advent_of_code_2025

commit d32cd88acd6b005a1c5cd24115b77e3f2ca36690
parent 73884c8c0620108bf6ceb73f153bcaf59eaa8309
Author: Eamon Caddigan <ec@eamoncaddigan.net>
Date:   Mon,  8 Dec 2025 13:34:58 -0800

Solve day 07, part 2

Diffstat:
M
src/day07.R
 | 
27
+++++++++++++++++++--------

1 file changed, 19 insertions(+), 8 deletions(-)
diff --git a/src/day07.R b/src/day07.R
@@ -39,36 +39,47 @@ rshift <- function(x, fill = vector(mode = class(x), length = 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
+  splits <- (manifold_row == "^") * beams_in
   list(
-    num_splits = sum(splits),
-    beams_out = manifold_row == "S" | manifold_row == "." & beams_in |
-      lshift(splits) | rshift(splits)
+    num_splits = sum(splits > 0),
+    beams_out = (manifold_row == "S") + ((manifold_row == ".") * beams_in) +
+      lshift(splits) + rshift(splits)
   )
 }
 
-count_splits <- function(manifold) {
+trace_beams <- function(manifold) {
   num_splits <- 0
-  beams <- rep(FALSE, ncol(manifold))
+  beams <- rep(0.0, 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
+  list(num_splits = num_splits, beams_out = beams)
 }
 
 part1 <- function(input_lines) {
-  count_splits(parse_input(input_lines))
+  trace_beams(parse_input(input_lines))$num_splits
 }
 
 test_part1 <- function() {
   stopifnot(part1(test_input) == 21)
 }
 
+part2 <- function(input_lines) {
+  sum(trace_beams(parse_input(input_lines))$beams_out)
+}
+
+test_part2 <- function() {
+  stopifnot(part2(test_input) == 40)
+}
+
 main <- function() {
+  # It's not remotely efficient to solve the puzzle twice, but it's fine
   test_part1()
   cat("Part 1 solution: ", part1(puzzle_input), "\n")
+  test_part2()
+  cat(sprintf("Part 2 solution: %0.0f\n", part2(puzzle_input)))
 }
 
 main()