Document day 21

src/year2023/day21.rs

@@ -1,3 +1,90 @@
+//! # Step Counter
+//!
+//! This solution uses a geometric approach. Looking at the input data reveals several crucial
+//! insights:
+//!
+//! * The sample data is a decoy and will not work with this solution.
+//! * The real input data has two special properties:
+//!     * Vertical and horizontal "roads" run from the center.
+//!     * The edge of the input is completely free of obstructions.
+//!
+//! These properties mean that we can always cross a tile in exactly 131 steps. We start in the
+//! middle of a tile and need 65 steps to reach the edge. Part two asks how many plots can be
+//! reached in 26501365 steps.
+//!
+//! ```none
+//!     26501365 => 65 + 131 * n => n = 202300
+//! ```
+//!
+//! The number of tiles that we can reach forms a rough diamond 202300 tiles wide.
+//! For example `n = 2` looks like:
+//!
+//! ```none
+//!       #
+//!      ###
+//!     #####
+//!      ###
+//!       #
+//! ```
+//!
+//! The next insight is that if we can reach a plot in `x` steps the we can also reach it in
+//! `x + 2, x + 4...` steps by repeatedly stepping back and forth 1 tile. This means the
+//! number of tiles reachable depends on the *parity* of a plot from the center,
+//! i.e. whether it is an odd or even number of steps. As the 131 width of the tile is an odd
+//! number of plots, the number of plots reachable flips from odd to even each time we cross a
+//! whole tile. There are `n²` even plots and `(n + 1)²` odd plots in the diamond.
+//!
+//! ```none
+//!       O
+//!      OEO
+//!     OEOEO
+//!      OEO
+//!       O
+//! ```
+//!
+//! Lastly, we can only partially reach some tiles on the edges. Solid triangles represent corners
+//! that can be reached and hollow triangle represents corners that are too far away.
+//!
+//! ```none
+//!          ┌--┐
+//!          |◸◹|
+//!         ◢|  |◣
+//!       ┌--┼--┼--┐
+//!       |◸ |  | ◹|
+//!      ◢|  |  |  |◣
+//!    ┌--┼--┼--┼--┼--┐
+//!    |◸ |  |  |  | ◹|
+//!    |◺ |  |  |  | ◿|
+//!    └--┼--┼--┼--┼--┘
+//!      ◥|  |  |  |◤
+//!       |◺ |  | ◿|
+//!       └--┼--┼--┘
+//!         ◥|  |◤
+//!          |◺◿|
+//!          └--┘
+//! ```
+//!
+//! The total area is adjusted by:
+//! * Adding `n` extra even corners
+//!     ```none
+//!         ◤◥
+//!         ◣◢
+//!     ```
+//! * Subtracting `n + 1` odd corners
+//!     ```none
+//!         ◸◹
+//!         ◺◿
+//!     ```
+//!
+//! To find the values for the total number of odd, even plots and the unreachable odd corners
+//! we BFS from the center tile, counting odd and even plots separately. Any plots more than
+//! 65 steps from the center will be unreachable at the edges of the diamond.
+//!
+//! One nuance is that to always correctly find the extra reachable even corner plots requires a
+//! *second* BFS starting from the corners and working inwards. All tiles within 64 steps are
+//! reachable at the edges of the diamond. For some inputs this happens to be the same as the number
+//! of tiles greater than 65 steps from the center by coincidence, however this is not guaranteed so
+//! a second BFS is more reliable solution.
 use crate::util::grid::*;
 use crate::util::point::*;
 use std::collections::VecDeque;
@@ -11,15 +98,18 @@ type Input = (u64, u64);
 pub fn parse(input: &str) -> Input {
     let grid = Grid::parse(input);
 
+    // Search from the center tile outwards.
     let (even_inner, even_outer, odd_inner, odd_outer) = bfs(&grid, &[CENTER], 130);
     let part_one = even_inner;
     let even_full = even_inner + even_outer;
     let odd_full = odd_inner + odd_outer;
     let remove_corners = odd_outer;
 
+    // Search from the 4 corners inwards.
     let (even_inner, ..) = bfs(&grid, &CORNERS, 64);
     let add_corners = even_inner;
 
+    // Sum the components of the diamond.
     let n = 202300;
     let first = n * n * even_full;
     let second = (n + 1) * (n + 1) * odd_full;
@@ -38,6 +128,7 @@ pub fn part2(input: &Input) -> u64 {
     input.1
 }
 
+/// Breadth first search from any number of starting locations with a limit on maximum steps.
 fn bfs(grid: &Grid<u8>, starts: &[Point], limit: u32) -> (u64, u64, u64, u64) {
     let mut grid = grid.clone();
     let mut todo = VecDeque::new();
@@ -53,6 +144,7 @@ fn bfs(grid: &Grid<u8>, starts: &[Point], limit: u32) -> (u64, u64, u64, u64) {
     }
 
     while let Some((position, cost)) = todo.pop_front() {
+        // First split by odd or even parity then by distance from the starting point.
         if cost % 2 == 1 {
             if position.manhattan(CENTER) <= 65 {
                 odd_inner += 1;