Day 11, Part 1!

Dijkstra's algorithm. Copied heavily from the Rust docs for its BinaryHeap.

2022/day12/Cargo.lock

@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "day12"
+version = "0.1.0"

2022/day12/src/main.rs

@@ -1,3 +1,166 @@
-fn main() {
+use std::cmp::Ordering;
-    println!("Hello, world!");
+use std::collections::BinaryHeap;
+use std::{env, fs};
+
+trait Elevation {
+    fn elevation(&self) -> Option<u32>;
+}
+
+impl Elevation for char {
+    fn elevation(&self) -> Option<u32> {
+        let value = match self {
+            'S' => 'a',
+            'E' => 'z',
+            _ => *self
+        };
+
+        Some((value as u32) - 'a' as u32)
+    }
+}
+
+#[derive(Debug)]
+struct Square {
+    symbol: char,
+    up: Option<usize>,
+    down: Option<usize>,
+    left: Option<usize>,
+    right: Option<usize>,
+}
+
+impl Square {
+    fn new(symbol: char) -> Square {
+        Square { symbol, up: None, down: None, left: None, right: None }
+    }
+
+    fn elevation(&self) -> u32 {
+        self.symbol.elevation().unwrap()
+    }
+}
+
+#[derive(Clone, Eq, PartialEq)]
+struct State {
+    node: usize,
+    cost: u32,
+}
+
+impl Ord for State {
+    fn cmp(&self, other: &Self) -> Ordering {
+        other.cost.cmp(&self.cost).then_with(|| self.node.cmp(&other.node))
+    }
+}
+
+impl PartialOrd for State {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+/// An implemetation of Dijkstra's algorithm to find the shortest path from start to end and return
+/// its length.
+fn find_length_of_shortest_path(squares: &Vec<Square>, start: usize, end: usize) -> u32 {
+    let mut distances: Vec<_> = (0..squares.len()).map(|_| u32::MAX).collect();
+    let mut heap = BinaryHeap::new();
+
+    // Initial state
+    distances[start] = 0;
+    heap.push(State { node: start, cost: 0 });
+
+    while let Some(State { node, cost }) = heap.pop() {
+        if node == end {
+            return cost;
+        }
+
+        if cost > distances[node] {
+            continue;
+        }
+
+        let square = &squares[node];
+        for edge in &[square.up, square.right, square.down, square.left] {
+            if edge.is_none() {
+                continue;
+            }
+
+            let edge = edge.unwrap();
+
+            let next_cost = cost + 1;
+            let next = State { node: edge, cost: next_cost };
+            if next.cost < distances[edge] {
+                heap.push(next);
+                distances[edge] = next_cost;
+            }
+        }
+    }
+
+    0
+}
+
+fn main() {
+    let args: Vec<String> = env::args().collect();
+
+    let filename = args.get(1).expect("Missing filename argument");
+
+    let file_contents = fs::read_to_string(&filename).expect("Unable to read file");
+    let character_grid = file_contents.lines().map(|l| l.chars().collect::<Vec<char>>()).collect::<Vec<Vec<char>>>();
+
+    // Assume a square grid
+    let width = character_grid[0].len();
+    let height = character_grid.len();
+
+    let index_into_squares_array = |x: usize, y: usize| -> usize {
+        y * width + x
+    };
+
+    let mut start: usize = 0;
+    let mut end: usize = 0;
+    let mut squares: Vec<Square> = Vec::new();
+
+    for y in 0..height {
+        for x in 0..width {
+            let symbol = character_grid[y][x];
+
+            match symbol {
+                'S' => start = index_into_squares_array(x, y),
+                'E' => end = index_into_squares_array(x, y),
+                _ => {},
+            }
+
+            let elevation = symbol.elevation().unwrap();
+            let mut square = Square::new(symbol);
+
+            if y > 0 {
+                let up_elevation = character_grid[y-1][x].elevation().unwrap();
+                if up_elevation <= elevation + 1 {
+                    square.up = Some(index_into_squares_array(x, y - 1));
+                }
+            }
+
+            if y < height - 1 {
+                let down_elevation = character_grid[y + 1][x].elevation().unwrap();
+                if down_elevation <= elevation + 1 {
+                    square.down = Some(index_into_squares_array(x, y + 1));
+                }
+            }
+
+            if x > 0 {
+                let left_elevation = character_grid[y][x - 1].elevation().unwrap();
+                if left_elevation <= elevation + 1 {
+                    square.left = Some(index_into_squares_array(x - 1, y));
+                }
+            }
+
+            if x < width - 1 {
+                let right_elevation = character_grid[y][x + 1].elevation().unwrap();
+                if right_elevation <= elevation + 1 {
+                    square.right = Some(index_into_squares_array(x + 1, y));
+                }
+            }
+
+            squares.push(square);
+        }
+    }
+
+    dbg!(&squares);
+
+    let length_of_shortest_path = find_length_of_shortest_path(&squares, start, end);
+    println!("Part 1: length of shortest path to location with best signal: {length_of_shortest_path}");
 }