Feature/renderer performance (#24)

docs/architecture/2026-03-15-renderer-performance-design.md

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+# Renderer Performance Optimizations
+
+**Date:** 2026-03-15
+**Status:** Draft
+
+## Goal
+
+Minimize the time between a subtitle line appearing and annotations being displayed. Three optimizations target different pipeline stages to achieve this.
+
+## Current Pipeline (Warm State)
+
+```text
+MPV subtitle change (0ms)
+  -> IPC to main (5ms)
+  -> Cache check (2ms)
+  -> [CACHE MISS] Yomitan parser (35-180ms)
+  -> Parallel: MeCab enrichment (20-80ms) + Frequency lookup (15-50ms)
+  -> Annotation stage: 4 sequential passes (25-70ms)
+  -> IPC to renderer (10ms)
+  -> DOM render: createElement per token (15-50ms)
+  ─────────────────────────────────
+  Total: ~200-320ms (cache miss)
+  Total: ~72ms (cache hit)
+```
+
+## Target Pipeline
+
+```text
+MPV subtitle change (0ms)
+  -> IPC to main (5ms)
+  -> Cache check (2ms)
+  -> [CACHE HIT via prefetch] (0ms)
+  -> IPC to renderer (10ms)
+  -> DOM render: cloneNode from template (10-30ms)
+  ─────────────────────────────────
+  Total: ~30-50ms (prefetch-warmed, normal playback)
+
+  [CACHE MISS, e.g. immediate seek]
+  -> Yomitan parser (35-180ms)
+  -> Parallel: MeCab enrichment + Frequency lookup
+  -> Annotation stage: 1 batched pass (10-25ms)
+  -> IPC to renderer (10ms)
+  -> DOM render: cloneNode from template (10-30ms)
+  ─────────────────────────────────
+  Total: ~150-260ms (cache miss, still improved)
+```
+
+---
+
+## Optimization 1: Subtitle Prefetching
+
+### Summary
+
+A new `SubtitlePrefetchService` parses external subtitle files and tokenizes upcoming lines in the background before they appear on screen. This converts most cache misses into cache hits during normal playback.
+
+### Scope
+
+External subtitle files only (SRT, VTT, ASS). Embedded subtitle tracks are out of scope since Japanese subtitles are virtually always external files.
+
+### Architecture
+
+#### Subtitle File Parsing
+
+A new cue parser that extracts both timing and text content from subtitle files. The existing `parseSrtOrVttStartTimes` in `subtitle-delay-shift.ts` only extracts timing; this needs a companion that also extracts the dialogue text.
+
+**Parsed cue structure:**
+```typescript
+interface SubtitleCue {
+  startTime: number;  // seconds
+  endTime: number;    // seconds
+  text: string;       // raw subtitle text
+}
+```
+
+**Supported formats:**
+- SRT/VTT: Regex-based parsing of timing lines + text content between timing blocks.
+- ASS: Parse `[Events]` section, extract `Dialogue:` lines, split on the first 9 commas only (ASS v4+ has 10 fields; the last field is Text which can itself contain commas). Strip ASS override tags (`{\...}`) from the text before storing.
+  ASS text fields contain inline override tags like `{\b1}`, `{\an8}`, `{\fad(200,300)}`. The cue parser strips these during extraction so the tokenizer receives clean text.
+
+#### Prefetch Service Lifecycle
+
+1. **Activation trigger:** When a subtitle track is activated (or changes), check if it's external via MPV's `track-list` property. If `external === true`, read the file via `external-filename` using the existing `loadSubtitleSourceText` infrastructure.
+2. **Parse phase:** Parse all cues from the file content. Sort by start time. Store as an ordered array.
+3. **Priority window:** Determine the current playback position. Identify the next 10 cues as the priority window.
+4. **Priority tokenization:** Tokenize the priority window cues sequentially, storing results into the `SubtitleProcessingController`'s tokenization cache.
+5. **Background tokenization:** After the priority window is done, tokenize remaining cues working forward from the current position, then wrapping around to cover earlier cues. The prefetcher stops once it has tokenized all cues or the cache is full (whichever comes first) to avoid wasteful eviction churn. For files with more cues than the cache limit, background tokenization focuses on cues ahead of the current position.
+6. **Seek handling:** On seek, re-compute the priority window from the new position. A seek is detected by observing MPV's `time-pos` property and checking if the delta from the last observed position exceeds a threshold (e.g., > 3 seconds forward or any backward jump). The current in-flight tokenization finishes naturally, then the new priority window takes over.
+7. **Teardown:** When the subtitle track changes or playback ends, stop all prefetch work and discard state.
+
+#### Live Priority
+
+The prefetcher and live subtitle handler share the Yomitan parser (single-threaded IPC). Live subtitle requests must always take priority. The prefetcher:
+
+- Checks a `paused` flag before each cue tokenization. The live handler sets `paused = true` on subtitle change and clears it after emission.
+- Yields between each background cue tokenization (via `setTimeout(0)` or equivalent) so the live handler can set the pause flag between cues.
+- When paused, the prefetcher waits (polling the flag on a short interval or awaiting a resume signal) before continuing with the next cue.
+
+#### Cache Integration
+
+The prefetcher calls the same `tokenizeSubtitle` function used by live processing to produce `SubtitleData` results, then stores them into the existing `SubtitleProcessingController` tokenization cache via a new method:
+
+```typescript
+// New methods on SubtitleProcessingController
+preCacheTokenization: (text: string, data: SubtitleData) => void;
+isCacheFull: () => boolean;
+```
+
+`preCacheTokenization` uses the same `setCachedTokenization` logic internally (LRU eviction, Map-based storage). `isCacheFull` returns `true` when the cache has reached its limit, allowing the prefetcher to stop background tokenization and avoid wasteful eviction churn.
+
+#### Cache Invalidation
+
+When the user marks a word as known (or any event triggers `invalidateTokenizationCache()`), all cached results are cleared -- including prefetched ones, since they share the same cache. After invalidation, the prefetcher re-computes the priority window from the current playback position and re-tokenizes those cues to restore warm cache state.
+
+#### Error Handling
+
+If the subtitle file is malformed or partially parseable, the cue parser uses what it can extract. A file that yields zero cues disables prefetching silently (falls back to live-only processing). Encoding errors from `loadSubtitleSourceText` are caught and logged; prefetching is skipped for that track.
+
+#### Integration Points
+
+- **MPV property subscriptions:** Needs `track-list` (to detect external subtitle file path) and `time-pos` (to track playback position for window calculation and seek detection).
+- **File loading:** Uses existing `loadSubtitleSourceText` dependency.
+- **Tokenization:** Calls the same `tokenizeSubtitle` function used by live processing.
+- **Cache:** Writes into `SubtitleProcessingController`'s cache.
+- **Cache invalidation:** Listens for cache invalidation events to re-prefetch the priority window.
+
+### Files Affected
+
+- **New:** `src/core/services/subtitle-prefetch.ts` -- the prefetch service
+- **New:** `src/core/services/subtitle-cue-parser.ts` -- SRT/VTT/ASS cue parser (text + timing)
+- **Modified:** `src/core/services/subtitle-processing-controller.ts` -- expose `preCacheTokenization` method
+- **Modified:** `src/main.ts` -- wire up the prefetch service, listen to track changes
+
+---
+
+## Optimization 2: Batched Annotation Pass
+
+### Summary
+
+Collapse the 4 sequential annotation passes (`applyKnownWordMarking` -> `applyFrequencyMarking` -> `applyJlptMarking` -> `markNPlusOneTargets`) into a single iteration over the token array, followed by N+1 marking.
+
+**Important context:** Frequency rank _values_ (`token.frequencyRank`) are already assigned at the parser level by `applyFrequencyRanks()` in `tokenizer.ts`, before the annotation stage is called. The annotation stage's `applyFrequencyMarking` only performs POS-based _filtering_ -- clearing `frequencyRank` to `undefined` for tokens that should be excluded (particles, noise tokens, etc.) and normalizing valid ranks. This optimization does not change the parser-level frequency rank assignment; it only batches the annotation-level filtering.
+
+### Current Flow (4 passes, 4 array copies)
+
+```text
+tokens (already have frequencyRank values from parser-level applyFrequencyRanks)
+  -> applyKnownWordMarking()   // .map() -> new array
+  -> applyFrequencyMarking()   // .map() -> new array (POS-based filtering only)
+  -> applyJlptMarking()        // .map() -> new array
+  -> markNPlusOneTargets()     // .map() -> new array
+```
+
+### Dependency Analysis
+
+All annotations either depend on MeCab POS data or benefit from running after it:
+- **Known word marking:** Needs base tokens (surface/headword). No POS dependency, but no reason to run separately.
+- **Frequency filtering:** Uses `pos1Exclusions` and `pos2Exclusions` to clear frequency ranks on excluded tokens (particles, noise). Depends on MeCab POS data.
+- **JLPT marking:** Uses `shouldIgnoreJlptForMecabPos1` to filter. Depends on MeCab POS data.
+- **N+1 marking:** Uses POS exclusion sets to filter candidates. Depends on known word status + MeCab POS.
+
+Since frequency filtering and JLPT marking both depend on POS data from MeCab enrichment, and MeCab enrichment already happens before the annotation stage, all four can run in a single pass after MeCab completes.
+
+### New Flow (1 pass + N+1)
+
+```typescript
+function annotateTokens(tokens, deps, options): MergedToken[] {
+  const pos1Exclusions = resolvePos1Exclusions(options);
+  const pos2Exclusions = resolvePos2Exclusions(options);
+
+  // Single pass: known word + frequency filtering + JLPT computed together
+  const annotated = tokens.map((token) => {
+    const isKnown = nPlusOneEnabled
+      ? token.isKnown || computeIsKnown(token, deps)
+      : false;
+
+    // Filter frequency rank using POS exclusions (rank values already set at parser level)
+    const frequencyRank = frequencyEnabled
+      ? filterFrequencyRank(token, pos1Exclusions, pos2Exclusions)
+      : undefined;
+
+    const jlptLevel = jlptEnabled
+      ? computeJlptLevel(token, deps.getJlptLevel)
+      : undefined;
+
+    return { ...token, isKnown, frequencyRank, jlptLevel };
+  });
+
+  // N+1 must run after known word status is set for all tokens
+  if (nPlusOneEnabled) {
+    return markNPlusOneTargets(annotated, minSentenceWords, pos1Exclusions, pos2Exclusions);
+  }
+
+  return annotated;
+}
+```
+
+### What Changes
+
+- The individual `applyKnownWordMarking`, `applyFrequencyMarking`, `applyJlptMarking` functions are refactored into per-token computation helpers (pure functions that compute a single field). The frequency helper is named `filterFrequencyRank` to clarify it performs POS-based exclusion, not rank computation.
+- The `annotateTokens` orchestrator runs one `.map()` call that invokes all three helpers per token.
+- `markNPlusOneTargets` remains a separate pass because it needs the full array with `isKnown` set (it examines sentence-level context).
+- The parser-level `applyFrequencyRanks()` call in `tokenizer.ts` is unchanged -- it remains a separate step outside the annotation stage.
+- Net: 4 array copies + 4 iterations become 1 array copy + 1 iteration + N+1 pass.
+
+### Expected Savings
+
+~15-45ms saved (3 fewer array allocations + 3 fewer full iterations). Annotation drops from ~25-70ms to ~10-25ms.
+
+### Files Affected
+
+- **Modified:** `src/core/services/tokenizer/annotation-stage.ts` -- refactor into batched single-pass
+
+---
+
+## Optimization 3: DOM Template Pooling
+
+### Summary
+
+Replace `document.createElement('span')` calls in the renderer with `templateSpan.cloneNode(false)` from a pre-created template element.
+
+### Current Behavior
+
+In `renderWithTokens` (`subtitle-render.ts`), each render cycle:
+1. Clears DOM with `innerHTML = ''`
+2. Creates a `DocumentFragment`
+3. Calls `document.createElement('span')` for each token (~10-15 per subtitle)
+4. Sets `className`, `textContent`, `dataset.*` individually
+5. Appends fragment to root
+
+### New Behavior
+
+1. At renderer initialization (`createSubtitleRenderer`), create a single template:
+   ```typescript
+   const templateSpan = document.createElement('span');
+   ```
+2. In `renderWithTokens`, replace every `document.createElement('span')` with:
+   ```typescript
+   const span = templateSpan.cloneNode(false) as HTMLSpanElement;
+   ```
+3. Replace all `innerHTML = ''` calls with `root.replaceChildren()` to avoid the HTML parser invocation on clear. This applies to `renderSubtitle` (primary subtitle root), `renderSecondarySub` (secondary subtitle root), and `renderCharacterLevel` if applicable.
+4. Everything else stays the same (setting className, textContent, dataset, appending to fragment).
+
+### Why cloneNode Over Full Node Recycling
+
+Full recycling (collecting old nodes, clearing attributes, reusing them) requires carefully resetting every `dataset.*` property that might have been set on a previous render. This is error-prone -- a stale `data-frequency-rank` from a previous subtitle appearing on a new token would cause incorrect styling. `cloneNode(false)` on a bare template is nearly as fast and produces a clean node every time.
+
+### Expected Savings
+
+`cloneNode(false)` is ~2-3x faster than `createElement` in most browser engines. For 10-15 tokens per subtitle: ~3-8ms saved per render cycle.
+
+### Files Affected
+
+- **Modified:** `src/renderer/subtitle-render.ts` -- template creation + cloneNode usage
+
+---
+
+## Combined Impact Summary
+
+| Scenario | Before | After | Improvement |
+|----------|--------|-------|-------------|
+| Normal playback (prefetch-warmed) | ~200-320ms | ~30-50ms | ~80-85% |
+| Cache hit (repeated subtitle) | ~72ms | ~55-65ms | ~10-20% |
+| Cache miss (immediate seek) | ~200-320ms | ~150-260ms | ~20-25% |
+
+---
+
+## Files Summary
+
+### New Files
+- `src/core/services/subtitle-prefetch.ts`
+- `src/core/services/subtitle-cue-parser.ts`
+
+### Modified Files
+- `src/core/services/subtitle-processing-controller.ts` (expose `preCacheTokenization`)
+- `src/core/services/tokenizer/annotation-stage.ts` (batched single-pass)
+- `src/renderer/subtitle-render.ts` (template cloneNode)
+- `src/main.ts` (wire up prefetch service)
+
+### Test Files
+- New tests for subtitle cue parser (SRT, VTT, ASS formats)
+- New tests for subtitle prefetch service (priority window, seek, pause/resume)
+- Updated tests for annotation stage (same behavior, new implementation)
+- Updated tests for subtitle render (template cloning)