package api

import (
	"net/http"
	"strconv"
	"strings"
	"sync"
	"time"
)

// Per-IP token bucket rate limiting (#2). Goroutine-safe via mutex.

type bucket struct {
	tokens float64
	last   time.Time
	rate   float64 // tokens per second
	burst  float64
}

type limiter struct {
	mu      sync.Mutex
	buckets map[string]*bucket
}

func newLimiter() *limiter {
	return &limiter{buckets: make(map[string]*bucket)}
}

func (l *limiter) allow(key string, rate, burst float64) bool {
	l.mu.Lock()
	defer l.mu.Unlock()
	now := time.Now()
	b, ok := l.buckets[key]
	if !ok {
		b = &bucket{tokens: burst, last: now, rate: rate, burst: burst}
		l.buckets[key] = b
	}
	elapsed := now.Sub(b.last).Seconds()
	b.tokens += elapsed * b.rate
	if b.tokens > b.burst {
		b.tokens = b.burst
	}
	b.last = now
	if b.tokens < 1 {
		return false
	}
	b.tokens--
	return true
}

// clientIP extracts the client IP for rate-limit keying (#85).
//
// Trust boundary: palette runs behind exactly ONE trusted reverse proxy
// (Traefik in the k3s pod network). Traefik APPENDS the real client IP to
// X-Forwarded-For, so the RIGHTMOST entry is the last value the trusted
// proxy observed and is unspoofable by the client (a client-supplied fake
// entry only lands on the LEFT and is ignored). This matches chi's
// middleware.RealIP semantics for a single trusted proxy hop.
//
// Direct connections (no XFF header) fall back to RemoteAddr. Directly
// reachable deployments must NOT expose the app to untrusted networks
// without a proxy in front, or attackers could forge the rightmost entry.
func clientIP(r *http.Request) string {
	if xff := r.Header.Get("X-Forwarded-For"); xff != "" {
		if i := strings.LastIndex(xff, ","); i >= 0 {
			return strings.TrimSpace(xff[i+1:])
		}
		return strings.TrimSpace(xff)
	}
	if xr := r.Header.Get("X-Real-Ip"); xr != "" {
		return strings.TrimSpace(xr)
	}
	host := r.RemoteAddr
	if i := strings.LastIndex(host, ":"); i > 0 {
		host = host[:i]
	}
	return host
}

var globalLimiter = newLimiter()

// globalSettingsFn is set at startup; tests can point it at fixed settings.
var globalSettingsFn func() Settings

func globalSettings() Settings {
	if globalSettingsFn != nil {
		return globalSettingsFn()
	}
	return defaultSettings(Config{})
}

// rateLimitCreate uses the admin-tunable burst and per-minute refill (#40).
func rateLimitCreate(r *http.Request, s Settings) bool {
	return globalLimiter.allow("create:"+clientIP(r), s.RateLimitPerMinute/60.0, s.RateLimitBurst)
}

// rateLimitGuess: 1 req/sec refill, burst 5, per IP.
func rateLimitGuess(r *http.Request) bool {
	return globalLimiter.allow("guess:"+clientIP(r), 1, 5)
}

// rateLimitUnlock: 5 per minute per IP+paste.
func rateLimitUnlock(id string, r *http.Request) bool {
	return globalLimiter.allow("unlock:"+id+":"+clientIP(r), 5.0/60.0, 5)
}

// rateLimitAdmin: 5 attempts per minute per IP on the admin key check (#66),
// same pattern as the unlock limiter (#34).
func rateLimitAdmin(r *http.Request) bool {
	return globalLimiter.allow("admin:"+clientIP(r), 5.0/60.0, 5)
}

// writeRateLimited responds 429 with Retry-After based on refill rate.
func writeRateLimited(w http.ResponseWriter, retryAfterSecs int) {
	w.Header().Set("Retry-After", strconv.Itoa(retryAfterSecs))
	writeErrCode(w, 429, "rate_limited", "rate limit exceeded")
}

// setRateLimitHeaders sets informational X-RateLimit headers for create/guess.
func setRateLimitHeaders(w http.ResponseWriter, limit, burst int) {
	w.Header().Set("X-RateLimit-Limit", strconv.Itoa(limit))
	w.Header().Set("X-RateLimit-Burst", strconv.Itoa(burst))
}