Solving Advent of Code 2020-09 with R & Neo4j.

[Disclaimer] Obviously, this post contains a big spoiler about Advent of Code.

Instructions

We have a XMAS code, which:

• Start with 25 numbers

• Any following number should be the sum of one of the possible pairs from the previous 25

• In step 2, find the consecutive set of number that sums to the result of part 1, and add its min and max

Find the complete instructions at: https://adventofcode.com/2020/day/9.

R solution

Part one

# Reading the data
input <- read.delim(
  sep = " ",
  "2020-09-aoc.txt", 
  header = FALSE, 
  stringsAsFactors = FALSE
)

for (i in 26:length(input$V1)){
  # The 25 starting numbers
  to_get <- (i - 25):(i-1)
  
  # The 26th number should be between the 
  # sum of the two lowest number and the sum 
  # of the two highest
  lower_bond <- sum(
    head(
      sort(input$V1[to_get]),
      2
    )
  )
  
  higher_bond <- sum(
    tail(
      sort(
        input$V1[to_get]
      ), 
      2
    )
  )
  
  if (
    !dplyr:::between(input$V1[i], lower_bond, higher_bond)
  ){
    output1 <<- input$V1[i]
    print(output1)
    break
    
  }
}

## [1] 530627549

Part Two

for (i in 1:length(input$V1)){
  # We will try to find any sequence in 1:n, then 2:n, 
  # then 3:n, etc, that sums to output1
  selection <- input$V1[i:length(input$V1)]
  c_s <- cumsum(selection) == output1
  if (
    any(c_s)
  ){
    bounds <- selection[1:which(c_s)]
    
    print(
      sum(
        min(bounds), 
        max(bounds)
      )
    )
    break
  }
}

## [1] 77730285