48 lines
1.8 KiB
JavaScript
48 lines
1.8 KiB
JavaScript
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import { InternalError } from "./errors.js";
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// An allocator of non-negative integer ids.
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//
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// This clever data structure has these "ideal" properties:
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// - It consumes memory proportional to the number of used ids (which is optimal).
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// - All operations are O(1) time.
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// - The allocated ids are small (with a slight modification, we could always provide the smallest possible
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// id).
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export class IdAlloc {
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// Set of all allocated ids
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#usedIds;
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// Set of all free ids lower than `#usedIds.size`
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#freeIds;
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constructor() {
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this.#usedIds = new Set();
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this.#freeIds = new Set();
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}
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// Returns an id that was free, and marks it as used.
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alloc() {
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// this "loop" is just a way to pick an arbitrary element from the `#freeIds` set
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for (const freeId of this.#freeIds) {
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this.#freeIds.delete(freeId);
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this.#usedIds.add(freeId);
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// maintain the invariant of `#freeIds`
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if (!this.#usedIds.has(this.#usedIds.size - 1)) {
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this.#freeIds.add(this.#usedIds.size - 1);
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}
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return freeId;
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}
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// the `#freeIds` set is empty, so there are no free ids lower than `#usedIds.size`
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// this means that `#usedIds` is a set that contains all numbers from 0 to `#usedIds.size - 1`,
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// so `#usedIds.size` is free
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const freeId = this.#usedIds.size;
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this.#usedIds.add(freeId);
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return freeId;
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}
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free(id) {
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if (!this.#usedIds.delete(id)) {
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throw new InternalError("Freeing an id that is not allocated");
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}
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// maintain the invariant of `#freeIds`
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this.#freeIds.delete(this.#usedIds.size);
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if (id < this.#usedIds.size) {
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this.#freeIds.add(id);
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}
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}
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}
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