package utils

import (
	"context"
	"errors"
	"sync"
	"time"

	"golang.org/x/time/rate"
)

var ErrDropOverflow = errors.New("drop overflow limit")

// RateLimiter implements per-chat and global rate limiting with optional blocking.
// It supports two modes:
//   - "drop" mode: immediately reject if limits are exceeded.
//   - "wait" mode: block until capacity is available.
type RateLimiter struct {
	globalLockUntil time.Time     // global cooldown timestamp (set by API errors)
	globalLimiter   *rate.Limiter // global token bucket (30 req/sec)
	globalMu        sync.RWMutex  // protects globalLockUntil and globalLimiter

	chatLocks    map[int64]time.Time     // per-chat cooldown timestamps
	chatLimiters map[int64]*rate.Limiter // per-chat token buckets (1 req/sec)
	chatMu       sync.Mutex              // protects chatLocks and chatLimiters
}

// NewRateLimiter creates a new RateLimiter with default limits.
// Global: 30 requests per second, burst 30.
// Per-chat: 1 request per second, burst 1.
func NewRateLimiter() *RateLimiter {
	return &RateLimiter{
		globalLimiter: rate.NewLimiter(30, 30),
		chatLimiters:  make(map[int64]*rate.Limiter),
		chatLocks:     make(map[int64]time.Time),
	}
}

// SetGlobalLock sets a global cooldown period (e.g., after receiving 429 from Telegram).
// If retryAfter <= 0, no lock is applied.
func (rl *RateLimiter) SetGlobalLock(retryAfter int) {
	if retryAfter <= 0 {
		return
	}
	rl.globalMu.Lock()
	defer rl.globalMu.Unlock()
	rl.globalLockUntil = time.Now().Add(time.Duration(retryAfter) * time.Second)
}

// SetChatLock sets a cooldown for a specific chat (e.g., after 429 for that chat).
// If retryAfter <= 0, no lock is applied.
func (rl *RateLimiter) SetChatLock(chatID int64, retryAfter int) {
	if retryAfter <= 0 {
		return
	}
	rl.chatMu.Lock()
	defer rl.chatMu.Unlock()
	rl.chatLocks[chatID] = time.Now().Add(time.Duration(retryAfter) * time.Second)
}

// GlobalWait blocks until a global request can be made.
// Waits for both global cooldown and token bucket availability.
func (rl *RateLimiter) GlobalWait(ctx context.Context) error {
	if err := rl.waitForGlobalUnlock(ctx); err != nil {
		return err
	}
	return rl.globalLimiter.Wait(ctx)
}

// Wait blocks until a request for the given chat can be made.
// Waits for: chat cooldown → global cooldown → chat token bucket.
// Note: Global limit is checked *before* chat limit to avoid overloading upstream.
func (rl *RateLimiter) Wait(ctx context.Context, chatID int64) error {
	if err := rl.waitForChatUnlock(ctx, chatID); err != nil {
		return err
	}
	if err := rl.waitForGlobalUnlock(ctx); err != nil {
		return err
	}
	limiter := rl.getChatLimiter(chatID)
	return limiter.Wait(ctx)
}

// GlobalAllow checks if a global request can be made without blocking.
// Returns false if either global cooldown is active or token bucket is exhausted.
func (rl *RateLimiter) GlobalAllow() bool {
	rl.globalMu.RLock()
	until := rl.globalLockUntil
	rl.globalMu.RUnlock()

	if !until.IsZero() && time.Now().Before(until) {
		return false
	}
	return rl.globalLimiter.Allow()
}

// Allow checks if a request for the given chat can be made without blocking.
// Returns false if: global cooldown, chat cooldown, global limiter, or chat limiter denies.
// Note: Global limiter is checked before chat limiter — upstream limits take priority.
func (rl *RateLimiter) Allow(chatID int64) bool {
	// Check global cooldown
	rl.globalMu.RLock()
	globalUntil := rl.globalLockUntil
	rl.globalMu.RUnlock()
	if !globalUntil.IsZero() && time.Now().Before(globalUntil) {
		return false
	}

	// Check chat cooldown
	rl.chatMu.Lock()
	chatUntil, ok := rl.chatLocks[chatID]
	rl.chatMu.Unlock()
	if ok && !chatUntil.IsZero() && time.Now().Before(chatUntil) {
		return false
	}

	// Check global token bucket
	if !rl.globalLimiter.Allow() {
		return false
	}

	// Check chat token bucket
	limiter := rl.getChatLimiter(chatID)
	return limiter.Allow()
}

// Check applies rate limiting based on configuration.
// If dropOverflow is true:
//   - Immediately returns ErrDropOverflow if either global or chat limit is exceeded.
//
// Else:
//   - If chatID != 0: waits for chat-specific capacity (including global limit).
//   - If chatID == 0: waits for global capacity only.
//
// chatID == 0 means no specific chat context (e.g., inline query, webhook without chat).
func (rl *RateLimiter) Check(ctx context.Context, dropOverflow bool, chatID int64) error {
	if dropOverflow {
		if chatID != 0 && !rl.Allow(chatID) {
			return ErrDropOverflow
		}
		if !rl.GlobalAllow() {
			return ErrDropOverflow
		}
	} else if chatID != 0 {
		if err := rl.Wait(ctx, chatID); err != nil {
			return err
		}
	} else {
		if err := rl.GlobalWait(ctx); err != nil {
			return err
		}
	}
	return nil
}

// waitForGlobalUnlock blocks until global cooldown expires or context is done.
// Does not check token bucket — only cooldown.
func (rl *RateLimiter) waitForGlobalUnlock(ctx context.Context) error {
	rl.globalMu.RLock()
	until := rl.globalLockUntil
	rl.globalMu.RUnlock()

	if until.IsZero() || time.Now().After(until) {
		return nil
	}

	select {
	case <-time.After(time.Until(until)):
		return nil
	case <-ctx.Done():
		return ctx.Err()
	}
}

// waitForChatUnlock blocks until the specified chat's cooldown expires or context is done.
// Does not check token bucket — only cooldown.
func (rl *RateLimiter) waitForChatUnlock(ctx context.Context, chatID int64) error {
	rl.chatMu.Lock()
	until, ok := rl.chatLocks[chatID]
	rl.chatMu.Unlock()

	if !ok || until.IsZero() || time.Now().After(until) {
		return nil
	}

	select {
	case <-time.After(time.Until(until)):
		return nil
	case <-ctx.Done():
		return ctx.Err()
	}
}

// getChatLimiter returns the rate limiter for the given chat, creating it if needed.
// Uses 1 request per second with burst of 1 — conservative for per-user limits.
// Must be called with rl.chatMu held.
func (rl *RateLimiter) getChatLimiter(chatID int64) *rate.Limiter {
	if lim, ok := rl.chatLimiters[chatID]; ok {
		return lim
	}
	lim := rate.NewLimiter(1, 1)
	rl.chatLimiters[chatID] = lim
	return lim
}