add day 5, part 2

day05/common.rs

@@ -98,6 +98,19 @@ impl Stacks {
         }
         Ok(())
     }
+
+    pub fn move_items_retain_order(&mut self, spec: Move) -> Result<(), &'static str> {
+        if spec.from >= self.inner.len()
+            || spec.to >= self.inner.len()
+            || spec.count > self.inner[spec.from].len()
+        {
+            return Err("invalid move");
+        }
+        let split_index = self.inner[spec.from].len() - spec.count;
+        let mut items = self.inner[spec.from].split_off(split_index);
+        self.inner[spec.to].append(&mut items);
+        Ok(())
+    }
 }
 
 impl Iterator for Stacks {

day05/part2.rs

@@ -0,0 +1,32 @@
+// 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::{Moves, Stacks};
+
+fn main() -> Result<(), &'static str> {
+    let mut stacks = Stacks::from_stdin()?;
+    for op in Moves::from_stdin() {
+        stacks.move_items_retain_order(op?)?;
+    }
+    let result: Result<String, _> = stacks
+        .map(|s| s.last().cloned().ok_or("at least one stack is empty"))
+        .collect();
+    println!("{}", result?);
+    Ok(())
+}