feature: Like a GIF For Your Yard (part 2)

day-18-like-a-gif-for-your-yard/README.md

@@ -74,5 +74,63 @@ After 4 steps, this example has four lights on.
 
 In your grid of 100x100 lights, given your initial configuration, **how many lights are on after 100 steps**?
 
+## Part Two
+
+You flip the instructions over; Santa goes on to point out that this is all just an implementation of [Conway's Game of Life](https://en.wikipedia.org/wiki/Conway's_Game_of_Life). At least, it was, until you notice that something's wrong with the grid of lights you bought: four lights, one in each corner, are **stuck on** and can't be turned off. The example above will actually run like this:
+
+```
+Initial state:
+##.#.#
+...##.
+#....#
+..#...
+#.#..#
+####.#
+
+After 1 step:
+#.##.#
+####.#
+...##.
+......
+#...#.
+#.####
+
+After 2 steps:
+#..#.#
+#....#
+.#.##.
+...##.
+.#..##
+##.###
+
+After 3 steps:
+#...##
+####.#
+..##.#
+......
+##....
+####.#
+
+After 4 steps:
+#.####
+#....#
+...#..
+.##...
+#.....
+#.#..#
+
+After 5 steps:
+##.###
+.##..#
+.##...
+.##...
+#.#...
+##...#
+```
+
+After `5` steps, this example now has `17` lights on.
+
+In your grid of 100x100 lights, given your initial configuration, but with the four corners always in the **on** state, **how many lights are on after 100 steps**?
+
 ## References
 - https://adventofcode.com/2015/day/18

day-18-like-a-gif-for-your-yard/gif2.js

@@ -0,0 +1,81 @@
+class Grid {
+  constructor (input) {
+    this._state = input
+      .split('\n')
+      .map((x) => x
+        .trim()
+        .split('')
+        .map((l) => l === '#')
+      );
+  }
+
+  getLight (x, y) {
+    return (this._state[y] || [])[x] || false;
+  }
+
+  getTurnedOnLights () {
+    return this._state.reduce((accumulator, currentValue) => {
+      return accumulator + currentValue.filter((x) => x).length;
+    }, 0);
+  }
+
+  _nextLightState (x, y) {
+    const getLight = this.getLight.bind(this);
+
+    if ((x === 0 && y === 0) ||
+        (x === 0 && y === this._state.length - 1) ||
+        (x === this._state[0].length - 1 && y === 0) ||
+        (x === this._state[0].length - 1 && y === this._state.length - 1)) {
+      return true;
+    }
+
+    const currentState = getLight(x, y);
+    const onNeighbors = [
+      getLight(x - 1, y - 1), getLight(x, y - 1), getLight(x + 1, y - 1),
+      getLight(x - 1, y),                         getLight(x + 1, y),
+      getLight(x - 1, y + 1), getLight(x, y + 1), getLight(x + 1, y + 1),
+    ].filter((l) => l).length;
+
+    return currentState
+      ? onNeighbors >= 2 && onNeighbors <= 3
+      : onNeighbors === 3;
+  }
+
+  tick () {
+    const nextState = [];
+
+    for (let y = 0; y < this._state.length; y++) {
+      const row = [];
+
+      for (let x = 0; x < this._state[0].length; x++) {
+        const light = this._nextLightState(x, y);
+
+        row.push(light);
+      }
+
+      nextState.push(row);
+    }
+
+    this._state = nextState;
+  }
+
+  print () {
+    return this._state
+      .map((row) => row
+        .map((col) => col ? '#' : '.')
+        .join(''))
+      .join('\n');
+  }
+}
+
+const gif = (input, steps) => {
+  const grid = new Grid(input);
+
+  for (let i = 0; i < steps; i++) {
+    grid.tick();
+  }
+
+  return grid.getTurnedOnLights();
+};
+
+module.exports = gif;

day-18-like-a-gif-for-your-yard/test.js

@@ -1,6 +1,7 @@
 const assert = require('assert');
 
 const gif = require('./gif');
+const gif2 = require('./gif2');
 
 describe('Day 18: Like a GIF For Your Yard', () => {
   it('should calculate final state lights', () => {
@@ -15,4 +16,19 @@ describe('Day 18: Like a GIF For Your Yard', () => {
 
     assert.strictEqual(gif(grid, steps), 4);
   });
+
+  describe('Part Two', () => {
+    it('should consider stuck lights', () => {
+      const grid =
+        `##.#.#
+         ...##.
+         #....#
+         ..#...
+         #.#..#
+         ####.#`;
+      const steps = 5;
+
+      assert.strictEqual(gif2(grid, steps), 17);
+    });
+  });
 });