Add minimal allback buffer for future use

lib/EpdFont/FontDecompressor.cpp

@@ -147,6 +147,19 @@ const uint8_t* FontDecompressor::getBitmap(const EpdFontData* fontData, const Ep
     break;  // Found the right slot but glyph wasn't in it; don't check other slots
   }
 
+  // Check fallback LRU cache
+  for (uint8_t i = 0; i < FALLBACK_CACHE_SLOTS; i++) {
+    if (_fallbackCache[i].fontData == fontData && _fallbackCache[i].glyphIndex == glyphIndex) {
+      _fallbackCache[i].lastUsedTick = ++_fallbackTick;
+      stats.cacheHits++;
+      stats.fallbackCacheHits++;
+      stats.getBitmapTimeUs += micros() - tStart;
+      return _fallbackCache[i].buffer;
+    }
+  }
+
+  stats.fallbackCacheMisses++;
+
   // Fallback: glyph wasn't in the page buffer — decompress its group transiently.
   // This is the rare path (prewarm should cover all glyphs on a normal page).
   uint16_t groupIndex = getGroupIndex(fontData, glyphIndex);
@@ -181,11 +194,25 @@ const uint8_t* FontDecompressor::getBitmap(const EpdFontData* fontData, const Ep
   }
 
   uint32_t alignedOff = getAlignedOffset(fontData, groupIndex, glyphIndex);
-  compactSingleGlyph(&groupBuf[alignedOff], _hotGlyphBuf, glyph->width, glyph->height);
+
+  uint8_t lruIndex = 0;
+  uint32_t oldestTick = UINT32_MAX;
+  for (uint8_t i = 0; i < FALLBACK_CACHE_SLOTS; i++) {
+    if (_fallbackCache[i].lastUsedTick < oldestTick) {
+      oldestTick = _fallbackCache[i].lastUsedTick;
+      lruIndex = i;
+    }
+  }
+
+  compactSingleGlyph(&groupBuf[alignedOff], _fallbackCache[lruIndex].buffer, glyph->width, glyph->height);
   free(groupBuf);
 
+  _fallbackCache[lruIndex].fontData = fontData;
+  _fallbackCache[lruIndex].glyphIndex = glyphIndex;
+  _fallbackCache[lruIndex].lastUsedTick = ++_fallbackTick;
+
   stats.getBitmapTimeUs += micros() - tStart;
-  return _hotGlyphBuf;
+  return _fallbackCache[lruIndex].buffer;
 }
 
 // --- Prewarm: pre-decompress glyph bitmaps for a page of text ---
@@ -537,5 +564,12 @@ void FontDecompressor::logStats(const char* label) {
     LOG_DBG("FDC", "[%s] getBitmap: %lu calls, %luus total, %luus/call avg", label, stats.getBitmapCalls,
             stats.getBitmapTimeUs, stats.getBitmapTimeUs / stats.getBitmapCalls);
   }
+
+  uint32_t lruTotal = stats.fallbackCacheHits + stats.fallbackCacheMisses;
+  if (lruTotal > 0) {
+    LOG_DBG("FDC", "[%s] LRU Fallback: hits=%lu misses=%lu (%.1f%%)", label, stats.fallbackCacheHits,
+            stats.fallbackCacheMisses, 100.0f * stats.fallbackCacheHits / lruTotal);
+  }
+
   resetStats();
 }

lib/EpdFont/FontDecompressor.h

@@ -41,6 +41,10 @@ class FontDecompressor {
     uint32_t peakTempBytes = 0;    // largest temp buffer in prewarm or getBitmap miss
     uint32_t getBitmapTimeUs = 0;  // cumulative getBitmap time (micros)
     uint32_t getBitmapCalls = 0;   // number of getBitmap calls
+
+    // LRU specific stats
+    uint32_t fallbackCacheHits = 0;
+    uint32_t fallbackCacheMisses = 0;
   };
   void logStats(const char* label = "FDC");
   void resetStats();
@@ -68,10 +72,18 @@ class FontDecompressor {
   uint8_t pageSlotCount = 0;
 
   static constexpr uint16_t HOT_GLYPH_BUF_SIZE = 512;  // largest packed single glyph
+  static constexpr uint8_t FALLBACK_CACHE_SLOTS = 1;
 
-  // Scratch buffer for compacting a single glyph after a getBitmap() miss.
-  // Valid until the next getBitmap() call.
-  uint8_t _hotGlyphBuf[HOT_GLYPH_BUF_SIZE] = {};
+  struct FallbackSlot {
+    const EpdFontData* fontData;
+    uint32_t glyphIndex;
+    uint32_t lastUsedTick;
+    uint8_t buffer[HOT_GLYPH_BUF_SIZE];
+  };
+
+  // LRU cache for fallback glyph decompresion
+  FallbackSlot _fallbackCache[FALLBACK_CACHE_SLOTS] = {};
+  uint32_t _fallbackTick = 0;
 
   void freePageBuffer();
   uint16_t getGroupIndex(const EpdFontData* fontData, uint32_t glyphIndex);