Day 12

Author: fuljo

Diff for: input/day12.txt

@@ -0,0 +1,41 @@
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+abccccccccccccccccccccccccccccccccaaaaaaaccccccccccccccaaaaaacccccccccccccccaacccccaaaaaaaccccccccccciiiqqqttttxxyyyyyvvvrrrnnneeecccccc
+abcaaaaccccccccccccccccccccccccccaaaaaaaaccccccccccccccccaacccccccccccccccccccccccccaaaaacccccccccccciiiqqqqttxxyyyyyyyvvrrnnnneeecccccc
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Diff for: src/bin/day12.rs

@@ -0,0 +1,121 @@
+use std::{
+    collections::{VecDeque},
+    fs::File,
+    io::{BufRead, BufReader},
+};
+
+const W: usize = 136;
+const H: usize = 41;
+
+struct Cell {
+    height: usize,
+    path_cost: usize,
+}
+
+struct SearchNode {
+    path_cost: usize,
+    pos: (usize, usize),
+}
+
+fn main() {
+    let f = File::open("input/day12.txt").unwrap();
+    let read = BufReader::new(f);
+    let mut lines = read.lines();
+
+    let mut cells: Vec<Cell> = vec![];
+    let mut start_pos: (usize, usize) = (0, 0);
+    let mut end_pos: (usize, usize) = (0, 0);
+
+    // Parse
+    let mut pos: usize = 0;
+    loop {
+        let Some(Ok(line)) = lines.next() else {
+            break;
+        };
+
+        assert_eq!(line.len(), W);
+
+        for c in line.chars() {
+            let height: usize = match c {
+                'S' => {
+                    start_pos = to_cartesian(pos);
+                    0
+                }
+                'E' => {
+                    end_pos = to_cartesian(pos);
+                    25
+                }
+                'a'..='z' => c as usize - 'a' as usize,
+                _ => panic!("invalid char"),
+            };
+            cells.push(Cell {
+                height,
+                path_cost: usize::MAX,
+            });
+            pos += 1;
+        }
+    }
+
+    // Build SPT backwards
+    let mut queue: VecDeque<SearchNode> = VecDeque::new();
+    queue.push_back(SearchNode {
+        path_cost: 0,
+        pos: end_pos,
+    });
+
+    while let Some(node) = queue.pop_front() {
+        // decide whether to visit
+        if cells[to_flat(node.pos)].path_cost > node.path_cost {
+            cells[to_flat(node.pos)].path_cost = node.path_cost;
+        } else {
+            continue;
+        }
+
+        let (i, j) = node.pos;
+        let height = cells[to_flat(node.pos)].height;
+        // expand
+        for (dx, dy) in [(-1, 0), (1, 0), (0, -1), (0, 1)] {
+            // move
+            let i_succ = i as isize + dx;
+            let j_succ = j as isize + dy;
+            if i_succ < 0 || i_succ >= H as isize || j_succ < 0 || j_succ >= W as isize {
+                continue;
+            }
+            let pos_succ = (i_succ as usize, j_succ as usize);
+
+            // check feasibility
+            let height_succ = cells[to_flat(pos_succ)].height;
+            if height as isize - height_succ as isize > 1 {
+                continue;
+            }
+
+            // enqueue node
+            let succ = SearchNode {
+                path_cost: node.path_cost + 1,
+                pos: pos_succ,
+            };
+            queue.push_back(succ);
+        }
+    }
+
+    // part 1
+    let min_cost = cells[to_flat(start_pos)].path_cost;
+    println!("{}", min_cost);
+
+    // part 2
+    let min_cost = cells
+        .iter()
+        .filter(|c| c.height == 0)
+        .map(|c| c.path_cost)
+        .min()
+        .unwrap();
+    println!("{}", min_cost)
+}
+
+fn to_flat(pos: (usize, usize)) -> usize {
+    pos.0 * W + pos.1
+}
+
+fn to_cartesian(pos: usize) -> (usize, usize) {
+    (pos / W, pos % W)
+}