add day 14, part 2

day14/common.rs

@@ -19,6 +19,7 @@ use std::ops::Range;
 pub struct CaveScan {
     tiles: Vec<Vec<bool>>,
     origin: (usize, usize),
+    bottom: Option<usize>,
     x_window: Range<usize>,
     y_window: Range<usize>,
 }
@@ -42,12 +43,13 @@ impl CaveScan {
         }
     }
 
-    pub fn from_stdin(origin: (usize, usize)) -> Result<CaveScan, &'static str> {
+    pub fn from_stdin(origin: (usize, usize), has_bottom: bool) -> Result<CaveScan, &'static str> {
         const ERROR: &str = "input file contains invalid line";
 
         let mut result = Self {
             tiles: vec![vec![false]],
             origin: origin,
+            bottom: None,
             x_window: origin.0..origin.0,
             y_window: origin.1..origin.1,
         };
@@ -78,14 +80,22 @@ impl CaveScan {
                 }
             }
         }
+        if has_bottom {
+            result.bottom = Some(result.tiles.len() + 1);
+        }
         Ok(result)
     }
 
-    fn bottom(&self) -> usize {
-        self.tiles.len()
+    fn dimensions(&self) -> (usize, usize) {
+        (self.tiles[0].len(), self.tiles.len())
     }
 
     fn tile(&self, pos: (usize, usize)) -> bool {
+        if let Some(bottom) = self.bottom {
+            if pos.1 == bottom {
+                return true;
+            }
+        }
         if pos.0 < self.x_window.start
             || pos.0 > self.x_window.end
             || pos.1 < self.y_window.start
@@ -120,24 +130,33 @@ impl CaveScan {
             self.tiles = [&head[..], &self.tiles[..], &tail[..]].concat();
             self.y_window = new_y_window;
         }
+
         self.tiles[pos.1 - self.y_window.start][pos.0 - self.x_window.start] = value;
     }
 
-    fn step_down(&self, origin: (usize, usize)) -> Option<(usize, usize)> {
+    fn step_down(&mut self, origin: (usize, usize)) -> Option<(usize, usize)> {
         let down_pos = (origin.0, origin.1.checked_add(1)?);
         let down_left_pos = (origin.0.checked_sub(1)?, down_pos.1);
         let down_right_pos = (origin.0.checked_add(1)?, down_pos.1);
+        let down = self.tile(down_pos);
+        let down_left = self.tile(down_left_pos);
+        let down_right = self.tile(down_right_pos);
 
-        let result = match (
-            self.tile(down_left_pos),
-            self.tile(down_pos),
-            self.tile(down_right_pos),
-        ) {
+        let result = match (down_left, down, down_right) {
             (_, false, _) => Some(down_pos),
             (false, true, _) => Some(down_left_pos),
             (true, true, false) => Some(down_right_pos),
-            (true, true, true) => None,
+            (true, true, true) => {
+                self.set_tile(origin, true);
+                None
+            }
         };
+
+        if let (None, Some(result)) = (self.bottom, result) {
+            if result.1 == self.dimensions().1 {
+                return Some((result.0, usize::MAX));
+            }
+        }
         result
     }
 
@@ -153,11 +172,7 @@ impl Iterator for Falls<'_> {
         let mut result = self.scan.origin;
         while let Some(pos) = self.scan.step_down(result) {
             result = pos;
-            if result.1 == self.scan.bottom() {
-                return Some((result.0, usize::MAX));
-            }
         }
-        self.scan.set_tile(result, true);
         Some(result)
     }
 }

day14/part1.rs

@@ -20,7 +20,7 @@ pub mod common;
 use common::CaveScan;
 
 fn main() -> Result<(), &'static str> {
-    let mut scan = CaveScan::from_stdin((500, 0))?;
+    let mut scan = CaveScan::from_stdin((500, 0), false)?;
     let mut result = 0;
     for end_pos in scan.falls() {
         match end_pos {

day14/part2.rs

@@ -0,0 +1,33 @@
+// Copyright 2022 Christian Ulrich
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+//
+// usage: ./part2 < input
+
+pub mod common;
+
+use common::CaveScan;
+
+fn main() -> Result<(), &'static str> {
+    let mut scan = CaveScan::from_stdin((500, 0), true)?;
+    let mut result = 1;
+    for end_pos in scan.falls() {
+        match end_pos {
+            (500, 0) => break,
+            _ => result += 1,
+        }
+    }
+    println!("{result}");
+    Ok(())
+}