Day 8

Advent of Code: Worked Solutions

Setup

Import libraries:

library(tidyverse)
library(unglue)

Read text input from file and parse into a dataframe of character operations and integer arguments:

input <- read_lines("../input/day08.txt") |> 
  unglue_data("{op} {arg}", convert = TRUE) |> 
  deframe()

Part 1

Define a function to loop through the instructions in the input, break whenever an instruction is visited for a second time, and return the accumulator value at that time:

find_loop <- function(instructions) {
  
  visited <- rep(FALSE, length(instructions))
  acc <- 0
  i <- 1
  
  while (i <= length(instructions)) {
    if (visited[i]) {
      return(acc)
    } else {
      visited[i] <- TRUE
    }
    
    op  <- names(instructions)[[i]]
    arg <- instructions[[i]]
    
    if (op == "nop") {
      i <- i + 1
    } else if (op == "acc") {
      acc <- acc + arg
      i <- i + 1
    } else if (op == "jmp") {
      i <- i + arg
    }
  }
  
  return(NA)
}

Run on puzzle input:

find_loop(input)

Part 2

Swap each jmp and nop operation one-by-one and see which swap results in a valid output (no loop):

instr_mod <- map(
  which(names(input) %in% c("jmp", "nop")),
  \(idx) {
    input |> 
      set_names(modify_at(
        names(input), 
        idx, 
        ~ case_match(.x, "jmp" ~ "nop", "nop" ~ "jmp")
      ))
  }
)

idx_mod <- instr_mod |> 
  map_dbl(find_loop) |> 
  detect_index(is.na)

Modify the find_loop function to run the instructions and return the final value of the accumulator:

run_instr <- function(instructions) {
  acc <- 0
  i <- 1
  
  while (i <= length(instructions)) {
    op  <- names(instructions)[[i]]
    arg <- instructions[[i]]
    
    if (op == "nop") {
      i <- i + 1
    } else if (op == "acc") {
      acc <- acc + arg
      i <- i + 1
    } else if (op == "jmp") {
      i <- i + arg
    }
  }
  
  return(acc)
}

Run on the valid modified index:

run_instr(instr_mod[[idx_mod]])