feature: All in a Single Night (part 2)

day-09-all-in-a-single-night/README.md

@@ -27,5 +27,15 @@ The shortest of these is `London -> Dublin -> Belfast = 605`, and so the answer
 
 What is the distance of the shortest route?
 
+## Part Two
+
+The next year, just to show off, Santa decides to take the route with the **longest distance** instead.
+
+He can still start and end at any two (different) locations he wants, and he still must visit each location exactly once.
+
+For example, given the distances above, the longest route would be `982` via (for example) `Dublin -> London -> Belfast`.
+
+What is the distance of the longest route?
+
 ## References
 - https://adventofcode.com/2015/day/9

day-09-all-in-a-single-night/shortest-distance2.js

@@ -0,0 +1,73 @@
+const getPermutations = (cities) => {
+  const permutations = [];
+
+  const permute = (array, permutation = []) => {
+    if (!array.length) {
+      permutations.push(permutation);
+
+      return;
+    }
+
+    for (let i = 0; i < array.length; i++) {
+      const rest = [...array];
+      const city = rest.splice(i, 1)[0];
+
+      permute(rest, [...permutation, city]);
+    }
+  };
+
+  permute(cities, []);
+
+  return permutations;
+};
+
+const shortestDistance = (input) => {
+  const cities = new Set();
+
+  const connections = input
+    .split('\n')
+    .map((distance) => {
+      const parts = distance
+        .trim()
+        .match(/(\w+) to (\w+) = (\d+)/);
+
+      return {
+        a: parts[1],
+        b: parts[2],
+        distance: parseInt(parts[3]),
+      };
+    });
+
+  connections.forEach((connection) => {
+    cities.add(connection.a);
+    cities.add(connection.b);
+  });
+
+  const permutations = getPermutations([...cities])
+    .map((permutation) => {
+      const distance = permutation.reduce((previousValue, currentValue, currentIndex, array) => {
+        const nextValue = array[currentIndex + 1];
+
+        if (nextValue) {
+          const connection = connections
+            .find((x) =>
+              (x.a === currentValue && x.b === nextValue) ||
+              (x.a === nextValue && x.b === currentValue)
+            );
+
+          return previousValue + connection.distance;
+        }
+
+        return previousValue;
+      }, 0);
+
+      return {
+        distance,
+        route: permutation,
+      };
+    });
+
+  return Math.max(...permutations.map((x) => x.distance));
+};
+
+module.exports = shortestDistance;

day-09-all-in-a-single-night/test.js

@@ -1,6 +1,7 @@
 const assert = require('assert');
 
 const distance = require('./shortest-distance');
+const distance2 = require('./shortest-distance2');
 
 describe('Day 9: All in a Single Night', () => {
   it('should calculate shortest distance', () => {
@@ -13,4 +14,17 @@ describe('Day 9: All in a Single Night', () => {
 
     assert.strictEqual(shortest, 605);
   });
+
+  describe('Part Two', () => {
+    it('should calculate longest distance', () => {
+      const distances =
+        `London to Dublin = 464
+         London to Belfast = 518
+         Dublin to Belfast = 141`;
+
+      const shortest = distance2(distances);
+
+      assert.strictEqual(shortest, 982);
+    });
+  });
 });