Method prewarm

lib/EpdFont/SdCardFont.cpp:660–762

// --- Prewarm ---

int SdCardFont::prewarm(const char* utf8Text, uint8_t styleMask, bool metadataOnly) {
  if (!loaded_) return -1;
  styleMask = resolveStyleMask(styleMask);
  if (styleMask == 0) return 0;

  unsigned long startMs = millis();

  // Step 1: Extract unique codepoints from UTF-8 text (shared across all styles).
  // Dedup uses O(n^2) linear scan — worst case is MAX_PAGE_GLYPHS (512) unique codepoints
  // = ~131K comparisons, but in practice pages contain far fewer unique codepoints so the
  // actual cost is much lower. This is dwarfed by SD I/O that follows. Alternatives (hash
  // set, bitmap) exceed the 256-byte stack limit or add template bloat.
  // Heap-allocated: MAX_PAGE_GLYPHS * 4 = 2048 bytes, too large for stack (limit < 256 bytes)
  std::unique_ptr<uint32_t[]> codepoints(new (std::nothrow) uint32_t[MAX_PAGE_GLYPHS]);
  if (!codepoints) {
    LOG_ERR("SDCF", "Failed to allocate codepoint buffer (%u bytes)", MAX_PAGE_GLYPHS * 4);
    return -1;
  }
  uint32_t cpCount = 0;

  const unsigned char* p = reinterpret_cast<const unsigned char*>(utf8Text);
  while (*p && cpCount < MAX_PAGE_GLYPHS) {
    uint32_t cp = utf8NextCodepoint(&p);
    if (cp == 0) break;

    bool found = false;
    for (uint32_t i = 0; i < cpCount; i++) {
      if (codepoints[i] == cp) {
        found = true;
        break;
      }
    }
    if (!found) {
      codepoints[cpCount++] = cp;
    }
  }

  // Always include the replacement character
  {
    bool hasReplacement = false;
    for (uint32_t i = 0; i < cpCount; i++) {
      if (codepoints[i] == REPLACEMENT_GLYPH) {
        hasReplacement = true;
        break;
      }
    }
    if (!hasReplacement && cpCount < MAX_PAGE_GLYPHS) {
      codepoints[cpCount++] = REPLACEMENT_GLYPH;
    }
  }

  // Add ligature output codepoints from all styles being prewarmed.
  // Skip during metadata-only prewarm (layout measurement) to avoid loading
  // kern/lig data for all styles upfront (~22KB per style). Kern/lig is
  // loaded per-style in prewarmStyle() during the full render prewarm instead.
  if (!metadataOnly) {
    for (uint8_t si = 0; si < MAX_STYLES; si++) {
      if (!(styleMask & (1 << si)) || !styles_[si].present) continue;