79 lines
2.6 KiB
Rust
79 lines
2.6 KiB
Rust
use crate::util::maps::*;
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const CHEAT_DURATION: u64 = 20;
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pub fn day20() {
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let input = World::from_string(&crate::input(20));
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let maze = input.map(|&c| c == '#');
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let (ex, ey, _) = input.enumerate().find(|(_, _, &c)| c == 'E').unwrap();
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let costs = path_costs(&maze, ex, ey);
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let free_spaces: Vec<(usize, usize)> = maze
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.enumerate()
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.filter_map(|(x, y, &b)| if !b { Some((x, y)) } else { None })
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.collect();
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let all_cheats: Vec<u64> = free_spaces
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.iter()
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.flat_map(|&(ax, ay)| {
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free_spaces
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.iter()
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.filter_map(|&(bx, by)| {
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if ax == bx && ay == by {
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return None;
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}
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let taxi_cost = taxicab_distance(ax, ay, bx, by) as u64;
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if taxi_cost > CHEAT_DURATION {
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return None;
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}
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let a_cost = costs.get(ax, ay).unwrap();
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let b_cost = costs.get(bx, by).unwrap();
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// a cheat's timesave is the cost of a - the cost of b, minus the time it takes to do the cheat
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// and we return None for cheats that have negative timesave (why would you cheat to do worse?)
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a_cost.checked_sub(b_cost)?.checked_sub(taxi_cost)
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})
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.collect::<Vec<u64>>()
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})
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.collect();
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let good_cheats = all_cheats
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.iter()
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.filter(|&&advantage| advantage >= 100)
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.count();
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println!("{good_cheats}");
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}
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fn path_costs(world: &World<bool>, x: usize, y: usize) -> World<u64> {
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fn calculate_path_costs(
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world: &World<bool>,
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x: usize,
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y: usize,
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cost_so_far: u64,
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costs: &mut World<u64>,
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) {
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let adjacents = world
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.adjacent_locations(x, y)
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.into_iter()
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.filter(|&(nx, ny)| !world.get(nx, ny).unwrap());
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for (nx, ny) in adjacents {
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let cost = cost_so_far + 1;
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let old_cost = costs.get(nx, ny).unwrap();
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if cost < old_cost {
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costs.set(nx, ny, cost);
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calculate_path_costs(world, nx, ny, cost, costs)
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}
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}
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}
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let mut costs = World::from_dimensions(world.width(), world.height(), u64::MAX);
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costs.set(x, y, 0);
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calculate_path_costs(world, x, y, 0, &mut costs);
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costs
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}
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fn taxicab_distance(ax: usize, ay: usize, bx: usize, by: usize) -> usize {
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let dx = if ax < bx { bx - ax } else { ax - bx };
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let dy = if ay < by { by - ay } else { ay - by };
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dx + dy
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}
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