R/day06.R

In tjmahr/adventofcode21:

#' Day 06: Lanternfish
#'
#' [Lanternfish](https://adventofcode.com/2021/day/6)
#'
#' @name day06
#' @rdname day06
#' @details
#'
#' **Part One**
#'
#' The sea floor is getting steeper. Maybe the sleigh keys got carried this
#' way?
#'
#' A massive school of glowing
#' [lanternfish](https://en.wikipedia.org/wiki/Lanternfish) swims past.
#' They must spawn quickly to reach such large numbers - maybe
#' *exponentially* quickly? You should model their growth rate to be sure.
#'
#' Although you know nothing about this specific species of lanternfish,
#' you make some guesses about their attributes. Surely, [each lanternfish
#' creates a new lanternfish]{title="I heard you like lanternfish."} once
#' every *7* days.
#'
#' However, this process isn\'t necessarily synchronized between every
#' lanternfish - one lanternfish might have 2 days left until it creates
#' another lanternfish, while another might have 4. So, you can model each
#' fish as a single number that represents *the number of days until it
#' creates a new lanternfish*.
#'
#' Furthermore, you reason, a *new* lanternfish would surely need slightly
#' longer before it\'s capable of producing more lanternfish: two more days
#' for its first cycle.
#'
#' So, suppose you have a lanternfish with an internal timer value of `3`:
#'
#' -   After one day, its internal timer would become `2`.
#' -   After another day, its internal timer would become `1`.
#' -   After another day, its internal timer would become `0`.
#' -   After another day, its internal timer would reset to `6`, and it
#'     would create a *new* lanternfish with an internal timer of `8`.
#' -   After another day, the first lanternfish would have an internal
#'     timer of `5`, and the second lanternfish would have an internal
#'     timer of `7`.
#'
#' A lanternfish that creates a new fish resets its timer to `6`, *not `7`*
#' (because `0` is included as a valid timer value). The new lanternfish
#' starts with an internal timer of `8` and does not start counting down
#' until the next day.
#'
#' Realizing what you\'re trying to do, the submarine automatically
#' produces a list of the ages of several hundred nearby lanternfish (your
#' puzzle input). For example, suppose you were given the following list:
#'
#'     3,4,3,1,2
#'
#' This list means that the first fish has an internal timer of `3`, the
#' second fish has an internal timer of `4`, and so on until the fifth
#' fish, which has an internal timer of `2`. Simulating these fish over
#' several days would proceed as follows:
#'
#'     Initial state: 3,4,3,1,2
#'     After  1 day:  2,3,2,0,1
#'     After  2 days: 1,2,1,6,0,8
#'     After  3 days: 0,1,0,5,6,7,8
#'     After  4 days: 6,0,6,4,5,6,7,8,8
#'     After  5 days: 5,6,5,3,4,5,6,7,7,8
#'     After  6 days: 4,5,4,2,3,4,5,6,6,7
#'     After  7 days: 3,4,3,1,2,3,4,5,5,6
#'     After  8 days: 2,3,2,0,1,2,3,4,4,5
#'     After  9 days: 1,2,1,6,0,1,2,3,3,4,8
#'     After 10 days: 0,1,0,5,6,0,1,2,2,3,7,8
#'     After 11 days: 6,0,6,4,5,6,0,1,1,2,6,7,8,8,8
#'     After 12 days: 5,6,5,3,4,5,6,0,0,1,5,6,7,7,7,8,8
#'     After 13 days: 4,5,4,2,3,4,5,6,6,0,4,5,6,6,6,7,7,8,8
#'     After 14 days: 3,4,3,1,2,3,4,5,5,6,3,4,5,5,5,6,6,7,7,8
#'     After 15 days: 2,3,2,0,1,2,3,4,4,5,2,3,4,4,4,5,5,6,6,7
#'     After 16 days: 1,2,1,6,0,1,2,3,3,4,1,2,3,3,3,4,4,5,5,6,8
#'     After 17 days: 0,1,0,5,6,0,1,2,2,3,0,1,2,2,2,3,3,4,4,5,7,8
#'     After 18 days: 6,0,6,4,5,6,0,1,1,2,6,0,1,1,1,2,2,3,3,4,6,7,8,8,8,8
#'
#' Each day, a `0` becomes a `6` and adds a new `8` to the end of the list,
#' while each other number decreases by 1 if it was present at the start of
#' the day.
#'
#' In this example, after 18 days, there are a total of `26` fish. After 80
#' days, there would be a total of `5934`.
#'
#' Find a way to simulate lanternfish. *How many lanternfish would there be
#' after 80 days?*
#'
#' **Part Two**
#'
#' Suppose the lanternfish live forever and have unlimited food and space.
#' Would they take over the entire ocean?
#'
#' After 256 days in the example above, there would be a total of
#' `26984457539` lanternfish!
#'
#' *How many lanternfish would there be after 256 days?*
#'
#' @param x some data
#' @param n number of days to simulate
#' @return For Part One and Part Two, `f06a_step_n_days(x)` returns the
#'   frequency table for the day timers. `sum()` for the solution.
#' @export
#' @examples
#' f06a_step_n_days(example_data_06())
#' f06a_step_n_days(example_data_06(), n = 256)
f06a_step_n_days <- function(x, n = 80) {
  # strategy: manipulating counts
  counts <- x |> f06_read_and_count()
  counts <- f_loop_n(counts, n, f06_step_day)
  counts
}

f06_step_day <- function(x) {
  x_new <- rep(0, 9)
  x_new[1:8] <- x[2:9]
  # 0 -> 8
  x_new[9] <- x_new[9] + x[1]
  # 0-> 6
  x_new[7] <- x_new[7] + x[1]
  x_new
}

# Used for unit testing
f06_uncount <- function(x) {
  x_new <- f06_step_day(x)
  rep(0:8, times = x)
}

f06_read_and_count <- function(x) {
  count <- x |>
    strsplit(",") |>
    unlist() |>
    factor(0:8) |>
    table()
  x <- as.vector(count)
  x
}


#' @param example Which example data to use (by position or name). Defaults to
#'   1.
#' @rdname day06
#' @export
example_data_06 <- function(example = 1) {
  l <- list(
    a = c("3,4,3,1,2")
  )
  l[[example]]
}