Day 9: All in a Single Night

Reference

library(tidyverse)

input <- read_lines(file = "inputs/2015/09.txt")

Part One

Solution

distances <- tibble(original = input) |> 
    separate_wider_delim(original,
                         delim = " = ",
                         names = c("trip", "distance")) |> 
    separate_wider_delim(trip,
                         delim = " to ",
                         names = c("from", "to")) |> 
    mutate(distance = as.integer(distance))

locations <- distances |>
    select(from, to) |> 
    unlist() |> 
    unique()

distance <- function(.from, .to) {
    if (.from == .to) {
        return(0)
    }
    
    tentative <- distances |> filter(from == .from) |> 
        filter(to == .to) |>  pull(distance)
    
    if (length(tentative > 0)) {
        return(tentative)
    } else {
        distances |> filter(from == .to, to == .from) |>  pull(distance)
    }
}

a <- matrix(ncol = 8, nrow = 8, dimnames = list(locations, locations))

for (location1 in locations) {
    for (location2 in locations) {
        a[location1, location2] <- distance(location1, location2)
    }
}

travels <- gtools::permutations(n = 8, r = 8, locations) |> 
    as_tibble() |> 
    mutate(total_distance = pmap_int(.l = list(V1, V2, V3, V4, V5, V6, V7, V8),
                                     .f = \(V1, V2, V3, V4, V5, V6, V7, V8)
                                     a[V1, V2] + a[V2, V3] + a[V3, V4] +
                                         a[V4, V5] + a[V5, V6] + 
                                         a[V6, V7] + a[V7, V8])) |> 
    arrange(total_distance)

travels |> 
    slice(1) |> 
    pull(total_distance) |> 
    cat()
## 141

Data

Part Two

Solution

travels |> 
    arrange(desc(total_distance)) |> 
    slice(1) |> 
    pull(total_distance) |> 
    cat()
## 736

Data