advent-of-code/source/year_2020/day_03/mod.rs

//! Day 03 of 2020.

use crate::prelude::*;

/// Get the solution for day 03 of 2020.
pub fn solution() -> Solution {
  Solution::new(Day::new(3, 2020), part_1, part_2)
    .with_expected(198, 5140884672_i64)
}

/// Generic solver that takes in a horizontal and vertical movement for the
/// slope.
fn solve(data: &str, (horizontal, vertical): (usize, usize)) -> usize {
  let line_length = data.find('\n').unwrap();
  let mut result = 0;

  let mut x_position = 0;
  for (y_position, line) in data.lines().enumerate() {
    if y_position % vertical != 0 {
      continue;
    }

    if line.chars().nth(x_position) == Some('#') {
      result += 1;
    }

    x_position += horizontal;
    if x_position >= line_length {
      x_position -= line_length;
    }
  }

  result
}

/// The logic to solve part one.
fn part_1(input: &str) -> Result<String> {
  Ok(solve(input, (3, 1)).to_string())
}

/// The logic to solve part two.
fn part_2(input: &str) -> Result<String> {
  Ok(
    [(1, 1), (3, 1), (5, 1), (7, 1), (1, 2)]
      .into_iter()
      .map(|increment| solve(input, increment))
      .product::<usize>()
      .to_string(),
  )
}