85 lines
2.2 KiB
TypeScript
85 lines
2.2 KiB
TypeScript
// Copyright 2024 Signal Messenger, LLC
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// SPDX-License-Identifier: AGPL-3.0-only
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import type { RequireAtLeastOne } from 'type-fest';
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export type MapKey<T extends Map<unknown, unknown>> =
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T extends Map<infer Key, unknown> ? Key : never;
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export type MapValue<T extends Map<unknown, unknown>> =
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T extends Map<unknown, infer Value> ? Value : never;
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export type MapEmplaceOptions<T extends Map<unknown, unknown>> =
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RequireAtLeastOne<{
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insert?: (key: MapKey<T>, map: T) => MapValue<T>;
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update?: (existing: MapValue<T>, key: MapKey<T>, map: T) => MapValue<T>;
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}>;
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/**
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* Lightweight polyfill of the `Map.prototype.emplace` TC39 Proposal:
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* @see https://github.com/tc39/proposal-upsert
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*
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* @throws If no `insert()` is provided and key is not present
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*
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* @example Get or Insert:
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* ```ts
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* let pagesByBook = new Map<BookId, Map<PageId, Page>>()
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* for (let page of pages) {
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* let bookPages = mapEmplace(pagesByBook, page.bookId, {
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* insert: () => new Map(),
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* })
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* bookPages.set(page.id, page)
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* }
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* ```
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*
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* @example Get+Update or Insert:
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* ```ts
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* let unreadPages = new Map<BookId, number>()
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* for (let page of pages) {
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* if (page.readAt == null) {
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* mapEmplace(unreadPages, page.bookId, {
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* insert: () => 1,
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* update: (prev) => prev + 1,
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* })
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* }
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* }
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* ```
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*
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* @example Get+Update or Throw
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* ```ts
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* let PagesCache = new Map<PageId, Page>()
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*
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* function onPageReadEvent(pageId: PageId, readAt: number) {
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* if (PagesCache.has(pageId)) {
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* mapEmplace(PagesCache, pageId, {
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* update(page) {
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* return { ...page, readAt }
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* },
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* })
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* } else {
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* // save for later
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* onEarlyPageReadEvent(pageId, readAt)
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* }
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* }
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* ```
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*/
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export function mapEmplace<T extends Map<unknown, unknown>>(
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map: T,
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key: MapKey<T>,
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options: MapEmplaceOptions<T>
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): MapValue<T> {
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if (map.has(key)) {
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let value = map.get(key) as MapValue<T>;
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if (options.update != null) {
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value = options.update(value, key, map);
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map.set(key, value);
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}
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return value;
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}
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if (options.insert != null) {
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const value = options.insert(key, map);
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map.set(key, value);
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return value;
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}
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throw new Error('Key was not present in map, and insert() was not provided');
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}
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