aoc-18.js

const fs = require('fs');

const [DEATH, LIFE, VOID] = [1, 2, 3];

const board = fs.readFileSync('aoc-input-18', 'utf-8')
  .trim()
  .split('\n')
  .map(line => line.split('')
    .map(c =>
        c === '|' ? LIFE
      : c === '#' ? DEATH
      : VOID));

const show = () => {
  console.log(board.map(l => l.map(c =>
      c === LIFE ? '|'
    : c === DEATH ? '#'
    : '.').join('')).join('\n'))
}

const cell = (b, x, y) => b[y] && b[y][x];
const count = (arr, type) => arr.reduce((acc, v) => acc + (v === type), 0);
const neighbours = (b, x, y) => [
  cell(b, x-1, y-1), cell(b, x, y-1), cell(b, x+1, y-1),
  cell(b, x-1, y),                    cell(b, x+1, y),
  cell(b, x-1, y+1), cell(b, x, y+1), cell(b, x+1, y+1),
].filter(Boolean);

const checksum = (b) => board.reduce((acc, row, y) => acc + 
  row.reduce((bcc, v, x) => bcc + (x+1) * (y+1) * v % 3123471, 0), 0);

console.log('init state');
show();

const values = [];
const sums = [];
let gen = 0;
while(++gen <= 1e9) {
  const prev = board.map(row => [...row]);

  for(let y = 0; y < board.length; ++y) {
    const row = board[y];
    for(let x = 0; x < row.length; ++x) {
      const neighs = neighbours(prev, x, y);
      const v = cell(prev, x, y);

      let n;

      switch (v) {
        case VOID:
          n = count(neighs, LIFE) >= 3 ? LIFE : VOID;
          break;
        case LIFE:
          n = count(neighs, DEATH) >= 3 ? DEATH : LIFE;
          break;
        case DEATH:
          n = (count(neighs, LIFE) >= 1 && count(neighs, DEATH) >= 1) ? DEATH : VOID;
          break;
      }

      board[y][x] = n;
    }
  }
  console.log(gen);
  //show();
  const allCells = [].concat(...board);
  const lifecount = count(allCells, LIFE);
  const deathcount = count(allCells, DEATH);
  const value = lifecount * deathcount;
  //console.log('alive: ' + lifecount);
  //console.log('dead: ' + deathcount);
  console.log('value: ' + value);

  const sum = checksum(board);
  const index = sums.indexOf(sum);
  if (index > -1) {
    console.log('found duplicate at ' + index);
    console.log('duplicate is ' + value);
    console.log('duplicate sum is ' + sum);
    break;
  }
  sums.push(sum);
  values.push(value);

  // found dupe from 496 (index) in 525 (cycle)
  // lookup value at 497 + (1e9-497)%(525-497)
  show();
}

aoc-input-18

||.|..#|.|.....|..|........|..#..|#...|.........#.
...#....|#..|......#.|||...#...|...#....|.......#.
##.#|.#..|.#|....||.|#.#....#.#.###.|..|...|#.#|..
..||.#|#|#..#.#|...|....||.|...##.....|.....#...#|
#|..#...#.##|.|.##||..|...|...#||.|...#.##.###|.|.
#.#||#...#.|#.....|#.......|#.||..|..|#..#.##...#.
|.||.|.|........|.|.#.|...##.|.||..#....#..|.|#..#
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|#..#..#.|.|#.||#...|.|...|#.||...#...##....|....|
..#...|......#.#...|#...#|.|......#.|||#.|#.|#.#.|
..##.#|#|...||....##|.|||.|##|#.....#|...#...###||
...||......#...|....#.....#|#......#......#..|.|#.
..........#|.|#..|..#.##.#...#|...#..#...|...#|.||
|.#..#|###..||...|#|....|...........|...#|#.#..#|.
|.#..##.........#||.....|#|.###.#.|.....#.|..|.##.
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...|#...##.#.#.|.||.......#...|.....###.||...##..|
..|...|#.|.#..|..#|..||.#.....#||.|..#..|..|.||.#.
#.##||#.#.....||.|.#.......#.|.##|##|#..#|.....|#.
...#..#.#..#.|.....|.#...|.....###||||..#..##...#.
...|...|.|.##|..#.#|#....#.#.|#||||....#...|||.#.#
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|#...#.#.......|...||...##.|.||#.....#.||.......#|
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.|#..#.|...||#..|.#..#....###.#...#.....||......##
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.|.||..|#.|...##.|.#...#..|.||||..##.##|#.##.|##.#
.|#.#.....#||#.#....||.|..##......|.#.|..#...##||.
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