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day_13.rs
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day_13.rs
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use common::{Answer, Solution};
pub struct Day13;
impl Solution for Day13 {
fn name(&self) -> &'static str {
"Point of Incidence"
}
fn part_a(&self, input: &str) -> Answer {
let valleys = parse(input);
solve(&valleys, 0).into()
}
fn part_b(&self, input: &str) -> Answer {
let valleys = parse(input);
solve(&valleys, 1).into()
}
}
fn solve(valleys: &[Valley], limit: usize) -> usize {
valleys
.iter()
.filter_map(|valley| {
valley
.horizontal_reflection(limit)
.map(|x| 100 * x)
.or_else(|| valley.vertical_reflection(limit))
})
.sum()
}
struct Valley {
tiles: Vec<Vec<char>>,
}
fn parse(input: &str) -> Vec<Valley> {
let mut out = Vec::new();
for set in input.split("\n\n") {
let tiles = set.lines().map(|line| line.chars().collect()).collect();
out.push(Valley { tiles });
}
out
}
impl Valley {
// Find a horizontal reflection in the valley.
// Horizontal reflections is from left to right.
fn horizontal_reflection(&self, error: usize) -> Option<usize> {
for mid in 1..=self.tiles.len() - 1 {
let side_len = mid.min(self.tiles.len() - mid);
let start = mid - side_len;
let mut diff = 0;
for a in start..mid {
let b = mid * 2 - a - 1;
diff += (0..self.tiles[a].len())
.filter(|&i| self.tiles[a][i] != self.tiles[b][i])
.count();
}
if diff == error {
return Some(mid);
}
}
None
}
fn vertical_reflection(&self, error: usize) -> Option<usize> {
for mid in 1..=self.tiles[0].len() - 1 {
let side_len = mid.min(self.tiles[0].len() - mid);
let start = mid - side_len;
let mut diff = 0;
for a in start..mid {
let b = mid * 2 - a - 1;
diff += (0..self.tiles.len())
.filter(|&i| self.tiles[i][a] != self.tiles[i][b])
.count();
}
if diff == error {
return Some(mid);
}
}
None
}
}
#[cfg(test)]
mod test {
use common::Solution;
use indoc::indoc;
use super::Day13;
const CASE: &str = indoc! {"
#.##..##.
..#.##.#.
##......#
##......#
..#.##.#.
..##..##.
#.#.##.#.
#...##..#
#....#..#
..##..###
#####.##.
#####.##.
..##..###
#....#..#
"};
#[test]
fn part_a() {
assert_eq!(Day13.part_a(CASE), 405.into());
}
#[test]
fn part_b() {
assert_eq!(Day13.part_b(CASE), 400.into());
}
}