namespace simdutf {
namespace SIMDUTF_IMPLEMENTATION {
namespace {
namespace utf8_to_utf16 {
using namespace simd;

simdutf_really_inline simd8<uint8_t>
check_special_cases(const simd8<uint8_t> input, const simd8<uint8_t> prev1) {
  // Bit 0 = Too Short (lead byte/ASCII followed by lead byte/ASCII)
  // Bit 1 = Too Long (ASCII followed by continuation)
  // Bit 2 = Overlong 3-byte
  // Bit 4 = Surrogate
  // Bit 5 = Overlong 2-byte
  // Bit 7 = Two Continuations
  constexpr const uint8_t TOO_SHORT = 1 << 0;  // 11______ 0_______
                                               // 11______ 11______
  constexpr const uint8_t TOO_LONG = 1 << 1;   // 0_______ 10______
  constexpr const uint8_t OVERLONG_3 = 1 << 2; // 11100000 100_____
  constexpr const uint8_t SURROGATE = 1 << 4;  // 11101101 101_____
  constexpr const uint8_t OVERLONG_2 = 1 << 5; // 1100000_ 10______
  constexpr const uint8_t TWO_CONTS = 1 << 7;  // 10______ 10______
  constexpr const uint8_t TOO_LARGE = 1 << 3;  // 11110100 1001____
                                               // 11110100 101_____
                                               // 11110101 1001____
                                               // 11110101 101_____
                                               // 1111011_ 1001____
                                               // 1111011_ 101_____
                                               // 11111___ 1001____
                                               // 11111___ 101_____
  constexpr const uint8_t TOO_LARGE_1000 = 1 << 6;
  // 11110101 1000____
  // 1111011_ 1000____
  // 11111___ 1000____
  constexpr const uint8_t OVERLONG_4 = 1 << 6; // 11110000 1000____

  const simd8<uint8_t> byte_1_high = prev1.shr<4>().lookup_16<uint8_t>(
      // 0_______ ________ <ASCII in byte 1>
      TOO_LONG, TOO_LONG, TOO_LONG, TOO_LONG, TOO_LONG, TOO_LONG, TOO_LONG,
      TOO_LONG,
      // 10______ ________ <continuation in byte 1>
      TWO_CONTS, TWO_CONTS, TWO_CONTS, TWO_CONTS,
      // 1100____ ________ <two byte lead in byte 1>
      TOO_SHORT | OVERLONG_2,
      // 1101____ ________ <two byte lead in byte 1>
      TOO_SHORT,
      // 1110____ ________ <three byte lead in byte 1>
      TOO_SHORT | OVERLONG_3 | SURROGATE,
      // 1111____ ________ <four+ byte lead in byte 1>
      TOO_SHORT | TOO_LARGE | TOO_LARGE_1000 | OVERLONG_4);
  constexpr const uint8_t CARRY =
      TOO_SHORT | TOO_LONG | TWO_CONTS; // These all have ____ in byte 1 .
  const simd8<uint8_t> byte_1_low =
      (prev1 & 0x0F)
          .lookup_16<uint8_t>(
              // ____0000 ________
              CARRY | OVERLONG_3 | OVERLONG_2 | OVERLONG_4,
              // ____0001 ________
              CARRY | OVERLONG_2,
              // ____001_ ________
              CARRY, CARRY,

              // ____0100 ________
              CARRY | TOO_LARGE,
              // ____0101 ________
              CARRY | TOO_LARGE | TOO_LARGE_1000,
              // ____011_ ________
              CARRY | TOO_LARGE | TOO_LARGE_1000,
              CARRY | TOO_LARGE | TOO_LARGE_1000,

              // ____1___ ________
              CARRY | TOO_LARGE | TOO_LARGE_1000,
              CARRY | TOO_LARGE | TOO_LARGE_1000,
              CARRY | TOO_LARGE | TOO_LARGE_1000,
              CARRY | TOO_LARGE | TOO_LARGE_1000,
              CARRY | TOO_LARGE | TOO_LARGE_1000,
              // ____1101 ________
              CARRY | TOO_LARGE | TOO_LARGE_1000 | SURROGATE,
              CARRY | TOO_LARGE | TOO_LARGE_1000,
              CARRY | TOO_LARGE | TOO_LARGE_1000);
  const simd8<uint8_t> byte_2_high = input.shr<4>().lookup_16<uint8_t>(
      // ________ 0_______ <ASCII in byte 2>
      TOO_SHORT, TOO_SHORT, TOO_SHORT, TOO_SHORT, TOO_SHORT, TOO_SHORT,
      TOO_SHORT, TOO_SHORT,

      // ________ 1000____
      TOO_LONG | OVERLONG_2 | TWO_CONTS | OVERLONG_3 | TOO_LARGE_1000 |
          OVERLONG_4,
      // ________ 1001____
      TOO_LONG | OVERLONG_2 | TWO_CONTS | OVERLONG_3 | TOO_LARGE,
      // ________ 101_____
      TOO_LONG | OVERLONG_2 | TWO_CONTS | SURROGATE | TOO_LARGE,
      TOO_LONG | OVERLONG_2 | TWO_CONTS | SURROGATE | TOO_LARGE,

      // ________ 11______
      TOO_SHORT, TOO_SHORT, TOO_SHORT, TOO_SHORT);
  return (byte_1_high & byte_1_low & byte_2_high);
}
simdutf_really_inline simd8<uint8_t>
check_multibyte_lengths(const simd8<uint8_t> input,
                        const simd8<uint8_t> prev_input,
                        const simd8<uint8_t> sc) {
  simd8<uint8_t> prev2 = input.prev<2>(prev_input);
  simd8<uint8_t> prev3 = input.prev<3>(prev_input);
  simd8<uint8_t> must23 =
      simd8<uint8_t>(must_be_2_3_continuation(prev2, prev3));
  simd8<uint8_t> must23_80 = must23 & uint8_t(0x80);
  return must23_80 ^ sc;
}

struct validating_transcoder {
  // If this is nonzero, there has been a UTF-8 error.
  simd8<uint8_t> error;

  validating_transcoder() : error(uint8_t(0)) {}
  //
  // Check whether the current bytes are valid UTF-8.
  //
  simdutf_really_inline void check_utf8_bytes(const simd8<uint8_t> input,
                                              const simd8<uint8_t> prev_input) {
    // Flip prev1...prev3 so we can easily determine if they are 2+, 3+ or 4+
    // lead bytes (2, 3, 4-byte leads become large positive numbers instead of
    // small negative numbers)
    simd8<uint8_t> prev1 = input.prev<1>(prev_input);
    simd8<uint8_t> sc = check_special_cases(input, prev1);
    this->error |= check_multibyte_lengths(input, prev_input, sc);
  }

  template <endianness endian>
  simdutf_really_inline size_t convert(const char *in, size_t size,
                                       char16_t *utf16_output) {
    size_t pos = 0;
    char16_t *start{utf16_output};
    // In the worst case, we have the haswell kernel which can cause an overflow
    // of 8 bytes when calling convert_masked_utf8_to_utf16. If you skip the
    // last 16 bytes, and if the data is valid, then it is entirely safe because
    // 16 UTF-8 bytes generate much more than 8 bytes. However, you cannot
    // generally assume that you have valid UTF-8 input, so we are going to go
    // back from the end counting 8 leading bytes, to give us a good margin.
    size_t leading_byte = 0;
    size_t margin = size;
    for (; margin > 0 && leading_byte < 8; margin--) {
      leading_byte += (int8_t(in[margin - 1]) > -65);
    }
    // If the input is long enough, then we have that margin-1 is the eight last
    // leading byte.
    const size_t safety_margin = size - margin + 1; // to avoid overruns!
    while (pos + 64 + safety_margin <= size) {
      simd8x64<int8_t> input(reinterpret_cast<const int8_t *>(in + pos));
      if (input.is_ascii()) {
        input.store_ascii_as_utf16<endian>(utf16_output);
        utf16_output += 64;
        pos += 64;
      } else {
        // you might think that a for-loop would work, but under Visual Studio,
        // it is not good enough.
        static_assert(
            (simd8x64<uint8_t>::NUM_CHUNKS == 2) ||
                (simd8x64<uint8_t>::NUM_CHUNKS == 4),
            "We support either two or four chunks per 64-byte block.");
        auto zero = simd8<uint8_t>{uint8_t(0)};
        if (simd8x64<uint8_t>::NUM_CHUNKS == 2) {
          this->check_utf8_bytes(input.chunks[0], zero);
          this->check_utf8_bytes(input.chunks[1], input.chunks[0]);
        } else if (simd8x64<uint8_t>::NUM_CHUNKS == 4) {
          this->check_utf8_bytes(input.chunks[0], zero);
          this->check_utf8_bytes(input.chunks[1], input.chunks[0]);
          this->check_utf8_bytes(input.chunks[2], input.chunks[1]);
          this->check_utf8_bytes(input.chunks[3], input.chunks[2]);
        }
        uint64_t utf8_continuation_mask = input.lt(-65 + 1);
        if (utf8_continuation_mask & 1) {
          return 0; // error
        }
        uint64_t utf8_leading_mask = ~utf8_continuation_mask;
        uint64_t utf8_end_of_code_point_mask = utf8_leading_mask >> 1;
        // We process in blocks of up to 12 bytes except possibly
        // for fast paths which may process up to 16 bytes. For the
        // slow path to work, we should have at least 12 input bytes left.
        size_t max_starting_point = (pos + 64) - 12;
        // Next loop is going to run at least five times.
        while (pos < max_starting_point) {
          // Performance note: our ability to compute 'consumed' and
          // then shift and recompute is critical. If there is a
          // latency of, say, 4 cycles on getting 'consumed', then
          // the inner loop might have a total latency of about 6 cycles.
          // Yet we process between 6 to 12 inputs bytes, thus we get
          // a speed limit between 1 cycle/byte and 0.5 cycle/byte
          // for this section of the code. Hence, there is a limit
          // to how much we can further increase this latency before
          // it seriously harms performance.
          size_t consumed = convert_masked_utf8_to_utf16<endian>(
              in + pos, utf8_end_of_code_point_mask, utf16_output);
          pos += consumed;
          utf8_end_of_code_point_mask >>= consumed;
        }
        // At this point there may remain between 0 and 12 bytes in the
        // 64-byte block. These bytes will be processed again. So we have an
        // 80% efficiency (in the worst case). In practice we expect an
        // 85% to 90% efficiency.
      }
    }
    if (errors()) {
      return 0;
    }
    if (pos < size) {
      size_t howmany = scalar::utf8_to_utf16::convert<endian>(
          in + pos, size - pos, utf16_output);
      if (howmany == 0) {
        return 0;
      }
      utf16_output += howmany;
    }
    return utf16_output - start;
  }

  template <endianness endian>
  simdutf_really_inline result convert_with_errors(const char *in, size_t size,
                                                   char16_t *utf16_output) {
    size_t pos = 0;
    char16_t *start{utf16_output};
    // In the worst case, we have the haswell kernel which can cause an overflow
    // of 8 bytes when calling convert_masked_utf8_to_utf16. If you skip the
    // last 16 bytes, and if the data is valid, then it is entirely safe because
    // 16 UTF-8 bytes generate much more than 8 bytes. However, you cannot
    // generally assume that you have valid UTF-8 input, so we are going to go
    // back from the end counting 8 leading bytes, to give us a good margin.
    size_t leading_byte = 0;
    size_t margin = size;
    for (; margin > 0 && leading_byte < 8; margin--) {
      leading_byte += (int8_t(in[margin - 1]) > -65);
    }
    // If the input is long enough, then we have that margin-1 is the eight last
    // leading byte.
    const size_t safety_margin = size - margin + 1; // to avoid overruns!
    while (pos + 64 + safety_margin <= size) {
      simd8x64<int8_t> input(reinterpret_cast<const int8_t *>(in + pos));
      if (input.is_ascii()) {
        input.store_ascii_as_utf16<endian>(utf16_output);
        utf16_output += 64;
        pos += 64;
      } else {
        // you might think that a for-loop would work, but under Visual Studio,
        // it is not good enough.
        static_assert(
            (simd8x64<uint8_t>::NUM_CHUNKS == 2) ||
                (simd8x64<uint8_t>::NUM_CHUNKS == 4),
            "We support either two or four chunks per 64-byte block.");
        auto zero = simd8<uint8_t>{uint8_t(0)};
        if (simd8x64<uint8_t>::NUM_CHUNKS == 2) {
          this->check_utf8_bytes(input.chunks[0], zero);
          this->check_utf8_bytes(input.chunks[1], input.chunks[0]);
        } else if (simd8x64<uint8_t>::NUM_CHUNKS == 4) {
          this->check_utf8_bytes(input.chunks[0], zero);
          this->check_utf8_bytes(input.chunks[1], input.chunks[0]);
          this->check_utf8_bytes(input.chunks[2], input.chunks[1]);
          this->check_utf8_bytes(input.chunks[3], input.chunks[2]);
        }
        uint64_t utf8_continuation_mask = input.lt(-65 + 1);
        if (errors() || (utf8_continuation_mask & 1)) {
          // rewind_and_convert_with_errors will seek a potential error from
          // in+pos onward, with the ability to go back up to pos bytes, and
          // read size-pos bytes forward.
          result res =
              scalar::utf8_to_utf16::rewind_and_convert_with_errors<endian>(
                  pos, in + pos, size - pos, utf16_output);
          res.count += pos;
          return res;
        }
        uint64_t utf8_leading_mask = ~utf8_continuation_mask;
        uint64_t utf8_end_of_code_point_mask = utf8_leading_mask >> 1;
        // We process in blocks of up to 12 bytes except possibly
        // for fast paths which may process up to 16 bytes. For the
        // slow path to work, we should have at least 12 input bytes left.
        size_t max_starting_point = (pos + 64) - 12;
        // Next loop is going to run at least five times.
        while (pos < max_starting_point) {
          // Performance note: our ability to compute 'consumed' and
          // then shift and recompute is critical. If there is a
          // latency of, say, 4 cycles on getting 'consumed', then
          // the inner loop might have a total latency of about 6 cycles.
          // Yet we process between 6 to 12 inputs bytes, thus we get
          // a speed limit between 1 cycle/byte and 0.5 cycle/byte
          // for this section of the code. Hence, there is a limit
          // to how much we can further increase this latency before
          // it seriously harms performance.
          size_t consumed = convert_masked_utf8_to_utf16<endian>(
              in + pos, utf8_end_of_code_point_mask, utf16_output);
          pos += consumed;
          utf8_end_of_code_point_mask >>= consumed;
        }
        // At this point there may remain between 0 and 12 bytes in the
        // 64-byte block. These bytes will be processed again. So we have an
        // 80% efficiency (in the worst case). In practice we expect an
        // 85% to 90% efficiency.
      }
    }
    if (errors()) {
      // rewind_and_convert_with_errors will seek a potential error from in+pos
      // onward, with the ability to go back up to pos bytes, and read size-pos
      // bytes forward.
      result res =
          scalar::utf8_to_utf16::rewind_and_convert_with_errors<endian>(
              pos, in + pos, size - pos, utf16_output);
      res.count += pos;
      return res;
    }
    if (pos < size) {
      // rewind_and_convert_with_errors will seek a potential error from in+pos
      // onward, with the ability to go back up to pos bytes, and read size-pos
      // bytes forward.
      result res =
          scalar::utf8_to_utf16::rewind_and_convert_with_errors<endian>(
              pos, in + pos, size - pos, utf16_output);
      if (res.error) { // In case of error, we want the error position
        res.count += pos;
        return res;
      } else { // In case of success, we want the number of word written
        utf16_output += res.count;
      }
    }
    return result(error_code::SUCCESS, utf16_output - start);
  }

  simdutf_really_inline bool errors() const {
    return this->error.any_bits_set_anywhere();
  }

}; // struct utf8_checker
} // namespace utf8_to_utf16
} // unnamed namespace
} // namespace SIMDUTF_IMPLEMENTATION
} // namespace simdutf
