François Michonneau, PhD

1 minute read

I started to have cold and my foggy brain had a difficult time with today’s problem. I’m pretty sure the code of part 2 would eventually produce the right answer, but it’s very slow and the answer is a high numnber. Maybe I’ll revisit if I find the time at a later point. One one star for me today…

Part 1

raw_input <- "0: 3
1: 2
4: 4
6: 4"
input <- cat(raw_input, file = "advent-data/2017-12-13-advent-day13-test.txt")

library(tidyverse)


build_sequence <- function(amplitude, element) {
    stopifnot(amplitude >= 0)
    if (amplitude <= 1)
        return(amplitude)
    res <- rep(c(1:amplitude, (amplitude-1):2) ,
               max(1, ceiling(element / amplitude)))
    if (length(res) < element) browser()
    res[element]
}

scanner_position <- function(lst, l) {
    map_int(lengths(lst), ~ build_sequence(., l))
}

get_severity <- function(input) {
    input <- read_delim(input, delim = ": ",
                        col_names = c("layer", "depth")) %>%
        mutate_all(as.numeric)
    full_input <- tibble(
        layer = full_seq(input$layer, 1)
    )
    full_input <- left_join(full_input, input, by = "layer") %>%
        mutate(depth = replace(depth, is.na(depth), 0))

    firewall <- pmap(full_input, function(layer, depth) integer(depth))

    severity <- 0
    for (i in seq_along(firewall)) {
        scan_pos <- scanner_position(firewall, i)
        if (scan_pos[i] == 1) {
            severity <- severity + (i-1) * full_input$depth[full_input$layer == (i - 1)]
        }
    }

    severity
}

get_severity("advent-data/2017-12-13-advent-day13-test.txt")

## Parsed with column specification:
## cols(
##   layer = col_integer(),
##   depth = col_character()
## )

## [1] 24

get_severity("advent-data/2017-12-13-advent-day13.txt")

## Parsed with column specification:
## cols(
##   layer = col_integer(),
##   depth = col_character()
## )

## [1] 2604

Part 2

get_delay <- function(input) {
    input <- read_delim(input, delim = ": ",
                        col_names = c("layer", "depth")) %>%
        mutate_all(as.numeric)
    full_input <- tibble(
        layer = full_seq(input$layer, 1)
    )
    full_input <- left_join(full_input, input, by = "layer") %>%
        mutate(depth = replace(depth, is.na(depth), 0))

    firewall <- pmap(full_input, function(layer, depth) integer(depth))

    delay <- 27060
    severity <- 1

    while (severity != 0) {
        severity <- 0
        delay <- delay + 1
        for (i in seq_along(firewall)) {
            scan_pos <- scanner_position(firewall, i + delay)
            if (scan_pos[i] == 1) {
                severity <- severity + (i-1) *
                    full_input$depth[full_input$layer == (i - 1)]
            }
        }
        if (delay %% 1000 == 0)
            message("delay: ", delay, " -- severity: ", severity)
    }
    delay
}

get_delay("advent-data/2017-12-13-advent-day13-test.txt")
get_delay("advent-data/2017-12-13-advent-day13.txt")

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