commit 73884c8c0620108bf6ceb73f153bcaf59eaa8309
parent 1ea5d8f5fc6c45d8830061bf700a9c9a79785550
Author: Eamon Caddigan <ec@eamoncaddigan.net>
Date: Sat, 6 Dec 2025 22:07:43 -0800
Solve day 07, part 1
Diffstat:
| A | src/day07.R | | | 74 | ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ |
1 file changed, 74 insertions(+), 0 deletions(-)
diff --git a/src/day07.R b/src/day07.R
@@ -0,0 +1,74 @@
+#!/usr/bin/env Rscript
+
+source("src/utils.R")
+
+test_input <- c(
+ ".......S.......",
+ "...............",
+ ".......^.......",
+ "...............",
+ "......^.^......",
+ "...............",
+ ".....^.^.^.....",
+ "...............",
+ "....^.^...^....",
+ "...............",
+ "...^.^...^.^...",
+ "...............",
+ "..^...^.....^..",
+ "...............",
+ ".^.^.^.^.^...^.",
+ "..............."
+)
+puzzle_input <- aoc_input(7)
+
+parse_input <- function(input_lines) {
+ input_lines |>
+ strsplit("", fixed = TRUE) |>
+ (\(x) do.call(rbind, x))()
+}
+
+lshift <- function(x, fill = vector(mode = class(x), length = 1)) {
+ c(x[2:length(x)], fill)
+}
+
+rshift <- function(x, fill = vector(mode = class(x), length = 1)) {
+ c(fill, x[1:(length(x) - 1)])
+}
+
+trace_beams_one_step <- function(manifold_row, beams_in) {
+ # This returns the number of splits as well as the updated beams in order to
+ # solve part 1
+ splits <- manifold_row == "^" & beams_in
+ list(
+ num_splits = sum(splits),
+ beams_out = manifold_row == "S" | manifold_row == "." & beams_in |
+ lshift(splits) | rshift(splits)
+ )
+}
+
+count_splits <- function(manifold) {
+ num_splits <- 0
+ beams <- rep(FALSE, ncol(manifold))
+ for (i in seq_len(nrow(manifold))) {
+ res <- trace_beams_one_step(manifold[i, ], beams)
+ num_splits <- num_splits + res$num_splits
+ beams <- res$beams_out
+ }
+ num_splits
+}
+
+part1 <- function(input_lines) {
+ count_splits(parse_input(input_lines))
+}
+
+test_part1 <- function() {
+ stopifnot(part1(test_input) == 21)
+}
+
+main <- function() {
+ test_part1()
+ cat("Part 1 solution: ", part1(puzzle_input), "\n")
+}
+
+main()
