// SPDX-License-Identifier: MIT
// SPDX-FileCopyrightText: 2026 Martino Ferrari

#include <plot/render/text.hpp>

namespace plot::render {

// The decoder lives with the interface, not with a font, because it is what keeps
// "what a string is worth" (`TextBackend::measure`) and "what a string draws"
// (`TextBatch::add_text`) the same thing: both walk a label with this function, so
// neither can disagree with the other about where a codepoint starts or how many
// bytes a malformed sequence costs.
u32 next_codepoint(const char*& p) noexcept {
  const auto b0 = static_cast<u32>(static_cast<unsigned char>(p[0]));
  if (b0 < 0x80u) {
    ++p;
    return b0;
  }
  u32 codepoint = 0;
  u32 extra = 0;
  if ((b0 & 0xE0u) == 0xC0u) {
    codepoint = b0 & 0x1Fu;
    extra = 1u;
  } else if ((b0 & 0xF0u) == 0xE0u) {
    codepoint = b0 & 0x0Fu;
    extra = 2u;
  } else if ((b0 & 0xF8u) == 0xF0u) {
    codepoint = b0 & 0x07u;
    extra = 3u;
  } else {
    ++p;
    return kInvalidCodepoint;
  }
  for (u32 i = 1; i <= extra; ++i) {
    const auto b = static_cast<u32>(static_cast<unsigned char>(p[i]));
    if ((b & 0xC0u) != 0x80u) {  // truncated: stop before the terminator
      ++p;
      return kInvalidCodepoint;
    }
    codepoint = (codepoint << 6) | (b & 0x3Fu);
  }
  p += extra + 1u;
  constexpr u32 kMinForLength[4] = {0u, 0x80u, 0x800u, 0x10000u};
  if (codepoint < kMinForLength[extra] || codepoint > 0x10FFFFu ||
      (codepoint >= 0xD800u && codepoint <= 0xDFFFu)) {
    return kInvalidCodepoint;  // overlong, out of range or a surrogate
  }
  return codepoint;
}

}  // namespace plot::render