Flyweight grid: 20,000,000 cells
Ground truth lives in columnar typed arrays; rendered cells are disposable flyweight facades created per render pass; a sparse reactive overlay materializes per observation and evicts with the viewport. ~55% of cells hold real Excel-syntax formulas. Everything costs proportional to what is observed — never to what exists.
Nothing downloads until you click — the model code and the formula parser load on demand, then one more click creates all 20,000,000 cells in your browser.
What to notice
- Creation fills ~95 MB of typed arrays — not 20 million objects. The reactive layer allocates only for cells something actually observes.
- Edit a cell at row 1,000,000 and the column totals react — the dependency graph is discovered per observation, not precomputed.
- The measured protocol, numbers and design live in
RESULTS.mdandDESIGN.md; the deeper story is in The Flyweight Pattern guide.
The source
The heart of the pattern — the sheet (columnar ground truth + sparse overlay) and the cell facade:
ts
/**
* FlyweightSheet — 20M cells with NO cell objects at rest.
*
* Ground truth is columnar (`kind` Uint8Array + lazily-allocated Float64Array
* per column + a sparse Map for text/edited-formula sources). Reactivity is a
* SPARSE OVERLAY that materializes per observation:
*
* - fine tier — a version ref per OBSERVED cell (rendered / onCell /
* small range). Precise: conditional dependencies shift.
* - coarse tier — a version ref per 4,096-row BLOCK, subscribed only by
* LARGE ranges. =SUM(A1:A1000000) costs 245 edges, not 1M.
* - formula computeds — cached on demand; each carries ONE sync watcher
* (the derived-write bridge) that bumps its own block when
* its value changes, so coarse subscribers see through
* formulas — including their out-of-range inputs.
*
* Writes are O(observers-of-that-cell): update the arrays, bump the fine ref
* IF IT EXISTS and the block ref IF IT EXISTS (peek-only). A write to a
* never-observed cell allocates nothing and notifies no one.
*
* The dependency graph is still DISCOVERED, never hand-built — the same
* onCell/onRange seam as the formula grid (reference: demo/formula/Sheet.ts),
* with the parser reading through this sheet's tracked accessors.
*/
import FormulaParser from 'fast-formula-parser';
import { pauseTracking, resetTracking } from '@vue/reactivity';
import {
computed,
ref,
watch,
type ComputedRef,
type Ref,
type WatchStopHandle,
} from 'vue';
import { Reactive } from '../../../ivue';
import {
BLOCK_ROWS,
BLOCK_SHIFT,
COLS,
FINE_RANGE_LIMIT,
Kind,
isDataCol,
isFormulaError,
isFormulaText,
matchSimpleAggregate,
numDataValue,
patternSource,
stripFormula,
type CellValue,
type SimpleAggregate,
} from '../flyweight-logic';
const { FormulaError } = FormulaParser as unknown as {
FormulaError: new (error: string, details?: unknown) => CellValue;
};
interface RangeRef {
from: { row: number; col: number };
to: { row: number; col: number };
}
interface Column {
kind: Uint8Array;
/** Allocated on the first numeric write — formula columns never pay. */
numbers: Float64Array | null;
/** Sparse: typed text AND user-edited formula sources (pattern overrides). */
text: Map<number, string>;
}
interface FormulaEntry {
value: ComputedRef<CellValue>;
/** Stops the derived-write bridge watcher (see formulaValue). */
stopBridge: WatchStopHandle;
}
class $FlyweightSheet {
readonly rows: number;
readonly cols: number;
// --- ground truth (plain, non-reactive) ---
private readonly columns: Column[];
// --- the sparse reactive overlay (empty until observed) ---
private readonly cellVersions = new Map<number, Ref<number>>();
private readonly blockVersions = new Map<number, Ref<number>>();
private readonly formulaCache = new Map<number, FormulaEntry>();
private readonly adHocCache = new Map<string, ComputedRef<CellValue>>();
/** Blocks per column (fine↔coarse key math). */
private readonly blockCount: number;
/** ONE parser for the whole sheet (reference: formula grid). */
private readonly parser: FormulaParser;
/** Cycle guard — a cell re-entered mid-evaluation is a cycle → #REF!. */
private readonly evaluating = new Set<number>();
/** When non-null, tracked reads record (row,col) — dep tracing. */
private tracer: Array<[number, number]> | null = null;
constructor(rows: number, cols: number = COLS) {
this.rows = rows;
this.cols = cols;
this.blockCount = Math.ceil(rows / BLOCK_ROWS);
// Seed the columnar ground truth. Data columns fill Float64Arrays
// numerically (no string round-trips — this is the whole creation cost);
// formula columns are a single Uint8Array.fill.
const columns: Column[] = new Array(cols);
for (let col = 0; col < cols; col++) {
const kind = new Uint8Array(rows);
let numbers: Float64Array | null = null;
if (isDataCol(col)) {
numbers = new Float64Array(rows);
for (let row = 0; row < rows; row++) {
const value = numDataValue(row, col);
if (value !== null) {
kind[row] = Kind.Number;
numbers[row] = value;
} // blanks stay Kind.Blank
}
} else {
kind.fill(Kind.Formula);
}
columns[col] = { kind, numbers, text: new Map() };
}
this.columns = columns;
this.parser = new FormulaParser({
onCell: (cellRef) => this.pointValue(cellRef.row, cellRef.col),
onRange: (rangeRef) => this.rangeValues(rangeRef as RangeRef),
});
}
// --- keys ---
private cellKey(row: number, col: number): number {
return col * this.rows + row;
}
private blockKey(row: number, col: number): number {
return col * this.blockCount + (row >> BLOCK_SHIFT);
}
// --- version-ref plumbing ---
/** Subscribe the current effect to a cell (get-OR-CREATE — observation). */
private trackCell(row: number, col: number): void {
const cellKey = this.cellKey(row, col);
let versionRef = this.cellVersions.get(cellKey);
if (!versionRef) {
versionRef = ref(0);
this.cellVersions.set(cellKey, versionRef);
}
void versionRef.value;
}
/** Subscribe the current effect to a block (get-or-create — observation). */
private trackBlock(blockKey: number): void {
let versionRef = this.blockVersions.get(blockKey);
if (!versionRef) {
versionRef = ref(0);
this.blockVersions.set(blockKey, versionRef);
}
void versionRef.value;
}
/** Notify a cell's observers — PEEK-ONLY (unobserved cells cost nothing). */
private bumpCell(row: number, col: number): void {
const versionRef = this.cellVersions.get(this.cellKey(row, col));
if (versionRef) versionRef.value++;
}
/** Notify a block's observers — peek-only. */
private bumpBlock(row: number, col: number): void {
const versionRef = this.blockVersions.get(this.blockKey(row, col));
if (versionRef) versionRef.value++;
}
// --- raw reads ---
/** UNTRACKED ground-truth value (blank→null). No refs, no observation. */
rawAt(row: number, col: number): CellValue {
const column = this.columns[col];
switch (column.kind[row]) {
case Kind.Number:
return column.numbers![row];
case Kind.Text:
return column.text.get(row) ?? '';
case Kind.Formula:
return this.sourceAt(row, col); // raw view of a formula = its source
default:
return null;
}
}
/** The literal text of a cell (formula source / number text / text). */
sourceAt(row: number, col: number): string {
const column = this.columns[col];
const override = column.text.get(row);
if (override !== undefined) return override;
switch (column.kind[row]) {
case Kind.Number:
return String(column.numbers![row]);
case Kind.Formula:
return patternSource(row, col) ?? '';
default:
return '';
}
}
kindAt(row: number, col: number): Kind {
return this.columns[col].kind[row] as Kind;
}
// --- tracked reads ---
/**
* The TRACKED point read — what rendered cells, facades and onCell use.
* Formula cells resolve through their cached computed (which carries its
* own fine ref for source edits); everything else takes a fine ref here.
*/
valueAt(row: number, col: number): CellValue {
if (row < 0 || row >= this.rows || col < 0 || col >= this.cols) return null;
if (this.columns[col].kind[row] === Kind.Formula) {
return this.formulaValue(row, col).value;
}
this.trackCell(row, col);
return this.rawAt(row, col);
}
/** onCell seam (1-based, like the parser). */
private pointValue(oneBasedRow: number, oneBasedCol: number): CellValue {
if (this.tracer) this.tracer.push([oneBasedRow, oneBasedCol]);
return this.valueAt(oneBasedRow - 1, oneBasedCol - 1);
}
/**
* onRange seam. Small ranges read per-cell (fine tier — precise).
* Large ranges subscribe BLOCKS, then read ground truth with tracking
* PAUSED; formula cells inside resolve through their cached computeds
* (transitive observation, priced) whose derived-write watchers keep the
* block tier truthful.
*/
private rangeValues(range: RangeRef): CellValue[][] {
const startRow = range.from.row - 1;
const startCol = range.from.col - 1;
const endRow = Math.min(range.to.row - 1, this.rows - 1);
const endCol = Math.min(range.to.col - 1, this.cols - 1);
const cellCount = (endRow - startRow + 1) * (endCol - startCol + 1);
if (this.tracer) {
for (let row = startRow; row <= endRow; row++)
for (let col = startCol; col <= endCol; col++)
this.tracer.push([row + 1, col + 1]);
}
const values: CellValue[][] = [];
if (cellCount <= FINE_RANGE_LIMIT) {
for (let row = startRow; row <= endRow; row++) {
const rowValues: CellValue[] = [];
for (let col = startCol; col <= endCol; col++)
rowValues.push(this.valueAt(row, col));
values.push(rowValues);
}
return values;
}
// Coarse tier: subscribe every covered block (tracked), …
const firstBlock = startRow >> BLOCK_SHIFT;
const lastBlock = endRow >> BLOCK_SHIFT;
for (let col = startCol; col <= endCol; col++) {
for (let block = firstBlock; block <= lastBlock; block++) {
this.trackBlock(col * this.blockCount + block);
}
}
// …then read with tracking paused (no fine edges from this range).
pauseTracking();
try {
for (let row = startRow; row <= endRow; row++) {
const rowValues: CellValue[] = [];
for (let col = startCol; col <= endCol; col++) {
rowValues.push(
this.columns[col].kind[row] === Kind.Formula
? this.formulaValue(row, col).value
: this.rawAt(row, col),
);
}
values.push(rowValues);
}
} finally {
resetTracking();
}
return values;
}
// --- formulas ---
/**
* The cached computed for a formula cell — created on first observation.
* THIN on purpose: the computed and watcher bodies are small pointers to
* named, directly testable methods on the prototype.
*/
private formulaValue(row: number, col: number): ComputedRef<CellValue> {
const cellKey = this.cellKey(row, col);
let entry = this.formulaCache.get(cellKey);
if (!entry) {
const value = computed<CellValue>(() => this.evaluateCell(row, col));
const stopBridge = watch(
value,
(newValue, oldValue) =>
this.onFormulaValueChanged(row, col, newValue, oldValue),
{ flush: 'sync' },
);
entry = { value, stopBridge };
this.formulaCache.set(cellKey, entry);
}
return entry.value;
}
/**
* The DERIVED-WRITE BRIDGE: when a formula's value changes, bump the
* cell's block so coarse subscribers invalidate even though the
* underlying write happened somewhere else entirely.
*/
private onFormulaValueChanged(
row: number,
col: number,
newValue: CellValue,
oldValue: CellValue,
): void {
if (newValue !== oldValue) this.bumpBlock(row, col);
}
/** Evaluate a cell by its CURRENT kind (formulas through the parser). */
private evaluateCell(row: number, col: number): CellValue {
// Source edits / kind flips invalidate this computed via the fine ref.
this.trackCell(row, col);
const kind = this.columns[col].kind[row];
if (kind !== Kind.Formula) return this.rawAt(row, col);
const source = this.sourceAt(row, col);
const body = stripFormula(source);
if (body.trim().length === 0) return null;
const cellKey = this.cellKey(row, col);
if (this.evaluating.has(cellKey)) return new FormulaError('#REF!');
this.evaluating.add(cellKey);
try {
// COLUMNAR FAST PATH: a bare aggregate over one range is computed
// linearly over ground truth with the SAME reactive semantics (fine
// tier small / block tier large, formulas via cached computeds). The
// general parser's range aggregation is O(n²) in range size —
// measured 27ms @ 10k cells → 40s @ 200k — so bulk aggregation
// belongs to the columnar layer, exactly as desktop engines
// special-case their range ops.
const aggregate = matchSimpleAggregate(body);
if (aggregate) return this.fastAggregate(aggregate);
return this.parser.parse(body, {
row: row + 1,
col: col + 1,
sheet: 'Sheet1',
}) as CellValue;
} catch (error) {
return error instanceof (FormulaError as unknown as Function)
? (error as CellValue)
: new FormulaError('#ERROR!');
} finally {
this.evaluating.delete(cellKey);
}
}
/**
* A live ad-hoc formula over the sheet (the demo's totals bar) — a cached
* computed evaluating `body` through the same parser/seams, so a large
* range inside it costs blocks, not cells. Thin: the computed is a pointer
* to the named, directly testable evaluateAdHocFormula method.
*/
liveFormula(body: string): ComputedRef<CellValue> {
let cached = this.adHocCache.get(body);
if (!cached) {
cached = computed<CellValue>(() => this.evaluateAdHocFormula(body));
this.adHocCache.set(body, cached);
}
return cached;
}
private evaluateAdHocFormula(body: string): CellValue {
try {
const aggregate = matchSimpleAggregate(body);
if (aggregate) return this.fastAggregate(aggregate);
return this.parser.parse(body, {
row: 1,
col: 1,
sheet: 'Sheet1',
}) as CellValue;
} catch (error) {
return error instanceof (FormulaError as unknown as Function)
? (error as CellValue)
: new FormulaError('#ERROR!');
}
}
/**
* Linear aggregation over a range with the same observation semantics as
* rangeValues: small ranges take fine per-cell tracking, large ranges take
* block subscriptions + paused reads (formula cells through their cached
* computeds; the derived-write bridge keeps blocks truthful). Numbers
* aggregate; blanks/text are skipped (COUNT counts numbers, Excel-style);
* an error value propagates.
*/
private fastAggregate(aggregate: SimpleAggregate): CellValue {
const startRow = aggregate.startRow - 1;
const startCol = aggregate.startCol - 1;
const endRow = Math.min(aggregate.endRow - 1, this.rows - 1);
const endCol = Math.min(aggregate.endCol - 1, this.cols - 1);
const cellCount = (endRow - startRow + 1) * (endCol - startCol + 1);
const isFineTier = cellCount <= FINE_RANGE_LIMIT;
if (!isFineTier) {
const firstBlock = startRow >> BLOCK_SHIFT;
const lastBlock = endRow >> BLOCK_SHIFT;
for (let col = startCol; col <= endCol; col++) {
for (let block = firstBlock; block <= lastBlock; block++) {
this.trackBlock(col * this.blockCount + block);
}
}
pauseTracking();
}
try {
let sum = 0;
let count = 0;
let min = Infinity;
let max = -Infinity;
for (let col = startCol; col <= endCol; col++) {
const column = this.columns[col];
for (let row = startRow; row <= endRow; row++) {
let cellValue: CellValue;
if (isFineTier) {
cellValue = this.valueAt(row, col);
} else if (column.kind[row] === Kind.Formula) {
cellValue = this.formulaValue(row, col).value;
} else {
cellValue = this.rawAt(row, col);
}
if (typeof cellValue === 'number') {
sum += cellValue;
count++;
if (cellValue < min) min = cellValue;
if (cellValue > max) max = cellValue;
} else if (isFormulaError(cellValue)) {
return cellValue; // errors propagate, Excel-style
}
}
}
switch (aggregate.fn) {
case 'SUM':
return sum;
case 'AVERAGE':
return count === 0 ? new FormulaError('#DIV/0!') : sum / count;
case 'COUNT':
return count;
case 'MIN':
return count === 0 ? 0 : min;
case 'MAX':
return count === 0 ? 0 : max;
default:
return new FormulaError('#VALUE!'); // unreachable — union is exhaustive
}
} finally {
if (!isFineTier) resetTracking();
}
}
// --- writes ---
/**
* THE single write path. O(1) storage update + O(observers) notification.
* Never allocates reactive state (peek-only bumps).
*/
write(row: number, col: number, input: string): void {
const column = this.columns[col];
const trimmed = input.trim();
if (isFormulaText(input)) {
column.kind[row] = Kind.Formula;
column.text.set(row, input);
} else if (trimmed.length === 0) {
column.kind[row] = Kind.Blank;
column.text.delete(row);
} else {
const numeric = Number(trimmed);
if (!Number.isNaN(numeric) && Number.isFinite(numeric)) {
if (!column.numbers) column.numbers = new Float64Array(this.rows);
column.kind[row] = Kind.Number;
column.numbers[row] = numeric;
column.text.delete(row);
} else {
column.kind[row] = Kind.Text;
column.text.set(row, input);
}
}
this.bumpCell(row, col);
this.bumpBlock(row, col);
}
// --- diagnostics ---
/**
* Which cells does (row,col)'s formula CURRENTLY read? Re-parses once with
* the read-tap on — it walks the same onCell/onRange path Vue tracks, so
* the set IS the live dependency set (and visibly SHIFTS across an IF's
* branch boundary). 1-based in/out, like the formula grid's traceDeps.
*/
traceDeps(oneBasedRow: number, oneBasedCol: number): Array<[number, number]> {
const row = oneBasedRow - 1;
const col = oneBasedCol - 1;
if (this.columns[col]?.kind[row] !== Kind.Formula) return [];
const body = stripFormula(this.sourceAt(row, col));
if (body.trim().length === 0) return [];
const previousTracer = this.tracer;
this.tracer = [];
pauseTracking();
try {
this.parser.parse(body, {
row: oneBasedRow,
col: oneBasedCol,
sheet: 'Sheet1',
});
} catch {
/* keep whatever reads happened before the error */
} finally {
resetTracking();
}
const recorded = this.tracer;
this.tracer = previousTracer;
const seenKeys = new Set<number>();
const dependencies: Array<[number, number]> = [];
for (const [row1, col1] of recorded) {
const dedupeKey = row1 * (this.cols + 1) + col1;
if (!seenKeys.has(dedupeKey)) {
seenKeys.add(dedupeKey);
dependencies.push([row1, col1]);
}
}
return dependencies;
}
/** The observation census — the law, measurable. */
stats() {
return {
fineRefs: this.cellVersions.size,
blockRefs: this.blockVersions.size,
formulaComputeds: this.formulaCache.size,
adHocFormulas: this.adHocCache.size,
};
}
/**
* Release a formula cell's cached computed (stops its derived-write
* watcher). Production ties this to viewport/refcount eviction — see
* DESIGN.md honest boundaries.
*/
releaseFormula(row: number, col: number): void {
const cellKey = this.cellKey(row, col);
const entry = this.formulaCache.get(cellKey);
if (entry) {
entry.stopBridge();
this.formulaCache.delete(cellKey);
}
}
/**
* Viewport-tied eviction: release overlay entries (fine refs + formula
* computeds) for all rows OUTSIDE [keepStart, keepEnd]. Row-scoped and
* column-agnostic. Block refs are kept (bounded: ≤ blockCount × cols).
*
* SAFETY relies on dependency LOCALITY: a released fine ref / computed
* must not have live dependents outside the kept range. In this layout
* the longest dependency reach is the running-sum chain (RUNSUM_BLOCK =
* 50 rows), so callers must keep a margin ≥ that around the viewport.
* A production impl replaces this with refcounts; documented boundary.
*
* Correctness after release is by re-materialization: the next
* observation of a released cell creates a fresh ref/computed over the
* unchanged ground truth.
*/
evictOutsideRows(keepStart: number, keepEnd: number): number {
let released = 0;
for (const [cellKey, entry] of this.formulaCache) {
const row = cellKey % this.rows;
if (row < keepStart || row > keepEnd) {
entry.stopBridge();
this.formulaCache.delete(cellKey);
released++;
}
}
for (const cellKey of this.cellVersions.keys()) {
const row = cellKey % this.rows;
if (row < keepStart || row > keepEnd) {
this.cellVersions.delete(cellKey);
released++;
}
}
return released;
}
/** Drop the entire overlay (watchers stopped). Ground truth untouched. */
releaseAll(): void {
for (const entry of this.formulaCache.values()) entry.stopBridge();
this.formulaCache.clear();
this.cellVersions.clear();
this.blockVersions.clear();
this.adHocCache.clear();
}
}
export namespace FlyweightSheet {
export const $Class = $FlyweightSheet;
export let Class = Reactive($Class);
export type Instance = typeof Class.Instance;
}ts
/**
* FlyweightCell — the disposable facade. THREE fields; everything else is
* plain getters delegating to the sheet's tracked accessors, so:
*
* - construction allocates one near-empty object (ivue runs nothing at
* `new`, and plain getters de-optimize to native prototype getters);
* - reads are tracked through whatever effect performs them — a facade in
* a template subscribes the component exactly like a real cell would;
* - the reactive state lives on the SHEET's sparse overlay, so facades are
* created per render and dropped on scroll with zero loss.
*
* The reference `FormulaCell` (demo/formula) holds its own ref + computed;
* this holds NOTHING — that is the flyweight move.
*/
import { Reactive } from '../../../ivue';
import { Kind, cssOf, displayOf, type CellValue } from '../flyweight-logic';
import type { FlyweightSheet } from './FlyweightSheet';
class $FlyweightCell {
readonly sheet: FlyweightSheet.Instance;
readonly row: number;
readonly col: number;
constructor(sheet: FlyweightSheet.Instance, row: number, col: number) {
this.sheet = sheet;
this.row = row;
this.col = col;
}
/** Resolved value — tracked point read through the sheet. */
get value(): CellValue {
return this.sheet.valueAt(this.row, this.col);
}
/** The literal text (formula source / number text). */
get source(): string {
return this.sheet.sourceAt(this.row, this.col);
}
get isFormula(): boolean {
return this.sheet.kindAt(this.row, this.col) === Kind.Formula;
}
get display(): string {
return displayOf(this.value);
}
get cssClass(): string {
return cssOf(this.value, this.isFormula);
}
write(input: string): void {
this.sheet.write(this.row, this.col, input);
}
}
export namespace FlyweightCell {
export const $Class = $FlyweightCell;
export let Class = Reactive($Class);
export type Instance = typeof Class.Instance;
}ts
/**
* Page controller for the flyweight grid, authored per the ivue operating
* manual. The composition-API version of this logic carried seven
* `computed()`s; under the doctrine exactly ONE survives (`visibleRows`,
* render suppression) — every other derivation is a plain getter at zero
* bytes per instance. The one computed is THIN: its body delegates to the
* directly testable `buildVisibleRows()` method.
*/
import {
computed,
getCurrentScope,
onMounted,
onScopeDispose,
ref,
shallowRef,
watch,
type ComputedRef,
} from 'vue';
import { Reactive } from '../../ivue';
import {
COLS,
OVERSCAN,
ROWS_1M,
ROW_HEIGHT,
VIEWPORT_HEIGHT,
colLabel,
type CellValue,
} from './flyweight-logic';
import { FlyweightCell } from './model/FlyweightCell';
import { FlyweightSheet } from './model/FlyweightSheet';
export interface PageRow {
row: number;
cells: FlyweightCell.Instance[];
}
/**
* Scroll has physical walls: Chrome's compositor does scroll math in
* FLOAT32 (dead past 2^24 = 16,777,216 px — a 28M px scroller stops at
* ~row 599,186); Firefox caps element height at ~17.9M px. Cap the
* physical height under both and map scroll ratio → virtual offset (the
* scaled scrollbar every big-grid engine uses; ~2.4:1 at 1M rows).
*/
const MAX_SCROLL_HEIGHT = 12_000_000;
/** Eviction margin ≫ the 50-row running-sum reach (dependency locality). */
const EVICT_MARGIN_ROWS = 512;
class $FlyweightGridPage {
constructor() {
// Viewport-tied eviction, debounced so a fast flick doesn't thrash.
// Plain watch: the constructor runs in setup() context, so the
// component scope owns and stops it on unmount.
watch(
() => this.startRow,
() => this.scheduleEviction(),
);
onMounted(() => {
this.censusTimer = setInterval(() => this.pollCensus(), 500);
this.installHarness();
});
// Timers die with the component (watchers are component-scoped already).
if (getCurrentScope()) {
onScopeDispose(() => {
if (this.censusTimer) clearInterval(this.censusTimer);
if (this.evictTimer) clearTimeout(this.evictTimer);
});
}
}
// --- state ---
get sheet() {
return shallowRef<FlyweightSheet.Instance | null>(null);
}
get creationMs() {
return ref(0);
}
get scrollTop() {
return ref(0);
}
/** Template-ref target — destructured by the SFC for ref="scrollEl". */
get scrollEl() {
return ref<HTMLElement | null>(null);
}
get editing() {
return ref<{ row: number; col: number } | null>(null);
}
get draft() {
return ref('');
}
/** Polled diagnostics, not model state — refreshed on an interval. */
get census() {
return ref({
fineRefs: 0,
blockRefs: 0,
formulaComputeds: 0,
adHocFormulas: 0,
});
}
// --- non-reactive infra (timers) ---
private censusTimer: ReturnType<typeof setInterval> | null = null;
private evictTimer: ReturnType<typeof setTimeout> | null = null;
// --- derived (plain getters) ---
get hasModel() {
return this.sheet.value !== null;
}
get modelCells() {
return this.sheet.value ? this.sheet.value.rows * COLS : 0;
}
get naturalHeight() {
return this.sheet.value ? this.sheet.value.rows * ROW_HEIGHT : 0;
}
get totalHeight() {
return Math.min(this.naturalHeight, MAX_SCROLL_HEIGHT);
}
get scrollScale() {
return this.naturalHeight > this.totalHeight
? (this.naturalHeight - VIEWPORT_HEIGHT) /
(this.totalHeight - VIEWPORT_HEIGHT)
: 1;
}
/** Position in CONTENT space (0 … naturalHeight − viewport). */
get virtualTop() {
return this.scrollTop.value * this.scrollScale;
}
get startRow() {
return Math.max(0, Math.floor(this.virtualTop / ROW_HEIGHT) - OVERSCAN);
}
get endRow() {
const visibleCount = Math.ceil(VIEWPORT_HEIGHT / ROW_HEIGHT);
return this.sheet.value
? Math.min(
this.sheet.value.rows,
this.startRow + visibleCount + OVERSCAN * 2,
)
: 0;
}
/** Pin the window band under the physical scroll position (degenerates
* to startRow × ROW_HEIGHT when scale = 1). */
get offsetY() {
return (
this.scrollTop.value - (this.virtualTop - this.startRow * ROW_HEIGHT)
);
}
/**
* The ONLY cell objects in existence — facades for the visible window.
* The one surgical computed() on this page: without the cache, the
* 500ms census poll would re-render the component and a plain getter
* would rebuild ~520 facades per poll; cached, an unchanged window
* returns the same array instance and the v-for never re-patches.
*/
get visibleRows(): ComputedRef<PageRow[]> {
return computed(() => this.buildVisibleRows());
}
private buildVisibleRows(): PageRow[] {
const sheet = this.sheet.value;
if (!sheet) return [];
const pageRows: PageRow[] = [];
for (let row = this.startRow; row < this.endRow; row++) {
const cells: FlyweightCell.Instance[] = new Array(COLS);
for (let col = 0; col < COLS; col++)
cells[col] = new FlyweightCell.Class(sheet, row, col);
pageRows.push({ row, cells });
}
return pageRows;
}
/** Live full-column totals (block tier: 245 edges each). liveFormula is
* cached on the sheet, so rebuilding this array per render is pointer
* work — a plain getter suffices. */
get totals(): { label: string; total: ComputedRef<CellValue> }[] {
const sheet = this.sheet.value;
if (!sheet) return [];
const lastRow = sheet.rows;
return [
{
label: `SUM(A1:A${lastRow})`,
total: sheet.liveFormula(`SUM(A1:A${lastRow})`),
},
{
label: `AVERAGE(B1:B${lastRow})`,
total: sheet.liveFormula(`AVERAGE(B1:B${lastRow})`),
},
{
label: `SUM(D1:D${lastRow})`,
total: sheet.liveFormula(`SUM(D1:D${lastRow})`),
},
];
}
get activeRef() {
const editing = this.editing.value;
return editing ? colLabel(editing.col) + (editing.row + 1) : '';
}
get activeSource() {
const editing = this.editing.value;
return editing && this.sheet.value
? this.sheet.value.sourceAt(editing.row, editing.col)
: '';
}
// --- methods ---
createModel() {
this.editing.value = null;
const startedAt = performance.now();
const sheet = new FlyweightSheet.Class(ROWS_1M, COLS);
this.creationMs.value = performance.now() - startedAt;
this.sheet.value = sheet;
this.pollCensus();
// eslint-disable-next-line no-console
console.log(
`[flyweight] created ${(ROWS_1M * COLS).toLocaleString()} cells in ${this.creationMs.value.toFixed(1)}ms`,
);
}
onScroll(event: Event) {
this.scrollTop.value = (event.target as HTMLElement).scrollTop;
}
isEditing(row: number, col: number) {
const editing = this.editing.value;
return !!editing && editing.row === row && editing.col === col;
}
edit(cell: FlyweightCell.Instance) {
this.editing.value = { row: cell.row, col: cell.col };
this.draft.value = cell.source;
}
commitEdit() {
const editing = this.editing.value;
if (editing && this.sheet.value)
this.sheet.value.write(editing.row, editing.col, this.draft.value);
this.editing.value = null;
}
pollCensus() {
const sheet = this.sheet.value;
if (sheet) this.census.value = sheet.stats();
}
scheduleEviction() {
if (this.evictTimer) clearTimeout(this.evictTimer);
this.evictTimer = setTimeout(() => {
const sheet = this.sheet.value;
if (!sheet) return;
sheet.evictOutsideRows(
Math.max(0, this.startRow - EVICT_MARGIN_ROWS),
this.endRow + EVICT_MARGIN_ROWS,
);
this.pollCensus();
}, 300);
}
scrollToRow(row: number) {
const scrollEl = this.scrollEl.value;
if (!this.sheet.value || !scrollEl) return;
const targetPx =
(row * ROW_HEIGHT - VIEWPORT_HEIGHT / 2) / this.scrollScale;
const clamped = Math.max(
0,
Math.min(targetPx, this.totalHeight - VIEWPORT_HEIGHT),
);
scrollEl.scrollTop = clamped;
this.scrollTop.value = clamped;
}
/** Measurement/verification harness (same idea as the reference grids). */
private installHarness() {
(window as unknown as { __fw: unknown }).__fw = {
rows: () => (this.sheet.value ? this.sheet.value.rows : 0),
cols: COLS,
createModel: () => this.createModel(),
hasModel: () => this.hasModel,
creationMs: () => this.creationMs.value,
stats: () => (this.sheet.value ? this.sheet.value.stats() : null),
scrollToRow: (row: number) => this.scrollToRow(row),
editCell: (row: number, col: number, input: string) =>
this.sheet.value?.write(row, col, input),
cellText: (row: number, col: number) => {
const cellEl = document.querySelector(
`[data-grid-cell][data-row="${row}"][data-col="${col}"]`,
);
return cellEl ? (cellEl.textContent || '').trim() : null;
},
cellValue: (row: number, col: number) => {
const value = this.sheet.value?.valueAt(row, col);
return value && typeof value === 'object'
? String(value)
: (value ?? null);
},
startRow: () => this.startRow,
};
}
}
export namespace FlyweightGridPage {
export const $Class = $FlyweightGridPage;
export let Class = Reactive($Class);
export type Instance = typeof Class.Instance;
}Open in StackBlitz ⚡ — the playground boots with this example's route and file active.