Make readback of alpha channel work for RGBA.

src/gpu/GrSwizzle.h

 /*
  * 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"
+#include "GrColor.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(const GrSwizzle& that) { *this = that; }
+
+    void setFromKey(uint8_t key) {
+        fKey = key;
+        for (int i = 0; i < 4; ++i) {
+            fSwiz[i] = IdxToChar(key & 3);
+            key >>= 2;
+        }
+        SkASSERT(fSwiz[4] == 0);
+    }
+
     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; }
 
+    GrColor applyTo(GrColor color) const {
+        int idx;
+        uint32_t key = fKey;
+
+        // Index of the input color that should be mapped to output r.
+        idx = (key & 3);
+        uint32_t outR = (color >> idx * 8)  & 0xFF;
+        key >>= 2;
+
+        idx = (key & 3);
+        uint32_t outG = (color >> idx * 8)  & 0xFF;
+        key >>= 2;
+
+        idx = (key & 3);
+        uint32_t outB = (color >> idx * 8)  & 0xFF;
+        key >>= 2;
+
+        idx = (key & 3);
+        uint32_t outA = (color >> idx * 8)  & 0xFF;
+
+        return GrColorPackRGBA(outR, outG, outB, outA);
+    }
+
     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;
+                return (GrColor_SHIFT_R / 8);
             case 'g':
-                return 1;
+                return (GrColor_SHIFT_G / 8);
             case 'b':
-                return 2;
+                return (GrColor_SHIFT_B/ 8);
             case 'a':
-                return 3;
+                return (GrColor_SHIFT_A / 8);
             default:
                 SkFAIL("Invalid swizzle char");
                 return 0;
         }
     }
 
+    static /* constexpr */ char IToC(int idx) {
+        return (8*idx) == GrColor_SHIFT_R ? 'r' :
+               (8*idx) == GrColor_SHIFT_G ? 'g' :
+               (8*idx) == GrColor_SHIFT_B ? 'b' : 
+               'a';
+    }
+
+    static char IdxToChar(int c) {
+        // Hopefully this array gets computed at compile time.
+        static const char gStr[4] = { IToC(0), IToC(1), IToC(2), IToC(3) };
+        return gStr[c];
+    }
+
     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