Day 5

Advent of Code: Worked Solutions

Setup

# Libraries
library(tidyverse)
library(unglue)
library(sets)

# Read input from file
input <- read_lines("../input/day05.txt", skip_empty_rows = FALSE)

Part 1

Parse input into sets of separate vector inputs and maps:

seeds <- head(input, 1) |> 
  str_remove("seeds: ") |> 
  str_split_1(" ") |> 
  as.numeric()

maps <- tail(input, -2) |> 
  enframe(name = NULL, value = "txt") |> 
  mutate(
    map_id = cumsum(txt == ""),
    map_name = case_when(str_detect(txt, ":") ~ txt)
  ) |> 
  fill(map_name, .direction = "down") |> 
  unglue_unnest(map_name, "{src_cat}-to-{dst_cat} map:") |> 
  filter(txt != "" & !str_detect(txt, ":")) |> 
  unglue_unnest(txt, "{dst_start} {src_start} {range_len}", convert = TRUE) |> 
  mutate(
    src_end = src_start + range_len - 1,
    dst_end = dst_start + range_len - 1,
    diff = dst_start - src_start
  ) |> 
  select(map_id, src_cat, dst_cat, src_start, src_end, dst_start, dst_end, diff) |> 
  group_split(map_id, .keep = FALSE)

Convert seed maps to interval notation:

# Add rows for the interval complement (neg infinity to infinity) to each map
maps_interval <- maps |> 
  map(\(df) {
    complement <- df |> 
      pmap(\(src_start, src_end, ...) {
        interval(src_start, src_end, domain = 'Z')
      }) |> 
      interval_union() |> 
      interval_complement() |> 
      as.list() |> 
      map_dfr(~ tibble(src_start = min(.x), src_end = max(.x))) |> 
      mutate(
        dst_start = src_start,
        dst_end = src_end,
        diff = 0
      )
    
    df |> 
      select(-ends_with("_cat")) |> 
      bind_rows(complement)
  }) 


# Join each map to one another in sequence to create one overall src/dest map
main_map <- maps_interval |> 
  reduce(
    ~ cross_join(.x, .y) |> 
      mutate(
        overlap_start = pmax(dst_start.x, src_start.y),
        overlap_end   = pmin(dst_end.x,   src_end.y),
        is_overlap    = overlap_start <= overlap_end
      ) |> 
      filter(is_overlap) |> 
      transmute(
        src_start = overlap_start - diff.x,
        src_end   = overlap_end   - diff.x,
        dst_start = overlap_start + diff.y,
        dst_end   = overlap_end   + diff.y,
        diff      = diff.x + diff.y
      )
  )

Find final location of all seeds, then take the minimum:

main_map |> 
  cross_join(tibble(seed = seeds)) |> 
  filter(seed >= src_start & seed <= src_end) |> 
  mutate(loc = seed + diff) |> 
  pull(loc) |> 
  min()

[1] 289863851

Part 2

Convert seed inputs to intervals:

seeds_interval <- tibble(seed = seeds) |> 
  mutate(
    type = case_match(
      row_number() %% 2, 
      1 ~ "seed_start", 
      0 ~ "range_len"
    ),
    pair_id = ceiling(row_number() / 2)
  ) |> 
  pivot_wider(names_from = type, values_from = seed) |> 
  transmute(seed_start, seed_end = seed_start + range_len - 1)

Join the seed intervals onto the mapper, filter to the overlap, and take the min result:

main_map |> 
  cross_join(seeds_interval) |> 
  mutate(
    overlap_start = pmax(seed_start, src_start),
    overlap_end   = pmin(seed_end,   src_end),
    is_overlap    = overlap_start <= overlap_end
  ) |> 
  filter(is_overlap) |> 
  mutate(loc_start = overlap_start + diff) |> 
  pull(loc_start) |> 
  min()

[1] 60568880