[Day 25](https://adventofcode.com/2018/day/25)[Day 18](https://adventofcode.com/2018/day/18)[Day 21](https://adventofcode.com/2018/day/21)Day 19[Day 16](https://adventofcode.com/2018/day/16)[Day 14](https://adventofcode.com/2018/day/14)[Day 13](https://adventofcode.com/2018/day/13)[Day 11](https://adventofcode.com/2018/day/11)[Day 10](https://adventofcode.com/2018/day/10)[Day 9](https://adventofcode.com/2018/day/9)[Day 8](https://adventofcode.com/2018/day/8)[Day 7](https://adventofcode.com/2018/day/7)[Day 6](https://adventofcode.com/2018/day/6)[Day 5](https://adventofcode.com/2018/day/5)[Day 4](https://adventofcode.com/2018/day/4)[Day 3](https://adventofcode.com/2018/day/3)Advent of Code: [Day 2](https://adventofcode.com/2018/day/2)[Day 1](https://adventofcode.com/2018/day/1)
[Day 12](https://adventofcode.com/2018/day/12)
function* evolve () {
let offset = 0;
let state = startState;
while (true) {
state = '.....' + state + '.....';
offset += 3;
let nextState = [];
for (let i = 2; i < state.length - 2; i++) {
let window = state.slice(i-2, i+3);
nextState.push(rules[window] || state[i]);
}
while (nextState[0] === '.') {
nextState = nextState.slice(1);
offset -= 1;
}
while (nextState[nextState.length - 1] === '.') {
nextState = nextState.slice(0, -1);
}
state = nextState.join('');
yield [state, offset];
}
}
function sumPotIndices (state, offset) {
let out = 0;
for (let i = 0; i < state.length; i++) {
if (state[i] === '#') {
out += (i - offset);
}
}
return out;
}
part1Sum = {
const gen = evolve();
let state, offset;
for (let g = 1; g <= 20; g++) {
[state, offset] = gen.next().value;
}
return sumPotIndices(state, offset);
}
part2Sum = {
const gen = evolve();
const numGenerations = 50000000000;
let prevSum = 0;
let prevDifference;
let repeatCount = 0;
for (let g = 1; g <= numGenerations; g += 1) {
const sum = sumPotIndices(...gen.next().value);
if (sum - prevSum === prevDifference) {
repeatCount++;
if (repeatCount > 10) {
return (sum + (numGenerations - g) * prevDifference);
}
} else {
prevDifference = sum - prevSum;
prevSum = sum;
}
}
}
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