compress/zlib: use binary.BigEndian consistently

One major reason to avoid binary.BigEndian is because
the binary package includes a transitive dependency on reflect.
See #54097.

Given that writer.go already depends on the binary package,
embrace use of it consistently where sensible.
We should either embrace use of binary or fully avoid it.

Change-Id: I5f2d27d0ed8cab5ac54be02362c7d33276dd4b9a
Reviewed-on: https://go-review.googlesource.com/c/go/+/452176
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>
Run-TryBot: Joseph Tsai <[email protected]>
Auto-Submit: Joseph Tsai <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: dsnet

src/compress/zlib/reader.go

@@ -26,6 +26,7 @@ package zlib
 import (
 	"bufio"
 	"compress/flate"
+	"encoding/binary"
 	"errors"
 	"hash"
 	"hash/adler32"
@@ -110,7 +111,7 @@ func (z *reader) Read(p []byte) (int, error) {
 		return n, z.err
 	}
 	// ZLIB (RFC 1950) is big-endian, unlike GZIP (RFC 1952).
-	checksum := uint32(z.scratch[0])<<24 | uint32(z.scratch[1])<<16 | uint32(z.scratch[2])<<8 | uint32(z.scratch[3])
+	checksum := binary.BigEndian.Uint32(z.scratch[:4])
 	if checksum != z.digest.Sum32() {
 		z.err = ErrChecksum
 		return n, z.err
@@ -145,7 +146,7 @@ func (z *reader) Reset(r io.Reader, dict []byte) error {
 		}
 		return z.err
 	}
-	h := uint(z.scratch[0])<<8 | uint(z.scratch[1])
+	h := binary.BigEndian.Uint16(z.scratch[:2])
 	if (z.scratch[0]&0x0f != zlibDeflate) || (z.scratch[0]>>4 > zlibMaxWindow) || (h%31 != 0) {
 		z.err = ErrHeader
 		return z.err
@@ -159,7 +160,7 @@ func (z *reader) Reset(r io.Reader, dict []byte) error {
 			}
 			return z.err
 		}
-		checksum := uint32(z.scratch[0])<<24 | uint32(z.scratch[1])<<16 | uint32(z.scratch[2])<<8 | uint32(z.scratch[3])
+		checksum := binary.BigEndian.Uint32(z.scratch[:4])
 		if checksum != adler32.Checksum(dict) {
 			z.err = ErrDictionary
 			return z.err

src/compress/zlib/writer.go

@@ -115,7 +115,7 @@ func (z *Writer) writeHeader() (err error) {
 	if z.dict != nil {
 		z.scratch[1] |= 1 << 5
 	}
-	z.scratch[1] += uint8(31 - (uint16(z.scratch[0])<<8+uint16(z.scratch[1]))%31)
+	z.scratch[1] += uint8(31 - binary.BigEndian.Uint16(z.scratch[:2])%31)
 	if _, err = z.w.Write(z.scratch[0:2]); err != nil {
 		return err
 	}