swiftney.js

'use strict';

function swiftney_impl(nums, expr, goal) {
  if (goal === eval(expr) && nums.length === 0) {
    console.log(`${expr} = ${goal}`);
  }
  if (nums.length > 0) {
    const num = nums[0];
    ['+', '-', '*', '/'].forEach(op => swiftney_impl(nums.slice(1), `(${expr}${op}${num})`, goal));
  }
}

function swiftney(nums, goal) {
  const num = nums[0];
  swiftney_impl(nums.slice(1), `${num}`, goal);
}

function permutations_impl(xs, res, acc) {
  if (xs.length === 0) {
    acc.push(res);
    return acc;
  }
  for (let i = 0; i < xs.length; i++) {
    const newres = res.slice();
    newres.push(xs[i]);
    permutations_impl(xs.slice(0, i).concat(xs.slice(i+1, xs.length)), newres, acc);
  }
  return acc;
}

function permutations(xs) {
  return permutations_impl(xs, [], []);
}

const nums = [1,7,7,2,3,2];
const goal = 131;
for (const p of permutations(nums)) {
  swiftney(p, goal);
}