Advent of Code: Day 8

• Problem

When I first read the puzzle, I thought it would be a good way to deepen my knowledge of rlang and tidy evaluation. The variables kand the operations are going to be storred as strings and need to be evaluated. Because of the nature of the problem, I couldn’t see a good way to vectorize the computation and resorted to using purrrlyr::by_row. Because the computations that happen for each row are contained in their own environment, I used the <<- operator.

Parts 1 and 2

library(tidyverse)
library(purrrlyr)

raw_input <- "b inc 5 if a > 1
a inc 1 if b < 5
c dec -10 if a >= 1
c inc -20 if c == 10"

input <- read.table(text = raw_input, stringsAsFactors = FALSE)
names(input) <- c("var", "dir", "amount", "if", "cond_var", "cond_test",
                  "cond_val")
input <- as_tibble(input)

read_input <- function(f) {
    out <- read.table(f, stringsAsFactors = FALSE)
    names(out) <-  c("var", "dir", "amount", "if", "cond_var", "cond_test",
                     "cond_val")
    as_tibble(out)
}

process_registers <- function(input) {

    ## create placeholder for all possible variables
    rgstr <- unique(input$cond_var)
    rgstr <- setNames(vector("integer", length(rgstr)),
                      rgstr)

    ## keep track of the max of the max for part 2
    total_max <- 0
    
     input %>%
         by_row(function(d) {
            assign(d$cond_var, rgstr[[d$cond_var]])
            test_res <- eval_tidy(sym(d$cond_test))(eval_tidy(sym(d$cond_var)), d$cond_val)
            f <- switch(d$dir, inc = `+`, dec = `-`)
            
            if (test_res) {
                rgstr[[d$var]] <<- f(rgstr[[d$var]], d$amount)
            }
            total_max <<- max(max(rgstr), total_max)
         })

    list(max_final = max(rgstr), max_seen = total_max)

}

puzzle_input <- read_input("advent-data/2017-12-08-advent-day8.txt")

process_registers(input)

## $max_final
## [1] 1
## 
## $max_seen
## [1] 10

process_registers(puzzle_input)

## $max_final
## [1] 7787
## 
## $max_seen
## [1] 8997