54 lines
1.6 KiB
JavaScript
54 lines
1.6 KiB
JavaScript
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//#region src/array/combinations.ts
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/**
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* Returns all `r`-length combinations of elements from the input array.
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*
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* Combinations are emitted in lexicographic order based on the position of elements in the input array.
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* Elements are treated as unique by position, not by value, so duplicates in the input may produce
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* combinations that look identical.
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*
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* The number of combinations is `n! / r! / (n - r)!` when `0 <= r <= n`, and zero when `r > n`.
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*
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* @template T
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* @param {readonly T[]} arr - The input array.
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* @param {number} r - The length of each combination. Must be a non-negative integer.
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* @returns {T[][]} An array of `r`-length combinations.
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* @throws {Error} If `r` is not a non-negative integer.
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*
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* @example
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* combinations(['A', 'B', 'C', 'D'], 2);
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* // => [['A','B'], ['A','C'], ['A','D'], ['B','C'], ['B','D'], ['C','D']]
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*
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* @example
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* combinations([1, 2, 3, 4], 3);
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* // => [[1,2,3], [1,2,4], [1,3,4], [2,3,4]]
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*
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* @example
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* combinations([1, 2, 3], 0);
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* // => [[]]
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*
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* @example
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* combinations([1, 2], 5);
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* // => []
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*/
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function combinations(arr, r) {
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if (!Number.isInteger(r) || r < 0) throw new Error("r must be a non-negative integer.");
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const n = arr.length;
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if (r > n) return [];
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if (r === 0) return [[]];
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const indices = Array(r);
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for (let i = 0; i < r; i++) indices[i] = i;
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const result = [];
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while (true) {
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const tuple = Array(r);
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for (let i = 0; i < r; i++) tuple[i] = arr[indices[i]];
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result.push(tuple);
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let i = r - 1;
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while (i >= 0 && indices[i] === i + n - r) i--;
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if (i < 0) return result;
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indices[i]++;
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for (let j = i + 1; j < r; j++) indices[j] = indices[j - 1] + 1;
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}
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}
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//#endregion
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exports.combinations = combinations;
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