feat: Day 18: Settlers of The North Pole (part 2)

Author: mariotacke

day-18-settlers-of-the-north-pole/README.md

@@ -156,5 +156,11 @@ After 10 minutes, there are `37` wooded acres and `31` lumberyards. Multiplying
 
 **What will the total resource value of the lumber collection area be after 10 minutes?**
 
+## Part Two
+
+This important natural resource will need to last for at least thousands of years. Are the Elves collecting this lumber sustainably?
+
+**What will the total resource value of the lumber collection area be after 1000000000 minutes?**
+
 ## References
 - https://adventofcode.com/2018/day/18

day-18-settlers-of-the-north-pole/settlers2.js

@@ -0,0 +1,112 @@
+const crypto = require('crypto');
+
+const countTrees = (acres) => acres.filter((a) => a === '|').length;
+const countLumberyards = (acres) => acres.filter((a) => a === '#').length;
+
+const computeNextState = (currentState, adjacentAcres) => {
+  const trees = countTrees(adjacentAcres);
+  const lumberyards = countLumberyards(adjacentAcres);
+
+  if (currentState === '.' && trees >= 3) {
+    return '|';
+  }
+
+  if (currentState === '|' && lumberyards >= 3) {
+    return '#';
+  }
+
+  if (currentState === '#') {
+    return lumberyards >= 1 && trees >= 1 ? '#' : '.';
+  }
+
+  return currentState;
+};
+
+const simulate = (area) => {
+  const getAdjacentAcres = (x, y) => {
+    const minimumX = x - 1 < 0 ? 0 : x - 1;
+    const maximumX = x + 2 > area.length ? area.length : x + 2;
+    const acres = [
+      ...(area[y - 1] || []).slice(minimumX, maximumX),
+      area[y][x - 1],
+      area[y][x + 1],
+      ...(area[y + 1] || []).slice(minimumX, maximumX),
+    ];
+
+    return acres.filter((acre) => acre);
+  };
+
+  const output = Array
+    .from({ length: area.length })
+    .map(() => Array.from({ length: area.length }));
+
+  for (let y = 0; y < area.length; y++) {
+    for (let x = 0; x < area.length; x++) {
+      const adjacentAcres = getAdjacentAcres(x, y);
+      const nextState = computeNextState(area[y][x], adjacentAcres);
+
+      output[y][x] = nextState;
+    }
+  }
+
+  return output;
+};
+
+const computeSimulationHash = (area) => {
+  const flattenedArea = area.map((row) => row.join('')).join('');
+  const sha1 = crypto.createHash('sha1');
+
+  sha1.update(flattenedArea);
+
+  const hash = sha1.digest('hex');
+
+  return hash;
+};
+
+module.exports = (input, simulations = 10) => {
+  const previousSimulations = [];
+
+  const simulationArea = input
+    .split('\n')
+    .map((line) => line
+      .trim()
+      .split('')
+    );
+
+  let output = simulationArea;
+  let additionalSimulations = 0;
+
+  const initialHash = computeSimulationHash(output);
+
+  previousSimulations.push(initialHash);
+
+  for (let i = 0; i < simulations; i++) {
+    output = simulate(output);
+
+    const hash = computeSimulationHash(output);
+
+    if (previousSimulations.includes(hash)) {
+      const firstOccurrence = previousSimulations.indexOf(hash);
+      const loopLength = previousSimulations.length - firstOccurrence;
+      const remainingSimulations = simulations - i;
+
+      additionalSimulations = remainingSimulations % loopLength;
+
+      break;
+    } else {
+      previousSimulations.push(hash);
+    }
+  }
+
+  // simulate remaining mutations
+  for (let i = 0; i < additionalSimulations - 1; i++) {
+    output = simulate(output);
+  }
+
+  const flattenedOutput = [].concat.apply([], output);
+
+  const trees = countTrees(flattenedOutput);
+  const lumberyards = countLumberyards(flattenedOutput);
+
+  return trees * lumberyards;
+};