Advent of Code: Day 7

1 minute read

Part 1

library(tidyverse)
library(igraph)

input <- "pbga (66)
xhth (57)
ebii (61)
havc (66)
ktlj (57)
fwft (72) -> ktlj, cntj, xhth
qoyq (66)
padx (45) -> pbga, havc, qoyq
tknk (41) -> ugml, padx, fwft
jptl (61)
ugml (68) -> gyxo, ebii, jptl
gyxo (61)
cntj (57)"

parse_input <- function(x) {
    strsplit(x, "\\n")[[1]] %>% 
        strsplit(" -> ") %>% 
        map_df(function(.x) {
            if (length(.x) == 1)
                tibble(ancestor = .x)
            else tibble(ancestor = .x[1],
                        descendant = .x[2])
        }) %>% 
        separate(ancestor, c("ancestor", "weight"), sep = " ") %>%
        mutate(weight = gsub("\\(|\\)", "", weight)) %>%
        mutate(descendant = strsplit(descendant, ", ")) %>%
        unnest() %>%
        select(ancestor, descendant, weight)
}

get_tree <- function(input) {
    tr_str <- parse_input(input)
    graph_from_data_frame(d = filter(tr_str, !is.na(descendant)))
}

get_root <- function(input) {
    g <- get_tree(input)
    deg <- degree(g, mode = "in")
    names(deg[deg == 0])
}

get_root(input)
## [1] "tknk"
puzzle_input <- readLines("advent-data/2017-12-07-advent-day7.txt") %>%
    paste(collapse = "\n")

get_root(puzzle_input)
## [1] "hmvwl"

Part 2

node_degrees <- function(g) {
    tibble(
        node = names(V(g)),
        depth = ego_size(g, vcount(g), mode = "out") - 1
    )
}

weights_from_node <- function(node, g, d) {
    sum(
        as.numeric(d[d$ancestor %in%
                     names(ego(g, vcount(g), node,
                               mode="out", mindist=0)[[1]]),
                     ]$weight)
    )
}

get_inbalance <- function(input) {
    g <- get_tree(input)
    d <- parse_input(input) %>%
        distinct(ancestor, weight) %>%
        mutate(weight = as.numeric(weight))
    
    nd <- node_degrees(g) %>%
        filter(depth > 0)

    weights <- set_names(map(nd$node, function(x) {
        nm <- names(ego(g, 1, x, mode = "out",
                        mindist = 1)[[1]])
        res <- map_dbl(nm, weights_from_node, g, d)
        names(res) <- nm
        res
        }), nd$node)
    inbalanced_node <- weights[map_lgl(weights, ~ n_distinct(.) ==  2L)]
    diff_weight <- diff(range(inbalanced_node[[1]]))    
    d$weight[d$ancestor == names(which.max(inbalanced_node[[1]]))] - diff_weight
    
}

get_inbalance(input)
## [1] 60
get_inbalance(puzzle_input)
## [1] 1853

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