hash/crc32: remove redundant code

Merge the CRC32 Update and Write functions using an unexported function, to avoid duplication of code and make it more readable.
The only difference between them is the check of the initialization of the IEEE table, and a boolean value specifies that.

Throughout the crc32.go file, in the switches the default value is inserted inside the switch statement, this change uniforms the style of the MakeTable function, making it like the other pieces of code.

Change-Id: I3889f6c6671210c82f0d7250cea67907bccf3ce7
GitHub-Last-Rev: b8777ee213
GitHub-Pull-Request: golang/go#55044
Reviewed-on: https://go-review.googlesource.com/c/go/+/430456
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Jenny Rakoczy <jenny@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Jenny Rakoczy <jenny@golang.org>
Auto-Submit: Jenny Rakoczy <jenny@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>

src/hash/crc32/crc32.go

@@ -128,8 +128,9 @@ func MakeTable(poly uint32) *Table {
 	case Castagnoli:
 		castagnoliOnce.Do(castagnoliInit)
 		return castagnoliTable
+	default:
+		return simpleMakeTable(poly)
 	}
-	return simpleMakeTable(poly)
 }
 
 // digest represents the partial evaluation of a checksum.
@@ -205,32 +206,31 @@ func readUint32(b []byte) uint32 {
 	return uint32(b[3]) | uint32(b[2])<<8 | uint32(b[1])<<16 | uint32(b[0])<<24
 }
 
-// Update returns the result of adding the bytes in p to the crc.
-func Update(crc uint32, tab *Table, p []byte) uint32 {
+func update(crc uint32, tab *Table, p []byte, checkInitIEEE bool) uint32 {
 	switch {
 	case haveCastagnoli.Load() && tab == castagnoliTable:
 		return updateCastagnoli(crc, p)
 	case tab == IEEETable:
-		// Unfortunately, because IEEETable is exported, IEEE may be used without a
-		// call to MakeTable. We have to make sure it gets initialized in that case.
-		ieeeOnce.Do(ieeeInit)
+		if checkInitIEEE {
+			ieeeOnce.Do(ieeeInit)
+		}
 		return updateIEEE(crc, p)
 	default:
 		return simpleUpdate(crc, tab, p)
 	}
 }
 
+// Update returns the result of adding the bytes in p to the crc.
+func Update(crc uint32, tab *Table, p []byte) uint32 {
+	// Unfortunately, because IEEETable is exported, IEEE may be used without a
+	// call to MakeTable. We have to make sure it gets initialized in that case.
+	return update(crc, tab, p, true)
+}
+
 func (d *digest) Write(p []byte) (n int, err error) {
-	switch {
-	case haveCastagnoli.Load() && d.tab == castagnoliTable:
-		d.crc = updateCastagnoli(d.crc, p)
-	case d.tab == IEEETable:
-		// We only create digest objects through New() which takes care of
-		// initialization in this case.
-		d.crc = updateIEEE(d.crc, p)
-	default:
-		d.crc = simpleUpdate(d.crc, d.tab, p)
-	}
+	// We only create digest objects through New() which takes care of
+	// initialization in this case.
+	d.crc = update(d.crc, d.tab, p, false)
 	return len(p), nil
 }
 

src/hash/crc32/crc32_test.go

@@ -329,11 +329,14 @@ func benchmark(b *testing.B, h hash.Hash32, n, alignment int64) {
 	h.Reset()
 	h.Write(data)
 	h.Sum(in)
+	// Avoid further allocations
+	in = in[:0]
 
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
 		h.Reset()
 		h.Write(data)
 		h.Sum(in)
+		in = in[:0]
 	}
 }