add size-variant

pkg/btree/node.go

@@ -5,6 +5,7 @@ import (
 	"encoding/binary"
 	"fmt"
 	"io"
+	"math/bits"
 )
 
 // MemoryPointer is a uint64 offset and uint32 length
@@ -81,13 +82,18 @@ func (n *BPTreeNode) NumPointers() int {
 	return len(n.InternalPointers) + len(n.LeafPointers)
 }
 
+func SizeVariant(v uint64) int {
+	return int(9*uint32(bits.Len64(v))+64) / 64
+}
+
 func (n *BPTreeNode) Size() int64 {
+
 	size := 4 // number of keys
 	for _, k := range n.Keys {
 		size += 8
 
-		lengthBytes := len(binary.AppendUvarint([]byte{}, uint64(k.DataPointer.Length)))
-		size += lengthBytes
+		lb := SizeVariant(uint64(k.DataPointer.Length))
+		size += lb
 
 		if n.Width != uint16(0) {
 			size += len(k.Value)
@@ -96,8 +102,8 @@ func (n *BPTreeNode) Size() int64 {
 	for _, n := range n.LeafPointers {
 		size += 8
 
-		lengthBytes := len(binary.AppendUvarint([]byte{}, uint64(n.Length)))
-		size += lengthBytes
+		lb := SizeVariant(uint64(n.Length))
+		size += lb
 	}
 	for range n.InternalPointers {
 		size += 8

pkg/btree/node_test.go

@@ -2,6 +2,7 @@ package btree
 
 import (
 	"bytes"
+	"encoding/binary"
 	"reflect"
 	"testing"
 )
@@ -103,3 +104,13 @@ func TestBPTreeNode_CompareReferencedValues(t *testing.T) {
 		}
 	}
 }
+
+func TestSizeVariant(t *testing.T) {
+
+	x := len(binary.AppendUvarint([]byte{}, uint64(123)))
+	y := SizeVariant(uint64(123))
+
+	if x != y {
+		t.Fatalf("expected x == y, got %v == %v", x, y)
+	}
+}