@onesy/utils/esm/combination.js

Utils

import is from './is';
import unique from './unique';
import combinationWithRepetition from './combinationWithRepetition';
const optionsDefault = {
  response: 'array'
}; // m - array, n - item length
// m! / n!(m - n)!

export default function combination(value_) {
  let items_ = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : 0;
  let options_ = arguments.length > 2 && arguments[2] !== undefined ? arguments[2] : {};
  const options = { ...optionsDefault,
    ...options_
  };

  if (is('array', value_)) {
    const value = unique(value_);
    const items = is('number', items_) ? items_ : 0;
    const length = value.length;
    if (items < 1) return [value];
    if (items === 1) return value.map(item_ => [item_]);
    if (items === length) return [value];
    if (items > length) return combinationWithRepetition(value, items, options);
    const response = [];
    let item = [...Array(items).keys()];
    let index = items - 2;
    const max = length - items;
    let updated = false;

    if (options.response === 'array') {
      while (index >= 0) {
        if (updated) {
          index = items - 2;
          updated = false;
        }

        while (item[item.length - 1] < length) {
          const item_ = item.slice().map(index_ => value[index_]);
          response.push(item_);
          item[item.length - 1]++;
        } // Increment index if it item index is less than max + index
        // else move on to previous one, etc.
        // and all following keys up to items length should be +1 index


        while (true) {
          if (item[index] === max + index) {
            index--;
            if (index < 0) break;
            updated = true;
          } else {
            // Increment item index
            item[index]++; // and make all following items up to items length previous + 1

            const part = item.slice(0, index + 1);
            item = [...part, ...Array(items - part.length).fill(0).map((_, i) => item[index] + i + 1)];
            break;
          }
        }
      }

      return response;
    }

    if (options.response === 'yield') return function* () {
      while (index >= 0) {
        if (updated) {
          index = items - 2;
          updated = false;
        }

        while (item[item.length - 1] < length) {
          const item_ = item.slice().map(index_ => value[index_]);
          yield item_;
          response.push(item_);
          item[item.length - 1]++;
        } // Increment index if it item index is less than max + index
        // else move on to previous one, etc.
        // and all following keys up to items length should be +1 index


        while (true) {
          if (item[index] === max + index) {
            index--;
            if (index < 0) break;
            updated = true;
          } else {
            // Increment item index
            item[index]++; // and make all following items up to items length previous + 1

            const part = item.slice(0, index + 1);
            item = [...part, ...Array(items - part.length).fill(0).map((_, i) => item[index] + i + 1)];
            break;
          }
        }
      }

      return response;
    };
  }
}