tools/relocation_packer/src/leb128.cc

   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
  12 namespace relocation_packer {
  13
  14 // Empty constructor and destructor to silence chromium-style.
  15 Leb128Encoder::Leb128Encoder() { }
  16 Leb128Encoder::~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.
  21 void 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.
  30 void 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.
  36 Leb128Decoder::Leb128Decoder(const std::vector<uint8_t>& encoding) {
  37   encoding_ = encoding;
  38   cursor_ = 0;
  39 }
  40
  41 // Empty destructor to silence chromium-style.
  42 Leb128Decoder::~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.
  47 ELF::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.
  64 void Leb128Decoder::DequeueAll(std::vector<ELF::Xword>* values) {
  65   while (cursor_ < encoding_.size())
  66     values->push_back(Dequeue());
  67 }
  68
  69 }  // namespace relocation_packer