s2: Optimise readFloat64 to speed up Rect decoding.

This speeds up this path by removing allocations.

name           old time/op    new time/op     delta
RectDecode-12     491ns ± 4%      275ns ± 4%  -44.07%   (p=0.000 n=9+10)

name           old alloc/op   new alloc/op    delta
RectDecode-12      112B ± 0%        56B ± 0%  -50.00%  (p=0.000 n=10+10)

name           old allocs/op  new allocs/op   delta
RectDecode-12      9.00 ± 0%       2.00 ± 0%  -77.78%  (p=0.000 n=10+10)

name           old speed      new speed       delta
RectDecode-12  66.8MB/s ± 5%  120.2MB/s ± 4%  +79.86%  (p=0.000 n=10+10)

Signed-off-by: David Symonds <dsymonds@golang.org>

Author: nes1983

s2/encode.go

@@ -17,6 +17,7 @@ package s2
 import (
 	"encoding/binary"
 	"io"
+	"math"
 )
 
 const (
@@ -145,6 +146,15 @@ func asByteReader(r io.Reader) byteReader {
 type decoder struct {
 	r   byteReader // the real reader passed to Decode
 	err error
+	buf []byte
+}
+
+// Get a buffer of size 8, to avoid allocating over and over.
+func (d *decoder) buffer() []byte {
+	if d.buf == nil {
+		d.buf = make([]byte, 8)
+	}
+	return d.buf
 }
 
 func (d *decoder) readBool() (x bool) {
@@ -164,22 +174,6 @@ func (d *decoder) readInt8() (x int8) {
 	return
 }
 
-func (d *decoder) readInt16() (x int16) {
-	if d.err != nil {
-		return
-	}
-	d.err = binary.Read(d.r, binary.LittleEndian, &x)
-	return
-}
-
-func (d *decoder) readInt32() (x int32) {
-	if d.err != nil {
-		return
-	}
-	d.err = binary.Read(d.r, binary.LittleEndian, &x)
-	return
-}
-
 func (d *decoder) readInt64() (x int64) {
 	if d.err != nil {
 		return
@@ -212,20 +206,13 @@ func (d *decoder) readUint64() (x uint64) {
 	return
 }
 
-func (d *decoder) readFloat32() (x float32) {
+func (d *decoder) readFloat64() float64 {
 	if d.err != nil {
-		return
+		return 0
 	}
-	d.err = binary.Read(d.r, binary.LittleEndian, &x)
-	return
-}
-
-func (d *decoder) readFloat64() (x float64) {
-	if d.err != nil {
-		return
-	}
-	d.err = binary.Read(d.r, binary.LittleEndian, &x)
-	return
+	buf := d.buffer()
+	_, d.err = io.ReadFull(d.r, buf)
+	return math.Float64frombits(binary.LittleEndian.Uint64(buf))
 }
 
 func (d *decoder) readUvarint() (x uint64) {