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Position-independent font shaper caching

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Use relative cluster positioning to allow identical texts runs in
different row positions to share the same cache entry.
This commit is contained in:
Jesse Miller
2025-09-06 11:26:23 -06:00
parent e4c3a56242
commit ecf3e2ad7d
2 changed files with 12 additions and 4 deletions
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10
src/font/shaper/run.zig
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@@ -17,6 +17,10 @@ pub const TextRun = struct {
/// lower the chance of hash collisions if they become a problem. If
/// there are hash collisions, it would result in rendering issues but
/// the core data would be correct.
///
/// The hash is position-independent within the row by using relative
/// cluster positions. This allows identical runs in different positions
/// to share the same cache entry, improving cache efficiency.
hash: u64,
/// The offset in the row where this run started
@@ -77,7 +81,11 @@ pub const RunIterator = struct {
// Go through cell by cell and accumulate while we build our run.
var j: usize = self.i;
while (j < max) : (j += 1) {
const cluster = j;
// Use relative cluster positions (offset from run start) to make
// the shaping cache position-independent. This ensures that runs
// with identical content but different starting positions in the
// row produce the same hash, enabling cache reuse.
const cluster = j - self.i;
const cell = &cells[j];
// If we have a selection and we're at a boundary point, then

6
src/renderer/generic.zig
View File

@@ -2530,7 +2530,7 @@ pub fn Renderer(comptime GraphicsAPI: type) type {
// Advance our index until we reach or pass
// our current x position in the shaper cells.
while (shaper_cells.?[shaper_cells_i].x < x) {
while (shaper_cells.?[shaper_cells_i].x + run.offset < x) {
shaper_cells_i += 1;
}
}
@@ -2769,13 +2769,13 @@ pub fn Renderer(comptime GraphicsAPI: type) type {
// If we encounter a shaper cell to the left of the current
// cell then we have some problems. This logic relies on x
// position monotonically increasing.
assert(cells[shaper_cells_i].x >= x);
assert(cells[shaper_cells_i].x + run.offset >= x);
// NOTE: An assumption is made here that a single cell will never
// be present in more than one shaper run. If that assumption is
// violated, this logic breaks.
while (shaper_cells_i < cells.len and cells[shaper_cells_i].x == x) : ({
while (shaper_cells_i < cells.len and cells[shaper_cells_i].x + run.offset == x) : ({
shaper_cells_i += 1;
}) {
self.addGlyph(