Combinations and Pairs in javascript

combos-and-pairs.js

const _ = require('lodash');

function getPairs(arr) {
  let rCount = 0;
  const results = arr.map(i => [i]);
  const f = (base, rest) => {
    for (let i = 0, max = rest.length; i < max; i += 1) {
      const entry = [base, rest[i]];
      results.push(entry);
    }
    const recurseRest = rest.slice(1);
    if (recurseRest.length) {
      f(rest[0], recurseRest);
    }
  }
  f(arr[0], arr.slice(1));
  return results;
}

function log(index, ...props) {
  let prefix = '';
  while (prefix.length < index) prefix += '.';
  console.log.apply(console, [prefix].concat(props));
}

function getCombinations(arr) {
  let rCount = 0;
  // create results, including the initial entries as single-item arrays
  const results = arr.map(i => [i]);
  // recursion function, takes a base entry and a list of others to include
  const f = (base, rest) => {
    // loop over each of the `rest`, appending a shortening list of them,
    // concatenated to `[base]`
    rest.forEach((entry, index) => {
      results.push(base.concat(rest.slice(0, rest.length - index)));
    })
    // if we have enough in `rest`, then do the recursion
    if (rest.length > 1) f(base, rest.slice(1));
  }
  // kick off the recursion once for each entry
  arr.forEach((entry, index, full) => {
    // don't bother with the last entry - it was added earlier, and will not
    // need any further concatenation
    if (index === full.length - 1) return;
    // go!
    f([entry], full.slice(index + 1));
  });
  // return the final set of results
  return results;
}
const topLevels = ["Country", "Division", "Industry", "Juliet", "Kilo", "Lima"];

const allCombinations = getCombinations(topLevels);

const sortedCombinations = _.orderBy(allCombinations, (item) => {
  return item.join('');
});
console.log(sortedCombinations);
console.log(sortedCombinations.length);

const allPairs = getPairs(topLevels);

const sortedPairs = _.orderBy(allPairs, (item) => {
  return item.join('');
});
console.log(sortedPairs);
console.log(sortedPairs.length);