2021 Day 18

— Day 18: Snailfish —

We need to help the snailfish with their math homework, but of course it is weird.

Add up all of the snailfish numbers from the homework assignment in the order they appear. What is the magnitude of the final sum?

library(tidyverse)

my_file <- here::here("2021", "data-2021-18.txt")
x <- readLines(my_file)

Loops! Loops and rules.

process <- function(w) {
    # explode and reduce
    
    # loop until we have no more operations
    while (TRUE) {
        depth = 0
        left = 0
        right = 0  
        op = ""

        # find the operation
        for (i in seq_len(nchar(w))) {
            cc = str_sub(w,i,i)
            if (cc == "[") {
                depth = depth + 1

            } else if (cc == "]") {
                depth = depth - 1
            } else if (cc == "0") { 
                # cc is a number, check for double digits
            } else if (cc == ",") {
                # comma, do nothing
            } else {
                # oops
                print("loop error")
            }
            
            if (depth == 5) {
                # skip out, operate on this
                break 
            }   
            
        }
        
        # no operation found, break out of the loop
        if (op == "") {
            break
        }
        
        # apply what we need to do
        

    }

    
    # return after we have finished
    return(w)
}

Create functions to deal with things, then add the numbers.

y = x[1]

for (z in 2:length(x)) {
    w = paste0("[", y, ",", x[z], "]")
    y = process(w)
}

And then we have to calculate the magnitude

# calculate the magnitude
# 3 * first element, 2 * second element

# [[1,2],[[3,4],5]]
# [7,[17,5]]
# [7,61]
# 143

# [[[[0,7],4],[[7,8],[6,0]]],[8,1]]
# [[[14,4],[37,18]],26]
# [[50,147], 26]
# [444,26]
# 1384

p = "\\[[0-9]+\\,[0-9]+\\]"
y = "[[[[6,6],[7,6]],[[7,7],[7,0]]],[[[7,7],[7,7]],[[7,8],[9,9]]]]"
    
while (str_detect(y, p)) {
    loc = str_locate(y, p)
    ext = str_extract(y, p)
    dig = as.integer(unlist(str_extract_all(ext, "[0-9]+")))
    sums = 3*dig[1] + 2*dig[2]
    str_sub(y,loc[1], loc[2]) <- sums
    # print(y)
}

as.integer(y)

[1] 4140