Day 8: Resonant Collinearity

library(tidyverse)
library(stringr)
library(tictoc)

dat <- read_lines("aoc08.txt")
mapmat <- str_split_fixed(dat, "", n = str_length(dat[1]))

dim(mapmat)

[1] 50 50

Part 1

Save the antinodes (not only do we have a while loop in this one but also a global variable; just need a goto statement):

antinodes <- matrix(0, nrow = nrow(mapmat), ncol = nrow(mapmat))

addnode <- function(r, c) {
  if (between(r, 1, nrow(antinodes)) && between(c, 1, ncol(antinodes))) {
    antinodes[r, c] <<- antinodes[r, c] + 1
  }
}

Frequencies:

freqs <- mapmat |> as.vector() |> unique() |> setdiff(".") |> sort()
freqs

 [1] "0" "1" "2" "3" "4" "5" "6" "7" "8" "9" "a" "A" "d" "D" "e" "E" "g"
[18] "G" "h" "H" "i" "I" "k" "K" "l" "L" "m" "M" "n" "N" "o" "O" "q" "Q"
[35] "r" "R" "s" "S" "t" "T" "u" "U" "v" "V" "w" "W" "x" "X" "y" "Y" "z"
[52] "Z"

find_antennas <- function(the_map, the_freq) {
  which(the_map == the_freq, arr.ind = TRUE)
}

find_antinodes <- function(p1, p2) {
  dist_row <- abs(p1[1] - p2[1])
  points <- bind_rows(p1, p2) |>
    as.data.frame() |>
    arrange(row)
  mod <- lm(col ~ row, data = points) # LEL
  stopifnot(!(mod |> coef() |> is.na() |> any()))

  min_row <- points[1,]$row - dist_row
  max_row <- points[2,]$row + dist_row
  min_col <- predict(mod, newdat = data.frame(row = min_row))
  max_col <- predict(mod, newdat = data.frame(row = max_row))
  
  addnode(min_row, min_col |> round())
  addnode(max_row, max_col |> round())
}

scan_pairs <- function(the_map, the_freq) {
  locations <- find_antennas(the_map, the_freq)
  pair_i    <- combn(1:nrow(locations), 2)
  
  for (col in 1:ncol(pair_i)) {
    first_r  <- pair_i[1, col]
    second_r <- pair_i[2, col]
    
    find_antinodes(locations[first_r,],
                   locations[second_r,])
  }
}

for (f in freqs) {
  scan_pairs(mapmat, f)
}

sum(antinodes > 0)

[1] 327

Part 2

I deleted this, started again with an even easier edit, and ran it again and it worked.

Reset the antinodes:

antinodes <- matrix(0, nrow = nrow(mapmat), ncol = nrow(mapmat))

Mild tweak to scan across all the rows.

find_antinodes <- function(p1, p2) {
  points <- bind_rows(p1, p2) |>
    as.data.frame() |>
    arrange(row)
  mod <- lm(col ~ row, data = points) # LEL
  stopifnot(!(mod |> coef() |> is.na() |> any()))
  
  for (r in 1:nrow(mapmat)) {
    c <- predict(mod, newdat = data.frame(row = r))
    if (abs(c - round(c)) < 1e-6)
      addnode(r, c |> round())
  }
}

tic()
for (f in freqs) {
  scan_pairs(mapmat, f)
}
toc()

6.08 sec elapsed

sum(antinodes > 0)

[1] 1233