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perf(tui): cache stringWidth/wrapText/sliceAnsi + skip-slice when line fits clip

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CPU profile (Apr 2026, real-user scroll on 11k-line session) showed three
hot loops in the per-frame render path:

  Output.get() per-frame walk:                 24% total
  └─ sliceAnsi(line, from, to) per write:     18% total
  stringWidth(line) chain (cached + JS):      14% total

All three were re-doing identical work every frame: same string → same
clipped slice → same width.

Fixes:

1. Memoize stringWidth (8k-entry LRU) for non-ASCII strings; ASCII fast-path
   skips the cache (inline scan beats Map.get for short ASCII, the >90%
   case). String.charCodeAt scan up to 64 chars is cheaper than the regex
   fallback.

2. Memoize wrapText (4k-entry LRU keyed by maxWidth|wrapType|text) — wrapAnsi
   is pure and the same content reflows identically every frame.

3. Memoize sliceAnsi (4k-entry LRU keyed by start|end|str) for the
   end-defined hot path used by Output.get().

4. Skip the slice entirely in Output.get() when the line already fits the
   clip box (startsBefore=false && endsAfter=false). Most transcript lines
   never exceed their container width, and tokenizing them just to slice
   (line, 0, width) was pure overhead. This single fast-path drops
   sliceAnsi from 18% → ~0% in the profile.

Also tighten virtualization constants (MAX_MOUNTED 260→120, OVERSCAN 40→20,
SLIDE_STEP 25→12) and cap historical-message render at 800 chars / 16
lines via HISTORY_RENDER_MAX_*; messages inside the FULL_RENDER_TAIL_ITEMS
window still render in full so reading-zone behavior is unchanged.

Validation, real-user CPU profile, page-up scroll on 11k-line session:

  Output.get() self-time:     24%   →   0.3%
  sliceAnsi total:            18%   →   not in top 25
  stringWidth family:         14%   →   ~3%
  idle:                     60.7%   →  77.3%

Frame timings (synthetic page-up profile harness):
  dur p95:   ~10ms   →  4.87ms
  dur p99:   25ms+   → 12.80ms
  yoga p99:  ~20ms   →  1.87ms

The remaining CPU in the profile is Yoga layoutNode + React commit,
which is the irreducible work for this UI tree size.
This commit is contained in:
Brooklyn Nicholson 2026-04-26 19:28:09 -05:00
parent 85e9a23efb
commit c370e2e1e5
14 changed files with 450 additions and 42 deletions
Download patch file Download diff file Expand all files Collapse all files

23
ui-tui/src/lib/text.ts
View file

@ -1,4 +1,10 @@
import { LIVE_RENDER_MAX_CHARS, LIVE_RENDER_MAX_LINES, THINKING_COT_MAX } from '../config/limits.js'
import {
HISTORY_RENDER_MAX_CHARS,
HISTORY_RENDER_MAX_LINES,
LIVE_RENDER_MAX_CHARS,
LIVE_RENDER_MAX_LINES,
THINKING_COT_MAX
} from '../config/limits.js'
import { VERBS } from '../content/verbs.js'
import type { ThinkingMode } from '../types.js'
@ -98,6 +104,17 @@ export const thinkingPreview = (reasoning: string, mode: ThinkingMode, max: numb
export const boundedLiveRenderText = (
text: string,
{ maxChars = LIVE_RENDER_MAX_CHARS, maxLines = LIVE_RENDER_MAX_LINES } = {}
) => boundedRenderText(text, 'showing live tail', { maxChars, maxLines })
export const boundedHistoryRenderText = (
text: string,
{ maxChars = HISTORY_RENDER_MAX_CHARS, maxLines = HISTORY_RENDER_MAX_LINES } = {}
) => boundedRenderText(text, 'showing tail', { maxChars, maxLines })
const boundedRenderText = (
text: string,
labelPrefix: string,
{ maxChars, maxLines }: { maxChars: number; maxLines: number }
) => {
if (text.length <= maxChars && text.split('\n', maxLines + 1).length <= maxLines) {
return text
@ -132,8 +149,8 @@ export const boundedLiveRenderText = (
const label =
omittedLines > 0
? `[showing live tail; omitted ${fmtK(omittedLines)} lines / ${fmtK(omittedChars)} chars]\n`
: `[showing live tail; omitted ${fmtK(omittedChars)} chars]\n`
? `[${labelPrefix}; omitted ${fmtK(omittedLines)} lines / ${fmtK(omittedChars)} chars]\n`
: `[${labelPrefix}; omitted ${fmtK(omittedChars)} chars]\n`
return `${label}${tail}`
}