2021 Day 6

— Day 6: Lanternfish —

There are some lanternfish swimming past, and we want to calculate their spawning rate for some reason

Find a way to simulate lanternfish. How many lanternfish would there be after 80 days?

library(tidyverse)

my_file <- here::here("2021", "data-2021-06.txt")
x <- readLines(my_file)

This is pretty straightforward

# split into separate fish
fishes = as.numeric(unlist(strsplit(x, split = ",")))

for (i in seq(80)) {
    # down one day
    fishes = fishes - 1

    # append to the end
    fishes = append(fishes, rep(8, sum(fishes < 0)))

    # replace -1 with 6
    fishes = replace(fishes, fishes < 0, 6)

}

# part one answer
length(fishes)

[1] 365862

— Part Two —

How many lanternfish would there be after 256 days?

# split into separate fish
fishes = as.numeric(unlist(strsplit(x, split = ",")))

# record days and count of fish
lantern = tibble(days = fishes) %>%
count(days)

# big loop for many respawns
for (k in seq(256)) {
    lantern$days = lantern$days - 1

    spawn = max(filter(lantern, days == -1)$n, 0)

    if (any(lantern$days == 6)) {
        lantern[lantern$days == 6, "n"] = lantern[lantern$days == 6, "n"] + spawn
    } else {
        lantern = rbind(lantern, c(6, spawn))
    }

    lantern = rbind(lantern, c(8, spawn))

    lantern = filter(lantern, days != -1)

}

# part two answer
format(sum(lantern$n), scientific=F)

[1] "1653250886439"