sum(nil))
}

// verifySession checks shape, then expiry, then the signature — in that order.
// The signature comparison is constant-time; the checks before it only look at
// data the holder already supplied, so their timing leaks nothing.
func verifySession(key []byte, value string, nowMs int64) bool {
	payload, sig, ok := strings.Cut(value, ".")
	if !ok {
		return false
	}
	expiry, err := strconv.ParseInt(payload, 10, 64)
	if err != nil || expiry <= nowMs {
		return false
	}
	want := sessionMAC(key, payload)
	return subtle.ConstantTimeCompare([]byte(sig), []byte(want)) == 1
}

// isHTTPS reports whether the browser's connection is encrypted. Behind Traefik
// the Go server itself speaks plain HTTP, so the forwarded header is the only
// signal; without this check the Secure cookie would never be set in
// production, and setting it unconditionally would break http://localhost dev.
func isHTTPS(r *http.Request) bool {
	return r.TLS != nil || r.Header.Get("X-Forwarded-Proto") == "https"
}

func setSessionCookie(w http.ResponseWriter, r *http.Request, key []byte) {
	http.SetCookie(w, &http.Cookie{
		Name:     sessionCookieName,
		Value:    signSession(key, time.Now().Add(sessionTTL).UnixMilli()),
		Path:     "/",
		MaxAge:   int(sessionTTL / time.Second),
		HttpOnly: true,
		Secure:   isHTTPS(r),
		SameSite: http.SameSiteLaxMode,
	})
}

func clearSessionCookie(w http.ResponseWriter, r *http.Request) {
	http.SetCookie(w, &http.Cookie{
		Name:     sessionCookieName,
		Value:    "",
		Path:     "/",
		MaxAge:   -1,
		HttpOnly: true,
		Secure:   isHTTPS(r),
		SameSite: http.SameSiteLaxMode,
	})
}