Day 2

Advent of Code: Worked Solutions

Setup

# Libraries
library(tidyverse)

# Read input from file
input <- read_lines("../input/day02.txt") |> 
  map(~parse_number(str_split_1(.x, " ")))

Part 1

# Compute difference between consecutive integers
seq_gaps <- function(seq)
  head(lead(seq) - seq, -1)

# Check whether the sequence is incr/decr with gaps between 1 and 3
gaps_are_valid <- function(gaps)
  (all(gaps < 0) | all(gaps > 0)) & all(between(abs(gaps), 1, 3))

# Count number of safe reports
input |> 
  map(seq_gaps) |> 
  map_lgl(gaps_are_valid) |> 
  sum()

[1] 306

Part 2

tibble(input) |> 
  
  # For each report, create a set of versions where each level is removed
  mutate(
    id = row_number(),
    mod = map(input, \(seq) map(1:length(seq), \(n) seq[-c(n)])),
  ) |> 
  unnest(mod) |> 
  
  # Check validity of each report and its altered versions
  mutate(
    report_is_safe = map_lgl(input, ~ gaps_are_valid(seq_gaps(.x))),
    mod_is_safe    = map_lgl(mod,   ~ gaps_are_valid(seq_gaps(.x))),
    is_safe = report_is_safe | mod_is_safe
  ) |> 
  summarize(is_safe = any(is_safe), .by = id) |> 
  
  # Count all safe reports
  summarize(total = sum(is_safe)) |> 
  pull()

[1] 366