SkBlockMemoryStream implements peek()

src/core/SkStream.cpp

 
 /*
  * Copyright 2006 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 "SkStream.h"
@@ skipping 706 matching lines @@
             fCurrentOffset = 0;
         }
         SkASSERT(false);
         return 0;
     }
 
     bool isAtEnd() const override {
         return fOffset == fSize;
     }
 
+    bool peek(void* buff, size_t size) const override {
+        SkASSERT(buff != NULL);
+        if (fOffset + size > fSize) {
+            return false;
+        }
+        char* buffer = static_cast<char*>(buff);
+        const SkDynamicMemoryWStream::Block* current = fCurrent;

[scroggo, 2015/05/21 14:13:18]

Patch set 2 separates out the code for dealing with the current block from the
code that deals with the rest. AFAICT, this means we skip the following
(unnecessary) calculations for all loop iterations but the first (which
technically is no longer an iteration of the loop):

- subtract 0
- add 0
- assign 0 to a variable (this is removed from the first as well)

On the other hand, we have some mostly duplicated code. Unless the original code
is noticeably slower, I'd prefer less duplication.

[hal.canary, 2015/05/21 14:35:47]

reverted

+        size_t bytesFromCurrent =
+            SkTMin(current->written() - fCurrentOffset, size);
+        memcpy(buffer, current->start() + fCurrentOffset, bytesFromCurrent);
+        size -= bytesFromCurrent;
+        buffer += bytesFromCurrent;
+        current = current->fNext;
+        while (size) {
+            SkASSERT(current);
+            bytesFromCurrent = SkTMin(current->written(), size);
+            memcpy(buffer, current->start(), bytesFromCurrent);
+            size -= bytesFromCurrent;
+            buffer += bytesFromCurrent;
+            current = current->fNext;
+        }
+        return true;
+    }
+
     bool rewind() override {
         fCurrent = fBlockMemory->fHead;
         fOffset = 0;
         fCurrentOffset = 0;
         return true;
     }
 
     SkBlockMemoryStream* duplicate() const override {
         return SkNEW_ARGS(SkBlockMemoryStream, (fBlockMemory.get(), fSize));
     }
@@ skipping 185 matching lines @@
     SkDynamicMemoryWStream tempStream;
     const size_t bufferSize = 4096;
     char buffer[bufferSize];
     do {
         size_t bytesRead = stream->read(buffer, bufferSize);
         tempStream.write(buffer, bytesRead);
     } while (!stream->isAtEnd());
     return tempStream.detachAsStream();  // returns a SkBlockMemoryStream,
                                          // cheaper than copying to SkData
 }