leb128.cc revision 5f1c94371a64b3196d4be9466099bb892df9b88e 
1// Copyright 2014 The Chromium Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4
5#include "leb128.h"
6
7#include <stdint.h>
8#include <vector>
9
10#include "elf_traits.h"
11
12namespace relocation_packer {
13
14// Empty constructor and destructor to silence chromium-style.
15Leb128Encoder::Leb128Encoder() { }
16Leb128Encoder::~Leb128Encoder() { }
17
18// Add a single value to the encoding.  Values are encoded with variable
19// length.  The least significant 7 bits of each byte hold 7 bits of data,
20// and the most significant bit is set on each byte except the last.
21void Leb128Encoder::Enqueue(ELF::Xword value) {
22  do {
23    const uint8_t byte = value & 127;
24    value >>= 7;
25    encoding_.push_back((value ? 128 : 0) | byte);
26  } while (value);
27}
28
29// Add a vector of values to the encoding.
30void Leb128Encoder::EnqueueAll(const std::vector<ELF::Xword>& values) {
31  for (size_t i = 0; i < values.size(); ++i)
32    Enqueue(values[i]);
33}
34
35// Create a new decoder for the given encoded stream.
36Leb128Decoder::Leb128Decoder(const std::vector<uint8_t>& encoding) {
37  encoding_ = encoding;
38  cursor_ = 0;
39}
40
41// Empty destructor to silence chromium-style.
42Leb128Decoder::~Leb128Decoder() { }
43
44// Decode and retrieve a single value from the encoding.  Read forwards until
45// a byte without its most significant bit is found, then read the 7 bit
46// fields of the bytes spanned to re-form the value.
47ELF::Xword Leb128Decoder::Dequeue() {
48  ELF::Xword value = 0;
49
50  size_t shift = 0;
51  uint8_t byte;
52
53  // Loop until we reach a byte with its high order bit clear.
54  do {
55    byte = encoding_[cursor_++];
56    value |= static_cast<ELF::Xword>(byte & 127) << shift;
57    shift += 7;
58  } while (byte & 128);
59
60  return value;
61}
62
63// Decode and retrieve all remaining values from the encoding.
64void Leb128Decoder::DequeueAll(std::vector<ELF::Xword>* values) {
65  while (cursor_ < encoding_.size())
66    values->push_back(Dequeue());
67}
68
69}  // namespace relocation_packer
70