add SkChecksum as a static class, for the replacement API

after this lands, plan to deprecate/remove the older APIs
Review URL: https://codereview.appspot.com/6356059

git-svn-id: http://skia.googlecode.com/svn/trunk@4457 2bbb7eff-a529-9590-31e7-b0007b416f81

bench/ChecksumBench.cpp

@@ -61,15 +61,33 @@ private:
 class ComputeChecksum64Bench : public ComputeChecksumBench {
 public:
     ComputeChecksum64Bench(void* param)
-        : INHERITED(param, "64") { }
-
+    : INHERITED(param, "64") { }
+    
 protected:
     virtual void computeChecksum(const uint64_t* data, size_t len) {
         for (int i = 0; i < N; i++) {
             volatile uint64_t result = SkComputeChecksum64(data, len);
         }
     }
+    
+private:
+    typedef ComputeChecksumBench INHERITED;
+};
 
+/*
+ *  Use SkComputeChecksum64 to compute a checksum on a datablock
+ */
+class ComputeChecksumXXBench : public ComputeChecksumBench {
+public:
+    ComputeChecksumXXBench(void* param) : INHERITED(param, "XX") { }
+    
+protected:
+    virtual void computeChecksum(const uint64_t* data, size_t len) {
+        for (int i = 0; i < N; i++) {
+            volatile uint32_t result = SkChecksum::Compute(reinterpret_cast<const uint32_t*>(data), len);
+        }
+    }
+    
 private:
     typedef ComputeChecksumBench INHERITED;
 };
@@ -78,6 +96,8 @@ private:
 
 static SkBenchmark* Fact0(void* p) { return new ComputeChecksum32Bench(p); }
 static SkBenchmark* Fact1(void* p) { return new ComputeChecksum64Bench(p); }
+static SkBenchmark* Fact2(void* p) { return new ComputeChecksumXXBench(p); }
 
 static BenchRegistry gReg0(Fact0);
 static BenchRegistry gReg1(Fact1);
+static BenchRegistry gReg2(Fact2);

include/core/SkChecksum.h

@@ -67,5 +67,78 @@ inline uint32_t SkComputeChecksum32(const uint32_t* ptr, size_t size) {
     }
     return result;
 }
+
+class SkChecksum : 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
+     *
+     *  @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.
+         */
+        int n4 = size / (sizeof(uintptr_t) << 2);
+        for (int 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