package api

import (
	"net"
	"net/http"
	"os"
	"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, #280).
//
// Trust boundary (revised in #280): the deployed ingress does NOT append the
// peer address to X-Forwarded-For, so every XFF entry (and X-Real-Ip) is
// client-controlled. Keying on any client-supplied header lets an attacker
// rotate the header per request and get a fresh rate-limit bucket every
// time. The bucket key therefore defaults to the PEER address (RemoteAddr),
// which the client cannot influence.
//
// Deployments that pin their proxy chain can restore per-client granularity:
// set PALETTE_CLIENT_IP_HEADER (e.g. "CF-Connecting-IP" once the proxy chain
// is pinned, e.g. Traefik forwardedHeaders.trustedIPs limited to Cloudflare
// ranges or an origin firewall locked to CF) and the header value is honored
// ONLY when the immediate peer (RemoteAddr) is inside PALETTE_TRUSTED_PROXIES
// (default: loopback + RFC1918 private ranges, i.e. the in-cluster Traefik
// hop). A public peer never triggers header trust, and the header value must
// parse as an IP.
func clientIP(r *http.Request) string {
	peer := hostOnly(r.RemoteAddr)
	if hdr := os.Getenv("PALETTE_CLIENT_IP_HEADER"); hdr != "" && trustedProxy(peer) {
		if v := strings.TrimSpace(r.Header.Get(hdr)); net.ParseIP(v) != nil {
			return v
		}
	}
	return peer
}

// trustedProxy reports whether peer (an IP without port) falls inside any of
// the configured trusted proxy CIDRs (PALETTE_TRUSTED_PROXIES, default
// loopback + RFC1918 private ranges). Parsed once and cached.
func trustedProxy(peer string) bool {
	trustedOnce.Do(func() {
		spec := os.Getenv("PALETTE_TRUSTED_PROXIES")
		if strings.TrimSpace(spec) == "" {
			spec = "127.0.0.0/8,::1/128,10.0.0.0/8,172.16.0.0/12,192.168.0.0/16"
		}
		for _, c := range strings.Split(spec, ",") {
			if _, cidr, err := net.ParseCIDR(strings.TrimSpace(c)); err == nil {
				trustedCIDRs = append(trustedCIDRs, cidr)
			}
		}
	})
	ip := net.ParseIP(peer)
	if ip == nil {
		return false
	}
	for _, cidr := range trustedCIDRs {
		if cidr.Contains(ip) {
			return true
		}
	}
	return false
}

var (
	trustedOnce  sync.Once
	trustedCIDRs []*net.IPNet
)

// hostOnly strips the port from a host:port address.
func hostOnly(addr string) string {
	host, _, err := net.SplitHostPort(addr)
	if err != nil {
		host = addr
	}
	return strings.TrimSpace(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))
}