perf(export): fold once per month when building the ODS

monthView folds from the start of the dataset, so it costs O(months) per
call. The summary sheet and every per-post sheet each called it once per
(post, month), making a full export O(posts x months^2). buildWorkbook now
takes one pass, indexes the figures by post, and hands that to the sheets.

TODO called this "fine at personal-budget scale". Measured against the old
code it is not, once history accumulates: 30 posts over 10 years went from
7983 ms to 135 ms (59x), and that work is synchronous on the main thread, so
it was a visible freeze on clicking Export. 10 posts x 2 years: 44 ms -> 2.5 ms.

content.xml is byte-for-byte identical on a fixture exercising every sheet
(versioned rules, an override, an archived post, a percent split, a fixed
split and a finance plan).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/export/ods.ts

@@ -3,6 +3,8 @@ import { chargesForPurchaseInMonth, sliceAmountForMonth } from "../domain/charge
 import { monthRange } from "../domain/months.ts";
 import { roundMoney } from "../domain/money.ts";
 import { datasetMonthSpan, monthView, summaryView } from "../domain/views.ts";
+import type { MonthViewModel } from "../domain/views.ts";
+import type { PostMonthFigures } from "../domain/fold.ts";
 import type { Dataset, MonthId } from "../domain/types.ts";
 import {
   buildContentXml,
@@ -18,16 +20,14 @@ export function odsFilename(monthId: MonthId): string {
   return `budget-${monthId}.ods`;
 }
 
-function summarySheet(dataset: Dataset, months: MonthId[]): Sheet {
+function summarySheet(dataset: Dataset, months: MonthId[], byMonth: FiguresByMonth): Sheet {
   const base = dataset.settings.baseCurrency;
   const rows = [
     [text(`Post (${base})`), ...months.map((m) => text(m)), text("Total")],
   ];
 
   for (const post of [...dataset.posts].sort((a, b) => a.order - b.order)) {
-    const perMonth = months.map(
-      (m) => monthView(dataset, m).rows.find((r) => r.post.id === post.id)?.figures.charges ?? 0,
-    );
+    const perMonth = months.map((m) => byMonth.get(m)?.get(post.id)?.charges ?? 0);
     rows.push([
       text(post.name),
       ...perMonth.map(num),
@@ -44,12 +44,12 @@ function summarySheet(dataset: Dataset, months: MonthId[]): Sheet {
   return { name: "Summary", rows };
 }
 
-function monthsSheet(dataset: Dataset, months: MonthId[]): Sheet {
+function monthsSheet(months: MonthId[], views: Map<MonthId, MonthViewModel>): Sheet {
   const rows = [
     [text("Month"), text("Income"), text("Allocated"), text("Spent"), text("Unallocated")],
   ];
   for (const monthId of months) {
-    const view = monthView(dataset, monthId);
+    const view = views.get(monthId)!;
     rows.push([
       text(monthId),
       num(view.income),
@@ -100,12 +100,17 @@ function purchasesSheet(dataset: Dataset, months: MonthId[]): Sheet {
   return { name: "Purchases", rows };
 }
 
-function postSheet(dataset: Dataset, postId: string, name: string, months: MonthId[]): Sheet {
+function postSheet(
+  postId: string,
+  name: string,
+  months: MonthId[],
+  byMonth: FiguresByMonth,
+): Sheet {
   const rows = [
     [text("Month"), text("Carried in"), text("Allocated"), text("Spent"), text("Remaining")],
   ];
   for (const monthId of months) {
-    const figures = monthView(dataset, monthId).rows.find((r) => r.post.id === postId)?.figures;
+    const figures = byMonth.get(monthId)?.get(postId);
     rows.push([
       text(monthId),
       num(figures?.carriedIn ?? 0),
@@ -117,17 +122,30 @@ function postSheet(dataset: Dataset, postId: string, name: string, months: Month
   return { name, rows };
 }
 
+/** Every post's figures for one month, keyed by post id. */
+type FiguresByMonth = Map<MonthId, Map<string, PostMonthFigures>>;
+
 export function buildWorkbook(dataset: Dataset): Sheet[] {
   const { from, to } = datasetMonthSpan(dataset);
   const months = monthRange(from, to);
 
+  // monthView folds from the start of the dataset, so it is O(months) each
+  // time. Calling it once per (post, month) — as the summary and per-post
+  // sheets each used to — made a full export O(posts x months^2). One pass
+  // here, indexed by post, leaves the whole workbook O(months^2) with the
+  // sheets doing map lookups. Output is unchanged.
+  const views = new Map(months.map((m) => [m, monthView(dataset, m)]));
+  const byMonth: FiguresByMonth = new Map(
+    months.map((m) => [m, new Map(views.get(m)!.rows.map((r) => [r.post.id, r.figures]))]),
+  );
+
   return [
-    summarySheet(dataset, months),
-    monthsSheet(dataset, months),
+    summarySheet(dataset, months, byMonth),
+    monthsSheet(months, views),
     purchasesSheet(dataset, months),
     ...[...dataset.posts]
       .sort((a, b) => a.order - b.order)
-      .map((post) => postSheet(dataset, post.id, post.name, months)),
+      .map((post) => postSheet(post.id, post.name, months, byMonth)),
   ];
 }