Interlaced gifs without the iterator

BUG=skia:

Review URL: https://codereview.chromium.org/1315583003

4 files changed, 29 insertions, 92 deletions

diff --git a/gyp/codec.gyp b/gyp/codec.gyp
index b34c85e7d9..a9a55d5638 100644
--- a/gyp/codec.gyp
+++ b/gyp/codec.gyp
@@ -41,7 +41,6 @@
         '../src/codec/SkCodec_libico.cpp',
         '../src/codec/SkCodec_libpng.cpp',
         '../src/codec/SkCodec_wbmp.cpp',
-        '../src/codec/SkGifInterlaceIter.cpp',
         '../src/codec/SkJpegCodec.cpp',
         '../src/codec/SkJpegDecoderMgr.cpp',
         '../src/codec/SkJpegUtility_codec.cpp',
diff --git a/src/codec/SkCodec_libgif.cpp b/src/codec/SkCodec_libgif.cpp
index 20a5572d14..3e78f23322 100644
--- a/src/codec/SkCodec_libgif.cpp
+++ b/src/codec/SkCodec_libgif.cpp
@@ -9,7 +9,6 @@
 #include "SkCodecPriv.h"
 #include "SkColorPriv.h"
 #include "SkColorTable.h"
-#include "SkGifInterlaceIter.h"
 #include "SkStream.h"
 #include "SkSwizzler.h"
 #include "SkUtils.h"
@@ -116,6 +115,31 @@ static uint32_t find_trans_index(const SavedImage& image) {
     return SK_MaxU32;
 }
 
+static inline uint32_t ceil_div(uint32_t a, uint32_t b) {
+    return (a + b - 1) / b;
+}
+
+/*
+ * Gets the output row corresponding to the encoded row for interlaced gifs
+ */
+static uint32_t get_output_row_interlaced(uint32_t encodedRow, uint32_t height) {
+    SkASSERT(encodedRow < height);
+    // First pass
+    if (encodedRow * 8 < height) {
+        return encodedRow * 8;
+    }
+    // Second pass
+    if (encodedRow * 4 < height) {
+        return 4 + 8 * (encodedRow - ceil_div(height, 8));
+    }
+    // Third pass
+    if (encodedRow * 2 < height) {
+        return 2 + 4 * (encodedRow - ceil_div(height, 4));
+    }
+    // Fourth pass
+    return 1 + 2 * (encodedRow - ceil_div(height, 2));
+}
+
 /*
  * Read enough of the stream to initialize the SkGifCodec.
  * Returns a bool representing success or failure.
@@ -409,18 +433,14 @@ SkCodec::Result SkGifCodec::onGetPixels(const SkImageInfo& dstInfo,
 
                 // Check the interlace flag and iterate over rows of the input
                 if (fGif->Image.Interlace) {
-                    // In interlace mode, the rows of input are rearranged in
-                    // the output image.  We use an iterator to take care of
-                    // the rearranging.
-                    SkGifInterlaceIter iter(innerHeight);
                     for (int32_t y = 0; y < innerHeight; y++) {
                         if (GIF_ERROR == DGifGetLine(fGif, buffer.get(), innerWidth)) {
                             // Recover from error by filling remainder of image
                             if (!skipBackground) {
                                 memset(buffer.get(), fillIndex, innerWidth);
                                 for (; y < innerHeight; y++) {
-                                    void* dstRow = SkTAddOffset<void>(dst,
-                                                                      dstRowBytes * iter.nextY());
+                                    void* dstRow = SkTAddOffset<void>(dst, dstRowBytes *
+                                            get_output_row_interlaced(y, innerHeight));
                                     swizzler->swizzle(dstRow, buffer.get());
                                 }
                             }
@@ -428,7 +448,8 @@ SkCodec::Result SkGifCodec::onGetPixels(const SkImageInfo& dstInfo,
                                     "Could not decode line %d of %d.\n",
                                     y, height - 1).c_str(), kIncompleteInput);
                         }
-                        void* dstRow = SkTAddOffset<void>(dst, dstRowBytes * iter.nextY());
+                        void* dstRow = SkTAddOffset<void>(dst,
+                                dstRowBytes * get_output_row_interlaced(y, innerHeight));
                         swizzler->swizzle(dstRow, buffer.get());
                     }
                 } else {
diff --git a/src/codec/SkGifInterlaceIter.cpp b/src/codec/SkGifInterlaceIter.cpp
deleted file mode 100644
index 3a87f6950d..0000000000
--- a/src/codec/SkGifInterlaceIter.cpp
+++ /dev/null
@@ -1,48 +0,0 @@
-/*
- * Copyright 2015 The Android Open Source Project
- *
- * Use of this source code is governed by a BSD-style license that can be
- * found in the LICENSE file.
- */
-
-#include "SkGifInterlaceIter.h"
-
-static const uint8_t kStartingInterlaceYValues[] = { 0, 4, 2, 1 };
-static const uint8_t kDeltaInterlaceYValues[] = { 8, 8, 4, 2 };
-
-SkGifInterlaceIter::SkGifInterlaceIter(int height) : fHeight(height) {
-    fStartYPtr = kStartingInterlaceYValues;
-    fDeltaYPtr = kDeltaInterlaceYValues;
-
-    fCurrY = *fStartYPtr++;
-    fDeltaY = *fDeltaYPtr++;
-}
-
-void SkGifInterlaceIter::prepareY() {
-    int32_t y = fCurrY + fDeltaY;
-
-    // Iterate through fStartYPtr until a valid row is found.
-    // This ensures that we do not move past the height of the small images.
-    while (y >= fHeight) {
-        if (kStartingInterlaceYValues +
-                SK_ARRAY_COUNT(kStartingInterlaceYValues) == fStartYPtr) {
-            // Now we have iterated over the entire image.  Forbid any
-            // subsequent calls to nextY().
-            SkDEBUGCODE(fStartYPtr = nullptr;)
-            SkDEBUGCODE(fDeltaYPtr = nullptr;)
-            y = 0;
-        } else {
-            y = *fStartYPtr++;
-            fDeltaY = *fDeltaYPtr++;
-        }
-    }
-    fCurrY = y;
-}
-
-int32_t SkGifInterlaceIter::nextY() {
-    SkASSERT(fStartYPtr);
-    SkASSERT(fDeltaYPtr);
-    int32_t y = fCurrY;
-    prepareY();
-    return y;
-}
diff --git a/src/codec/SkGifInterlaceIter.h b/src/codec/SkGifInterlaceIter.h
deleted file mode 100644
index 970cd4950f..0000000000
--- a/src/codec/SkGifInterlaceIter.h
+++ /dev/null
@@ -1,35 +0,0 @@
-/*
- * Copyright 2015 The Android Open Source Project
- *
- * Use of this source code is governed by a BSD-style license that can be
- * found in the LICENSE file.
- */
-
-#include "SkTypes.h"
-
-/*
- * Helper class to determine the destination y-values for interlaced gifs
- */
-class SkGifInterlaceIter : SkNoncopyable {
-public:
-
-    explicit SkGifInterlaceIter(int32_t height);
-
-    /*
-     * Get the next destination y-value
-     */
-    int32_t nextY();
-
-private:
-
-    /*
-     * Updates the iterator to prepare the next y-value
-     */
-    void prepareY();
-
-    const int32_t fHeight;
-    int32_t        fCurrY;
-    int32_t        fDeltaY;
-    const uint8_t* fStartYPtr;
-    const uint8_t* fDeltaYPtr;
-};