unicode.cpp

#include <cstdint>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <boost/range/iterator_range.hpp>
#include <boost/spirit/home/support/char_encoding/unicode.hpp>

namespace {
struct multi_byte_info {
  std::uint8_t id_mask;
  std::uint8_t id_matcher;
  std::uint8_t data_mask;
};

constexpr const std::uint8_t multi_byte_id_mask = 0xC0;
constexpr const std::uint8_t multi_byte_id_matcher = 0x80;
constexpr const std::uint8_t multi_byte_data_mask = 0x3F;
constexpr const std::uint8_t multi_byte_bits = 6;
constexpr const multi_byte_info multi_byte_infos[] = {
    // skip 1 byte info
    {0xE0, 0xC0, 0x1F},
    {0xF0, 0xE0, 0x0F},
    {0xF8, 0xF0, 0x07}};
constexpr const unsigned max_length =
    (sizeof(multi_byte_infos) / sizeof(multi_byte_info));

constexpr const std::uint32_t overlong[] = {0x80, 0x800, 0x10000};
constexpr const std::uint32_t max_code_point = 0x10FFFF;
}

enum class extraction : std::uint8_t { success, failure };

struct extraction_attempt {
  std::uint32_t code_point;
  std::uint8_t bytes_processed;
  extraction status;
};

template <typename Iterator>
constexpr extraction_attempt next_code_point(Iterator position,
                                             const Iterator &end) {
  static_assert(
      std::is_same<typename std::iterator_traits<Iterator>::iterator_category,
                   std::random_access_iterator_tag>{},
      "bad iterator type");

  extraction_attempt result{0, 0, extraction::failure};

  if (end - position) {
    result.code_point = std::uint8_t(*position);
    ++position;
    ++result.bytes_processed;

    if (0x7F < result.code_point) {
      unsigned expected_length = 1;

      for (const auto info : multi_byte_infos) {
        if ((result.code_point & info.id_mask) == info.id_matcher) {
          result.code_point &= info.data_mask;
          break;
        }
        ++expected_length;
      }

      if (max_length < expected_length || (end - position) < expected_length) {
        return result;
      }

      for (unsigned byte = 0; byte < expected_length; ++byte) {
        const std::uint8_t next_byte = *(position + byte);
        if ((next_byte & multi_byte_id_mask) != multi_byte_id_matcher) {
          return result;
        }

        result.code_point <<= multi_byte_bits;
        result.code_point |= (next_byte & multi_byte_data_mask);
        ++result.bytes_processed;
      }

      if (max_code_point < result.code_point) {
        return result;
      }

      if (overlong[expected_length - 1] > result.code_point) {
        return result;
      }
    }

    result.status = extraction::success;
  } // end multi-byte processing

  return result;
}

template <typename Range>
constexpr extraction_attempt next_code_point(const Range &range) {
  return next_code_point(std::begin(range), std::end(range));
}

template <typename T>
boost::iterator_range<T>
next_character_bytes(const boost::iterator_range<T> &range,
                     const extraction_attempt result) {
  return boost::make_iterator_range(range.begin(),
                                    range.begin() + result.bytes_processed);
}

template <std::size_t Length>
constexpr bool test(const char (&range)[Length],
                    const extraction expected_status,
                    const std::uint32_t expected_code_point,
                    const std::uint8_t expected_bytes_processed) {
  const extraction_attempt result =
      next_code_point(std::begin(range), std::end(range) - 1);
  switch (expected_status) {
  case extraction::success:
    return result.status == extraction::success &&
           result.bytes_processed == expected_bytes_processed &&
           result.code_point == expected_code_point;
  case extraction::failure:
    return result.status == extraction::failure &&
           result.bytes_processed == expected_bytes_processed;
  default:
    return false;
  }
}

int main() {
  static_assert(test("F", extraction::success, 'F', 1), "");
  static_assert(test("\0", extraction::success, 0, 1), "");
  static_assert(test("\x7F", extraction::success, 0x7F, 1), "");
  static_assert(test("\xFF\xFF", extraction::failure, 0, 1), "");

  static_assert(test("\xDF", extraction::failure, 0, 1), "");
  static_assert(test("\xDF\xFF", extraction::failure, 0, 1), "");
  static_assert(test("\xC1\xBF", extraction::failure, 0, 2), "");
  static_assert(test("\xC2\x80", extraction::success, 0x80, 2), "");
  static_assert(test("\xDF\xBF", extraction::success, 0x07FF, 2), "");

  static_assert(test("\xEF\xBF", extraction::failure, 0, 1), "");
  static_assert(test("\xEF\xBF\xFF", extraction::failure, 0, 2), "");
  static_assert(test("\xE0\x9F\xBF", extraction::failure, 0, 3), "");
  static_assert(test("\xE0\xA0\x80", extraction::success, 0x800, 3), "");
  static_assert(test("\xEF\xBF\xBF", extraction::success, 0xFFFF, 3), "");

  static_assert(test("\xF7\xBF\xBF", extraction::failure, 0, 1), "");
  static_assert(test("\xF7\xBF\xBF\xFF", extraction::failure, 0, 3), "");
  static_assert(test("\xF0\x8F\xBF\xBF", extraction::failure, 0, 4), "");
  static_assert(test("\xF0\x90\x80\x80", extraction::success, 0x10000, 4), "");
  static_assert(test("\xF4\x8F\xBF\xBF", extraction::success, 0x10FFFF, 4), "");
  static_assert(test("\xF7\xBF\xBF\xBF", extraction::failure, 0, 4), "");

  static_assert(test("𝕫", extraction::success, 0x1D56B, 4), "");

  constexpr const static char text[] =
      "Hello あにま ➦ 👙 𝕫⊆𝕢 \x02\x01\b \xff\xff\xff ";

  std::cout << text << std::endl;

  auto data = boost::make_iterator_range(text);
  while (!data.empty()) {
    const extraction_attempt result = next_code_point(data);
    switch (result.status) {
    case extraction::success:
      if (boost::spirit::char_encoding::unicode::isprint(result.code_point)) {
        std::cout << next_character_bytes(data, result);
        break;
      }

    default:
    case extraction::failure:
      std::cout << "[";
      std::cout << std::hex << std::setw(2) << std::setfill('0');
      for (const auto byte : next_character_bytes(data, result)) {
        std::cout << int(std::uint8_t(byte));
      }
      std::cout << "]";
      break;
    }

    data.advance_begin(result.bytes_processed);
  }

  return 0;
}