1 files changed, 76 insertions, 16 deletions
diff --git a/libgo/go/hash/crc32/crc32_generic.go b/libgo/go/hash/crc32/crc32_generic.go
index e9096cef786..abacbb663d4 100644
--- a/libgo/go/hash/crc32/crc32_generic.go
+++ b/libgo/go/hash/crc32/crc32_generic.go
@@ -2,28 +2,88 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// -build !amd64,!amd64p32,!s390x
+// This file contains CRC32 algorithms that are not specific to any architecture
+// and don't use hardware acceleration.
+//
+// The simple (and slow) CRC32 implementation only uses a 256*4 bytes table.
+//
+// The slicing-by-8 algorithm is a faster implementation that uses a bigger
+// table (8*256*4 bytes).
 
 package crc32
 
-// This file contains the generic version of updateCastagnoli which does
-// slicing-by-8, or uses the fallback for very small sizes.
+// simpleMakeTable allocates and constructs a Table for the specified
+// polynomial. The table is suitable for use with the simple algorithm
+// (simpleUpdate).
+func simpleMakeTable(poly uint32) *Table {
+	t := new(Table)
+	simplePopulateTable(poly, t)
+	return t
+}
+
+// simplePopulateTable constructs a Table for the specified polynomial, suitable
+// for use with simpleUpdate.
+func simplePopulateTable(poly uint32, t *Table) {
+	for i := 0; i < 256; i++ {
+		crc := uint32(i)
+		for j := 0; j < 8; j++ {
+			if crc&1 == 1 {
+				crc = (crc >> 1) ^ poly
+			} else {
+				crc >>= 1
+			}
+		}
+		t[i] = crc
+	}
+}
+
+// simpleUpdate uses the simple algorithm to update the CRC, given a table that
+// was previously computed using simpleMakeTable.
+func simpleUpdate(crc uint32, tab *Table, p []byte) uint32 {
+	crc = ^crc
+	for _, v := range p {
+		crc = tab[byte(crc)^v] ^ (crc >> 8)
+	}
+	return ^crc
+}
+
+// Use slicing-by-8 when payload >= this value.
+const slicing8Cutoff = 16
 
-func updateCastagnoli(crc uint32, p []byte) uint32 {
-	// Use slicing-by-8 on larger inputs.
-	if len(p) >= sliceBy8Cutoff {
-		return updateSlicingBy8(crc, castagnoliTable8, p)
+// slicing8Table is array of 8 Tables, used by the slicing-by-8 algorithm.
+type slicing8Table [8]Table
+
+// slicingMakeTable constructs a slicing8Table for the specified polynomial. The
+// table is suitable for use with the slicing-by-8 algorithm (slicingUpdate).
+func slicingMakeTable(poly uint32) *slicing8Table {
+	t := new(slicing8Table)
+	simplePopulateTable(poly, &t[0])
+	for i := 0; i < 256; i++ {
+		crc := t[0][i]
+		for j := 1; j < 8; j++ {
+			crc = t[0][crc&0xFF] ^ (crc >> 8)
+			t[j][i] = crc
+		}
 	}
-	return update(crc, castagnoliTable, p)
+	return t
 }
 
-func updateIEEE(crc uint32, p []byte) uint32 {
-	// Use slicing-by-8 on larger inputs.
-	if len(p) >= sliceBy8Cutoff {
-		ieeeTable8Once.Do(func() {
-			ieeeTable8 = makeTable8(IEEE)
-		})
-		return updateSlicingBy8(crc, ieeeTable8, p)
+// slicingUpdate uses the slicing-by-8 algorithm to update the CRC, given a
+// table that was previously computed using slicingMakeTable.
+func slicingUpdate(crc uint32, tab *slicing8Table, p []byte) uint32 {
+	if len(p) >= slicing8Cutoff {
+		crc = ^crc
+		for len(p) > 8 {
+			crc ^= uint32(p[0]) | uint32(p[1])<<8 | uint32(p[2])<<16 | uint32(p[3])<<24
+			crc = tab[0][p[7]] ^ tab[1][p[6]] ^ tab[2][p[5]] ^ tab[3][p[4]] ^
+				tab[4][crc>>24] ^ tab[5][(crc>>16)&0xFF] ^
+				tab[6][(crc>>8)&0xFF] ^ tab[7][crc&0xFF]
+			p = p[8:]
+		}
+		crc = ^crc
+	}
+	if len(p) == 0 {
+		return crc
 	}
-	return update(crc, IEEETable, p)
+	return simpleUpdate(crc, &tab[0], p)
 }