fpdf_parser_decode.cpp

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// Copyright 2014 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/fpdfapi/parser/fpdf_parser_decode.h"
 
#include <ctype.h>
#include <limits.h>
#include <stddef.h>
 
#include <algorithm>
#include <utility>
 
#include "build/build_config.h"
#include "constants/stream_dict_common.h"
#include "core/fpdfapi/parser/cpdf_array.h"
#include "core/fpdfapi/parser/cpdf_dictionary.h"
#include "core/fpdfapi/parser/fpdf_parser_utility.h"
#include "core/fxcodec/fax/faxmodule.h"
#include "core/fxcodec/flate/flatemodule.h"
#include "core/fxcodec/scanlinedecoder.h"
#include "core/fxcrt/fx_extension.h"
#include "core/fxcrt/fx_safe_types.h"
#include "core/fxcrt/span_util.h"
#include "core/fxcrt/utf16.h"
#include "third_party/base/check.h"
#include "third_party/base/containers/contains.h"
 
namespace {
 
const uint32_t kMaxStreamSize = 20 * 1024 * 1024;
 
bool CheckFlateDecodeParams(int Colors, int BitsPerComponent, int Columns) {
  if (Colors < 0 || BitsPerComponent < 0 || Columns < 0)
    return false;
 
  FX_SAFE_INT32 check = Columns;
  check *= Colors;
  check *= BitsPerComponent;
  if (!check.IsValid())
    return false;
 
  return check.ValueOrDie() <= INT_MAX - 7;
}
 
uint8_t GetA85Result(uint32_t res, size_t i) {
  return static_cast<uint8_t>(res >> (3 - i) * 8);
}
 
}  // namespace
 
const uint16_t kPDFDocEncoding[256] = {
    0x0000, 0x0001, 0x0002, 0x0003, 0x0004, 0x0005, 0x0006, 0x0007, 0x0008,
    0x0009, 0x000a, 0x000b, 0x000c, 0x000d, 0x000e, 0x000f, 0x0010, 0x0011,
    0x0012, 0x0013, 0x0014, 0x0015, 0x0016, 0x0017, 0x02d8, 0x02c7, 0x02c6,
    0x02d9, 0x02dd, 0x02db, 0x02da, 0x02dc, 0x0020, 0x0021, 0x0022, 0x0023,
    0x0024, 0x0025, 0x0026, 0x0027, 0x0028, 0x0029, 0x002a, 0x002b, 0x002c,
    0x002d, 0x002e, 0x002f, 0x0030, 0x0031, 0x0032, 0x0033, 0x0034, 0x0035,
    0x0036, 0x0037, 0x0038, 0x0039, 0x003a, 0x003b, 0x003c, 0x003d, 0x003e,
    0x003f, 0x0040, 0x0041, 0x0042, 0x0043, 0x0044, 0x0045, 0x0046, 0x0047,
    0x0048, 0x0049, 0x004a, 0x004b, 0x004c, 0x004d, 0x004e, 0x004f, 0x0050,
    0x0051, 0x0052, 0x0053, 0x0054, 0x0055, 0x0056, 0x0057, 0x0058, 0x0059,
    0x005a, 0x005b, 0x005c, 0x005d, 0x005e, 0x005f, 0x0060, 0x0061, 0x0062,
    0x0063, 0x0064, 0x0065, 0x0066, 0x0067, 0x0068, 0x0069, 0x006a, 0x006b,
    0x006c, 0x006d, 0x006e, 0x006f, 0x0070, 0x0071, 0x0072, 0x0073, 0x0074,
    0x0075, 0x0076, 0x0077, 0x0078, 0x0079, 0x007a, 0x007b, 0x007c, 0x007d,
    0x007e, 0x0000, 0x2022, 0x2020, 0x2021, 0x2026, 0x2014, 0x2013, 0x0192,
    0x2044, 0x2039, 0x203a, 0x2212, 0x2030, 0x201e, 0x201c, 0x201d, 0x2018,
    0x2019, 0x201a, 0x2122, 0xfb01, 0xfb02, 0x0141, 0x0152, 0x0160, 0x0178,
    0x017d, 0x0131, 0x0142, 0x0153, 0x0161, 0x017e, 0x0000, 0x20ac, 0x00a1,
    0x00a2, 0x00a3, 0x00a4, 0x00a5, 0x00a6, 0x00a7, 0x00a8, 0x00a9, 0x00aa,
    0x00ab, 0x00ac, 0x0000, 0x00ae, 0x00af, 0x00b0, 0x00b1, 0x00b2, 0x00b3,
    0x00b4, 0x00b5, 0x00b6, 0x00b7, 0x00b8, 0x00b9, 0x00ba, 0x00bb, 0x00bc,
    0x00bd, 0x00be, 0x00bf, 0x00c0, 0x00c1, 0x00c2, 0x00c3, 0x00c4, 0x00c5,
    0x00c6, 0x00c7, 0x00c8, 0x00c9, 0x00ca, 0x00cb, 0x00cc, 0x00cd, 0x00ce,
    0x00cf, 0x00d0, 0x00d1, 0x00d2, 0x00d3, 0x00d4, 0x00d5, 0x00d6, 0x00d7,
    0x00d8, 0x00d9, 0x00da, 0x00db, 0x00dc, 0x00dd, 0x00de, 0x00df, 0x00e0,
    0x00e1, 0x00e2, 0x00e3, 0x00e4, 0x00e5, 0x00e6, 0x00e7, 0x00e8, 0x00e9,
    0x00ea, 0x00eb, 0x00ec, 0x00ed, 0x00ee, 0x00ef, 0x00f0, 0x00f1, 0x00f2,
    0x00f3, 0x00f4, 0x00f5, 0x00f6, 0x00f7, 0x00f8, 0x00f9, 0x00fa, 0x00fb,
    0x00fc, 0x00fd, 0x00fe, 0x00ff};
 
bool ValidateDecoderPipeline(const CPDF_Array* pDecoders) {
  size_t count = pDecoders->size();
  if (count == 0)
    return true;
 
  for (size_t i = 0; i < count; ++i) {
    RetainPtr<const CPDF_Object> object = pDecoders->GetDirectObjectAt(i);
    if (!object || !object->IsName()) {
      return false;
    }
  }
 
  if (count == 1)
    return true;
 
  // TODO(thestig): Consolidate all the places that use these filter names.
  static const char kValidDecoders[][16] = {
      "FlateDecode",    "Fl",  "LZWDecode",       "LZW", "ASCII85Decode", "A85",
      "ASCIIHexDecode", "AHx", "RunLengthDecode", "RL"};
  for (size_t i = 0; i < count - 1; ++i) {
    if (!pdfium::Contains(kValidDecoders, pDecoders->GetByteStringAt(i)))
      return false;
  }
  return true;
}
 
uint32_t A85Decode(pdfium::span<const uint8_t> src_span,
                   std::unique_ptr<uint8_t, FxFreeDeleter>* dest_buf,
                   uint32_t* dest_size) {
  *dest_size = 0;
  if (src_span.empty()) {
    dest_buf->reset();
    return 0;
  }
 
  // Count legal characters and zeros.
  uint32_t zcount = 0;
  uint32_t pos = 0;
  while (pos < src_span.size()) {
    uint8_t ch = src_span[pos];
    if (ch == 'z') {
      zcount++;
    } else if ((ch < '!' || ch > 'u') && !PDFCharIsLineEnding(ch) &&
               ch != ' ' && ch != '\t') {
      break;
    }
    pos++;
  }
  // No content to decode.
  if (pos == 0)
    return 0;
 
  // Count the space needed to contain non-zero characters. The encoding ratio
  // of Ascii85 is 4:5.
  uint32_t space_for_non_zeroes = (pos - zcount) / 5 * 4 + 4;
  FX_SAFE_UINT32 size = zcount;
  size *= 4;
  size += space_for_non_zeroes;
  if (!size.IsValid())
    return FX_INVALID_OFFSET;
 
  dest_buf->reset(FX_Alloc(uint8_t, size.ValueOrDie()));
  uint8_t* dest_buf_ptr = dest_buf->get();
  size_t state = 0;
  uint32_t res = 0;
  pos = 0;
  while (pos < src_span.size()) {
    uint8_t ch = src_span[pos++];
    if (PDFCharIsLineEnding(ch) || ch == ' ' || ch == '\t')
      continue;
 
    if (ch == 'z') {
      memset(dest_buf_ptr + *dest_size, 0, 4);
      state = 0;
      res = 0;
      *dest_size += 4;
      continue;
    }
 
    // Check for the end or illegal character.
    if (ch < '!' || ch > 'u')
      break;
 
    res = res * 85 + ch - 33;
    if (state < 4) {
      ++state;
      continue;
    }
 
    for (size_t i = 0; i < 4; ++i) {
      dest_buf_ptr[(*dest_size)++] = GetA85Result(res, i);
    }
    state = 0;
    res = 0;
  }
  // Handle partial group.
  if (state) {
    for (size_t i = state; i < 5; ++i)
      res = res * 85 + 84;
    for (size_t i = 0; i < state - 1; ++i)
      dest_buf_ptr[(*dest_size)++] = GetA85Result(res, i);
  }
  if (pos < src_span.size() && src_span[pos] == '>')
    ++pos;
  return pos;
}
 
uint32_t HexDecode(pdfium::span<const uint8_t> src_span,
                   std::unique_ptr<uint8_t, FxFreeDeleter>* dest_buf,
                   uint32_t* dest_size) {
  *dest_size = 0;
  if (src_span.empty()) {
    dest_buf->reset();
    return 0;
  }
 
  uint32_t i = 0;
  // Find the end of data.
  while (i < src_span.size() && src_span[i] != '>')
    ++i;
 
  dest_buf->reset(FX_Alloc(uint8_t, i / 2 + 1));
  uint8_t* dest_buf_ptr = dest_buf->get();
  bool bFirst = true;
  for (i = 0; i < src_span.size(); ++i) {
    uint8_t ch = src_span[i];
    if (PDFCharIsLineEnding(ch) || ch == ' ' || ch == '\t')
      continue;
 
    if (ch == '>') {
      ++i;
      break;
    }
    if (!isxdigit(ch))
      continue;
 
    int digit = FXSYS_HexCharToInt(ch);
    if (bFirst)
      dest_buf_ptr[*dest_size] = digit * 16;
    else
      dest_buf_ptr[(*dest_size)++] += digit;
    bFirst = !bFirst;
  }
  if (!bFirst)
    ++(*dest_size);
  return i;
}
 
uint32_t RunLengthDecode(pdfium::span<const uint8_t> src_span,
                         std::unique_ptr<uint8_t, FxFreeDeleter>* dest_buf,
                         uint32_t* dest_size) {
  size_t i = 0;
  *dest_size = 0;
  while (i < src_span.size()) {
    if (src_span[i] == 128)
      break;
 
    uint32_t old = *dest_size;
    if (src_span[i] < 128) {
      *dest_size += src_span[i] + 1;
      if (*dest_size < old)
        return FX_INVALID_OFFSET;
      i += src_span[i] + 2;
    } else {
      *dest_size += 257 - src_span[i];
      if (*dest_size < old)
        return FX_INVALID_OFFSET;
      i += 2;
    }
  }
  if (*dest_size >= kMaxStreamSize)
    return FX_INVALID_OFFSET;
 
  dest_buf->reset(FX_Alloc(uint8_t, *dest_size));
  pdfium::span<uint8_t> dest_span(dest_buf->get(), *dest_size);
  i = 0;
  int dest_count = 0;
  while (i < src_span.size()) {
    if (src_span[i] == 128)
      break;
 
    if (src_span[i] < 128) {
      uint32_t copy_len = src_span[i] + 1;
      uint32_t buf_left = src_span.size() - i - 1;
      if (buf_left < copy_len) {
        uint32_t delta = copy_len - buf_left;
        copy_len = buf_left;
        fxcrt::spanclr(dest_span.subspan(dest_count + copy_len, delta));
      }
      auto copy_span = src_span.subspan(i + 1, copy_len);
      fxcrt::spancpy(dest_span.subspan(dest_count), copy_span);
      dest_count += src_span[i] + 1;
      i += src_span[i] + 2;
    } else {
      const uint8_t fill = i + 1 < src_span.size() ? src_span[i + 1] : 0;
      const size_t fill_size = 257 - src_span[i];
      fxcrt::spanset(dest_span.subspan(dest_count, fill_size), fill);
      dest_count += fill_size;
      i += 2;
    }
  }
  return std::min(i + 1, src_span.size());
}
 
std::unique_ptr<ScanlineDecoder> CreateFaxDecoder(
    pdfium::span<const uint8_t> src_span,
    int width,
    int height,
    const CPDF_Dictionary* pParams) {
  int K = 0;
  bool EndOfLine = false;
  bool ByteAlign = false;
  bool BlackIs1 = false;
  int Columns = 1728;
  int Rows = 0;
  if (pParams) {
    K = pParams->GetIntegerFor("K");
    EndOfLine = !!pParams->GetIntegerFor("EndOfLine");
    ByteAlign = !!pParams->GetIntegerFor("EncodedByteAlign");
    BlackIs1 = !!pParams->GetIntegerFor("BlackIs1");
    Columns = pParams->GetIntegerFor("Columns", 1728);
    Rows = pParams->GetIntegerFor("Rows");
    if (Rows > USHRT_MAX)
      Rows = 0;
  }
  return FaxModule::CreateDecoder(src_span, width, height, K, EndOfLine,
                                  ByteAlign, BlackIs1, Columns, Rows);
}
 
std::unique_ptr<ScanlineDecoder> CreateFlateDecoder(
    pdfium::span<const uint8_t> src_span,
    int width,
    int height,
    int nComps,
    int bpc,
    const CPDF_Dictionary* pParams) {
  int predictor = 0;
  int Colors = 0;
  int BitsPerComponent = 0;
  int Columns = 0;
  if (pParams) {
    predictor = pParams->GetIntegerFor("Predictor");
    Colors = pParams->GetIntegerFor("Colors", 1);
    BitsPerComponent = pParams->GetIntegerFor("BitsPerComponent", 8);
    Columns = pParams->GetIntegerFor("Columns", 1);
    if (!CheckFlateDecodeParams(Colors, BitsPerComponent, Columns))
      return nullptr;
  }
  return FlateModule::CreateDecoder(src_span, width, height, nComps, bpc,
                                    predictor, Colors, BitsPerComponent,
                                    Columns);
}
 
uint32_t FlateOrLZWDecode(bool bLZW,
                          pdfium::span<const uint8_t> src_span,
                          const CPDF_Dictionary* pParams,
                          uint32_t estimated_size,
                          std::unique_ptr<uint8_t, FxFreeDeleter>* dest_buf,
                          uint32_t* dest_size) {
  int predictor = 0;
  int Colors = 0;
  int BitsPerComponent = 0;
  int Columns = 0;
  bool bEarlyChange = true;
  if (pParams) {
    predictor = pParams->GetIntegerFor("Predictor");
    bEarlyChange = !!pParams->GetIntegerFor("EarlyChange", 1);
    Colors = pParams->GetIntegerFor("Colors", 1);
    BitsPerComponent = pParams->GetIntegerFor("BitsPerComponent", 8);
    Columns = pParams->GetIntegerFor("Columns", 1);
    if (!CheckFlateDecodeParams(Colors, BitsPerComponent, Columns))
      return FX_INVALID_OFFSET;
  }
  return FlateModule::FlateOrLZWDecode(bLZW, src_span, bEarlyChange, predictor,
                                       Colors, BitsPerComponent, Columns,
                                       estimated_size, dest_buf, dest_size);
}
 
absl::optional<DecoderArray> GetDecoderArray(
    RetainPtr<const CPDF_Dictionary> pDict) {
  RetainPtr<const CPDF_Object> pFilter = pDict->GetDirectObjectFor("Filter");
  if (!pFilter)
    return DecoderArray();
 
  if (!pFilter->IsArray() && !pFilter->IsName())
    return absl::nullopt;
 
  RetainPtr<const CPDF_Object> pParams =
      pDict->GetDirectObjectFor(pdfium::stream::kDecodeParms);
 
  DecoderArray decoder_array;
  if (const CPDF_Array* pDecoders = pFilter->AsArray()) {
    if (!ValidateDecoderPipeline(pDecoders))
      return absl::nullopt;
 
    RetainPtr<const CPDF_Array> pParamsArray = ToArray(pParams);
    for (size_t i = 0; i < pDecoders->size(); ++i) {
      decoder_array.emplace_back(
          pDecoders->GetByteStringAt(i),
          pParamsArray ? pParamsArray->GetDictAt(i) : nullptr);
    }
  } else {
    DCHECK(pFilter->IsName());
    decoder_array.emplace_back(pFilter->GetString(),
                               pParams ? pParams->GetDict() : nullptr);
  }
 
  return decoder_array;
}
 
bool PDF_DataDecode(pdfium::span<const uint8_t> src_span,
                    uint32_t last_estimated_size,
                    bool bImageAcc,
                    const DecoderArray& decoder_array,
                    std::unique_ptr<uint8_t, FxFreeDeleter>* dest_buf,
                    uint32_t* dest_size,
                    ByteString* ImageEncoding,
                    RetainPtr<const CPDF_Dictionary>* pImageParams) {
  std::unique_ptr<uint8_t, FxFreeDeleter> result;
  // May be changed to point to |result| in the for-loop below. So put it below
  // |result| and let it get destroyed first.
  pdfium::span<const uint8_t> last_span = src_span;
  size_t nSize = decoder_array.size();
  for (size_t i = 0; i < nSize; ++i) {
    int estimated_size = i == nSize - 1 ? last_estimated_size : 0;
    ByteString decoder = decoder_array[i].first;
    RetainPtr<const CPDF_Dictionary> pParam =
        ToDictionary(decoder_array[i].second);
    std::unique_ptr<uint8_t, FxFreeDeleter> new_buf;
    uint32_t new_size = 0xFFFFFFFF;
    uint32_t offset = FX_INVALID_OFFSET;
    if (decoder == "Crypt")
      continue;
    if (decoder == "FlateDecode" || decoder == "Fl") {
      if (bImageAcc && i == nSize - 1) {
        *ImageEncoding = "FlateDecode";
        *dest_buf = std::move(result);
        *dest_size = last_span.size();
        *pImageParams = std::move(pParam);
        return true;
      }
      offset = FlateOrLZWDecode(false, last_span, pParam, estimated_size,
                                &new_buf, &new_size);
    } else if (decoder == "LZWDecode" || decoder == "LZW") {
      offset = FlateOrLZWDecode(true, last_span, pParam, estimated_size,
                                &new_buf, &new_size);
    } else if (decoder == "ASCII85Decode" || decoder == "A85") {
      offset = A85Decode(last_span, &new_buf, &new_size);
    } else if (decoder == "ASCIIHexDecode" || decoder == "AHx") {
      offset = HexDecode(last_span, &new_buf, &new_size);
    } else if (decoder == "RunLengthDecode" || decoder == "RL") {
      if (bImageAcc && i == nSize - 1) {
        *ImageEncoding = "RunLengthDecode";
        *dest_buf = std::move(result);
        *dest_size = last_span.size();
        *pImageParams = std::move(pParam);
        return true;
      }
      offset = RunLengthDecode(last_span, &new_buf, &new_size);
    } else {
      // If we get here, assume it's an image decoder.
      if (decoder == "DCT")
        decoder = "DCTDecode";
      else if (decoder == "CCF")
        decoder = "CCITTFaxDecode";
      *ImageEncoding = std::move(decoder);
      *pImageParams = std::move(pParam);
      *dest_buf = std::move(result);
      *dest_size = last_span.size();
      return true;
    }
    if (offset == FX_INVALID_OFFSET)
      return false;
 
    last_span = {new_buf.get(), new_size};
    result = std::move(new_buf);
  }
  ImageEncoding->clear();
  *pImageParams = nullptr;
  *dest_buf = std::move(result);
  *dest_size = last_span.size();
  return true;
}
 
static size_t StripLanguageCodes(pdfium::span<wchar_t> s, size_t n) {
  size_t dest_pos = 0;
  for (size_t i = 0; i < n; ++i) {
    // 0x001B is a begin/end marker for language metadata region that
    // should not be in the decoded text.
    if (s[i] == 0x001B) {
      for (++i; i < n && s[i] != 0x001B; ++i) {
        // No for-loop body. The loop searches for the terminating 0x001B.
      }
      continue;
    }
    s[dest_pos++] = s[i];
  }
  return dest_pos;
}
 
WideString PDF_DecodeText(pdfium::span<const uint8_t> span) {
  size_t dest_pos = 0;
  WideString result;
  if (span.size() >= 2 && ((span[0] == 0xfe && span[1] == 0xff) ||
                           (span[0] == 0xff && span[1] == 0xfe))) {
    if (span[0] == 0xfe) {
      result = WideString::FromUTF16BE(span.subspan(2));
    } else {
      result = WideString::FromUTF16LE(span.subspan(2));
    }
    pdfium::span<wchar_t> dest_buf = result.GetBuffer(result.GetLength());
    dest_pos = StripLanguageCodes(dest_buf, result.GetLength());
  } else if (span.size() >= 3 && span[0] == 0xef && span[1] == 0xbb &&
             span[2] == 0xbf) {
    result = WideString::FromUTF8(span.subspan(3));
    pdfium::span<wchar_t> dest_buf = result.GetBuffer(result.GetLength());
    dest_pos = StripLanguageCodes(dest_buf, result.GetLength());
  } else {
    pdfium::span<wchar_t> dest_buf = result.GetBuffer(span.size());
    for (size_t i = 0; i < span.size(); ++i)
      dest_buf[i] = kPDFDocEncoding[span[i]];
    dest_pos = span.size();
  }
  result.ReleaseBuffer(dest_pos);
  return result;
}
 
ByteString PDF_EncodeText(WideStringView str) {
  size_t i = 0;
  size_t len = str.GetLength();
  ByteString result;
  {
    pdfium::span<char> dest_buf = result.GetBuffer(len);
    for (i = 0; i < len; ++i) {
      int code;
      for (code = 0; code < 256; ++code) {
        if (kPDFDocEncoding[code] == str[i])
          break;
      }
      if (code == 256)
        break;
 
      dest_buf[i] = code;
    }
  }
  result.ReleaseBuffer(i);
  if (i == len)
    return result;
 
  if (len > INT_MAX / 2 - 1) {
    result.ReleaseBuffer(0);
    return result;
  }
 
  size_t dest_index = 0;
  {
#if defined(WCHAR_T_IS_32_BIT)
    // 2 or 4 bytes required per UTF-32 code unit.
    pdfium::span<uint8_t> dest_buf =
        pdfium::as_writable_bytes(result.GetBuffer(len * 4 + 2));
#else
    // 2 bytes required per UTF-16 code unit.
    pdfium::span<uint8_t> dest_buf =
        pdfium::as_writable_bytes(result.GetBuffer(len * 2 + 2));
#endif  // defined(WCHAR_T_IS_32_BIT)
 
    dest_buf[dest_index++] = 0xfe;
    dest_buf[dest_index++] = 0xff;
    for (size_t j = 0; j < len; ++j) {
#if defined(WCHAR_T_IS_32_BIT)
      if (pdfium::IsSupplementary(str[j])) {
        pdfium::SurrogatePair pair(str[j]);
        dest_buf[dest_index++] = pair.high() >> 8;
        dest_buf[dest_index++] = static_cast<uint8_t>(pair.high());
        dest_buf[dest_index++] = pair.low() >> 8;
        dest_buf[dest_index++] = static_cast<uint8_t>(pair.low());
        continue;
      }
#endif  // defined(WCHAR_T_IS_32_BIT)
      dest_buf[dest_index++] = str[j] >> 8;
      dest_buf[dest_index++] = static_cast<uint8_t>(str[j]);
    }
  }
  result.ReleaseBuffer(dest_index);
  return result;
}
 
ByteString PDF_EncodeString(ByteStringView src) {
  ByteString result;
  result.Reserve(src.GetLength() + 2);
  result += '(';
  for (size_t i = 0; i < src.GetLength(); ++i) {
    uint8_t ch = src[i];
    if (ch == 0x0a) {
      result += "\\n";
      continue;
    }
    if (ch == 0x0d) {
      result += "\\r";
      continue;
    }
    if (ch == ')' || ch == '\\' || ch == '(')
      result += '\\';
    result += static_cast<char>(ch);
  }
  result += ')';
  return result;
}
 
ByteString PDF_HexEncodeString(ByteStringView src) {
  ByteString result;
  result.Reserve(2 * src.GetLength() + 2);
  result += '<';
  for (size_t i = 0; i < src.GetLength(); ++i) {
    char buf[2];
    FXSYS_IntToTwoHexChars(src[i], buf);
    result += buf[0];
    result += buf[1];
  }
  result += '>';
  return result;
}