Vastly optimize the MRU inventory handling.

Profiling showed the MRU inventory handling was taking 5% of the total
block handling time.  Upon inspection this is because the original
implementation was extremely inefficient using iteration to find the
oldest node for eviction.

This commit reworks it to use a map and list in order to achieve close to
O(1) performance for lookups, insertion, and eviction.

The following benchmark results show the difference:

Before: BenchmarkMruInventoryList          100     1069168 ns/op
After:  BenchmarkMruInventoryList     10000000         152 ns/op

Closes #21

mruinvmap.go

@@ -5,16 +5,17 @@
 package main
 
 import (
+	"container/list"
 	"fmt"
 	"github.com/conformal/btcwire"
-	"time"
 )
 
 // MruInventoryMap provides a map that is limited to a maximum number of items
 // with eviction for the oldest entry when the limit is exceeded.
 type MruInventoryMap struct {
-	invMap map[btcwire.InvVect]int64 // Use int64 for time for less mem.
-	limit  uint
+	invMap  map[btcwire.InvVect]*list.Element // nearly O(1) lookups
+	invList *list.List                        // O(1) insert, update, delete
+	limit   uint
 }
 
 // String returns the map as a human-readable string.
@@ -34,39 +35,51 @@ func (m *MruInventoryMap) Exists(iv *btcwire.InvVect) bool {
 // item if adding the new item would exceed the max limit.
 func (m *MruInventoryMap) Add(iv *btcwire.InvVect) {
 	// When the limit is zero, nothing can be added to the map, so just
-	// return
+	// return.
 	if m.limit == 0 {
 		return
 	}
 
-	// When the entry already exists update its last seen time.
-	if m.Exists(iv) {
-		m.invMap[*iv] = time.Now().Unix()
+	// When the entry already exists move it to the front of the list
+	// thereby marking it most recently used.
+	if node, exists := m.invMap[*iv]; exists {
+		m.invList.MoveToFront(node)
 		return
 	}
 
-	// Evict the oldest entry if the the new entry would exceed the size
-	// limit for the map.
+	// Evict the least recently used entry (back of the list) if the the new
+	// entry would exceed the size limit for the map.  Also reuse the list
+	// node so a new one doesn't have to be allocated.
 	if uint(len(m.invMap))+1 > m.limit {
-		var oldestEntry btcwire.InvVect
-		var oldestTime int64
-		for iv, lastUpdated := range m.invMap {
-			if oldestTime == 0 || lastUpdated < oldestTime {
-				oldestEntry = iv
-				oldestTime = lastUpdated
-			}
+		node := m.invList.Back()
+		lru, ok := node.Value.(*btcwire.InvVect)
+		if !ok {
+			return
 		}
 
-		m.Delete(&oldestEntry)
+		// Evict least recently used item.
+		delete(m.invMap, *lru)
+
+		// Reuse the list node of the item that was just evicted for the
+		// new item.
+		node.Value = iv
+		m.invList.MoveToFront(node)
+		m.invMap[*iv] = node
+		return
 	}
 
-	m.invMap[*iv] = time.Now().Unix()
+	// The limit hasn't been reached yet, so just add the new item.
+	node := m.invList.PushFront(iv)
+	m.invMap[*iv] = node
 	return
 }
 
 // Delete deletes the passed inventory item from the map (if it exists).
 func (m *MruInventoryMap) Delete(iv *btcwire.InvVect) {
-	delete(m.invMap, *iv)
+	if node, exists := m.invMap[*iv]; exists {
+		m.invList.Remove(node)
+		delete(m.invMap, *iv)
+	}
 }
 
 // NewMruInventoryMap returns a new inventory map that is limited to the number
@@ -75,8 +88,9 @@ func (m *MruInventoryMap) Delete(iv *btcwire.InvVect) {
 // new entry..
 func NewMruInventoryMap(limit uint) *MruInventoryMap {
 	m := MruInventoryMap{
-		invMap: make(map[btcwire.InvVect]int64),
-		limit:  limit,
+		invMap:  make(map[btcwire.InvVect]*list.Element),
+		invList: list.New(),
+		limit:   limit,
 	}
 	return &m
 }