Add a class representing texture swizzle.

src/gpu/GrSwizzle.h

Index: src/gpu/GrSwizzle.h
diff --git a/src/gpu/GrSwizzle.h b/src/gpu/GrSwizzle.h
new file mode 100644
index 0000000000000000000000000000000000000000..87ea9534b9e021bfc6d5b1d0e942b1c5168307fd
--- /dev/null
+++ b/src/gpu/GrSwizzle.h
@@ -0,0 +1,92 @@
+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef GrSwizzle_DEFINED
+#define GrSwizzle_DEFINED
+
+#include "GrTypes.h"
+
+/** Represents a rgba swizzle. It can be converted either into a string or a eight bit int.
+    Currently there is no way to specify an arbitrary swizzle, just some static swizzles and an
+    assignment operator. That could be relaxed. */
+class GrSwizzle {
+public:
+    GrSwizzle() { *this = RGBA(); }
+
+    GrSwizzle& operator=(const GrSwizzle& that) {
+        memcpy(this, &that, sizeof(GrSwizzle));
+        return *this;
+    }
+
+    bool operator==(const GrSwizzle& that) const { return this->asUInt() == that.asUInt(); }
+
+    bool operator!=(const GrSwizzle& that) const { return !(*this == that); }
+
+    /** Compact representation of the swizzle suitable for a key. */
+    uint8_t asKey() const { return fKey; }
+
+    /** 4 char null terminated string consisting only of chars 'r', 'g', 'b', 'a'. */
+    const char* c_str() const { return fSwiz; }
+
+    static const GrSwizzle& RGBA() {
+        static GrSwizzle gRGBA("rgba");
+        return gRGBA;
+    }
+
+    static const GrSwizzle& AAAA() {
+        static GrSwizzle gAAAA("aaaa");
+        return gAAAA;
+    }
+
+    static const GrSwizzle& RRRR() {
+        static GrSwizzle gRRRR("rrrr");
+        return gRRRR;
+    }
+
+    static const GrSwizzle& BGRA() {
+        static GrSwizzle gBGRA("bgra");
+        return gBGRA;
+    }
+
+private:
+    char fSwiz[5];
+    uint8_t fKey;
+
+    static int CharToIdx(char c) {
+        switch (c) {
+            case 'r':
+                return 0;
+            case 'g':
+                return 1;
+            case 'b':
+                return 2;
+            case 'a':
+                return 3;
+            default:
+                SkFAIL("Invalid swizzle char");
+                return 0;
+        }
+    }
+
+    explicit GrSwizzle(const char* str) {
+        SkASSERT(strlen(str) == 4);
+        fSwiz[0] = str[0];
+        fSwiz[1] = str[1];
+        fSwiz[2] = str[2];
+        fSwiz[3] = str[3];
+        fSwiz[4] = 0;
+        fKey = SkToU8(CharToIdx(fSwiz[0]) | (CharToIdx(fSwiz[1]) << 2) |
+                      (CharToIdx(fSwiz[2]) << 4) | (CharToIdx(fSwiz[3]) << 6));
+    }
+
+    uint32_t* asUIntPtr() { return SkTCast<uint32_t*>(fSwiz); }
+    uint32_t asUInt() const { return *SkTCast<const uint32_t*>(fSwiz); }
+
+    GR_STATIC_ASSERT(sizeof(char[4]) == sizeof(uint32_t));
+};
+
+#endif