W pierwszym liczyłem każdą rybkę osobno ale przeglądarka nie dała rady i nawet node wywaliło po 1.5GB zużytej pamięci w drugim dopiero wtedy rozkminiłem, że można policzyć ile ma się rybek w jakim wieku i je grupami mnożyć różnica w chuj. :D

```
init = [3,4,3,1,2]

days = [...(Array(81))].map((_, index) => index);

populationAges = [...init].reduce((stack, age) => {
    stack[age] = stack[age] ? stack[age] + 1 : 1;

    return stack;
}, {});

days.forEach((day) => {
    const population = Object.values(populationAges).reduce((sum, val) => (sum + val), 0);
    console.log(`Day #${day} population: ${population}`, populationAges);

    const newPopulationAges = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9].reduce((stack, age) => {
        const newAge = age - 1;
        const agePopulation = populationAges[age] || 0;

        if (newAge !== -1) {
            stack[newAge] = stack[newAge] ? (stack[newAge] + agePopulation) : agePopulation;
        } else {
            stack[6] = stack[newAge] ? (stack[6] + agePopulation) : agePopulation;
            stack[8] = stack[newAge] ? (stack[8] + agePopulation) : agePopulation;
        }

        return stack;
    }, {});

    populationAges = { ...newPopulationAges };
});
```