Move gif image decoder to SkCodec

third_party/WebKit/Source/platform/image-decoders/SegmentStream.cpp

+// Copyright (c) 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "platform/image-decoders/SegmentStream.h"
+
+namespace blink {
+
+SegmentStream::SegmentStream()
+    : reader_(),
+      position_(0),
+      has_read_all_contents_(true),
+      is_cleared_(true) {}
+
+void SegmentStream::SetReader(SegmentReader* reader) {
+  reader_ = reader;
+  UpdateInternals(position_);
+}
+
+size_t SegmentStream::read(void* buffer, size_t size) {
+  if (is_cleared_)
+    return 0;
+
+  size = std::min(size, reader_->size() - position_);
+
+  size_t bytes_advanced = 0;
+  if (!buffer) {  // skipping, not reading
+    bytes_advanced = size;
+  } else {
+    bytes_advanced = peek(buffer, size);
+  }
+
+  position_ += bytes_advanced;
+  UpdateInternals(position_);
+
+  return bytes_advanced;
+}
+
+size_t SegmentStream::peek(void* buffer, size_t size) const {
+  if (is_cleared_)
+    return 0;
+
+  size = std::min(size, reader_->size() - position_);
+
+  size_t peek_position = position_;
+  size_t total_bytes_peeked = 0;
+  char* buffer_as_char_ptr = reinterpret_cast<char*>(buffer);
+  while (size) {
+    const char* segment = nullptr;
+    size_t bytes_peeked = reader_->GetSomeData(segment, peek_position);
+    if (!bytes_peeked)
+      break;
+    if (bytes_peeked > size)
+      bytes_peeked = size;
+
+    memcpy(buffer_as_char_ptr, segment, bytes_peeked);
+    buffer_as_char_ptr += bytes_peeked;
+    size -= bytes_peeked;
+    total_bytes_peeked += bytes_peeked;
+    peek_position += bytes_peeked;
+  }
+
+  return total_bytes_peeked;
+}
+
+bool SegmentStream::rewind() {
+  UpdateInternals(0);
+  return true;
+}
+
+bool SegmentStream::seek(size_t position) {
+  // If we do not have a reader, we can either:
+  // * ignore this input, or
+  // * blindly follow it without checking against bounds.
+  // Given those two options, blindly following sounds better.

[scroggo_chromium, 2017/04/24 20:25:07]

What would happen if we blindly followed in the case where there is a reader? I
think peek/read will already properly fail silently if position_ is beyond the
end of reader_->size().

What if we went through the following:

- seek(position) (where position > reader_->size())
- setReader(reader) (where the new reader's size() > position_, but < the
previously supplied position)

Then we want the previously supplied position, right?

(Maybe this cannot happen anyway?)

[cblume, 2017/04/24 21:35:53]

I like your idea. That would simplify things a lot.
I'll try it out.

+  if (reader_) {
+    position = std::min(position, reader_->size());
+  }
+  UpdateInternals(position);
+  return true;
+}
+
+bool SegmentStream::move(long offset) {
+  long absolute_position = position_ + offset;
+
+  // clamp inside the bounds of the buffer size
+  absolute_position = std::max(absolute_position, 0l);
+  absolute_position =
+      std::min(static_cast<size_t>(absolute_position), reader_->size());

[scroggo_chromium, 2017/04/24 20:25:07]

Should this one check for a null reader_, too?

Oh, I was going to suggest that this relied on UpdateInternals to handle the
case where the position is > reader_->size(), but UpdateInternals behaves
differently from this one. Why?

[cblume, 2017/04/24 21:35:53]

I don't think there is a good reason why. It is probably a mistake.

I like your idea of allowing going past the end without caring about reader_
existing or not. I'll update here, too.

+
+  UpdateInternals(absolute_position);
+  return true;
+}
+
+void SegmentStream::UpdateInternals(size_t new_position) {
+  position_ = new_position;
+  if (reader_) {
+    const size_t reader_size = reader_->size();
+    has_read_all_contents_ = position_ >= reader_size;
+    is_cleared_ = position_ > reader_size;
+  } else {
+    has_read_all_contents_ = true;
+    is_cleared_ = true;
+  }
+}
+
+}  // namespace blink