locutus/esm/haskell/list/subsequences.js

Locutus other languages' standard libraries to JavaScript for fun and educational purposes

export function subsequences(items) {
    //  discuss at: https://locutus.io/haskell/list/subsequences/
    // original by: Kevin van Zonneveld (https://kvz.io)
    //      note 1: Returns all subsequences (power set) while preserving element order, matching Haskell Data.List.subsequences.
    //   example 1: subsequences([1, 2, 3])
    //   returns 1: [[], [1], [2], [1, 2], [3], [1, 3], [2, 3], [1, 2, 3]]
    //   example 2: subsequences(['a', 'b'])
    //   returns 2: [[], ['a'], ['b'], ['a', 'b']]
    //   example 3: subsequences([])
    //   returns 3: [[]]
    if (!Array.isArray(items)) {
        return [[]];
    }
    const out = [[]];
    for (const item of items) {
        const currentLength = out.length;
        for (let i = 0; i < currentLength; i++) {
            const prefix = out[i];
            if (prefix) {
                out.push([...prefix, item]);
            }
        }
    }
    return out;
}