day08.md

— Day 8: Handheld Halting —

Fleur Kelpin Dec 8, 2020

library(tidyverse)
input <- readr::read_lines("day08.txt") %>%
  tibble(instruction=.)
input

## # A tibble: 617 x 1
##    instruction
##    <chr>      
##  1 acc -5     
##  2 nop +333   
##  3 acc +45    
##  4 jmp +288   
##  5 acc -9     
##  6 jmp +1     
##  7 acc +27    
##  8 jmp +464   
##  9 acc +34    
## 10 jmp +478   
## # … with 607 more rows

Part 1

First let’s parse the instructions.

program <- input %>%
  extract(instruction,
          into=c("operation", "argument"),
          regex="(\\w+) ([+-]\\d+)",
          convert = TRUE)
program

## # A tibble: 617 x 2
##    operation argument
##    <chr>        <int>
##  1 acc             -5
##  2 nop            333
##  3 acc             45
##  4 jmp            288
##  5 acc             -9
##  6 jmp              1
##  7 acc             27
##  8 jmp            464
##  9 acc             34
## 10 jmp            478
## # … with 607 more rows

Then implement the instructions

state <- list(acc=0, ip=0)
nop <- function(acc, ip) {
  list(acc = acc, ip = ip + 1)
}
acc <- function(arg, acc, ip) {
  list(acc = acc + arg, ip = ip + 1)
}
jmp <- function(arg, acc, ip) {
  list(acc = acc, ip = ip + arg)
}

step <- function(program, s) {
  op <- program$operation[[s$ip + 1]]
  arg <- program$argument[[s$ip + 1]]
  switch(op,
         "nop" = nop(s$acc, s$ip),
         "acc" = acc(arg, s$acc, s$ip),
         "jmp" = jmp(arg, s$acc, s$ip)
         )
}
step(program, state)

## $acc
## [1] -5
## 
## $ip
## [1] 1

Then we’re ready to solve part 1:

findLoop <- function(program) {
  state <- list(acc=0, ip=0)
  visited <- c()
  while(!state$ip %in% visited) {
    visited <- append(visited, state$ip)
    if (state$ip > nrow(program)-1) {
      print("Terminated!")
      print(state)
      return(state)
    }
    state <- step(program, state)
  }
  state
}
findLoop(program)

## $acc
## [1] 1709
## 
## $ip
## [1] 439

Part 2

fix <- function (program, i) {
  op <- program$operation[[i]]
  fixed <- program
  if (op == 'jmp'){
    fixed$operation[[i]] = 'nop'
  }
  if (op == 'nop'){
    fixed$operation[[i]] = 'jmp'
  }
  fixed
}

for(i in 1:nrow(program)) {
  program %>%
    fix(i) %>%
    findLoop
}

## [1] "Terminated!"
## $acc
## [1] 1976
## 
## $ip
## [1] 617