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FontDecompressor: heap-allocate hot group buffer sized to active font

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Replace the 50 KB static BSS _hotGroupBuf array with a heap pointer
allocated once per font (ensureHotGroupBuf). The buffer is sized to the
largest group in the active font — 6 KB for small fonts, ~50 KB for 18pt
— so no memory is wasted when rendering body text at typical sizes.

The buffer persists across pages (freed only in freeHotGroup/deinit),
so there is exactly one malloc per font session rather than one per page.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
This commit is contained in:
jpirnay
2026-04-22 08:14:04 +02:00
co-authored by Claude Sonnet 4.6
parent e4170e2272
commit 80722118a2
2 changed files with 52 additions and 20 deletions
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lib/EpdFont/FontDecompressor.h
+7 -9
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@@ -63,19 +63,16 @@ class FontDecompressor {
PageSlot pageSlots[MAX_PAGE_SLOTS] = {};
uint8_t pageSlotCount = 0;
// Measured maxima across all built-in fonts:
// uncompressedSize: 50 KB (notosans_18 / bookerly_18)
// glyph dataLength: 500 B (bookerly_18)
// Static BSS arrays eliminate per-page heap alloc/free and the fragmentation it causes.
static constexpr uint32_t HOT_GROUP_BUF_SIZE = 51200; // 50 KB uncompressed group
static constexpr uint16_t HOT_GLYPH_BUF_SIZE = 512; // largest packed single glyph
static constexpr uint16_t HOT_GLYPH_BUF_SIZE = 512; // largest packed single glyph
// Hot group: last decompressed group (byte-aligned) for non-prewarmed fallback path.
// Kept in byte-aligned format; individual glyphs are compacted on demand into _hotGlyphBuf.
// Hot group: single heap buffer sized to the largest group of the active font.
// Allocated once per font (lazily in ensureHotGroupBuf) and freed in freeHotGroup().
// Single malloc per font session — no repeated alloc/free per page.
const EpdFontData* hotGroupFont = nullptr;
uint16_t hotGroupIndex = UINT16_MAX;
uint32_t _hotGroupBufUsed = 0;
uint8_t _hotGroupBuf[HOT_GROUP_BUF_SIZE];
uint8_t* _hotGroupBuf = nullptr;
uint32_t _hotGroupBufSize = 0;
// Scratch buffer for compacting a single glyph out of the byte-aligned hot group.
// Valid until the next getBitmap() call.
@@ -83,6 +80,7 @@ class FontDecompressor {
void freePageBuffer();
void freeHotGroup();
bool ensureHotGroupBuf(const EpdFontData* fontData);
uint16_t getGroupIndex(const EpdFontData* fontData, uint32_t glyphIndex);
uint32_t getAlignedOffset(const EpdFontData* fontData, uint16_t groupIndex, uint32_t glyphIndex);
bool decompressGroup(const EpdFontData* fontData, uint16_t groupIndex, uint8_t* outBuf, uint32_t outSize);