Split off SkConsistentChecksum from SkChecksum
as part of https://goto.google.com/ImprovingTheSkiaRebaseliningProcess
Review URL: https://codereview.appspot.com/6820074
git-svn-id: http://skia.googlecode.com/svn/trunk@6270 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/utils/SkConsistentChecksum.h b/src/utils/SkConsistentChecksum.h
new file mode 100644
index 0000000..8b7c53d
--- /dev/null
+++ b/src/utils/SkConsistentChecksum.h
@@ -0,0 +1,95 @@
+/*
+ * Copyright 2012 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkConsistentChecksum_DEFINED
+#define SkConsistentChecksum_DEFINED
+
+#include "SkTypes.h"
+
+class SkConsistentChecksum : SkNoncopyable {
+private:
+ /*
+ * Our Rotate and Mash helpers are meant to automatically do the right
+ * thing depending if sizeof(uintptr_t) is 4 or 8.
+ */
+ enum {
+ ROTR = 17,
+ ROTL = sizeof(uintptr_t) * 8 - ROTR,
+ HALFBITS = sizeof(uintptr_t) * 4
+ };
+
+ static inline uintptr_t Mash(uintptr_t total, uintptr_t value) {
+ return ((total >> ROTR) | (total << ROTL)) ^ value;
+ }
+
+public:
+ /**
+ * Compute a 32-bit checksum for a given data block
+ *
+ * WARNING: As of 1 Nov 2012, this algorithm is still in
+ * flux... but once we get it doing what we want, it will be:
+ * 1. consistent across revisions of the library (for a given set
+ * of bytes, the checksum generated at one revision of the Skia
+ * library will match the one generated on any other revision of
+ * the Skia library)
+ * 2. consistent across platforms (for a given
+ * set of bytes, the checksum generated on one platform will
+ * match the one generated on any other platform)
+ *
+ * @param data Memory address of the data block to be processed. Must be
+ * 32-bit aligned.
+ * @param size Size of the data block in bytes. Must be a multiple of 4.
+ * @return checksum result
+ */
+ static uint32_t Compute(const uint32_t* data, size_t size) {
+ SkASSERT(SkIsAlign4(size));
+
+ /*
+ * We want to let the compiler use 32bit or 64bit addressing and math
+ * so we use uintptr_t as our magic type. This makes the code a little
+ * more obscure (we can't hard-code 32 or 64 anywhere, but have to use
+ * sizeof()).
+ */
+ uintptr_t result = 0;
+ const uintptr_t* ptr = reinterpret_cast<const uintptr_t*>(data);
+
+ /*
+ * count the number of quad element chunks. This takes into account
+ * if we're on a 32bit or 64bit arch, since we use sizeof(uintptr_t)
+ * to compute how much to shift-down the size.
+ */
+ size_t n4 = size / (sizeof(uintptr_t) << 2);
+ for (size_t i = 0; i < n4; ++i) {
+ result = Mash(result, *ptr++);
+ result = Mash(result, *ptr++);
+ result = Mash(result, *ptr++);
+ result = Mash(result, *ptr++);
+ }
+ size &= ((sizeof(uintptr_t) << 2) - 1);
+
+ data = reinterpret_cast<const uint32_t*>(ptr);
+ const uint32_t* stop = data + (size >> 2);
+ while (data < stop) {
+ result = Mash(result, *data++);
+ }
+
+ /*
+ * smash us down to 32bits if we were 64. Note that when uintptr_t is
+ * 32bits, this code-path should go away, but I still got a warning
+ * when I wrote
+ * result ^= result >> 32;
+ * since >>32 is undefined for 32bit ints, hence the wacky HALFBITS
+ * define.
+ */
+ if (8 == sizeof(result)) {
+ result ^= result >> HALFBITS;
+ }
+ return static_cast<uint32_t>(result);
+ }
+};
+
+#endif