package main

import (
	"crypto/hmac"
	"crypto/sha256"
	"crypto/subtle"
	"encoding/base64"
	"net"
	"net/http"
	"strconv"
	"strings"
	"sync"
	"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,
	})
}

const (
	loginMaxFailures = 10
	loginWindow      = 20 * time.Minute
)

// clientIP returns the address the reverse proxy actually observed.
//
// Traefik appends the peer address to whatever X-Forwarded-For the client sent,
// so the leftmost entry is attacker-controlled and the rightmost is not. Go's
// Header.Get would only read the first header line, which a client can preempt
// by sending its own; Values covers every line so the true last hop is found.
// RemoteAddr is useless behind the proxy — it is always the Traefik container —
// so it serves only as the direct-connection fallback for local development.
func clientIP(r *http.Request) string {
	if vals := r.Header.Values("X-Forwarded-For"); len(vals) > 0 {
		hops := strings.Split(vals[len(vals)-1], ",")
		if ip := strings.TrimSpace(hops[len(hops)-1]); ip != "" {
			return ip
		}
	}
	host, _, err := net.SplitHostPort(r.RemoteAddr)
	if err != nil {
		return r.RemoteAddr
	}
	return host
}

// loginLimiter throttles password guessing: loginMaxFailures failures inside a
// rolling loginWindow blocks further attempts from that IP until the oldest one
// ages out. There is no permanent ban and no unlock step.
//
// Behind carrier-grade NAT this budget is shared with every other subscriber on
// the same public address, so a stranger can lock the owner out for up to one
// window. That is accepted: the block self-heals, and ten attempts is generous
// for a mistyped password.
//
// State is in memory and per-process, so a restart clears it. Entries are
// pruned lazily on access; for a single-user deployment the map cannot grow
// past the handful of addresses that ever attempt a login.
type loginLimiter struct {
	mu       sync.Mutex
	failures map[string][]time.Time
}

func newLoginLimiter() *loginLimiter {
	return &loginLimiter{failures: make(map[string][]time.Time)}
}

// retryAfter returns how long ip must wait, or zero when it may try now.
func (l *loginLimiter) retryAfter(ip string, now time.Time) time.Duration {
	l.mu.Lock()
	defer l.mu.Unlock()

	recent := l.pruneLocked(ip, now)
	if len(recent) < loginMaxFailures {
		return 0
	}
	return recent[0].Add(loginWindow).Sub(now)
}

func (l *loginLimiter) fail(ip string, now time.Time) {
	l.mu.Lock()
	defer l.mu.Unlock()
	l.failures[ip] = append(l.pruneLocked(ip, now), now)
}

func (l *loginLimiter) reset(ip string) {
	l.mu.Lock()
	defer l.mu.Unlock()
	delete(l.failures, ip)
}

// pruneLocked drops attempts older than the window and returns what is left.
// The caller must hold l.mu.
func (l *loginLimiter) pruneLocked(ip string, now time.Time) []time.Time {
	cutoff := now.Add(-loginWindow)
	kept := l.failures[ip][:0]
	for _, at := range l.failures[ip] {
		if at.After(cutoff) {
			kept = append(kept, at)
		}
	}
	if len(kept) == 0 {
		delete(l.failures, ip)
		return nil
	}
	l.failures[ip] = kept
	return kept
}