formats/gif/reader.cc

// Copyright 2026 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//    https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//

#include "formats/gif/reader.h"

#include <cstdint>
#include <string>
#include <utility>

#include "absl/status/status.h"
#include "absl/status/status_macros.h"
#include "absl/status/statusor.h"
#include "absl/strings/str_format.h"
#include "absl/strings/string_view.h"
#include "formats/gif/constants.h"
#include "riegeli/bytes/reader.h"

namespace credentio {

namespace {

struct ColorTable {
  bool has_table;
  uint8_t size;

  static ColorTable FromPacked(uint8_t packed) {
    return {
        .has_table = (packed & 0x80) != 0,
        .size = static_cast<uint8_t>(packed & 0x07),
    };
  }
};

absl::Status SeekToEndOfData(riegeli::Reader& input) {
  uint8_t size = 0x00;
  do {
    if (!input.ReadByte(size)) {
      return input.StatusOrAnnotate(absl::DataLossError("Failed to read byte"));
    }
    if (!input.ok()) {
      return input.status();
    }

    if (size > 0) {
      if (!input.Skip(size)) {
        return absl::InternalError("Failed to seek to offset");
      }
      if (!input.ok()) {
        return input.status();
      }
    }
  } while (size > 0);

  return absl::OkStatus();
}

absl::StatusOr<GifBlock> ReadHeader(riegeli::Reader& input) {
  GifBlock block = {.offset = input.pos()};

  std::string data;
  if (!input.Read(kGifHeaderBlock.size(), data)) {
    return input.StatusOrAnnotate(
        absl::DataLossError("Failed to read GIF header block"));
  }
  if (data != kGifHeaderBlock) {
    return absl::InvalidArgumentError("Invalid GIF header block");
  }

  block.length = data.size();
  block.type = data;
  return std::move(block);
}

absl::Status PopulateDescriptor(riegeli::Reader& input,
                                int64_t packed_byte_offset, GifBlock& block) {
  std::string data;
  if (!input.Read(block.length, data)) {
    return input.StatusOrAnnotate(
        absl::DataLossError("Failed to read descriptor"));
  }

  ColorTable packed = ColorTable::FromPacked(data[packed_byte_offset]);

  if (packed.has_table) {
    int64_t ct_length = 3 * (1 << (packed.size + 1));
    block.length += ct_length;
  }

  return absl::OkStatus();
}

absl::Status PopulateLogicalDescriptor(riegeli::Reader& input,
                                       GifBlock& block) {
  block.length = 7;
  block.type = std::string(kGifLogicalScreenDescriptorLabel);
  return PopulateDescriptor(input, 4, block);
}

absl::Status PopulateImageDescriptor(riegeli::Reader& input, GifBlock& block) {
  // Backup one byte so we can read the introducer byte.
  if (!input.Seek(input.pos() - 1)) {
    return absl::InternalError("Failed to seek to offset");
  }
  block.length = 10;
  block.type = std::string(kGifImageDescriptorLabel);
  return PopulateDescriptor(input, 9, block);
}

absl::Status PopulateBlock(riegeli::Reader& input, GifBlock& block) {
  ABSL_RETURN_IF_ERROR(SeekToEndOfData(input));
  block.length = input.pos() - block.offset;
  return absl::OkStatus();
}

absl::Status PopulateApplicationExtension(riegeli::Reader& input,
                                          GifBlock& block) {
  int64_t next_size_offset = input.pos();

  uint8_t extension_info_size = 0x00;
  if (!input.ReadByte(extension_info_size)) {
    return input.StatusOrAnnotate(
        absl::DataLossError("Failed to read extension info size"));
  }

  next_size_offset += 1 + extension_info_size;

  if (extension_info_size == kGifC2paExtensionInfoSize) {
    std::string data;
    if (!input.Read(kGifC2paIdentifier.size(), data)) {
      return input.StatusOrAnnotate(
          absl::DataLossError("Failed to read c2pa identifier"));
    }
    if (data == kGifC2paIdentifier) {
      block.type = kGifC2paLabel;
    }
  }

  // Seek past the remaining Application Extension Information
  if (!input.Seek(next_size_offset)) {
    return absl::InternalError("Failed to seek to offset");
  }

  return PopulateBlock(input, block);
}

}  // namespace

absl::Status IterateOverGifBlocks(riegeli::Reader& input,
                                  GifBlockProcessor processor,
                                  int64_t end_offset) {
  if (!input.SupportsSize() || !input.Size().has_value()) {
    return absl::InvalidArgumentError(
        "Input does not support size or size is unknown");
  }

  uint64_t local_end_offset =
      end_offset < 0 ? input.Size().value() : end_offset;

  ABSL_ASSIGN_OR_RETURN(auto header, ReadHeader(input));
  ABSL_ASSIGN_OR_RETURN(bool should_continue, processor(header));
  ABSL_RETURN_IF_ERROR(input.status());
  if (!should_continue) {
    return absl::OkStatus();
  }

  if (input.pos() >= local_end_offset) {
    return absl::OkStatus();
  }

  GifBlock logical_descriptor = {.offset = input.pos()};
  ABSL_RETURN_IF_ERROR(PopulateLogicalDescriptor(input, logical_descriptor));
  ABSL_ASSIGN_OR_RETURN(should_continue, processor(logical_descriptor));
  ABSL_RETURN_IF_ERROR(input.status());
  if (!should_continue) {
    return absl::OkStatus();
  }

  if (!input.Seek(logical_descriptor.offset + logical_descriptor.length)) {
    return input.StatusOrAnnotate(absl::InternalError(
        "Failed to seek to offset after logical descriptor"));
  }

  while (input.pos() < local_end_offset) {
    GifBlock block = {.offset = input.pos()};

    uint8_t introducer = 0x00;
    if (!input.ReadByte(introducer)) {
      return input.StatusOrAnnotate(
          absl::DataLossError("Failed to read introducer"));
    }

    switch (introducer) {
      case kGifExtensionIntroducer: {
        // is extension
        uint8_t extension_type = 0x00;
        if (!input.ReadByte(extension_type)) {
          return input.StatusOrAnnotate(
              absl::DataLossError("Failed to read extension type"));
        }

        switch (extension_type) {
          case kGifExtensionPlainText:
            block.type = std::string(kGifExtensionPlainTextLabel);
            ABSL_RETURN_IF_ERROR(PopulateBlock(input, block));
            break;
          case kGifExtensionGraphicsControl:
            block.type = std::string(kGifExtensionGraphicsControlLabel);
            ABSL_RETURN_IF_ERROR(PopulateBlock(input, block));
            break;
          case kGifExtensionComment:
            block.type = std::string(kGifExtensionCommentLabel);
            ABSL_RETURN_IF_ERROR(PopulateBlock(input, block));
            break;
          case kGifExtensionApplication:
            block.type = std::string(kGifExtensionApplicationLabel);
            ABSL_RETURN_IF_ERROR(PopulateApplicationExtension(input, block));
            break;
          default:
            return absl::UnimplementedError(absl::StrFormat(
                "Extension Type not supported: %#04x", extension_type));
        }
        break;
      }
      case kGifImageDescriptorIntroducer: {
        ABSL_RETURN_IF_ERROR(PopulateImageDescriptor(input, block));
        break;
      }
      case kGifTrailerIntroducer: {
        block.length = 1;
        block.type = std::string(kGifTrailerLabel);
        break;
      }
      default:
        block.type = std::string(kGifImageDataLabel);
        ABSL_RETURN_IF_ERROR(PopulateBlock(input, block));
    }

    ABSL_ASSIGN_OR_RETURN(should_continue, processor(block));
    ABSL_RETURN_IF_ERROR(input.status());
    if (!should_continue) {
      return absl::OkStatus();
    }

    if (!input.Seek(block.offset + block.length)) {
      return input.StatusOrAnnotate(
          absl::InternalError("Failed to seek to offset"));
    }

    if (block.type == kGifTrailerLabel) {
      break;
    }
  }

  if (input.pos() < local_end_offset) {
    GifBlock after_label = {.offset = input.pos(),
                            .length = local_end_offset - input.pos(),
                            .type = "c2pa.after"};
    return processor(after_label).status();
  }

  return absl::OkStatus();
}

}  // namespace credentio