feat: Day 12: Subterranean Sustainability (part 2)

Author: mariotacke

day-12-subterranean-sustainability/README.md

@@ -71,5 +71,11 @@ In this example, after 20 generations, the pots shown as `#` contain plants, the
 
 **After `20` generations, what is the sum of the numbers of all pots which contain a plant?**
 
+## Part Two
+
+You realize that 20 generations aren't enough. After all, these plants will need to last another 1500 years to even reach your timeline, not to mention your future.
+
+**After fifty billion (`50000000000`) generations, what is the sum of the numbers of all pots which contain a plant?**
+
 ## References
 - https://adventofcode.com/2018/day/12

day-12-subterranean-sustainability/sustainability2.js

@@ -0,0 +1,69 @@
+const sustainability = (input) => {
+  const lines = input.split('\n');
+  const initialState = lines[0]
+    .replace('initial state:', '')
+    .trim()
+    .split('');
+
+  const notes = lines
+    .slice(2, lines.length)
+    .map((line) => {
+      const parts = line.trim().split(' => ');
+
+      return {
+        pattern: parts[0],
+        outcome: parts[1],
+      };
+    });
+
+  const mutations = [];
+
+  let offset = 10;
+  let state = [
+    ...'.'.repeat(offset).split(''),
+    ...initialState,
+    ...'.'.repeat(offset).split(''),
+  ];
+
+  let generation = 1;
+
+  // eslint-disable-next-line no-constant-condition
+  while (true) {
+    let nextState = [...'.'.repeat(state.length).split('')];
+
+    for (let i = 0; i < state.length; i++) {
+      const pot = state.slice(i, i + 5).join('');
+      const note = notes.find(({ pattern }) => pattern === pot);
+
+      if (note) {
+        nextState[i + 2] = note.outcome;
+      }
+    }
+
+    const i = [...nextState].reverse().indexOf('#');
+    const padding = i < 5 ? 5 - i : 0;
+
+    nextState = [...nextState, ...'.'.repeat(padding).split('')];
+
+    state = nextState;
+
+    const mutation = state
+      .join('')
+      .replace(/\./g, ' ')
+      .trim();
+
+    if (mutations.includes(mutation)) {
+      break;
+    } else {
+      mutations.push(mutation);
+    }
+
+    generation++;
+  }
+
+  return state
+    .map((x, i) => x === '#' ? i - offset + 50000000000 - generation : 0)
+    .reduce((a, b) => a + b, 0);
+};
+
+module.exports = sustainability;