Split SkBmpCodec into three separate classes

src/codec/SkCodecPriv.h

Index: src/codec/SkCodecPriv.h
diff --git a/src/codec/SkCodecPriv.h b/src/codec/SkCodecPriv.h
index 90b16323a76969dc691f00844bf00963a162fbf2..7bf75c69d8a7208d8df27aea2ee1f7a9d70dd7e3 100644
--- a/src/codec/SkCodecPriv.h
+++ b/src/codec/SkCodecPriv.h
@@ -8,6 +8,7 @@
 #ifndef SkCodecPriv_DEFINED
 #define SkCodecPriv_DEFINED
 
+#include "SkBmpCodec.h"
 #include "SkColorTable.h"
 #include "SkImageInfo.h"
 #include "SkSwizzler.h"
@@ -30,10 +31,30 @@
 #define COMPUTE_RESULT_ALPHA                    \
     SkSwizzler::GetResult(zeroAlpha, maxAlpha);
 
+static inline bool valid_alpha(SkAlphaType dstAlpha, SkAlphaType srcAlpha) {
+    // Check for supported alpha types
+    if (srcAlpha != dstAlpha) {
+        if (kOpaque_SkAlphaType == srcAlpha) {
+            // If the source is opaque, we must decode to opaque
+            return false;
+        }
+
+        // The source is not opaque
+        switch (dstAlpha) {
+            case kPremul_SkAlphaType:
+            case kUnpremul_SkAlphaType:
+                // The source is not opaque, so either of these is okay
+                break;
+            default:
+                // We cannot decode a non-opaque image to opaque (or unknown)
+                return false;
+        }
+    }
+    return true;
+}
+
 /*
- *
  * Copy the codec color table back to the client when kIndex8 color type is requested
- *
  */
 static inline void copy_color_table(const SkImageInfo& dstInfo, SkColorTable* colorTable,
         SkPMColor* inputColorPtr, int* inputColorCount) {
@@ -46,27 +67,21 @@ static inline void copy_color_table(const SkImageInfo& dstInfo, SkColorTable* co
 }
 
 /*
- *
  * Compute row bytes for an image using pixels per byte
- *
  */
 static inline size_t compute_row_bytes_ppb(int width, uint32_t pixelsPerByte) {
     return (width + pixelsPerByte - 1) / pixelsPerByte;
 }
 
 /*
- *
  * Compute row bytes for an image using bytes per pixel
- *
  */
 static inline size_t compute_row_bytes_bpp(int width, uint32_t bytesPerPixel) {
     return width * bytesPerPixel;
 }
 
 /*
- *
  * Compute row bytes for an image
- *
  */
 static inline size_t compute_row_bytes(int width, uint32_t bitsPerPixel) {
     if (bitsPerPixel < 16) {
@@ -81,20 +96,16 @@ static inline size_t compute_row_bytes(int width, uint32_t bitsPerPixel) {
 }
 
 /*
- *
  * Get a byte from a buffer
  * This method is unsafe, the caller is responsible for performing a check
- *
  */
 static inline uint8_t get_byte(uint8_t* buffer, uint32_t i) {
     return buffer[i];
 }
 
 /*
- *
  * Get a short from a buffer
  * This method is unsafe, the caller is responsible for performing a check
- *
  */
 static inline uint16_t get_short(uint8_t* buffer, uint32_t i) {
     uint16_t result;
@@ -107,10 +118,8 @@ static inline uint16_t get_short(uint8_t* buffer, uint32_t i) {
 }
 
 /*
- *
  * Get an int from a buffer
  * This method is unsafe, the caller is responsible for performing a check
- *
  */
 static inline uint32_t get_int(uint8_t* buffer, uint32_t i) {
     uint32_t result;