#include "utf16.hh"

#include "utf_error.hh"

namespace utf {

namespace {

inline bool is_high_surrogate(uint16_t c) {
  return c >= 0xd800 && c <= 0xdbff;
}

inline bool is_low_surrogate(uint16_t c) {
  return c >= 0xdc00 && c <= 0xdfff;
}

}  // namespace

uint32_t read16be(std::span<uint8_t const> data, std::size_t& offset) {
  if (offset > data.size() || data.size() - offset < 2)
    return NEED_MORE;
  uint16_t c = static_cast<uint16_t>(data[offset]) << 8
               | static_cast<uint16_t>(data[offset + 1] & 0xff);
  if (is_high_surrogate(c)) {
    if (data.size() - offset < 4)
      return NEED_MORE;
    uint16_t d = static_cast<uint16_t>(data[offset + 2]) << 8
                 | static_cast<uint16_t>(data[offset + 3] & 0xff);
    if (is_low_surrogate(d)) {
      offset += 4;
      return 0x10000
          + (static_cast<uint32_t>(c & 0x3ff) << 10
             | (d & 0x3ff));
    }
    return INVALID;
  } else if (is_low_surrogate(c)) {
    return INVALID;
  }
  offset += 2;
  return c;
}

uint32_t read16le(std::span<uint8_t const> data, std::size_t& offset) {
  if (offset > data.size() || data.size() - offset < 2)
    return NEED_MORE;
  uint16_t c = static_cast<uint16_t>(data[offset + 1]) << 8
               | static_cast<uint16_t>(data[offset] & 0xff);
  if (is_high_surrogate(c)) {
    if (data.size() - offset < 4)
      return NEED_MORE;
    uint16_t d = static_cast<uint16_t>(data[offset + 3]) << 8
                 | static_cast<uint16_t>(data[offset + 2] & 0xff);
    if (is_low_surrogate(d)) {
      offset += 4;
      return 0x10000
          + (static_cast<uint32_t>(c & 0x3ff) << 10
             | (d & 0x3ff));
    }
    return INVALID;
  } else if (is_low_surrogate(c)) {
    return INVALID;
  }
  offset += 2;
  return c;
}

}  // namespace utf