@electric-sql/d2mini/dist/operators/topK.js

D2Mini is a minimal implementation of Differential Dataflow for performing in-memory incremental view maintenance.

import { reduce } from './reduce.js';
import { MultiSet } from '../multiset.js';
/**
 * Limits the number of results based on a comparator, with optional offset.
 * This works on a keyed stream, where the key is the first element of the tuple
 * The ordering is withing a key group, i.e. elements are sorted within a key group
 * and the limit + offset is applied to that sorted group.
 * To order the entire stream, key by the same value for all elements such as null.
 *
 * @param comparator - A function that compares two elements
 * @param options - An optional object containing limit and offset properties
 * @returns A piped operator that limits the number of results
 */
export function topK(comparator, options) {
    const limit = options?.limit ?? Infinity;
    const offset = options?.offset ?? 0;
    return (stream) => {
        const reduced = stream.pipe(reduce((values) => {
            // `values` is a list of tuples, first element is the value, second is the multiplicity
            const consolidated = new MultiSet(values).consolidate();
            const sortedValues = consolidated
                .getInner()
                .sort((a, b) => comparator(a[0], b[0]));
            return sortedValues.slice(offset, offset + limit);
        }));
        return reduced;
    };
}
/**
 * Limits the number of results based on a comparator, with optional offset.
 * This works on a keyed stream, where the key is the first element of the tuple
 * The ordering is withing a key group, i.e. elements are sorted within a key group
 * and the limit + offset is applied to that sorted group.
 * To order the entire stream, key by the same value for all elements such as null.
 * Adds the index of the element to the result as [key, [value, index]]
 *
 * @param comparator - A function that compares two elements
 * @param options - An optional object containing limit and offset properties
 * @returns A piped operator that orders the elements and limits the number of results
 */
export function topKWithIndex(comparator, options) {
    const limit = options?.limit ?? Infinity;
    const offset = options?.offset ?? 0;
    return (stream) => {
        const reduced = stream.pipe(reduce((values) => {
            // `values` is a list of tuples, first element is the value, second is the multiplicity
            const consolidated = new MultiSet(values).consolidate();
            let i = offset;
            const sortedValues = consolidated
                .getInner()
                .sort((a, b) => comparator(a[0], b[0]))
                .slice(offset, offset + limit)
                .map(([value, multiplicity]) => [
                [value, i++],
                multiplicity,
            ]);
            return sortedValues;
        }));
        return reduced;
    };
}
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