Avoid OOM during prewarm by loading only ligature data, not kern pairs

This is particularly important when using SD fonts with many kern pairs

lib/EpdFont/SdCardFont.cpp

@@ -66,11 +66,12 @@ void SdCardFont::freeStyleMiniData(PerStyle& s) {
 void SdCardFont::freeStyleKernLigatureData(PerStyle& s) {
   delete[] s.kernLeftClasses;
   s.kernLeftClasses = nullptr;
+  s.kernClassesLoaded = false;
   delete[] s.kernRightClasses;
   s.kernRightClasses = nullptr;
   delete[] s.ligaturePairs;
   s.ligaturePairs = nullptr;
-  s.kernLigLoaded = false;
+  s.ligLoaded = false;
 }
 
 void SdCardFont::freeStyleMiniKern(PerStyle& s) {
@@ -134,14 +135,17 @@ void SdCardFont::applyKernLigaturePointers(const PerStyle& s, EpdFontData& data)
   data.ligaturePairCount = s.header.ligaturePairCount;
 }
 
-bool SdCardFont::loadStyleKernLigatureData(PerStyle& s) {
+bool SdCardFont::loadStyleKernLigatureData(PerStyle& s, bool ligatureOnly) {
-  if (s.kernLigLoaded) return true;
+  // During metadata-only (layout) prewarms, skip the kern class tables: the kern
-  bool hasKern = s.header.kernLeftEntryCount > 0;
+  // matrix is never built at layout time so getKerning() returns 0 regardless.
-  bool hasLig = s.header.ligaturePairCount > 0;
+  // Skipping them saves ~4KB per style (~17KB total for 4 styles), preventing OOM
-  if (!hasKern && !hasLig) {
+  // on low-heap devices when long paragraphs try to grow their word vector.
-    s.kernLigLoaded = true;
+  const bool wantKern = !ligatureOnly && s.header.kernLeftEntryCount > 0;
-    return true;
+  const bool wantLig = s.header.ligaturePairCount > 0;
-  }
+
+  const bool kernDone = !wantKern || s.kernClassesLoaded;
+  const bool ligDone = !wantLig || s.ligLoaded;
+  if (kernDone && ligDone) return true;
 
   FsFile file;
   if (!Storage.openFileForRead("SDCF", filePath_, file)) {
@@ -149,7 +153,7 @@ bool SdCardFont::loadStyleKernLigatureData(PerStyle& s) {
     return false;
   }
 
-  if (hasKern) {
+  if (wantKern && !s.kernClassesLoaded) {
     // Load only the small class-lookup tables (~3KB each). The full matrix
     // (~36KB contiguous for Literata) is built per-page from SD in
     // buildMiniKernMatrix().
@@ -179,9 +183,10 @@ bool SdCardFont::loadStyleKernLigatureData(PerStyle& s) {
       file.close();
       return false;
     }
+    s.kernClassesLoaded = true;
   }
 
-  if (hasLig) {
+  if (wantLig && !s.ligLoaded) {
     s.ligaturePairs = new (std::nothrow) EpdLigaturePair[s.header.ligaturePairCount];
     if (!s.ligaturePairs) {
       LOG_ERR("SDCF", "Failed to allocate ligature pairs");
@@ -202,19 +207,21 @@ bool SdCardFont::loadStyleKernLigatureData(PerStyle& s) {
       file.close();
       return false;
     }
+    s.ligLoaded = true;
+
+    // Make ligatures visible to the stub (used when no mini data built yet).
+    // Kern stays nullptr on the stub — it is only wired in miniData via
+    // applyKernLigaturePointers() after buildMiniKernMatrix() runs.
+    s.stubData.ligaturePairs = s.ligaturePairs;
+    s.stubData.ligaturePairCount = s.header.ligaturePairCount;
   }
 
   file.close();
-  s.kernLigLoaded = true;
+  LOG_DBG("SDCF", "Kern/lig loaded: kernL=%u kernR=%u ligs=%u ligOnly=%d",
-
+          s.kernClassesLoaded ? s.header.kernLeftEntryCount : 0u,
-  // Make ligatures visible to the stub (used when no mini data built yet).
+          s.kernClassesLoaded ? s.header.kernRightEntryCount : 0u,
-  // Kern stays nullptr on the stub — it is only wired in miniData via
+          s.ligLoaded ? s.header.ligaturePairCount : 0u,
-  // applyKernLigaturePointers() after buildMiniKernMatrix() runs.
+          ligatureOnly);
-  s.stubData.ligaturePairs = s.ligaturePairs;
-  s.stubData.ligaturePairCount = s.header.ligaturePairCount;
-
-  LOG_DBG("SDCF", "Kern classes + lig loaded: kernL=%u, kernR=%u, ligs=%u", s.header.kernLeftEntryCount,
-          s.header.kernRightEntryCount, s.header.ligaturePairCount);
   return true;
 }
 
@@ -653,7 +660,7 @@ int SdCardFont::prewarm(const char* utf8Text, uint8_t styleMask, bool metadataOn
       if (!(styleMask & (1 << si)) || !styles_[si].present) continue;
       auto& s = styles_[si];
 
-      loadStyleKernLigatureData(s);
+      loadStyleKernLigatureData(s, /*ligatureOnly=*/true);
       if (s.ligaturePairs && s.header.ligaturePairCount > 0) {
         for (uint8_t li = 0; li < s.header.ligaturePairCount && cpCount < MAX_PAGE_GLYPHS; li++) {
           uint32_t leftCp = s.ligaturePairs[li].pair >> 16;
@@ -1143,7 +1150,7 @@ int SdCardFont::prewarmStyle(uint8_t styleIdx, const uint32_t* codepoints, uint3
       kernLigOk = buildMiniKernMatrix(s, codepoints, cpCount);
     }
   } else if (loadKernLigatureData) {
-    loadStyleKernLigatureData(s);
+    loadStyleKernLigatureData(s, /*ligatureOnly=*/true);
     // Don't set kernLigOk → mini kern matrix stays null on miniData, but
     // ligatures are still resident on stubData (set in loadStyleKernLigatureData).
   }
@@ -1161,7 +1168,7 @@ int SdCardFont::prewarmStyle(uint8_t styleIdx, const uint32_t* codepoints, uint3
   if (kernLigOk) {
     // Full prewarm: wire mini kern matrix + class tables + ligatures.
     applyKernLigaturePointers(s, s.miniData);
-  } else if (loadKernLigatureData && s.kernLigLoaded) {
+  } else if (loadKernLigatureData && s.ligLoaded) {
     // Layout-only prewarm: wire ligatures so applyLigatures() works (e.g. "fi"
     // measures correctly). Skip the kern matrix — getKerning() returns 0
     // cleanly when kernMatrix is null. Per-pair kern is applied at render time.
@@ -1173,6 +1180,8 @@ int SdCardFont::prewarmStyle(uint8_t styleIdx, const uint32_t* codepoints, uint3
 
   s.epdFont.data = &s.miniData;
   s.miniMode = metadataOnly ? PerStyle::MiniMode::METADATA : PerStyle::MiniMode::FULL;
+  LOG_DBG("SDCF", "prewarmStyle %u: mode→%s glyphs=%u bitmap=%p",
+          styleIdx, metadataOnly ? "METADATA" : "FULL", validCount, s.miniBitmap);
 
   // Accumulate stats
   stats_.sdReadTimeMs += sdTime;
@@ -1236,6 +1245,10 @@ const EpdGlyph* SdCardFont::onGlyphMiss(void* ctx, uint32_t codepoint) {
   const auto& s = self->styles_[styleIdx];
   if (!s.fullIntervals) return nullptr;
 
+  // Diagnostic: log first miss per codepoint+style to show why it bypassed prewarm
+  LOG_DBG("SDCF", "onGlyphMiss: U+%04X style %u miniMode=%u miniIntervals=%u bitmap=%p",
+          codepoint, styleIdx, (uint8_t)s.miniMode, s.miniIntervalCount, s.miniBitmap);
+
   // Check overflow cache first (matching both codepoint and style)
   for (uint32_t i = 0; i < self->overflowCount_; i++) {
     if (self->overflow_[i].codepoint == codepoint && self->overflow_[i].styleIdx == styleIdx) {
@@ -1309,6 +1322,8 @@ const EpdGlyph* SdCardFont::onGlyphMiss(void* ctx, uint32_t codepoint) {
 
   // All reads succeeded — commit to slot (evict old entry if at capacity)
   if (wasAtCapacity) {
+    LOG_DBG("SDCF", "Overflow: evicting U+%04X style %u from slot %u",
+            self->overflow_[slot].codepoint, self->overflow_[slot].styleIdx, slot);
     delete[] self->overflow_[slot].bitmap;
   }
   self->overflow_[slot].glyph = tempGlyph;

lib/EpdFont/SdCardFont.h

@@ -118,7 +118,8 @@ class SdCardFont {
     EpdKernClassEntry* kernLeftClasses = nullptr;
     EpdKernClassEntry* kernRightClasses = nullptr;
     EpdLigaturePair* ligaturePairs = nullptr;
-    bool kernLigLoaded = false;
+    bool ligLoaded = false;        ///< ligaturePairs resident
+    bool kernClassesLoaded = false; ///< kernLeft/RightClasses resident (skipped during metadata-only prewarm)
 
     // Stub EpdFontData returned when not prewarmed
     EpdFontData stubData{};
@@ -204,7 +205,7 @@ class SdCardFont {
   void freeStyleAll(PerStyle& s);
   void freeStyleKernLigatureData(PerStyle& s);
   void freeStyleMiniKern(PerStyle& s);
-  bool loadStyleKernLigatureData(PerStyle& s);
+  bool loadStyleKernLigatureData(PerStyle& s, bool ligatureOnly = false);
   bool buildMiniKernMatrix(PerStyle& s, const uint32_t* codepoints, uint32_t cpCount);
   void applyKernLigaturePointers(const PerStyle& s, EpdFontData& data) const;
   void applyGlyphMissCallback(uint8_t styleIdx);