// Package queue provides a simple, type-safe, concurrent FIFO queue
// with support for parallel handlers, graceful shutdown, optional
// Dead Letter Queue (DLQ), hooks, and rate limiting.
package queue

import (
	"context"
	"fmt"
	"sync"
	"time"
)

// QueueHooks defines optional callbacks that are invoked during queue
// operations. Hooks can be used for metrics, logging, and monitoring.
//
// All hooks are invoked best-effort and must be fast and panic-safe in user code.
//
//   - OnProduced is called after an item is successfully enqueued.
//   - OnHandled is called after a handler returns without panic.
//   - OnDropped is called when an item is intentionally dropped due to
//     a canceled context or a closed queue.
type QueueHooks[T any] struct {
	OnProduced func()
	OnHandled  func()
	OnDropped  func(T)
}

// Queue is a generic, concurrency-safe FIFO queue with optional DLQ, parallel
// handling, rate limiting and graceful shutdown without data races.
type Queue[T any] struct {
	ch       chan T
	capacity int

	mu       sync.RWMutex // protects: handlers, errorFunc, dlq, hooks, closed, rateTicker
	sendMu   sync.RWMutex // synchronizes sends to the channel and its closing
	handlers []func(T)

	errorFunc func(error)
	dlq       *Queue[T]
	hooks     QueueHooks[T]

	closeOnce sync.Once
	closed    bool

	wg         sync.WaitGroup
	rateTicker *time.Ticker
}

// NewQueue creates a new queue with the given buffer capacity.
// Capacity must be > 0.
func NewQueue[T any](capacity int) *Queue[T] {
	if capacity <= 0 {
		panic("queue capacity must be > 0")
	}
	return &Queue[T]{
		ch:       make(chan T, capacity),
		capacity: capacity,
	}
}

// Register adds a handler function that will be invoked for each item.
// Multiple handlers may be registered; each item will be passed to each
// registered handler in registration order. Handlers must be concurrency-safe.
func (q *Queue[T]) Register(handler func(T)) {
	if handler == nil {
		panic("handler cannot be nil")
	}
	q.mu.Lock()
	q.handlers = append(q.handlers, handler)
	q.mu.Unlock()
}

// OnError registers an error callback which is invoked when a handler panics.
// The error contains panic details. Set to nil to disable.
func (q *Queue[T]) OnError(fn func(error)) {
	q.mu.Lock()
	q.errorFunc = fn
	q.mu.Unlock()
}

// SetDLQ sets a Dead Letter Queue which receives items that caused a handler panic.
// Items are enqueued to the DLQ in a non-blocking best-effort manner.
func (q *Queue[T]) SetDLQ(dlq *Queue[T]) {
	q.mu.Lock()
	q.dlq = dlq
	q.mu.Unlock()
}

// SetHooks installs optional hooks for produced/handled/dropped events.
// Passing a zero-value struct clears previously set hooks.
func (q *Queue[T]) SetHooks(h QueueHooks[T]) {
	q.mu.Lock()
	q.hooks = h
	q.mu.Unlock()
}

// SetRateLimit sets a per-queue processing rate limit in requests-per-second.
// If rps <= 0, rate limiting is disabled. Calling SetRateLimit replaces any
// previous ticker and will be applied by active workers.
func (q *Queue[T]) SetRateLimit(rps int) {
	q.mu.Lock()
	defer q.mu.Unlock()
	// Stop previous ticker if exists
	if q.rateTicker != nil {
		q.rateTicker.Stop()
		q.rateTicker = nil
	}
	if rps > 0 {
		q.rateTicker = time.NewTicker(time.Second / time.Duration(rps))
	}
}

// HandleParallel starts worker goroutines that consume items from the queue
// and invoke registered handlers. Workers exit when the channel is closed or
// when the provided context is canceled. If workers <= 0, a single worker
// is started.
func (q *Queue[T]) HandleParallel(ctx context.Context, workers int) {
	if workers <= 0 {
		workers = 1
	}
	q.wg.Add(workers)
	for i := 0; i < workers; i++ {
		go func() {
			defer q.wg.Done()
			for {
				select {
				case item, ok := <-q.ch:
					if !ok {
						return
					}
					// copy ticker safely
					q.mu.RLock()
					t := q.rateTicker
					q.mu.RUnlock()
					if t != nil {
						<-t.C
					}
					q.processItem(item)
				case <-ctx.Done():
					return
				}
			}
		}()
	}
}

// sendBlocking performs a blocking send into the channel (used by Produce).
// It holds a read lock across the send to prevent Close() from closing the
// channel concurrently.
func (q *Queue[T]) sendBlocking(item T) bool {
	// Быстрая проверка закрытия под RLock
	q.sendMu.RLock()
	q.mu.RLock()
	closed := q.closed
	hooks := q.hooks
	q.mu.RUnlock()
	if closed {
		q.sendMu.RUnlock()
		if hooks.OnDropped != nil {
			hooks.OnDropped(item)
		}
		return false
	}
	defer q.sendMu.RUnlock()

	defer func() { _ = recover() }() // guard against external races/panics
	q.ch <- item
	if hooks.OnProduced != nil {
		hooks.OnProduced()
	}
	return true
}

// sendNonBlocking attempts a non-blocking send (used by TryProduce and DLQ).
func (q *Queue[T]) sendNonBlocking(item T) bool {
	q.sendMu.RLock()
	q.mu.RLock()
	closed := q.closed
	hooks := q.hooks
	q.mu.RUnlock()
	if closed {
		q.sendMu.RUnlock()
		if hooks.OnDropped != nil {
			hooks.OnDropped(item)
		}
		return false
	}
	defer q.sendMu.RUnlock()

	defer func() { _ = recover() }()
	select {
	case q.ch <- item:
		if hooks.OnProduced != nil {
			hooks.OnProduced()
		}
		return true
	default:
		return false
	}
}

// sendWithContext blocks until the item is sent or the context is canceled.
func (q *Queue[T]) sendWithContext(ctx context.Context, item T) bool {
	q.sendMu.RLock()
	q.mu.RLock()
	closed := q.closed
	hooks := q.hooks
	q.mu.RUnlock()
	if closed {
		q.sendMu.RUnlock()
		if hooks.OnDropped != nil {
			hooks.OnDropped(item)
		}
		return false
	}
	defer q.sendMu.RUnlock()

	defer func() { _ = recover() }()
	select {
	case q.ch <- item:
		if hooks.OnProduced != nil {
			hooks.OnProduced()
		}
		return true
	case <-ctx.Done():
		if hooks.OnDropped != nil {
			hooks.OnDropped(item)
		}
		return false
	}
}

// Produce enqueues an item and blocks until the item is placed into the buffer.
// If the queue is closed, the item is dropped and the method returns immediately.
func (q *Queue[T]) Produce(item T) {
	_ = q.sendBlocking(item)
}

// ProduceWithContext enqueues an item, blocking until the item is accepted or
// the provided context is canceled. If the context is already canceled or the
// queue is closed, the item is dropped and OnDropped is invoked (if set).
func (q *Queue[T]) ProduceWithContext(ctx context.Context, item T) {
	// Если контекст уже отменён — не блокируемся и помечаем как dropped
	select {
	case <-ctx.Done():
		q.mu.RLock()
		if q.hooks.OnDropped != nil {
			q.hooks.OnDropped(item)
		}
		q.mu.RUnlock()
		return
	default:
	}
	_ = q.sendWithContext(ctx, item)
}

// TryProduce attempts to enqueue an item without blocking. It retries until
// either the item is accepted or the timeout elapses. Returns true on success.
func (q *Queue[T]) TryProduce(item T, timeout time.Duration) bool {
	ctx, cancel := context.WithTimeout(context.Background(), timeout)
	defer cancel()

	for {
		if ok := q.sendNonBlocking(item); ok {
			return true
		}
		select {
		case <-ctx.Done():
			return false
		default:
			// короткая уступка планировщику
			time.Sleep(time.Microsecond)
		}
	}
}

// Shutdown closes the queue for new items and waits for workers to finish
// processing or until the provided context is canceled. Returns ctx.Err() on
// timeout/cancel.
func (q *Queue[T]) Shutdown(ctx context.Context) error {
	q.Close()
	done := make(chan struct{})
	go func() {
		defer close(done)
		q.wg.Wait()
	}()
	select {
	case <-done:
		return nil
	case <-ctx.Done():
		return ctx.Err()
	}
}

func (q *Queue[T]) Close() {
	q.closeOnce.Do(func() {
		// forbid new sends: exclusive lock supersedes send RLock
		q.sendMu.Lock()
		q.mu.Lock()
		if !q.closed {
			q.closed = true
			close(q.ch)
		}
		// Остановим тикер, если включён
		if q.rateTicker != nil {
			q.rateTicker.Stop()
			q.rateTicker = nil
		}
		q.mu.Unlock()
		q.sendMu.Unlock()
	})
}

// Wait blocks until all worker goroutines started by HandleParallel exit.
func (q *Queue[T]) Wait() { q.wg.Wait() }

// Len returns the number of buffered items currently in the queue.
func (q *Queue[T]) Len() int { return len(q.ch) }

// Cap returns the configured buffer capacity of the queue.
func (q *Queue[T]) Cap() int { return q.capacity }

// IsEmpty reports whether the queue buffer is empty.
func (q *Queue[T]) IsEmpty() bool { return len(q.ch) == 0 }

// Closed reports whether the queue has been closed for sending.
func (q *Queue[T]) Closed() bool {
	q.mu.RLock()
	defer q.mu.RUnlock()
	return q.closed
}

// String returns a human-readable summary of the queue state.
func (q *Queue[T]) String() string {
	return fmt.Sprintf("Queue{len=%d, cap=%d, closed=%t}", q.Len(), q.Cap(), q.Closed())
}

// processItem executes all registered handlers for the item, recovering from
// panics. On panic, it calls the error callback (if any) and forwards the
// item to the DLQ using a best-effort non-blocking send.
func (q *Queue[T]) processItem(item T) {
	q.mu.RLock()
	handlers := append([]func(T){}, q.handlers...)
	errFn := q.errorFunc
	dlq := q.dlq
	hooks := q.hooks
	q.mu.RUnlock()
	for _, h := range handlers {
		func() {
			defer func() {
				if r := recover(); r != nil {
					// сообщим об ошибке
					if errFn != nil {
						errFn(fmt.Errorf("handler panic: %v", r))
					}
					// отправим в DLQ неблокирующе и безопасно
					if dlq != nil {
						_ = dlq.sendNonBlocking(item)
					}
				}
			}()
			h(item)
			if hooks.OnHandled != nil {
				hooks.OnHandled()
			}
		}()
	}
}