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// Copyright 2015 The Chromium OS 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 <stdio.h>
#include <memory>
#include <vector>
#include "file_interface.h"
* Extent files.
* This modules provides a familiar interface for handling files through an
* indirection layer of extents, which are contiguous chunks of variable length
* at arbitrary offsets within a file. Once an extent file handle is obtained,
* users may read, write and seek as they do with ordinary files, having the I/O
* with the underlying file done for them by the extent file implementation. The
* implementation supports "sparse extents", which are assumed to contain zeros
* but otherwise have no actual representation in the underlying file; these are
* denoted by negative offset values.
* Unlike ordinary files, the size of an extent file is fixed; it is not
* truncated on open, nor is writing past the extent span allowed. Also, writing
* to a sparse extent has no effect and will not raise an error.
namespace bsdiff {
/* An extent, defined by an offset and a length. */
struct ex_t {
off_t off; // the extent offset; negative indicates a sparse extent.
uint64_t len; // the extent length.
class ExtentsFile : public FileInterface {
// Creates an ExtentsFile based on the underlying |file| passed. The positions
// in the ExtentsFile will be linearly mapped to the extents provided in
// |extents|. The created ExtentsFile takes ownership of the |file| will close
// it on destruction.
ExtentsFile(std::unique_ptr<FileInterface> file,
const std::vector<ex_t>& extents);
~ExtentsFile() override;
// FileInterface overrides.
bool Read(void* buf, size_t count, size_t* bytes_read) override;
bool Write(const void* buf, size_t count, size_t* bytes_written) override;
bool Seek(off_t pos) override;
bool Close() override;
bool GetSize(uint64_t* size) override;
void AdvancePos(uint64_t size);
// Performs an I/O operation (either read or write). This template shares the
// code for both Read() and Write() implementations.
template <typename T>
bool IOOperation(bool (FileInterface::*io_op)(T*, size_t, size_t*),
T* buf,
size_t count,
size_t* bytes_processed);
// The underlying FileInterace instance.
std::unique_ptr<FileInterface> file_;
// The list of extents mapping this instance to |file_|.
const std::vector<ex_t> extents_;
// The accumulated length of the extents. The i-th element contains the sum of
// the length of all the extents from 0 up to but not including the i-th
// extent. This reduces the complexity for random-access Seek() calls.
std::vector<uint64_t> acc_len_;
// Current extent index.
size_t curr_ex_idx_{0};
// Current logical file position.
uint64_t curr_pos_{0};
// Total length of all extents (constant).
uint64_t total_ex_len_{0};
} // namespace bsdiff