feature: It Hangs in the Balance (part 2)

day-24-it-hangs-in-the-balance/README.md

@@ -33,5 +33,28 @@ Of these, although `10 9 1` has the smallest quantum entanglement (`90`), the co
 
 What is the **quantum entanglement** of the first group of packages in the ideal configuration?
 
+## Part Two
+
+That's weird... the sleigh still isn't balancing.
+
+"Ho ho ho", Santa muses to himself. "I forgot the trunk".
+
+Balance the sleigh again, but this time, separate the packages into **four groups** instead of three. The other constraints still apply.
+
+Given the example packages above, this would be some of the new unique first groups, their quantum entanglements, and one way to divide the remaining packages:
+
+```
+11 4    (QE=44); 10 5;   9 3 2 1; 8 7
+10 5    (QE=50); 11 4;   9 3 2 1; 8 7
+9 5 1   (QE=45); 11 4;   10 3 2;  8 7
+9 4 2   (QE=72); 11 3 1; 10 5;    8 7
+9 3 2 1 (QE=54); 11 4;   10 5;    8 7
+8 7     (QE=56); 11 4;   10 5;    9 3 2 1
+```
+
+Of these, there are three arrangements that put the minimum (two) number of packages in the first group: `11 4`, `10 5`, and `8 7`. Of these, `11 4` has the lowest quantum entanglement, and so it is selected.
+
+Now, what is the **quantum entanglement** of the first group of packages in the ideal configuration?
+
 ## References
 - https://adventofcode.com/2015/day/24

day-24-it-hangs-in-the-balance/quantum2.js

@@ -0,0 +1,54 @@
+const findCombinations = (packages = [], currentCombination = [], maximumWeight = Infinity, maximumPackages = Infinity) => {
+  const availablePackages = [...packages];
+  const currentWeight = currentCombination.reduce((a, b) => a + b, 0);
+
+  if (currentWeight === maximumWeight && currentCombination.length <= maximumPackages) {
+    return [currentCombination];
+  }
+
+  let combinations = [];
+
+  while (availablePackages.length) {
+    const weight = availablePackages.shift();
+
+    if (currentWeight + weight <= maximumWeight) {
+      const nextCombination = [...currentCombination, weight];
+      const otherCombinations = findCombinations([...availablePackages], nextCombination, maximumWeight, maximumPackages);
+
+      otherCombinations.forEach((combination) => combinations.push(combination));
+    }
+  }
+
+  return combinations;
+};
+
+const findSmallestGroupSize = (packages, maximumWeight) => {
+  let weight = 0;
+  let numberOfPackages = 0;
+
+  for (let i = 0; i < packages.length; i++) {
+    if (weight + packages[i] <= maximumWeight) {
+      weight += packages[i];
+      numberOfPackages++;
+    }
+  }
+
+  return numberOfPackages;
+};
+
+module.exports = (list) => {
+  const packages = list
+    .split('\n')
+    .map((item) => parseInt(item))
+    // sort package weights descending
+    .sort((a, b) => b - a);
+
+  const totalWeight = packages.reduce((a, b) => a + b, 0);
+  const maximumGroupWeight = totalWeight / 4;
+
+  const maximumPackages = findSmallestGroupSize(packages, maximumGroupWeight);
+
+  return findCombinations(packages, [], maximumGroupWeight, maximumPackages)
+    .map((group) => group.reduce((a, b) => a * b, 1))
+    .sort((a, b) => a - b)[0];
+};

day-24-it-hangs-in-the-balance/test.js

@@ -1,6 +1,7 @@
 const assert = require('assert');
 
 const quantum = require('./quantum');
+const quantum2 = require('./quantum2');
 
 describe('Day 24: It Hangs in the Balance', () => {
   it('should calculate quantum entanglement', () => {
@@ -20,4 +21,24 @@ describe('Day 24: It Hangs in the Balance', () => {
 
     assert.strictEqual(entanglement, 99);
   });
+
+  describe('Part Two', () => {
+    it('should calculate quantum entanglement with four compartments', () => {
+      const packages =
+        `1
+         2
+         3
+         4
+         5
+         7
+         8
+         9
+         10
+         11`;
+
+      const entanglement = quantum2(packages);
+
+      assert.strictEqual(entanglement, 44);
+    });
+  });
 });