Per-Frame O(N) Cache Elimination

May 18, 2026 · View on GitHub

Eliminates 7 per-frame O(N) operations on tab.content using the existing content_version: u64 counter on Tab for cache invalidation.

Problem

Several code paths scanned the entire tab.content string every frame:

Site	Operation	Cost (50MB file)
`status_bar.rs`	`TextStats::from_text()`	~25ms (word/char/line counting)
`central_panel.rs` / `mod.rs`	`tab.title()` → `is_modified()`	~15ms (hash or string compare)
`mod.rs`	`needs_cjk()` / `needs_complex_script_fonts()`	~10ms (char scan)
`mod.rs`	`should_auto_save()` → `hash_content()`	~10ms
`mod.rs`	frontmatter `update_from_content()` clone + hash	~30ms
`editor.rs`	`show_raw_editor()` content clone	~25ms
`editor.rs`	`show_rendered_editor()` content clone	~25ms

Solution

Cache Pattern

Each cached value uses a version guard:

if tab.content_version != tab.cached_xxx_version {
    tab.cached_xxx = compute_xxx(&tab.content);
    tab.cached_xxx_version = tab.content_version;
}

content_version increments on undo, redo, and increment_content_version() calls — never during scroll or idle frames.

Cached Fields on `Tab` (state.rs)

Field	Guards	Source
`cached_text_stats`	`cached_text_stats_version`	`TextStats::from_text()`
`cached_is_modified`	`cached_is_modified_version` + `cached_is_modified_save_version`	`is_modified_uncached()`
`cached_needs_cjk`	`cached_needs_cjk_version`	`fonts::needs_cjk()`
`cached_needs_complex_script`	`cached_needs_complex_script_version`	`fonts::needs_complex_script_fonts()`
`last_auto_save_content_version`	—	Replaces `hash_content()` call

Dual-Version Guard for is_modified

is_modified() depends on both content and save state. It uses two version counters:

content_version — changes on edits
save_version — changes on mark_saved()

Both must match for the cache to be valid.

AppState::warm_tab_caches() is called at the start of each update() frame to pre-compute is_modified_cached() for all tabs. This ensures subsequent &self calls to title() and is_modified() are O(1).

MarkdownEditor Clone Elimination

Raw editor: Replaced content.clone() + string compare with response.changed() from egui's TextEdit output.
Rendered editor: Eliminated the full content clone. rebuild_markdown() already ignores the original parameter. get_focused_element() now uses the current content directly (self-corrects on the next frame if an edit happened).

Frontmatter Panel

Replaced DefaultHasher content hashing with a (tab_id, content_version) cache key. The content clone only happens when the active tab or its content actually changes (not every frame).

Note (v0.3.x): The first cut of this change keyed on content_version alone, which is unsafe because the counter is per-tab and starts at 0 for every new tab — two unedited tabs share key 0 and the panel never re-parses on tab switch. Pairing with the stable Tab.id is required; see docs/technical/ui/frontmatter-panel.md (Caching).

Files Changed

File	Change
`src/state.rs`	Added 12 cache fields to `Tab`, `is_modified_cached()`, `text_stats()`, `needs_cjk_cached()`, `needs_complex_script_cached()`, `warm_tab_caches()`
`src/app/mod.rs`	Call `warm_tab_caches()` at frame start; use cached font detection
`src/app/status_bar.rs`	Use `tab.text_stats()` instead of `TextStats::from_text()`
`src/ui/frontmatter_panel.rs`	`update_from_content_versioned(content, tab_id, content_version)` with `(tab_id, content_version)` cache key (initial version used `content_version` alone — see note above)
`src/markdown/editor.rs`	Eliminate content clones in both editor modes