advent_of_code_2025

commit 2667cb9f1c74814e781af0a7dbf2a4693da555c4
parent aa551c12b65a4f1adad9a7b2a233f37efd52629e
Author: Eamon Caddigan <ec@eamoncaddigan.net>
Date:   Mon,  8 Dec 2025 15:24:46 -0800

Solve day 08, part 2

Diffstat:
M
src/day08.R
 | 
25
++++++++++++++++++++-----

1 file changed, 20 insertions(+), 5 deletions(-)
diff --git a/src/day08.R b/src/day08.R
@@ -38,7 +38,7 @@ order_distances <- function(distmat) {
   which(lower.tri(distmat), arr.ind = TRUE)[order(distmat), ]
 }
 
-greedy_connect <- function(coords, steps) {
+greedy_connect <- function(coords, max_steps = NULL) {
   # Circuit IDs for each junction box
   coord_circuits <- seq_len(nrow(coords))
   # Junction box (coordinate) IDs for each circuit
@@ -47,29 +47,31 @@ greedy_connect <- function(coords, steps) {
   pair_order <- coords |>
     dist() |>
     order_distances()
-  stopifnot(steps <= nrow(pair_order))
 
-  for (i in seq_len(steps)) {
+  max_steps <- min(max_steps, nrow(pair_order))
+  for (i in seq_len(max_steps)) {
     c1 <- coord_circuits[pair_order[i, 1]]
     c2 <- coord_circuits[pair_order[i, 2]]
     if (c1 < c2) {
       coord_circuits[circuit_coords[[c2]]] <- c1
       circuit_coords[[c1]] <- union(circuit_coords[[c1]], circuit_coords[[c2]])
       circuit_coords[c2] <- list(NULL)
+      if (length(circuit_coords[[c1]]) == nrow(coords)) break
     } else if (c1 > c2) {
       coord_circuits[circuit_coords[[c1]]] <- c2
       circuit_coords[[c2]] <- union(circuit_coords[[c1]], circuit_coords[[c2]])
       circuit_coords[c1] <- list(NULL)
+      if (length(circuit_coords[[c2]]) == nrow(coords)) break
     }
   }
 
-  circuit_coords
+  list(circuits = circuit_coords, last_pair = pair_order[i, ])
 }
 
 part1 <- function(input_lines, steps) {
   input_lines |>
     parse_input() |>
-    greedy_connect(steps) |>
+    (\(x) greedy_connect(x, steps)$circuits)() |>
     vapply(length, integer(1)) |>
     sort(decreasing = TRUE) |>
     (\(x) prod(x[1:3]))()
@@ -79,9 +81,22 @@ test_part1 <- function() {
   stopifnot(part1(test_input, 10) == 40)
 }
 
+part2 <- function(input_lines) {
+  coords <- parse_input(input_lines)
+  last_pair <- greedy_connect(coords)$last_pair
+
+  prod(coords[last_pair, ][, 1])
+}
+
+test_part2 <- function() {
+  stopifnot(part2(test_input) == 25272)
+}
+
 main <- function() {
   test_part1()
   cat("Part 1 solution: ", part1(puzzle_input, 1000), "\n")
+  test_part2()
+  cat("Part 2 solution: ", part2(puzzle_input), "\n")
 }
 
 main()