day03.Rmd

---
title: "--- Day 3: Toboggan Trajectory ---"
author: Fleur Kelpin
date: Dec 3, 2020
output: github_document
---

```{r message=FALSE, warning=FALSE}
library(tidyverse)
input <- read_lines("day03.txt")
```

# Part 1
Start by **counting all the trees** you would encounter for the slope right 3, down 1

```{r}
hasTreeOnRow <- function(rowNum, stepRight) {
  row <- input [[rowNum]]
  col <- ((stepRight * (rowNum - 1)) %% str_length(row)) + 1
  char <- str_sub(row, col, col)
  char == "#"
}
```
```{r}
seq(along.with = input) %>%
  sapply(function(rowNum) {
    hasTreeOnRow(rowNum, 3)
  }) %>%
  sum()
```

# Part 2
Determine the number of trees you would encounter if, for each of the following slopes, you start at the top-left corner and traverse the map all the way to the bottom:

- Right 1, down 1.
- Right 3, down 1. (This is the slope you already checked.)
- Right 5, down 1.
- Right 7, down 1.
- Right 1, down 2.

**What do you get if you multiply together the number of trees encountered on each of the listed slopes?**

```{r}
treesOnSlope <- function(stepRight, stepDown) {
  seq(from = 1, to = length(input), by = stepDown) %>%
    sapply(function(rowNum) {
      hasTreeOnRow(rowNum, stepRight / stepDown)
    }) %>%
    sum()
}
counts <- (c(
  treesOnSlope(1, 1),
  treesOnSlope(3, 1),
  treesOnSlope(5, 1),
  treesOnSlope(7, 1),
  treesOnSlope(1, 2)
))
prod(counts)
```