day08.Rmd

---
title: "--- Day 8: Handheld Halting ---"
author: Fleur Kelpin
date: Dec 8, 2020
output: github_document
---
```{r message=FALSE, warning=FALSE}
library(tidyverse)
input <- readr::read_lines("day08.txt") %>%
  tibble(instruction=.)
input
```
# Part 1

First let's parse the instructions.
```{r}
program <- input %>%
  extract(instruction,
          into=c("operation", "argument"),
          regex="(\\w+) ([+-]\\d+)",
          convert = TRUE)
program
```
Then implement the instructions
```{r}
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)
```
Then we're ready to solve part 1:
```{r}
findLoop <- function(program) {
  state <- list(acc=0, ip=0)
  visited <- c()
  while(!state$ip %in% visited) {
    visited <- append(visited, state$ip)
    if (state$ip > nrow(program)-1) {
      print("Terminated!")
      print(state)
      return(state)
    }
    state <- step(program, state)
  }
  state
}
findLoop(program)
```
# Part 2
```{r error=FALSE}
fix <- function (program, i) {
  op <- program$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
}
```