day10.md

— Day 10: Adapter Array —

Fleur Kelpin Dec 10, 2020

library(tidyverse)
input <- readr::read_csv("day10.txt", col_names = FALSE)[[1]] %>%
  prepend(0) %>%
  {append(., max(.) + 3)} %>%
  sort()
  
as_tibble(input)

## # A tibble: 104 x 1
##    value
##    <dbl>
##  1     0
##  2     1
##  3     2
##  4     3
##  5     4
##  6     7
##  7    10
##  8    11
##  9    12
## 10    15
## # … with 94 more rows

Part 1

Find a chain that uses all of your adapters to connect the charging outlet to your device’s built-in adapter and count the joltage differences between the charging outlet, the adapters, and your device. What is the number of 1-jolt differences multiplied by the number of 3-jolt differences?

input %>%
  diff() %>%
  { sum(. == 1) * sum(. == 3) }

## [1] 2346

Part 2

What is the total number of distinct ways you can arrange the adapters to connect the charging outlet to your device?

Use dynamic programming to reuse the number of ways you can connect the lower jolt adapters to the outlet to compute the number of ways for the next adapter.

adapters <- tibble(jolt = input, ways=as.numeric(NA))
adapters[1, 'ways'] = 1

while((index <- detect_index(adapters$ways, is.na)) > 0 ){
  joltage <- adapters[index, 'jolt']
  prev_adapters <- filter(adapters, jolt %>% between(joltage-3, joltage-1))
  adapters[index, 'ways'] = sum(prev_adapters$ways)
}
adapters

## # A tibble: 104 x 2
##     jolt  ways
##    <dbl> <dbl>
##  1     0     1
##  2     1     1
##  3     2     2
##  4     3     4
##  5     4     7
##  6     7     7
##  7    10     7
##  8    11     7
##  9    12    14
## 10    15    14
## # … with 94 more rows

options(digits = 15)
max(adapters$ways)

## [1] 6044831973376