Advent of Code day 10

aoc2023-day10.r

library(tidyverse)

d <- read_table("./inst/input10.txt", col_names = FALSE) |>
  separate_longer_position(X1, 1) |>
  mutate(dist = if_else(X1 == "S", 0, NA)) |>
  mutate(x = if_else(X1 == "S", "L", X1)) # eyeballing

NCOL <- sqrt(nrow(d))

top <- c("7", "|", "F")
bottom <- c("J", "|", "L")
left <- c("L", "-", "F")
right <- c("J", "-", "7")

follow_pipe <- function(dta, i){
  mutate(dta, dist = case_when(
    x == "|" & lag(dist, NCOL) == i & (lag(x, NCOL) %in% top) ~ i + 1,
    x == "|" & lead(dist, NCOL) == i & (lead(x, NCOL) %in% bottom) ~ i + 1,
    x == "-" & lag(dist, 1) == i & (lag(x, 1) %in% left) ~ i + 1,
    x == "-" & lead(dist, 1) == i & (lead(x, 1) %in% right) ~ i + 1,
    x == "F" & lead(dist, 1) == i & (lead(x, 1) %in% right) ~ i + 1,
    x == "F" & lead(dist, NCOL) == i & (lead(x, NCOL) %in% bottom) ~ i + 1,
    x == "7" & lag(dist, 1) == i & (lag(x, 1) %in% left) ~ i + 1,
    x == "7" & lead(dist, NCOL) == i & (lead(x, NCOL) %in% bottom) ~ i + 1,
    x == "J" & lag(dist, NCOL) == i & (lag(x, NCOL) %in% top) ~ i + 1,
    x == "J" & lag(dist, 1) == i & (lag(x, 1) %in% left) ~ i + 1,
    x == "L" & lag(dist, NCOL) == i & (lag(x, NCOL) %in% top) ~ i + 1,
    x == "L" & lead(dist, 1) == i & (lead(x, 1) %in% right) ~ i + 1,
    .default = dist))
}

measure_pipe <- function(x){
  x <- x |> filter(!is.na(dist))
  nrow(x) / 2 # we only get an even number when loop is closed
}

i <- 0 # for-loop is safer but slower
while (floor(measure_pipe(d)) != measure_pipe(d)) {
  d <- follow_pipe(d, i)
  i <- i + 1
}

# part 2 - a ray crossing will be inside at odd counts
p <- d |> mutate(pipe = if_else(!is.na(dist), x, NA)) |>
  pull(pipe) |> matrix(ncol = NCOL, byrow = TRUE)

inner_ray <- p |>
  # pain: the corners
  apply(2, \(x) cumsum(x %in% left)) |>
  apply(2, \(x) x %% 2 != 0)  # odd ray count

not_pipe <- p |> apply(2, is.na)

reduce(inner_ray * not_pipe, sum) # NOT matrix multiplication