Day 06

Posted on Mon 06 December 2021 in aoc2021

See problem here.

My initial approach was to track all of the fish. We decrease number of days for each fish by 1 until it reaches 0. Then we reset it to 6 for existing fish and add a new fish with value 8. This works fine but it is too slow for part 2.

def part1(arr):
    fishes = [int(e) for e in arr[0].split(',')]
    pop = copy.deepcopy(fishes)
    for _ in range(80):
        n = len(pop)
        for i in range(n):
            if pop[i] > 0:
                pop[i] -= 1
            else:
                pop[i] = 6
                pop.append(8)
    ans = len(pop)
    print(ans)

Then I realized that all fish with value x are the same so we can just keep track of the count of each fish.

The idea would be if there are v fish with value k, then the next day there will by v more fish with value k-1 if k > 0. Similarly, if k == 0, then there will be v more fish the next with value 6 and v more fish the next value with value 8. This solution is much faster.

function count_fish(pi, days) {
    var initial = pi.split(',').map(Number);
    var count = {};
    for (var fish of initial) {
        if (count[fish] === undefined) count[fish] = 0;
        count[fish] += 1;
    }
    for (var day = 0; day < days; day++) {
        var new_count = {};
        for (const k in count) {
            const v = count[k];
            if (k > 0) {
                if (new_count[k-1] === undefined) new_count[k-1] = 0;
                new_count[k-1] += v;
            }
            else {
                if (new_count[6] === undefined) new_count[6] = 0;
                if (new_count[8] === undefined) new_count[8] = 0;
                new_count[6] += v;
                new_count[8] += v;
            }
        }
        count = new_count;
    }
    return Object.values(count).reduce(function(a, b) { return a + b; });
}

For part1, we want the result after 80 days and for part2, we want the result after 256 days.

function part1(pi) {
    var ans = count_fish(pi, 80);
    console.log(`part1: ${ans}`);
}

function part2(pi) {
    var ans = count_fish(pi, 256);
    console.log(`part2: ${ans}`);
}

This one was easier than day 5, at least for me.

js python