commit 98b333587165f6f2b97b5d74e40ed9d36132d43c
parent ce1530a4675d326b66cadabe566364326c963740
Author: Eamon Caddigan <ec@eamoncaddigan.net>
Date: Thu, 4 Dec 2025 12:40:00 -0800
Solve day 03, part 1
Diffstat:
| A | src/day04.R | | | 64 | ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ |
1 file changed, 64 insertions(+), 0 deletions(-)
diff --git a/src/day04.R b/src/day04.R
@@ -0,0 +1,64 @@
+#!/usr/bin/env Rscript
+
+source("src/utils.R")
+
+test_input <- c(
+ "..@@.@@@@.",
+ "@@@.@.@.@@",
+ "@@@@@.@.@@",
+ "@.@@@@..@.",
+ "@@.@@@@.@@",
+ ".@@@@@@@.@",
+ ".@.@.@.@@@",
+ "@.@@@.@@@@",
+ ".@@@@@@@@.",
+ "@.@.@@@.@."
+)
+puzzle_input <- aoc_input(4)
+
+parse_input <- function(input_lines) {
+ input_lines |>
+ strsplit("", fixed = TRUE) |>
+ (\(x) matrix(unlist(x), nrow = length(x), byrow = TRUE))()
+}
+
+count_neighbors <- function(diagram) {
+ nc <- ncol(diagram)
+ nr <- nrow(diagram)
+ neighbors <- matrix(0, nrow = nr, ncol = nc)
+ is_occupied <- ifelse(diagram == "@", 1, 0)
+
+ # shift = -1: 1 to nr-1 |-> 2 to nr-0
+ # shift = +1: 2 to nr-0 |-> 1 to nr-1
+ # shift = 0: 1 to nr-0 |-> 1 to nr-0
+ for (r_shift in (-1:1)) {
+ for (c_shift in (-1:1)) {
+ if (r_shift != 0 || c_shift != 0) {
+ neighbors[(1 + max(0, r_shift)):(nr + min(0, r_shift)), (1 + max(0, c_shift)):(nc + min(0, c_shift))] <-
+ neighbors[(1 + max(0, r_shift)):(nr + min(0, r_shift)), (1 + max(0, c_shift)):(nc + min(0, c_shift))] +
+ is_occupied[(1 - min(0, r_shift)):(nr - max(0, r_shift)), (1 - min(0, c_shift)):(nc - max(0, c_shift))]
+ }
+ }
+ }
+ neighbors
+}
+
+part1 <- function(input_lines) {
+ diagram <- parse_input(input_lines)
+ neighbors <- count_neighbors(diagram)
+ is_movable <- (diagram == "@") & (neighbors < 4)
+ sum(is_movable)
+}
+
+test_part1 <- function() {
+ stopifnot(part1(test_input) == 13)
+}
+
+main <- function() {
+ test_part1()
+ cat("Part 1 solution: ", part1(puzzle_input), "\n")
+ # test_part2()
+ # cat("Part 2 solution: ", sprintf("%0.0f", part2(puzzle_input)), "\n")
+}
+
+main()
