import Numbers from "../Numbers.js";
import SequentialCombinations from "./SequentialCombinations.js";

/**
 * Generates sequential sequences of combinations, according to the given number of elements.
 */
export default class Combinations extends SequentialCombinations {
  elements: number[];
  combinationSize: number;
  numCombinations: number;
  currentCombinations: number = 0;

  constructor(elements: number[], combinationSize: number) {
    super(elements.length);

    this.elements = elements;
    this.combinationSize = combinationSize;

    // Number of combinations
    // n!/(r!(n−r)!)
    const nFact = Numbers.factorial(elements.length);
    const rFact = Numbers.factorial(combinationSize);
    const nrDiff = elements.length - combinationSize;
    const nrDiffFact = Numbers.factorial(nrDiff);

    this.numCombinations = nFact / (rFact * nrDiffFact);
  }

  /**
   * @returns The next sequence
   */
  next() {
    // Check if there are combinations left
    if (!this.hasNext()) {
      throw new Error(
        `Combinations.next: Reached maximum number of combinations (${this.numCombinations})`
      );
    }

    // Get new values, until one with length of combinationSize appear
    let sequence: number[] = [];
    while (sequence.length !== this.combinationSize) {
      sequence = super.next();
    }

    // Found sequence, create combination
    const combination = [];
    for (const index of sequence) {
      combination.push(this.elements[index]);
    }

    this.currentCombinations++;

    return combination;
  }

  /**
   * @returns True if there are stil combinations to generate
   */
  hasNext(): boolean {
    return this.currentCombinations < this.numCombinations;
  }
}