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

1 files changed, 95 insertions, 0 deletions

diff --git a/src/utils/SkConsistentChecksum.h b/src/utils/SkConsistentChecksum.h
new file mode 100644
index 0000000000..8b7c53dde8
--- /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