AoC Day 6 Part 2

export const calculateLantarns = (input, iterations) => {
    if (!input) {
        return []
    }

    const reSpawnRate = 7;
    const newSpawnRate = 9;
    const fishes = input.split(',').map(num => parseInt(num));

    const knownFish = fishes.length; // start length

    // Create array with days from current
    const dayBuffer = Array.from({length: iterations + 1}, () => 0)

    // Count how often a known fish is spawn and add to buffer
    fishes.forEach((fish) => {

        // amount of spawns of all remaining days
        const spawns = Math.floor((iterations - fish - 1) / reSpawnRate)
        for (let i = 0; i <= spawns; i++) {
            const spawnDay = fish + (reSpawnRate * i) + 1
            dayBuffer[spawnDay]++;
        }

    })

    // Increase the daily buffer with new fish
    // see per day how many fish will be added in remaining days
    for (let day = 0; day < iterations; day++) {
        const newFishesToday = dayBuffer[day]

        if (dayBuffer[day] > 0) {

            const spawns = Math.floor((iterations - day - newSpawnRate) / reSpawnRate)

            for (let i = 0; i <= spawns; i++) {

                const spawnDay = day + newSpawnRate + (reSpawnRate * i)
                dayBuffer[spawnDay] += newFishesToday;
            }
        }
    }
    // Sum of dayBuffer (amount of fish that are spawned each day)
    const spawnedFish = dayBuffer.reduce((accumulative, current) => accumulative + current);
    return knownFish + spawnedFish
}