From 9b3f113716dd073c653fe16dd8a6bc9303a852be Mon Sep 17 00:00:00 2001
From: Sky Johnson <skylearj@gmail.com>
Date: Mon, 21 Jul 2025 13:50:28 -0500
Subject: [PATCH] combiner middleware

---
 internal/udp/middleware/combiner.go | 244 ++++++++++++++++++++++++++++
 1 file changed, 244 insertions(+)
 create mode 100644 internal/udp/middleware/combiner.go

diff --git a/internal/udp/middleware/combiner.go b/internal/udp/middleware/combiner.go
new file mode 100644
index 0000000..d0cb395
--- /dev/null
+++ b/internal/udp/middleware/combiner.go
@@ -0,0 +1,244 @@
+package middleware
+
+import (
+	"encoding/binary"
+	"net"
+	"sync"
+	"time"
+)
+
+// CombinerConfig holds configuration for packet combination
+type CombinerConfig struct {
+	MaxCombinedSize  int
+	FlushInterval    time.Duration
+	MaxQueuedPackets int
+}
+
+// DefaultCombinerConfig returns default combiner configuration
+func DefaultCombinerConfig() *CombinerConfig {
+	return &CombinerConfig{
+		MaxCombinedSize:  1200,
+		FlushInterval:    250 * time.Millisecond,
+		MaxQueuedPackets: 16,
+	}
+}
+
+type queuedPacket struct {
+	data      []byte
+	timestamp time.Time
+	callback  func([]byte) (int, error)
+	result    chan combineResult
+}
+
+type combineResult struct {
+	n   int
+	err error
+}
+
+// Combiner implements packet combination middleware
+type Combiner struct {
+	config    *CombinerConfig
+	queue     []*queuedPacket
+	queueMux  sync.Mutex
+	flushChan chan struct{}
+	done      chan struct{}
+	closeOnce sync.Once
+}
+
+// NewCombiner creates a new packet combining middleware
+func NewCombiner(config *CombinerConfig) *Combiner {
+	if config == nil {
+		config = DefaultCombinerConfig()
+	}
+
+	c := &Combiner{
+		config:    config,
+		flushChan: make(chan struct{}, 1),
+		done:      make(chan struct{}),
+	}
+
+	go c.flushLoop()
+	return c
+}
+
+// ProcessOutbound implements Middleware.ProcessOutbound
+func (c *Combiner) ProcessOutbound(data []byte, next func([]byte) (int, error)) (int, error) {
+	if len(data) == 0 {
+		return 0, nil
+	}
+
+	// Large packets bypass combination
+	if len(data) > c.config.MaxCombinedSize/2 {
+		return next(data)
+	}
+
+	c.queueMux.Lock()
+	defer c.queueMux.Unlock()
+
+	result := make(chan combineResult, 1)
+
+	c.queue = append(c.queue, &queuedPacket{
+		data:      append([]byte(nil), data...),
+		timestamp: time.Now(),
+		callback:  next,
+		result:    result,
+	})
+
+	shouldFlush := len(c.queue) >= c.config.MaxQueuedPackets
+	if !shouldFlush {
+		totalSize := c.calculateCombinedSize()
+		shouldFlush = totalSize > c.config.MaxCombinedSize
+	}
+
+	if shouldFlush {
+		c.flushQueueLocked()
+	} else {
+		select {
+		case c.flushChan <- struct{}{}:
+		default:
+		}
+	}
+
+	select {
+	case res := <-result:
+		return res.n, res.err
+	case <-c.done:
+		return 0, net.ErrClosed
+	}
+}
+
+// ProcessInbound implements Middleware.ProcessInbound
+func (c *Combiner) ProcessInbound(data []byte, next func([]byte) (int, error)) (int, error) {
+	if len(data) < 2 {
+		return next(data)
+	}
+
+	packetCount := binary.BigEndian.Uint16(data[0:2])
+
+	// Single packet or invalid format
+	if packetCount == 1 {
+		if len(data) < 4 {
+			return next(data)
+		}
+
+		firstLen := binary.BigEndian.Uint16(data[2:4])
+		if int(firstLen)+4 == len(data) {
+			return next(data[4 : 4+firstLen])
+		}
+		return next(data)
+	}
+
+	// Multiple packets - return first one
+	if packetCount > 1 && len(data) >= 4 {
+		firstLen := binary.BigEndian.Uint16(data[2:4])
+		if len(data) >= 4+int(firstLen) {
+			return next(data[4 : 4+firstLen])
+		}
+	}
+
+	return next(data)
+}
+
+func (c *Combiner) calculateCombinedSize() int {
+	size := 2 // count field
+	for _, pkt := range c.queue {
+		size += 2 + len(pkt.data)
+	}
+	return size
+}
+
+func (c *Combiner) flushLoop() {
+	ticker := time.NewTicker(c.config.FlushInterval)
+	defer ticker.Stop()
+
+	for {
+		select {
+		case <-ticker.C:
+			c.flushIfNeeded()
+		case <-c.flushChan:
+			c.flushIfNeeded()
+		case <-c.done:
+			c.queueMux.Lock()
+			c.flushQueueLocked()
+			c.queueMux.Unlock()
+			return
+		}
+	}
+}
+
+func (c *Combiner) flushIfNeeded() {
+	c.queueMux.Lock()
+	defer c.queueMux.Unlock()
+
+	if len(c.queue) == 0 {
+		return
+	}
+
+	now := time.Now()
+	oldestAge := now.Sub(c.queue[0].timestamp)
+
+	if oldestAge >= c.config.FlushInterval || len(c.queue) >= c.config.MaxQueuedPackets {
+		c.flushQueueLocked()
+	}
+}
+
+func (c *Combiner) flushQueueLocked() {
+	if len(c.queue) == 0 {
+		return
+	}
+
+	queue := c.queue
+	c.queue = nil
+
+	go c.processQueue(queue)
+}
+
+func (c *Combiner) processQueue(queue []*queuedPacket) {
+	if len(queue) == 1 {
+		pkt := queue[0]
+		n, err := pkt.callback(pkt.data)
+		pkt.result <- combineResult{n, err}
+		return
+	}
+
+	// Combine multiple packets
+	combined := c.combinePackets(queue)
+	_, err := queue[0].callback(combined)
+
+	// Distribute result to all packets
+	for _, pkt := range queue {
+		pkt.result <- combineResult{len(pkt.data), err}
+	}
+}
+
+func (c *Combiner) combinePackets(packets []*queuedPacket) []byte {
+	totalSize := 2 // count
+	for _, pkt := range packets {
+		totalSize += 2 + len(pkt.data)
+	}
+
+	combined := make([]byte, totalSize)
+	offset := 0
+
+	// Write packet count
+	binary.BigEndian.PutUint16(combined[offset:], uint16(len(packets)))
+	offset += 2
+
+	// Write each packet
+	for _, pkt := range packets {
+		binary.BigEndian.PutUint16(combined[offset:], uint16(len(pkt.data)))
+		offset += 2
+		copy(combined[offset:], pkt.data)
+		offset += len(pkt.data)
+	}
+
+	return combined
+}
+
+// Close implements Middleware.Close
+func (c *Combiner) Close() error {
+	c.closeOnce.Do(func() {
+		close(c.done)
+	})
+	return nil
+}