lib/mergeset: skip common prefixes when comparing inmemoryBlock items

This should improve the performance for items sorting inside inmemoryBlock.MarshalUnsortedData
if they have common prefix.

While at it, improve the performance for inmemoryBlock.updateCommonPrefix for sorted items.
This should improve performance for inmemoryBlock.MarshalSortedData during background merge.

lib/mergeset/encoding.go

@@ -28,9 +28,10 @@ type Item struct {
 //
 // The returned bytes representation belongs to data.
 func (it Item) Bytes(data []byte) []byte {
+	n := int(it.End - it.Start)
 	sh := (*reflect.SliceHeader)(unsafe.Pointer(&data))
-	sh.Cap = int(it.End - it.Start)
-	sh.Len = int(it.End - it.Start)
+	sh.Cap = n
+	sh.Len = n
 	sh.Data += uintptr(it.Start)
 	return data
 }
@@ -48,8 +49,13 @@ func (it Item) String(data []byte) string {
 func (ib *inmemoryBlock) Len() int { return len(ib.items) }
 
 func (ib *inmemoryBlock) Less(i, j int) bool {
-	data := ib.data
 	items := ib.items
+	a := items[i]
+	b := items[j]
+	cpLen := uint32(len(ib.commonPrefix))
+	a.Start += cpLen
+	b.Start += cpLen
+	data := ib.data
 	return string(items[i].Bytes(data)) < string(items[j].Bytes(data))
 }
 
@@ -59,9 +65,15 @@ func (ib *inmemoryBlock) Swap(i, j int) {
 }
 
 type inmemoryBlock struct {
+	// commonPrefix contains common prefix for all the items stored in the inmemoryBlock
 	commonPrefix []byte
-	data         []byte
-	items        []Item
+
+	// data contains source data for items
+	data []byte
+
+	// items contains items stored in inmemoryBlock.
+	// Every item contains the prefix specified at commonPrefix.
+	items []Item
 }
 
 func (ib *inmemoryBlock) SizeBytes() int {
@@ -74,17 +86,29 @@ func (ib *inmemoryBlock) Reset() {
 	ib.items = ib.items[:0]
 }
 
-func (ib *inmemoryBlock) updateCommonPrefix() {
+func (ib *inmemoryBlock) updateCommonPrefixSorted() {
 	ib.commonPrefix = ib.commonPrefix[:0]
-	if len(ib.items) == 0 {
+	items := ib.items
+	if len(items) == 0 {
 		return
 	}
-	items := ib.items
 	data := ib.data
 	cp := items[0].Bytes(data)
-	if len(cp) == 0 {
+	if len(items) > 1 {
+		cpLen := commonPrefixLen(cp, items[len(items)-1].Bytes(data))
+		cp = cp[:cpLen]
+	}
+	ib.commonPrefix = append(ib.commonPrefix[:0], cp...)
+}
+
+func (ib *inmemoryBlock) updateCommonPrefixUnsorted() {
+	ib.commonPrefix = ib.commonPrefix[:0]
+	items := ib.items
+	if len(items) == 0 {
 		return
 	}
+	data := ib.data
+	cp := items[0].Bytes(data)
 	for _, it := range items[1:] {
 		cpLen := commonPrefixLen(cp, it.Bytes(data))
 		if cpLen == 0 {
@@ -176,9 +200,11 @@ func (ib *inmemoryBlock) isSorted() bool {
 // - returns the marshal type used for the encoding.
 func (ib *inmemoryBlock) MarshalUnsortedData(sb *storageBlock, firstItemDst, commonPrefixDst []byte, compressLevel int) ([]byte, []byte, uint32, marshalType) {
 	if !ib.isSorted() {
+		ib.updateCommonPrefixUnsorted()
 		sort.Sort(ib)
+	} else {
+		ib.updateCommonPrefixSorted()
 	}
-	ib.updateCommonPrefix()
 	return ib.marshalData(sb, firstItemDst, commonPrefixDst, compressLevel)
 }
 
@@ -197,7 +223,7 @@ func (ib *inmemoryBlock) MarshalSortedData(sb *storageBlock, firstItemDst, commo
 	if isInTest && !ib.isSorted() {
 		logger.Panicf("BUG: %d items must be sorted; items:\n%s", len(ib.items), ib.debugItemsString())
 	}
-	ib.updateCommonPrefix()
+	ib.updateCommonPrefixSorted()
 	return ib.marshalData(sb, firstItemDst, commonPrefixDst, compressLevel)
 }
 
@@ -218,7 +244,7 @@ func (ib *inmemoryBlock) debugItemsString() string {
 
 // Preconditions:
 // - ib.items must be sorted.
-// - updateCommonPrefix must be called.
+// - updateCommonPrefix* must be called.
 func (ib *inmemoryBlock) marshalData(sb *storageBlock, firstItemDst, commonPrefixDst []byte, compressLevel int) ([]byte, []byte, uint32, marshalType) {
 	if len(ib.items) <= 0 {
 		logger.Panicf("BUG: inmemoryBlock.marshalData must be called on non-empty blocks only")