105 lines
3.3 KiB
TypeScript
105 lines
3.3 KiB
TypeScript
export class Cache<T> {
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// The actual "database" that holds the cache entries, along with their
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// insertion time.
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// Insertion order is the same as the order of elements expiring. This is
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// important because the expiration logic relies on the insertion order.
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public cache: Map<string, { date: number; value: T; }>;
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// The lifetime of each cache member.
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//
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// This must not be changed after setup because it may upset
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// the expiration logic.
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private lifetime: number;
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// Function of which the results should be cached.
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public fetcher: (key: string) => Promise<T | undefined>;
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private timeoutScheduled: boolean;
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constructor(lifetime: number, fetcher: Cache<T>['fetcher']) {
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this.cache = new Map();
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this.lifetime = lifetime;
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this.fetcher = fetcher;
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this.timeoutScheduled = false;
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}
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public set(key: string, value: T): void {
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this.cache.set(key, {
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date: Date.now(),
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value,
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});
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// make sure the expiration timeout is in place
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this.expire();
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}
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public get(key: string): T | undefined {
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const cached = this.cache.get(key);
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if (cached == null) return undefined;
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else return cached.value;
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}
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public delete(key: string): void {
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this.cache.delete(key);
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}
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// If the value is cached, it is returned. Otherwise the fetcher is
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// run to get the value. If the fetcher returns undefined, it is
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// returned but not cached.
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public async fetch(key: string): Promise<T | undefined> {
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// Check if this value is cached
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const cached = this.get(key);
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if (cached !== undefined) {
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// The value was cached, return it.
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return cached;
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} else {
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// The value was not cached, need to call the original function
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// to get its result and then cache it.
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const value = await this.fetcher(key);
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// `undefined` is not cached
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if (value !== undefined) {
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this.set(key, value);
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}
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return value;
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}
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}
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// Handling the expiration of cached values.
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// This is done using a timeout.
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private expire(): void {
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// If there already is a timeout scheduled, it will be appropriate
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// for the first inserted element of the cache.
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// If the first element of the cache was removed, it will reschedule
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// to the appropriate time when it runs out.
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//
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// If the cache is empty, there is nothing to expire either.
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if (this.timeoutScheduled) return;
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// Otherwise, this must mean this is the previously scheduled timeout.
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// Since it is running now, it is no longer scheduled.
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this.timeoutScheduled = false;
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// Check if the first element is actually due for expiration.
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//
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// Items may have been removed in the meantime or this may be
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// the initial call for the first key inserted into the cache.
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const [expiredKey, expiredValue] = this.cache.entries().next().value;
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if (expiredValue.date + this.lifetime <= Date.now()) {
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// This item is due for expiration, so remove it.
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this.cache.delete(expiredKey);
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}
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// If there are no further elements in the cache, there is nothing to
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// expire at a later time. The timeout will be set up again later by
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// a call from `this.set`.
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if (this.cache.size === 0) return;
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// Check when the next key is due for removal and schedule
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// an appropriate timeout.
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const [nextKey, nextValue] = this.cache.entries().next().value;
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setTimeout(
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() => this.expire(),
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nextValue.date + this.lifetime - Date.now()
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);
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this.timeoutScheduled = true;
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}
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}
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