Add a method to read a stream without advancing it.

src/utils/SkFrontBufferedStream.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkFrontBufferedStream.h"
 #include "SkStream.h"
 #include "SkTemplates.h"
 
 class FrontBufferedStream : public SkStreamRewindable {
 public:
     // Called by Create.
     FrontBufferedStream(SkStream*, size_t bufferSize);
 
     size_t read(void* buffer, size_t size) override;
 
+    bool peek(void* buffer, size_t size) const override;
+
     bool isAtEnd() const override;
 
     bool rewind() override;
 
     bool hasPosition() const override { return true; }
 
     size_t getPosition() const override { return fOffset; }
 
     bool hasLength() const override { return fHasLength; }
 
@@ skipping 119 matching lines @@
 
     // If we have read past the end of the buffer, rewinding is no longer
     // supported, so we can go ahead and free the memory.
     if (bytesReadDirectly > 0) {
         sk_free(fBuffer.detach());
     }
 
     return bytesReadDirectly;
 }
 
+bool FrontBufferedStream::peek(void* dst, size_t size) const {
+    // Keep track of the offset so we can return to it.
+    const size_t start = fOffset;
+    if (start + size > fBufferSize) {
+        // This stream is not able to buffer enough.
+        return false;
+    }
+    FrontBufferedStream* nonConstThis = const_cast<FrontBufferedStream*>(this);
+#ifdef SK_DEBUG
+    const size_t bytesRead =
+#endif
+    nonConstThis->read(dst, size);
+    SkASSERT(bytesRead == size);
+    nonConstThis->fOffset = start;
+    return bytesRead;

[scroggo, 2015/04/02 19:49:43]

Oops, this should be true. Good thing I tested in Release!

+}
+
 size_t FrontBufferedStream::read(void* voidDst, size_t size) {
     // Cast voidDst to a char* for easy addition.
     char* dst = reinterpret_cast<char*>(voidDst);
     SkDEBUGCODE(const size_t totalSize = size;)
     const size_t start = fOffset;
 
     // First, read any data that was previously buffered.
     if (fOffset < fBufferedSoFar) {
         const size_t bytesCopied = this->readFromBuffer(dst, size);
 
@@ skipping 21 matching lines @@
     }
 
     if (size > 0 && !fStream->isAtEnd()) {
         SkDEBUGCODE(const size_t bytesReadDirectly =) this->readDirectlyFromStream(dst, size);
         SkDEBUGCODE(size -= bytesReadDirectly;)
         SkASSERT(size + (fOffset - start) == totalSize);
     }
 
     return fOffset - start;
 }