core/fxcodec/gif/lzw_decompressor.cpp

// Copyright 2017 The PDFium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com

#include "core/fxcodec/gif/lzw_decompressor.h"

#include <algorithm>
#include <memory>
#include <type_traits>
#include <utility>

#include "core/fxcrt/compiler_specific.h"
#include "core/fxcrt/fx_safe_types.h"
#include "core/fxcrt/numerics/safe_conversions.h"
#include "core/fxcrt/ptr_util.h"

namespace fxcodec {

std::unique_ptr<LZWDecompressor> LZWDecompressor::Create(uint8_t color_exp,
                                                         uint8_t code_exp) {
  // |color_exp| generates 2^(n + 1) codes, where as the code_exp reserves 2^n.
  // This is a quirk of the GIF spec.
  if (code_exp > GIF_MAX_LZW_EXP || code_exp < color_exp + 1) {
    return nullptr;
  }

  // Private ctor.
  return pdfium::WrapUnique(new LZWDecompressor(color_exp, code_exp));
}

LZWDecompressor::LZWDecompressor(uint8_t color_exp, uint8_t code_exp)
    : code_size_(code_exp),
      code_color_end_(static_cast<uint16_t>(1 << (color_exp + 1))),
      code_clear_(static_cast<uint16_t>(1 << code_exp)),
      code_end_(static_cast<uint16_t>((1 << code_exp) + 1)) {
  ClearTable();
}

LZWDecompressor::~LZWDecompressor() = default;

LZWDecompressor::Status LZWDecompressor::Decode(uint8_t* dest_buf,
                                                uint32_t* dest_size) {
  if (!dest_buf || !dest_size) {
    return Status::kError;
  }

  if (avail_input_.empty()) {
    return Status::kUnfinished;
  }

  if (*dest_size == 0) {
    return Status::kInsufficientDestSize;
  }

  FX_SAFE_UINT32 i = 0;
  if (decompressed_next_ != 0) {
    size_t extracted_size =
        ExtractData(UNSAFE_TODO(pdfium::span(dest_buf, *dest_size)));
    if (decompressed_next_ != 0) {
      return Status::kInsufficientDestSize;
    }

    UNSAFE_TODO(dest_buf += extracted_size);
    i += extracted_size;
  }

  while (i.ValueOrDie() <= *dest_size &&
         (!avail_input_.empty() || bits_left_ >= code_size_cur_)) {
    if (code_size_cur_ > GIF_MAX_LZW_EXP) {
      return Status::kError;
    }

    if (!avail_input_.empty()) {
      if (bits_left_ > 31) {
        return Status::kError;
      }

      FX_SAFE_UINT32 safe_code = avail_input_.front();
      safe_code <<= bits_left_;
      safe_code |= code_store_;
      if (!safe_code.IsValid()) {
        return Status::kError;
      }

      code_store_ = safe_code.ValueOrDie();
      avail_input_ = avail_input_.subspan(1u);
      bits_left_ += 8;
    }

    while (bits_left_ >= code_size_cur_) {
      uint16_t code =
          static_cast<uint16_t>(code_store_) & ((1 << code_size_cur_) - 1);
      code_store_ >>= code_size_cur_;
      bits_left_ -= code_size_cur_;
      if (code == code_clear_) {
        ClearTable();
        continue;
      }
      if (code == code_end_) {
        *dest_size = i.ValueOrDie();
        return Status::kSuccess;
      }

      if (code_old_ != static_cast<uint16_t>(-1)) {
        if (code_next_ < GIF_MAX_LZW_CODE) {
          if (code == code_next_) {
            AddCode(code_old_, code_first_);
            if (!DecodeString(code)) {
              return Status::kError;
            }
          } else if (code > code_next_) {
            return Status::kError;
          } else {
            if (!DecodeString(code)) {
              return Status::kError;
            }

            uint8_t append_char = decompressed_[decompressed_next_ - 1];
            AddCode(code_old_, append_char);
          }
        }
      } else {
        if (!DecodeString(code)) {
          return Status::kError;
        }
      }

      code_old_ = code;
      size_t extracted_size = ExtractData(
          UNSAFE_TODO(pdfium::span(dest_buf, (*dest_size - i).ValueOrDie())));
      if (decompressed_next_ != 0) {
        return Status::kInsufficientDestSize;
      }
      UNSAFE_TODO(dest_buf += extracted_size);
      i += extracted_size;
    }
  }

  if (!avail_input_.empty()) {
    return Status::kError;
  }
  *dest_size = i.ValueOrDie();
  return Status::kUnfinished;
}

void LZWDecompressor::ClearTable() {
  code_size_cur_ = code_size_ + 1;
  code_next_ = code_end_ + 1;
  code_old_ = static_cast<uint16_t>(-1);
  std::ranges::fill(code_table_, CodeEntry{});  // Aggregate initialization.
  static_assert(std::is_aggregate_v<CodeEntry>);
  for (uint16_t i = 0; i < code_clear_; i++) {
    code_table_[i].suffix = static_cast<uint8_t>(i);
  }
  decompressed_.resize(code_next_ - code_clear_ + 1);
  decompressed_next_ = 0;
}

void LZWDecompressor::AddCode(uint16_t prefix_code, uint8_t append_char) {
  if (code_next_ == GIF_MAX_LZW_CODE) {
    return;
  }

  code_table_[code_next_].prefix = prefix_code;
  code_table_[code_next_].suffix = append_char;
  if (++code_next_ < GIF_MAX_LZW_CODE) {
    if (code_next_ >> code_size_cur_) {
      code_size_cur_++;
    }
  }
}

bool LZWDecompressor::DecodeString(uint16_t code) {
  decompressed_.resize(code_next_ - code_clear_ + 1);
  decompressed_next_ = 0;

  while (code >= code_clear_ && code <= code_next_) {
    if (code == code_table_[code].prefix ||
        decompressed_next_ >= decompressed_.size()) {
      return false;
    }

    decompressed_[decompressed_next_++] = code_table_[code].suffix;
    code = code_table_[code].prefix;
  }

  if (code >= code_color_end_) {
    return false;
  }

  decompressed_[decompressed_next_++] = static_cast<uint8_t>(code);
  code_first_ = static_cast<uint8_t>(code);
  return true;
}

size_t LZWDecompressor::ExtractData(pdfium::span<uint8_t> dest_span) {
  if (dest_span.empty()) {
    return 0;
  }
  size_t copy_size = std::min(dest_span.size(), decompressed_next_);
  UNSAFE_TODO(std::reverse_copy(
      decompressed_.data() + decompressed_next_ - copy_size,
      decompressed_.data() + decompressed_next_, dest_span.data()));
  decompressed_next_ -= copy_size;
  return copy_size;
}

}  // namespace fxcodec