// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: PolyForm-Noncommercial-1.0.0

// Package ratelimit implements sliding-window request counters keyed by
// client IP, with LRU-bounded storage and periodic sweeps.
package ratelimit

import (
	"container/list"
	"sync"
	"time"
)

const (
	apiMaxTracked       = 20_000
	apiCleanupInterval  = 300 * time.Second
	loginWindow         = 60 * time.Second
	loginMaxAttempts    = 10
	loginMaxTracked     = 10_000
	loginCleanupEvery   = 300 * time.Second
	attemptsPerKeyLimit = 64
)

// window is a sliding-window counter over opaque string keys. Both the
// public API limiter and the login limiter are one of these with
// different limits, which is why the storage, the sweep and the
// eviction live here once.
type window struct {
	limit        int
	duration     time.Duration
	maxTracked   int
	cleanupEvery time.Duration
	now          func() time.Time

	mu          sync.Mutex
	hits        map[string][]time.Time
	order       *list.List // front = most recently used key
	positions   map[string]*list.Element
	lastCleanup time.Time
	haveCleaned bool
}

func newWindow(limit int, duration time.Duration, maxTracked int, cleanupEvery time.Duration, now func() time.Time) *window {
	if now == nil {
		now = time.Now
	}
	return &window{
		limit:        limit,
		duration:     duration,
		maxTracked:   maxTracked,
		cleanupEvery: cleanupEvery,
		now:          now,
		hits:         map[string][]time.Time{},
		order:        list.New(),
		positions:    map[string]*list.Element{},
	}
}

// hit records one attempt for key and returns the number of attempts in
// the window, including this one.
func (w *window) hit(key string) int {
	w.mu.Lock()
	defer w.mu.Unlock()
	now := w.now()
	w.sweepLocked(now)
	kept := w.pruneLocked(key, now)
	kept = append(kept, now)
	w.hits[key] = kept
	w.touchLocked(key)
	w.evictLocked()
	return len(kept)
}

// count returns the number of attempts for key inside the window,
// without recording one.
func (w *window) count(key string) int {
	w.mu.Lock()
	defer w.mu.Unlock()
	return len(w.pruneLocked(key, w.now()))
}

// retryAfter returns how long the caller should wait before the oldest
// tracked attempt for key leaves the window.
func (w *window) retryAfter(key string) time.Duration {
	w.mu.Lock()
	defer w.mu.Unlock()
	now := w.now()
	kept := w.pruneLocked(key, now)
	if len(kept) == 0 {
		return 0
	}
	if wait := kept[0].Add(w.duration).Sub(now); wait > 0 {
		return wait
	}
	return 0
}

// pruneLocked drops the attempts for key that have left the window and
// stores what is left. It also caps how many attempts one key may keep:
// a key that is refused on every request would otherwise grow a slice of
// timestamps for the whole window, unbounded by anything. The cap is far
// above the limit, so the count a caller sees is unaffected; the retry
// delay of a hammering client becomes approximate, which is all it can
// be once it is already being refused. The caller must hold w.mu.
func (w *window) pruneLocked(key string, now time.Time) []time.Time {
	cutoff := now.Add(-w.duration)
	hits := w.hits[key]
	kept := hits[:0]
	for _, t := range hits {
		if t.After(cutoff) {
			kept = append(kept, t)
		}
	}
	if len(kept) == 0 {
		delete(w.hits, key)
		return kept
	}
	if len(kept) > w.limit+attemptsPerKeyLimit {
		kept = kept[len(kept)-w.limit-attemptsPerKeyLimit:]
	}
	w.hits[key] = kept
	return kept
}

func (w *window) touchLocked(key string) {
	if elem, ok := w.positions[key]; ok {
		w.order.MoveToFront(elem)
		return
	}
	w.positions[key] = w.order.PushFront(key)
}

// evictLocked drops the least recently used keys once the bound is
// exceeded. It runs on every insert, so the maps cannot grow between
// sweeps. The caller must hold w.mu.
func (w *window) evictLocked() {
	for w.order.Len() > w.maxTracked {
		oldest := w.order.Back()
		if oldest == nil {
			return
		}
		key := oldest.Value.(string)
		w.order.Remove(oldest)
		delete(w.positions, key)
		delete(w.hits, key)
	}
}

// sweepLocked drops every attempt that has left its window and trims
// the key set. It is rate-limited by cleanupEvery so a busy server does
// not walk the map on every request. The caller must hold w.mu.
func (w *window) sweepLocked(now time.Time) {
	if w.haveCleaned && now.Sub(w.lastCleanup) < w.cleanupEvery {
		return
	}
	cutoff := now.Add(-w.duration)
	for key, hits := range w.hits {
		kept := hits[:0]
		for _, t := range hits {
			if t.After(cutoff) {
				kept = append(kept, t)
			}
		}
		if len(kept) == 0 {
			delete(w.hits, key)
			if elem, ok := w.positions[key]; ok {
				w.order.Remove(elem)
				delete(w.positions, key)
			}
			continue
		}
		w.hits[key] = kept
	}
	w.evictLocked()
	w.lastCleanup = now
	w.haveCleaned = true
}

// Limiter is a thread-safe sliding-window request counter.
type Limiter struct {
	window *window
}

// New creates a limiter allowing limit hits per window.
func New(limit int, window time.Duration) *Limiter {
	return &Limiter{window: newWindow(limit, window, apiMaxTracked, apiCleanupInterval, time.Now)}
}

// Limit returns the configured request budget per window.
func (l *Limiter) Limit() int { return l.window.limit }

// Check records a hit for key and reports whether it is allowed, how
// many requests remain, and the retry-after delay in seconds.
func (l *Limiter) Check(key string) (allowed bool, remaining, retryAfter int) {
	count := l.window.hit(key)
	allowed = count <= l.window.limit
	remaining = max(0, l.window.limit-count)
	if !allowed {
		retryAfter = int(l.window.retryAfter(key).Seconds()) + 1
	}
	return allowed, remaining, retryAfter
}

// LoginLimiter counts login attempts per IP. Record returns the number
// of attempts in the current window; Blocked reports whether the
// threshold was exceeded together with the retry-after delay.
type LoginLimiter struct {
	window *window
}

// NewLoginLimiter creates the shared login-attempt tracker.
func NewLoginLimiter() *LoginLimiter {
	return &LoginLimiter{window: newWindow(loginMaxAttempts, loginWindow, loginMaxTracked, loginCleanupEvery, time.Now)}
}

// Record logs an attempt for ip and returns the count in the window.
func (l *LoginLimiter) Record(ip string) int { return l.window.hit(ip) }

// Blocked reports whether ip exceeded the login budget and, when it
// did, the whole-second wait before the oldest attempt expires.
func (l *LoginLimiter) Blocked(ip string) (bool, int) {
	if l.window.count(ip) <= l.window.limit {
		return false, 0
	}
	retryAfter := l.window.retryAfter(ip)
	if retryAfter <= 0 {
		retryAfter = l.window.duration
	}
	return true, int(retryAfter.Seconds()) + 1
}