optimized buffer refill

No extra allocation when refilling the buffer
Closes go-sql-driver#52

buffer.go

@@ -31,11 +31,14 @@ func newBuffer(rd io.Reader) *buffer {
  }
 }
 
-// fill reads at least _need_ bytes in the buffer
-// existing data in the buffer gets lost
+// fill reads into the buffer until at least _need_ bytes are in it
 func (b *buffer) fill(need int) (err error) {
+ // move existing data to the beginning
+ if b.length > 0 && b.idx > 0 {
+ copy(b.buf[0:b.length], b.buf[b.idx:])
+ }
+
  b.idx = 0
- b.length = 0
 
  var n int
  for b.length < need {
@@ -62,27 +65,30 @@ func (b *buffer) readNext(need int) (p []byte, err error) {
  return
 
  } else {
- p = make([]byte, need)
- has := 0
-
- // copy data that is already in the buffer
- if b.length > 0 {
- copy(p[0:b.length], b.buf[b.idx:])
- has = b.length
- need -= has
- b.idx = 0
- b.length = 0
- }
 
  // does the data fit into the buffer?
  if need < len(b.buf) {
+ // refill
  err = b.fill(need) // err deferred
- copy(p[has:has+need], b.buf[b.idx:])
+ p = b.buf[:need]
  b.idx += need
  b.length -= need
  return
 
  } else {
+ p = make([]byte, need)
+ has := 0
+
+ // copy data that is already in the buffer
+ if b.length > 0 {
+ copy(p[0:b.length], b.buf[b.idx:])
+ has = b.length
+ need -= has
+ b.idx = 0
+ b.length = 0
+ }
+
+ // read rest directly into the new slice
  var n int
  for err == nil && need > 0 {
  n, err = b.rd.Read(p[has:])