advent_of_code_2025

commit 180e6d60336ea92ff5c3c0b9835723e1125ee511
parent 1f053c4aebdebb8ac7a82bce5a9be77b9dda3d8e
Author: Eamon Caddigan <ec@eamoncaddigan.net>
Date:   Mon,  1 Dec 2025 16:19:39 -0800

Solve day 01, parts 1 and 2

I meant to commit the part 1 solution separately.

Diffstat:
A
src/day01.R
 | 
72
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
A
src/utils.R
 | 
12
++++++++++++

2 files changed, 84 insertions(+), 0 deletions(-)
diff --git a/src/day01.R b/src/day01.R
@@ -0,0 +1,72 @@
+source("src/utils.R")
+
+apply_turn <- function(position_and_zero_crossings, turn_instruction) {
+  turn_size <- as.integer(substr(turn_instruction, 2, nchar(turn_instruction)))
+  old_pos <- position_and_zero_crossings[1]
+
+  if (substr(turn_instruction, 1, 1) == "L") {
+    new_pos <- old_pos - turn_size
+    new_zero_crossings <- abs(new_pos %/% 100) +
+      (new_pos %% 100 == 0) - (old_pos %% 100 == 0)
+  } else {
+    new_pos <- old_pos + turn_size
+    new_zero_crossings <- new_pos %/% 100
+  }
+
+  c(new_pos %% 100, position_and_zero_crossings[2] + new_zero_crossings)
+}
+
+count_zeros <- function(x) {
+  sum(vapply(x, \(x) (x[1] %% 100) == 0, logical(1)))
+}
+
+part1 <- function(input_lines) {
+  Reduce(apply_turn, input_lines, c(50, 0), accumulate = TRUE) |>
+    count_zeros()
+}
+
+test_part1 <- function() {
+  part1_test_input <- "L68
+L30
+R48
+L5
+R60
+L55
+L1
+L99
+R14
+L82"
+  stopifnot(
+    part1(strsplit(part1_test_input, "\n")[[1]]) == 3
+  )
+}
+
+part2 <- function(input_lines) {
+  Reduce(apply_turn, input_lines, c(50, 0), accumulate = FALSE)[2]
+}
+
+test_part2 <- function() {
+  part2_test_input <- "L68
+L30
+R48
+L5
+R60
+L55
+L1
+L99
+R14
+L82"
+  stopifnot(
+    part2(strsplit(part2_test_input, "\n")[[1]]) == 6
+  )
+}
+
+main <- function() {
+  puzzle_input <- aoc_input(1)
+  test_part1()
+  cat("Part 1 solution:", part1(puzzle_input), "\n")
+  test_part2()
+  cat("Part 2 solution:", part2(puzzle_input), "\n")
+}
+
+main()
diff --git a/src/utils.R b/src/utils.R
@@ -0,0 +1,12 @@
+aoc_input <- function(day, year = 2025) {
+  if (!file.exists("~/.aoc")) {
+    stop("AoC session ID must be present in ~/.aoc")
+  }
+  con <- url(
+    sprintf("https://adventofcode.com/%d/day/%d/input", year, day),
+    headers = c("Cookie" = paste0("session=", readLines("~/.aoc")))
+  )
+  puzzle_input <- readLines(con)
+  close(con)
+  puzzle_input
+}