xbufio: put buf/pos to head of struct; avoid 1 temporaray

t/neo/storage/fs1/xbufio.go

@@ -13,11 +13,11 @@ import (
 // SeqBufReader implements buffering for a io.ReaderAt optimized for sequential access
 // XXX -> xbufio.SeqReader
 type SeqBufReader struct {
-	r io.ReaderAt
-
 	// buffer for data at pos. cap(buf) - whole buffer capacity
-	pos	int64
 	buf	[]byte
+	pos	int64
+
+	r io.ReaderAt
 }
 
 // TODO text about syscall / memcpy etc
@@ -95,8 +95,7 @@ func (sb *SeqBufReader) ReadAt(p []byte, pos int64) (n int, err error) {
 		}
 	}
 
-	buf := sb.buf[:cap(sb.buf)]
-	nn, err := sb.r.ReadAt(buf, xpos)
+	nn, err := sb.r.ReadAt(sb.buf[:cap(sb.buf)], xpos)
 
 	// nothing read - just return the error
 	if nn == 0 {
@@ -105,7 +104,7 @@ func (sb *SeqBufReader) ReadAt(p []byte, pos int64) (n int, err error) {
 
 	// even if there was an error, but data partly read, we remember it in the buffer
 	sb.pos = xpos
-	sb.buf = buf[:nn]
+	sb.buf = sb.buf[:nn]
 
 	// here we know:
 	// - some data was read

t/neo/storage/fs1/xbufio_test.go

@@ -38,12 +38,9 @@ func (r *XReader) ReadAt(p []byte, pos int64) (n int, err error) {
 	return n, err
 }
 
-func TestSeqBufReader(t *testing.T) {
-	r := &XReader{}
-	rb := NewSeqBufReaderSize(r, 10) // with 10 it is easier to do/check math for a human
 
-	// read @pos/len -> rb.pos, len(rb.buf)
-	testv := []struct {pos int64; Len int; bufPos int64; bufLen int} {
+// read @pos/len -> rb.pos, len(rb.buf)
+var xSeqBufTestv = []struct {pos int64; Len int; bufPos int64; bufLen int} {
 	{40,  5, 40, 10},	// 1st access, forward by default
 	{45,  7, 50, 10},	// part taken from buf, part read next, forward
 	{52,  5, 50, 10},	// everything taken from buf
@@ -68,9 +65,14 @@ func TestSeqBufReader(t *testing.T) {
 	{254, 5, 250,  6},	// access overlapping EOF - EOF returned
 	{256, 1, 250,  6},	// access past EOF -> EOF
 	{257, 1, 250,  6},	// ----//----
-	}
+}
+
+
+func TestSeqBufReader(t *testing.T) {
+	r := &XReader{}
+	rb := NewSeqBufReaderSize(r, 10) // with 10 it is easier to do/check math for a human
 
-	for _, tt := range testv {
+	for _, tt := range xSeqBufTestv {
 		pOk := make([]byte, tt.Len)
 		pB  := make([]byte, tt.Len)
 
@@ -91,3 +93,15 @@ func TestSeqBufReader(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkSeqBufReader(b *testing.B) {
+	r := &XReader{}
+	rb := NewSeqBufReaderSize(r, 10) // same as in TestSeqBufReader
+	buf := make([]byte, 128 /* > all .Len in xSeqBufTestv */)
+
+	for i := 0; i < b.N; i++ {
+		for _, tt := range xSeqBufTestv {
+			rb.ReadAt(buf[:tt.Len], tt.pos)
+		}
+	}
+}