#ifndef SIMDUTF_VALID_UTF8_TO_UTF16_H
#define SIMDUTF_VALID_UTF8_TO_UTF16_H

namespace simdutf {
namespace scalar {
namespace {
namespace utf8_to_utf16 {

template <endianness big_endian, typename InputPtr>
#if SIMDUTF_CPLUSPLUS20
  requires simdutf::detail::indexes_into_byte_like<InputPtr>
#endif
simdutf_constexpr23 size_t convert_valid(InputPtr data, size_t len,
                                         char16_t *utf16_output) {
  size_t pos = 0;
  char16_t *start{utf16_output};
  while (pos < len) {
#if SIMDUTF_CPLUSPLUS23
    if !consteval
#endif
    {                       // try to convert the next block of 8 ASCII bytes
      if (pos + 8 <= len) { // if it is safe to read 8 more bytes, check that
                            // they are ascii
        uint64_t v;
        ::memcpy(&v, data + pos, sizeof(uint64_t));
        if ((v & 0x8080808080808080) == 0) {
          size_t final_pos = pos + 8;
          while (pos < final_pos) {
            const char16_t byte = uint8_t(data[pos]);
            *utf16_output++ =
                !match_system(big_endian) ? u16_swap_bytes(byte) : byte;
            pos++;
          }
          continue;
        }
      }
    }

    auto leading_byte = uint8_t(data[pos]); // leading byte
    if (leading_byte < 0b10000000) {
      // converting one ASCII byte !!!
      *utf16_output++ = !match_system(big_endian)
                            ? char16_t(u16_swap_bytes(leading_byte))
                            : char16_t(leading_byte);
      pos++;
    } else if ((leading_byte & 0b11100000) == 0b11000000) {
      // We have a two-byte UTF-8, it should become
      // a single UTF-16 word.
      if (pos + 1 >= len) {
        break;
      } // minimal bound checking
      uint16_t code_point = uint16_t(((leading_byte & 0b00011111) << 6) |
                                     (uint8_t(data[pos + 1]) & 0b00111111));
      if constexpr (!match_system(big_endian)) {
        code_point = u16_swap_bytes(uint16_t(code_point));
      }
      *utf16_output++ = char16_t(code_point);
      pos += 2;
    } else if ((leading_byte & 0b11110000) == 0b11100000) {
      // We have a three-byte UTF-8, it should become
      // a single UTF-16 word.
      if (pos + 2 >= len) {
        break;
      } // minimal bound checking
      uint16_t code_point =
          uint16_t(((leading_byte & 0b00001111) << 12) |
                   ((uint8_t(data[pos + 1]) & 0b00111111) << 6) |
                   (uint8_t(data[pos + 2]) & 0b00111111));
      if constexpr (!match_system(big_endian)) {
        code_point = u16_swap_bytes(uint16_t(code_point));
      }
      *utf16_output++ = char16_t(code_point);
      pos += 3;
    } else if ((leading_byte & 0b11111000) == 0b11110000) { // 0b11110000
      // we have a 4-byte UTF-8 word.
      if (pos + 3 >= len) {
        break;
      } // minimal bound checking
      uint32_t code_point = ((leading_byte & 0b00000111) << 18) |
                            ((uint8_t(data[pos + 1]) & 0b00111111) << 12) |
                            ((uint8_t(data[pos + 2]) & 0b00111111) << 6) |
                            (uint8_t(data[pos + 3]) & 0b00111111);
      code_point -= 0x10000;
      uint16_t high_surrogate = uint16_t(0xD800 + (code_point >> 10));
      uint16_t low_surrogate = uint16_t(0xDC00 + (code_point & 0x3FF));
      if constexpr (!match_system(big_endian)) {
        high_surrogate = u16_swap_bytes(high_surrogate);
        low_surrogate = u16_swap_bytes(low_surrogate);
      }
      *utf16_output++ = char16_t(high_surrogate);
      *utf16_output++ = char16_t(low_surrogate);
      pos += 4;
    } else {
      // we may have a continuation but we do not do error checking
      return 0;
    }
  }
  return utf16_output - start;
}

} // namespace utf8_to_utf16
} // unnamed namespace
} // namespace scalar
} // namespace simdutf

#endif
