2021: day09: part 2
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@ -1,3 +1,4 @@
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use std::collections::HashSet;
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use std::fmt::Write;
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use std::fmt::Write;
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use anyhow::{Context, Result};
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use anyhow::{Context, Result};
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@ -8,6 +9,7 @@ pub fn run() -> Result<String> {
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let mut res = String::with_capacity(128);
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let mut res = String::with_capacity(128);
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writeln!(res, "part 1: {}", part1(INPUT)?)?;
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writeln!(res, "part 1: {}", part1(INPUT)?)?;
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writeln!(res, "part 2: {}", part2(INPUT)?)?;
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Ok(res)
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Ok(res)
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}
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}
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@ -21,6 +23,25 @@ fn part1(input: &str) -> Result<u64> {
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.sum())
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.sum())
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}
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}
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fn part2(input: &str) -> Result<u64> {
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let mut height_map: HeightMap = input.parse()?;
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let low_points: Vec<_> = height_map.low_points().collect();
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let mut bassin_sizes: Vec<_> = low_points
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.iter()
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.map(|&(x, y)| height_map.fill_basin(x, y))
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.collect();
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bassin_sizes.sort_unstable();
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bassin_sizes
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.iter()
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.copied()
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.skip(bassin_sizes.len() - 3)
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.reduce(|acc, elem| acc * elem)
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.context("couldn't find 3 bassins")
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}
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#[derive(Clone, Copy)]
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#[derive(Clone, Copy)]
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enum Neighbour {
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enum Neighbour {
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Up,
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Up,
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@ -53,9 +74,28 @@ struct HeightMap {
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heights: Vec<u8>,
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heights: Vec<u8>,
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width: usize,
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width: usize,
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height: usize,
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height: usize,
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filled_points: HashSet<(usize, usize)>,
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}
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}
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impl HeightMap {
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impl HeightMap {
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fn fill_basin(&mut self, x: usize, y: usize) -> u64 {
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if self.get(x, y) == 9 {
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return 0;
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}
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if self.filled_points.contains(&(x, y)) {
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return 0;
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}
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self.filled_points.insert((x, y));
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let neighbours: Vec<_> = self.neighbours(x, y).collect();
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neighbours
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.iter()
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.map(|&(nx, ny)| self.fill_basin(nx, ny))
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.sum::<u64>()
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+ 1
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}
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fn low_points(&self) -> impl Iterator<Item = (usize, usize)> + '_ {
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fn low_points(&self) -> impl Iterator<Item = (usize, usize)> + '_ {
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(0..self.height)
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(0..self.height)
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.flat_map(|y| (0..self.width).map(move |x| (x, y)))
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.flat_map(|y| (0..self.width).map(move |x| (x, y)))
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@ -111,6 +151,7 @@ impl std::str::FromStr for HeightMap {
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heights,
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heights,
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width: width.context("0 lines parsed, width never computed")?,
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width: width.context("0 lines parsed, width never computed")?,
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height,
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height,
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filled_points: HashSet::new(),
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})
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})
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}
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}
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}
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}
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@ -130,4 +171,14 @@ mod tests {
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fn part1_real() {
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fn part1_real() {
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assert_eq!(part1(INPUT).unwrap(), 522);
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assert_eq!(part1(INPUT).unwrap(), 522);
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}
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}
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#[test]
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fn part2_provided() {
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assert_eq!(part2(PROVIDED).unwrap(), 1134);
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}
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#[test]
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fn part2_real() {
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assert_eq!(part2(INPUT).unwrap(), 916688);
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}
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}
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}
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