Solving Advent of Code 2020-03 with R

[Disclaimer] Obviously, this post contains a big spoiler about Advent of Code.

## Instructions

### Step 1

The input is a map (x, y), where

`.`

is a square,`#`

is a tree.We can move this way: 3 right (x + 3), 1 down (y - 1).

The y repeats itself indefinitely.

We start from the top left corner (coordinate x = nrow(map), and y = 1).

We move until we’ve reached x == 1, i.e by taking nrow(map) steps.

How many trees

`#`

have we met along the way?

### Step 2

Repeat this but modify the step schema.

Find the complete instructions at:https://adventofcode.com/2020/day/3.

## R solution

### Part one

```
library(purrr)
n_trees <- function(
by_x = 1,
by_y = 3
){
ipt <- read.delim( "2020-03-aoc.txt", header = FALSE, stringsAsFactors = FALSE)
# Building the path
# x need to go from 1 to nrow(ipt), 1 by 1
x <- seq(1, nrow(ipt), by = by_x)
# y needs to start at 1, and go by 3 steps until we have length(x) steps
y <- seq(1, by = by_y, length.out = length(x))
list(
x = x,
y = y
) %>%
pmap_dbl(~ {
# Which x step are we in?
row <- ipt[..1,]
# If the row isn't wide enough, expand it until it is
# There is probably a better way to do that but I'm not
# sure I want to spend 5 minutes trying stuff that will
# save me half a micro second
while(nchar(row) < ..2){
row <- paste0(row, row)
}
# Split the damn thing
row <- strsplit(row, "")[[1]]
# look for the y, is it a tree?
if (row[..2] == ".") return(0)
if (row[..2] == "#") return(1)
}
) %>% sum()
}
n_trees()
## [1] 278
```

### Part two

```
data.frame(
x = c(1, 1, 1, 1, 2),
y = c(1, 3, 5, 7, 1)
) %>%
pmap_dbl(~ n_trees(..1, ..2)) %>%
reduce(`*`)
## [1] 9709761600
```

## Discussion (0)