package notify

import (
	"fmt"
	"net"
	"sync"
)

const defaultInboundDispatchSource = "_notify.default_inbound_source"

// Inbound dispatch runs one serial worker per source so messages from the same
// connection keep their relative order. The pending queue is bounded both per
// source and in total: a slow handler can never let one connection grow the
// queue without limit, and a saturated connection can never starve the others.
// Callers block until there is room, which pushes backpressure to the transport
// reader instead of growing memory; CloseAndWait unblocks every caller.
const (
	defaultInboundDispatchQueueLimit = 4096
	defaultInboundDispatchQueueBytes = 64 << 20
	defaultInboundSourceQueueLimit   = 512
	defaultInboundSourceQueueBytes   = 16 << 20
)

type inboundDispatchItem struct {
	size int
	fn   func()
}

type inboundDispatcher struct {
	mu          sync.Mutex
	closed      bool
	closeCh     chan struct{}
	roomCh      chan struct{}
	queued      int
	queuedBytes int
	maxItems    int
	maxBytes    int
	sourceItems int
	sourceBytes int
	workers     map[string]*inboundDispatchWorker
	wg          sync.WaitGroup
}

type inboundDispatchWorker struct {
	queue       []inboundDispatchItem
	running     bool
	queued      int
	queuedBytes int
}

func newInboundDispatcher() *inboundDispatcher {
	return newInboundDispatcherWithCaps(
		defaultInboundDispatchQueueLimit,
		defaultInboundDispatchQueueBytes,
		defaultInboundSourceQueueLimit,
		defaultInboundSourceQueueBytes,
	)
}

// newInboundDispatcherWithLimits sizes a dispatcher for a single source: the
// per-source caps equal the global caps.
func newInboundDispatcherWithLimits(maxItems int, maxBytes int) *inboundDispatcher {
	return newInboundDispatcherWithCaps(maxItems, maxBytes, maxItems, maxBytes)
}

func newInboundDispatcherWithCaps(maxItems int, maxBytes int, sourceItems int, sourceBytes int) *inboundDispatcher {
	if maxItems <= 0 {
		maxItems = defaultInboundDispatchQueueLimit
	}
	if maxBytes <= 0 {
		maxBytes = defaultInboundDispatchQueueBytes
	}
	if sourceItems <= 0 || sourceItems > maxItems {
		sourceItems = maxItems
	}
	if sourceBytes <= 0 || sourceBytes > maxBytes {
		sourceBytes = maxBytes
	}
	return &inboundDispatcher{
		closeCh:     make(chan struct{}),
		roomCh:      make(chan struct{}, 1),
		maxItems:    maxItems,
		maxBytes:    maxBytes,
		sourceItems: sourceItems,
		sourceBytes: sourceBytes,
		workers:     make(map[string]*inboundDispatchWorker),
	}
}

// Dispatch queues fn for the given source without byte accounting. Prefer
// DispatchSized when the queued payload size is known. Like DispatchSized it
// blocks while the queue is at its limit.
func (d *inboundDispatcher) Dispatch(source string, fn func()) bool {
	return d.DispatchSized(source, 0, fn)
}

// DispatchSized queues fn for the given source. It blocks while the source or
// the dispatcher is at its item or byte limit, and returns false once the
// dispatcher is closed. A parked caller is released by CloseAndWait, so the
// owner of the reader must keep CloseAndWait reachable (a concurrent closer, or
// the reader's own stop path once the wait unblocks).
func (d *inboundDispatcher) DispatchSized(source string, size int, fn func()) bool {
	if d == nil || fn == nil {
		return false
	}
	if source == "" {
		source = defaultInboundDispatchSource
	}
	if size < 0 {
		size = 0
	}
	for {
		d.mu.Lock()
		if d.closed {
			d.mu.Unlock()
			return false
		}
		worker := d.workers[source]
		if worker == nil {
			worker = &inboundDispatchWorker{}
			d.workers[source] = worker
		}
		if d.roomLocked(worker, size) {
			worker.queue = append(worker.queue, inboundDispatchItem{size: size, fn: fn})
			worker.queued++
			worker.queuedBytes += size
			d.queued++
			d.queuedBytes += size
			if worker.running {
				d.mu.Unlock()
				return true
			}
			worker.running = true
			d.wg.Add(1)
			d.mu.Unlock()
			go d.run(source, worker)
			return true
		}
		d.mu.Unlock()
		// roomCh is signalled whenever a queued item is consumed; closeCh
		// releases the caller during shutdown.
		select {
		case <-d.roomCh:
		case <-d.closeCh:
			return false
		}
	}
}

func (d *inboundDispatcher) roomLocked(worker *inboundDispatchWorker, size int) bool {
	if d.maxItems > 0 && d.queued >= d.maxItems {
		return false
	}
	if d.sourceItems > 0 && worker.queued >= d.sourceItems {
		return false
	}
	// Always admit at least one item per scope so an oversized payload cannot
	// deadlock the reader behind an empty queue.
	if d.maxBytes > 0 && d.queued > 0 && d.queuedBytes+size > d.maxBytes {
		return false
	}
	if d.sourceBytes > 0 && worker.queued > 0 && worker.queuedBytes+size > d.sourceBytes {
		return false
	}
	return true
}

func (d *inboundDispatcher) signalRoomLocked() {
	select {
	case d.roomCh <- struct{}{}:
	default:
	}
}

func (d *inboundDispatcher) run(source string, worker *inboundDispatchWorker) {
	defer d.wg.Done()
	for {
		d.mu.Lock()
		if len(worker.queue) == 0 {
			worker.running = false
			if current := d.workers[source]; current == worker {
				delete(d.workers, source)
			}
			d.signalRoomLocked()
			d.mu.Unlock()
			return
		}
		item := worker.queue[0]
		worker.queue[0] = inboundDispatchItem{}
		worker.queue = worker.queue[1:]
		worker.queued--
		worker.queuedBytes -= item.size
		d.queued--
		d.queuedBytes -= item.size
		d.signalRoomLocked()
		d.mu.Unlock()
		item.fn()
	}
}

func (d *inboundDispatcher) CloseAndWait() {
	if d == nil {
		return
	}
	d.mu.Lock()
	if !d.closed {
		d.closed = true
		close(d.closeCh)
	}
	d.signalRoomLocked()
	d.mu.Unlock()
	d.wg.Wait()
}

func clientInboundDispatchSource() string {
	return "client"
}

func serverInboundDispatchSource(source interface{}) string {
	switch data := source.(type) {
	case serverInboundSource:
		return serverInboundDispatchSourceKey(data)
	case *serverInboundSource:
		if data == nil {
			return defaultInboundDispatchSource
		}
		return serverInboundDispatchSourceKey(*data)
	case net.Conn:
		return fmt.Sprintf("conn:%p", data)
	case string:
		if data == "" {
			return defaultInboundDispatchSource
		}
		return "peer:" + data
	default:
		return defaultInboundDispatchSource
	}
}

func serverInboundDispatchSourceKey(source serverInboundSource) string {
	if source.Conn != nil {
		return fmt.Sprintf("conn:%p:%d", source.Conn, source.TransportGeneration)
	}
	if source.Logical != nil {
		return fmt.Sprintf("logical:%s:%d", source.Logical.ID(), source.TransportGeneration)
	}
	if source.Source != "" {
		return fmt.Sprintf("peer:%s:%d", source.Source, source.TransportGeneration)
	}
	if source.RemoteAddr != nil {
		return fmt.Sprintf("addr:%s:%d", source.RemoteAddr.String(), source.TransportGeneration)
	}
	return defaultInboundDispatchSource
}