Diff each scene revision once when loading a review

src/lib/review-anchor.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect } from 'vitest';
-import { reanchorPoint, reanchorRange } from './review-anchor';
+import { createAnchorMapper, reanchorPoint, reanchorRange } from './review-anchor';
 
 const BASE = 'The quick brown fox jumps over the lazy dog.';
 // Anchor "brown fox" = [10, 19).
@@ -84,3 +84,61 @@ describe('reanchorPoint', () => {
 		expect(reanchorPoint(BASE, BASE, BASE.length + 1)).toBeNull();
 	});
 });
+
+describe('createAnchorMapper', () => {
+	// The mapper reuses one diff per pair of texts, so what it returns has to
+	// stay identical to re-anchoring each anchor on its own.
+	const CASES: Array<[string, string]> = [
+		[BASE, BASE],
+		[BASE, 'A very ' + BASE],
+		[BASE, BASE.replace('lazy dog', 'sleeping cat')],
+		[BASE, BASE.replace('brown fox', 'brown furry fox')],
+		[BASE, BASE.replace('brown fox', 'red vixen')],
+		[BASE, BASE.replace('brown fox ', '')]
+	];
+
+	it('matches reanchorRange on every offset', () => {
+		const mapper = createAnchorMapper();
+		for (const [base, current] of CASES) {
+			for (let start = 0; start < base.length; start++) {
+				for (let end = start; end <= base.length + 1; end++) {
+					expect(mapper.range(base, current, start, end)).toEqual(
+						reanchorRange(base, current, start, end)
+					);
+				}
+			}
+		}
+	});
+
+	it('matches reanchorPoint on every position', () => {
+		const mapper = createAnchorMapper();
+		for (const [base, current] of CASES) {
+			for (let position = -1; position <= base.length + 1; position++) {
+				expect(mapper.point(base, current, position)).toEqual(
+					reanchorPoint(base, current, position)
+				);
+			}
+		}
+	});
+
+	it('keeps separate results for different current texts against one base', () => {
+		const mapper = createAnchorMapper();
+		const shifted = 'A very ' + BASE;
+		const rewritten = BASE.replace('brown fox', 'red vixen');
+		expect(mapper.range(BASE, shifted, START, END)).toEqual({ start: START + 7, end: END + 7 });
+		expect(mapper.range(BASE, rewritten, START, END)).toBeNull();
+		// Repeating the first pair still maps through its own diff, not the second's.
+		expect(mapper.range(BASE, shifted, START, END)).toEqual({ start: START + 7, end: END + 7 });
+	});
+
+	it('diffs each pair of texts once however many anchors map through it', () => {
+		const mapper = createAnchorMapper();
+		const current = BASE.replace('lazy dog', 'sleeping cat');
+		const first = mapper.range(BASE, current, START, END);
+		const t0 = performance.now();
+		for (let i = 0; i < 1000; i++) mapper.range(BASE, current, START, END);
+		// A thousand cached maps cost far less than a thousand diffs would.
+		expect(performance.now() - t0).toBeLessThan(50);
+		expect(mapper.range(BASE, current, START, END)).toEqual(first);
+	});
+});

src/lib/review-anchor.ts

@@ -1,4 +1,4 @@
-import { diffChars } from 'diff';
+import { diffChars, type Change } from 'diff';
 
 // Re-anchors a review thread's character range after the text changed.
 // Comments pin a [start, end) range against the revision they were made on;
@@ -15,12 +15,31 @@ export function reanchorRange(
 ): { start: number; end: number } | null {
 	if (start < 0 || end > baseText.length || start >= end) return null;
 	if (baseText === currentText) return { start, end };
+	return mapRange(diffChars(baseText, currentText), start, end);
+}
+
+// Re-anchors a single position (a pure insertion point). The position
+// survives if it lies in or at the boundary of unchanged text.
+export function reanchorPoint(
+	baseText: string,
+	currentText: string,
+	position: number
+): number | null {
+	if (position < 0 || position > baseText.length) return null;
+	if (baseText === currentText) return position;
+	return mapPoint(diffChars(baseText, currentText), position);
+}
 
+function mapRange(
+	parts: Change[],
+	start: number,
+	end: number
+): { start: number; end: number } | null {
 	let basePos = 0;
 	let currentPos = 0;
 	let newStart = -1;
 	let newEnd = -1;
-	for (const part of diffChars(baseText, currentText)) {
+	for (const part of parts) {
 		const length = part.value.length;
 		if (part.added) {
 			currentPos += length;
@@ -45,19 +64,10 @@ export function reanchorRange(
 	return { start: newStart, end: newEnd };
 }
 
-// Re-anchors a single position (a pure insertion point). The position
-// survives if it lies in or at the boundary of unchanged text.
-export function reanchorPoint(
-	baseText: string,
-	currentText: string,
-	position: number
-): number | null {
-	if (position < 0 || position > baseText.length) return null;
-	if (baseText === currentText) return position;
-
+function mapPoint(parts: Change[], position: number): number | null {
 	let basePos = 0;
 	let currentPos = 0;
-	for (const part of diffChars(baseText, currentText)) {
+	for (const part of parts) {
 		const length = part.value.length;
 		if (part.added) {
 			currentPos += length;
@@ -75,3 +85,50 @@ export function reanchorPoint(
 	}
 	return null;
 }
+
+// Every thread and suggestion anchored to the same scene revision re-anchors
+// against the same two texts, and a character diff of a long scene costs
+// hundreds of milliseconds. A mapper diffs each pair of texts once and maps
+// every anchor through that one result. Make one per request and hand it to
+// everything that re-anchors, so the cost is per revision rather than per
+// comment.
+export type AnchorMapper = {
+	range(
+		baseText: string,
+		currentText: string,
+		start: number,
+		end: number
+	): { start: number; end: number } | null;
+	point(baseText: string, currentText: string, position: number): number | null;
+};
+
+export function createAnchorMapper(): AnchorMapper {
+	// Nested by base text then current text, so neither key has to be
+	// concatenated into a third copy of the scene.
+	const cache = new Map<string, Map<string, Change[]>>();
+	const diff = (baseText: string, currentText: string): Change[] => {
+		let byCurrent = cache.get(baseText);
+		if (!byCurrent) {
+			byCurrent = new Map();
+			cache.set(baseText, byCurrent);
+		}
+		let parts = byCurrent.get(currentText);
+		if (!parts) {
+			parts = diffChars(baseText, currentText);
+			byCurrent.set(currentText, parts);
+		}
+		return parts;
+	};
+	return {
+		range(baseText, currentText, start, end) {
+			if (start < 0 || end > baseText.length || start >= end) return null;
+			if (baseText === currentText) return { start, end };
+			return mapRange(diff(baseText, currentText), start, end);
+		},
+		point(baseText, currentText, position) {
+			if (position < 0 || position > baseText.length) return null;
+			if (baseText === currentText) return position;
+			return mapPoint(diff(baseText, currentText), position);
+		}
+	};
+}

src/lib/server/export-reviews.ts

@@ -2,7 +2,7 @@ import { and, eq, isNull } from 'drizzle-orm';
 import type { Database } from './auth.ts';
 import { scenes } from './db/schema.ts';
 import { listThreads } from './review.ts';
-import { reanchorRange } from '../review-anchor.ts';
+import { createAnchorMapper } from '../review-anchor.ts';
 import type { ExportReviewThread, ReviewLoader } from './export.ts';
 
 // Long selections are trimmed: the excerpt locates the thread, the scene
@@ -15,7 +15,7 @@ const EXCERPT_MAX = 240;
 // export builders does not drag in the review module.
 export function reviewLoader(db: Database): ReviewLoader {
 	return async (storyId) => {
-		const threads = await listThreads(db, storyId, reanchorRange);
+		const threads = await listThreads(db, storyId, createAnchorMapper().range);
 		if (threads.length === 0) return [];
 		const sceneRows = await db
 			.select({ id: scenes.id, title: scenes.title, bodyMd: scenes.bodyMd })

src/lib/server/review.ts

@@ -15,7 +15,7 @@ import {
 import { hashToken } from './tokens.ts';
 import { signToken, verifyToken } from './crypto.ts';
 import { recordRevision } from './revisions.ts';
-import { reanchorPoint, reanchorRange } from '../review-anchor.ts';
+import { createAnchorMapper, type AnchorMapper } from '../review-anchor.ts';
 import { wordCount } from '../word-count.ts';
 import { normaliseAssistantName } from './llm/prompts/persona.ts';
 import { EMAIL_RE } from './signup.ts';
@@ -634,24 +634,26 @@ export type SuggestionView = {
 };
 
 // Re-anchors the proposed range onto the current text: a real range maps
-// through reanchorRange, a pure insertion point through reanchorPoint.
+// through the mapper's range, a pure insertion point through its point.
 function mapSuggestionRange(
+	mapper: AnchorMapper,
 	baseText: string,
 	currentText: string,
 	start: number,
 	end: number
 ): { start: number; end: number } | null {
 	if (start === end) {
-		const point = reanchorPoint(baseText, currentText, start);
+		const point = mapper.point(baseText, currentText, start);
 		return point === null ? null : { start: point, end: point };
 	}
-	return reanchorRange(baseText, currentText, start, end);
+	return mapper.range(baseText, currentText, start, end);
 }
 
 // Where a decided suggestion now sits, so the Done view can point at it: the
 // accepted replacement (the new text it left behind) or the rejected original
 // (still in place). Null when there is nothing to mark or it cannot be found.
 function decidedAnchor(
+	mapper: AnchorMapper,
 	status: 'accepted' | 'rejected',
 	baseText: string,
 	currentText: string,
@@ -662,20 +664,24 @@ function decidedAnchor(
 	if (status === 'rejected') {
 		// Rejection left the text untouched, so the original passage maps as-is.
 		if (start === end) return null; // a rejected insertion left no text
-		return reanchorRange(baseText, currentText, start, end);
+		return mapper.range(baseText, currentText, start, end);
 	}
 	// Accepted: the replacement now occupies the spot the original held.
 	if (replacement.length === 0) return null; // a deletion leaves nothing to mark
-	const point = reanchorPoint(baseText, currentText, start);
+	const point = mapper.point(baseText, currentText, start);
 	if (point === null) return null;
 	const at = { start: point, end: point + replacement.length };
 	return at.end <= currentText.length ? at : null;
 }
 
+// The mapper is shared with listThreads when both run for the same story, so
+// each scene revision is diffed once for the whole page rather than once per
+// thread and once per suggestion.
 export async function listSuggestions(
 	db: Database,
 	storyId: string,
-	viewer?: ReviewViewer
+	viewer?: ReviewViewer,
+	mapper: AnchorMapper = createAnchorMapper()
 ): Promise<SuggestionView[]> {
 	const rows = await db
 		.select({
@@ -701,8 +707,9 @@ export async function listSuggestions(
 		// anchors to where it now sits, so the Done view can point at it.
 		const anchor =
 			row.suggestion.status === 'pending'
-				? mapSuggestionRange(row.baseBody, row.currentBody, rangeStart, rangeEnd)
+				? mapSuggestionRange(mapper, row.baseBody, row.currentBody, rangeStart, rangeEnd)
 				: decidedAnchor(
+						mapper,
 						row.suggestion.status,
 						row.baseBody,
 						row.currentBody,
@@ -819,7 +826,10 @@ export async function decideSuggestion(
 				.where(eq(scenes.id, row.scene.id))
 				.for('update');
 			if (!scene) return { ok: false, reason: 'That scene does not exist.' };
+			// One anchor against the text just read under the lock, so there is
+			// nothing for a shared mapper to reuse.
 			const anchor = mapSuggestionRange(
+				createAnchorMapper(),
 				row.baseBody,
 				scene.bodyMd,
 				row.suggestion.rangeStart,

src/routes/api/assistant/review-reply/+server.ts

@@ -13,7 +13,7 @@ import { assembleContext, buildSystemMessage } from '$lib/server/llm/context/ass
 import { buildReviewReplyMessage, excerptAround } from '$lib/server/llm/prompts/review-reply';
 import { complete } from '$lib/server/llm/gateway';
 import { addComment, listSuggestions, listThreads } from '$lib/server/review';
-import { reanchorRange } from '$lib/review-anchor';
+import { createAnchorMapper } from '$lib/review-anchor';
 import { isUuid } from '$lib/slug';
 import type { ChatMessage } from '$lib/server/llm/providers/types';
 
@@ -48,16 +48,21 @@ export const POST: RequestHandler = async ({ request, locals }) => {
 
 	const layout = await requireAssistantGate(userId, story.id);
 
-	// The full thread view carries the re-anchored range and display names.
-	const threads = await listThreads(db, story.id, reanchorRange, { userId });
+	// The full thread view carries the re-anchored range and display names. One
+	// mapper covers the threads and the suggestion lookup below, so a scene
+	// revision is diffed once rather than once per anchor.
+	const anchors = createAnchorMapper();
+	const threads = await listThreads(db, story.id, anchors.range, { userId });
 	const thread = threads.find((t) => t.id === threadId);
 	if (!thread) error(404, 'That thread does not exist.');
 	if (!thread.comments[0]?.isAssistant) {
 		error(400, 'The Assistant only replies in threads it opened.');
 	}
 
 	const suggestion = thread.suggestionId
-		? ((await listSuggestions(db, story.id)).find((s) => s.id === thread.suggestionId) ?? null)
+		? ((await listSuggestions(db, story.id, undefined, anchors)).find(
+				(s) => s.id === thread.suggestionId
+			) ?? null)
 		: null;
 
 	const [scene] = await db

src/routes/review/[token]/+page.server.ts

@@ -22,7 +22,7 @@ import {
 } from '$lib/server/review';
 import { gatherStory } from '$lib/server/export';
 import { reviewMentionData } from '$lib/server/mention-entities';
-import { reanchorRange } from '$lib/review-anchor';
+import { createAnchorMapper } from '$lib/review-anchor';
 import { rateLimit } from '$lib/server/rate-limit';
 import { notifyReviewActivity } from '$lib/server/notify';
 
@@ -88,15 +88,18 @@ export const load: PageServerLoad = async ({ params, cookies, locals }) => {
 		sceneIds: scenes.map((scene) => scene.id),
 		restrictToMentioned: true
 	});
+	// Threads and suggestions re-anchor against the same scene revisions, so one
+	// mapper covers both and each revision is diffed once for the page.
+	const anchors = createAnchorMapper();
 	return {
 		state: 'review' as const,
 		storyTitle: resolved.storyTitle,
 		reviewerName: reviewer.displayName,
 		canSuggest: resolved.invitation.canSuggest,
 		chapters: content.chapters,
 		scenes,
-		threads: await listThreads(db, storyId, reanchorRange, { reviewerId: reviewer.id }),
-		suggestions: await listSuggestions(db, storyId, { reviewerId: reviewer.id }),
+		threads: await listThreads(db, storyId, anchors.range, { reviewerId: reviewer.id }),
+		suggestions: await listSuggestions(db, storyId, { reviewerId: reviewer.id }, anchors),
 		mentionEntities: mentions.entities,
 		mentionMembers: mentions.storyMembers,
 		mentionPins: mentions.pins

src/routes/stories/[id]/review/+page.server.ts

@@ -20,7 +20,7 @@ import { gatherStory } from '$lib/server/export';
 import { storyPreferences } from '$lib/server/preferences';
 import { storyPageSetup } from '$lib/server/page-setup';
 import { reviewMentionData } from '$lib/server/mention-entities';
-import { reanchorRange } from '$lib/review-anchor';
+import { createAnchorMapper } from '$lib/review-anchor';
 import { queueSceneMentions } from '$lib/server/jobs';
 import { assistantLayout, saveStoryLlmOverride } from '$lib/server/llm/config';
 import { listChat } from '$lib/server/llm/chat-history';
@@ -38,6 +38,9 @@ export const load: PageServerLoad = async ({ params, locals }) => {
 	// navigation as the editor. The author's review takes the full cast
 	// (restrictToMentioned is false), so reviewMentionData needs no scene ids and
 	// nothing here waits on gatherStory.
+	// Threads and suggestions re-anchor against the same scene revisions, so one
+	// mapper covers both and each revision is diffed once for the page.
+	const anchors = createAnchorMapper();
 	const [
 		content,
 		mentions,
@@ -56,8 +59,8 @@ export const load: PageServerLoad = async ({ params, locals }) => {
 			restrictToMentioned: false
 		}),
 		listTrashedScenes(db, story.id),
-		listThreads(db, story.id, reanchorRange, { userId: locals.user!.id }),
-		listSuggestions(db, story.id, { userId: locals.user!.id }),
+		listThreads(db, story.id, anchors.range, { userId: locals.user!.id }),
+		listSuggestions(db, story.id, { userId: locals.user!.id }, anchors),
 		storyPreferences(db, locals.user!.id, story.id),
 		storyPageSetup(db, story.id),
 		assistantLayout(db, locals.user!.id, story.id)