diff --git a/day11/common.rs b/day11/common.rs
new file mode 100644
index 0000000..35f2cdc
--- /dev/null
+++ b/day11/common.rs
@@ -0,0 +1,197 @@
+// 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/>.
+
+use std::io;
+
+#[derive(Debug, Clone)]
+enum Operation {
+    Add { arg: u64 },
+    Multiply { arg: u64 },
+    Square,
+}
+
+#[derive(Debug, Clone)]
+pub struct Monkey {
+    items: Vec<u64>,
+    operation: Operation,
+    modulus: u64,
+    target_devisible: usize,
+    target_indevisible: usize,
+    item_count: usize,
+}
+
+pub struct Game {
+    monkeys: Vec<Monkey>,
+    divisor: u64,
+}
+
+pub struct Round {
+    monkeys: Vec<Monkey>,
+}
+
+impl Monkey {
+    fn from_stdin() -> Result<Option<Self>, &'static str> {
+        const ERROR: &str = "cannot parse monkey";
+        let mut line = String::new();
+        let _ = io::stdin().read_line(&mut line).map_err(|_| ERROR)?;
+        if line.len() == 0 {
+            return Ok(None);
+        }
+        if !line.starts_with("Monkey ") {
+            return Err(ERROR);
+        }
+        line.clear();
+
+        // parse starting items
+        let _ = io::stdin().read_line(&mut line).map_err(|_| ERROR)?;
+        if !line.starts_with("  Starting items: ") {
+            return Err(ERROR);
+        }
+        let mut items = vec![];
+        for part in line[18..].split(',') {
+            items.push(part.trim().parse::<u64>().map_err(|_| ERROR)?);
+        }
+        line.clear();
+
+        // parse operation
+        let _ = io::stdin().read_line(&mut line).map_err(|_| ERROR)?;
+        if !line.starts_with("  Operation: new = old ") {
+            return Err(ERROR);
+        }
+        let operation = match line[23..].trim() {
+            "* old" => Operation::Square,
+            arg if { arg.starts_with("+ ") } => Operation::Add {
+                arg: arg[2..].parse().map_err(|_| ERROR)?,
+            },
+            arg if { arg.starts_with("* ") } => Operation::Multiply {
+                arg: arg[2..].parse().map_err(|_| ERROR)?,
+            },
+            _ => return Err(ERROR),
+        };
+        line.clear();
+
+        // parse modulus
+        let _ = io::stdin().read_line(&mut line).map_err(|_| ERROR)?;
+        if !line.starts_with("  Test: divisible by ") {
+            return Err(ERROR);
+        }
+        let modulus = line[21..].trim().parse().map_err(|_| ERROR)?;
+        line.clear();
+
+        // parse action if devisible
+        let _ = io::stdin().read_line(&mut line).map_err(|_| ERROR)?;
+        if !line.starts_with("    If true: throw to monkey ") {
+            return Err(ERROR);
+        }
+        let target_devisible = line[29..].trim().parse().map_err(|_| ERROR)?;
+        line.clear();
+
+        // parse action if indevisible
+        let _ = io::stdin().read_line(&mut line).map_err(|_| ERROR)?;
+        if !line.starts_with("    If false: throw to monkey ") {
+            return Err(ERROR);
+        }
+        let target_indevisible = line[30..].trim().parse().map_err(|_| ERROR)?;
+        line.clear();
+        let _ = io::stdin().read_line(&mut line).map_err(|_| ERROR)?;
+        if line.trim().len() > 0 {
+            return Err(ERROR);
+        }
+        Ok(Some(Self {
+            items: items,
+            operation: operation,
+            modulus: modulus,
+            target_devisible: target_devisible,
+            target_indevisible: target_indevisible,
+            item_count: 0,
+        }))
+    }
+
+    fn throw_item(&mut self, divisor: u64) -> Result<Option<(u64, usize)>, &'static str> {
+        const ERROR: &str = "integer overflow";
+        if self.items.len() == 0 {
+            return Ok(None);
+        }
+        let mut item = self.items.remove(0);
+        match self.operation {
+            Operation::Add { arg } => item = item.checked_add(arg).ok_or(ERROR)?,
+            Operation::Multiply { arg } => item = item.checked_mul(arg).ok_or(ERROR)?,
+            Operation::Square => item = item.checked_mul(item).ok_or(ERROR)?,
+        };
+        item /= divisor;
+        let target = match item % self.modulus {
+            0 => self.target_devisible,
+            _ => self.target_indevisible,
+        };
+        self.item_count += 1;
+        Ok(Some((item, target)))
+    }
+
+    pub fn add_item(&mut self, item: u64) {
+        self.items.push(item);
+    }
+
+    pub fn items(&self) -> &[u64] {
+        &self.items[..]
+    }
+
+    pub fn item_count(&self) -> usize {
+        self.item_count
+    }
+}
+
+impl Game {
+    pub fn from_stdin(divisor: u64) -> Result<Self, &'static str> {
+        let mut monkeys = vec![];
+        while let Some(monkey) = Monkey::from_stdin()? {
+            monkeys.push(monkey);
+        }
+        Ok(Self {
+            monkeys: monkeys,
+            divisor: divisor,
+        })
+    }
+
+    pub fn monkeys(&self) -> &[Monkey] {
+        &self.monkeys[..]
+    }
+}
+
+impl Iterator for Game {
+    type Item = Result<Round, &'static str>;
+
+    fn next<'a>(&'a mut self) -> Option<Self::Item> {
+        for i in 0..self.monkeys.len() {
+            loop {
+                match self.monkeys[i].throw_item(self.divisor) {
+                    Ok(None) => break,
+                    Ok(Some((item, target))) => {
+                        self.monkeys[target].add_item(item);
+                    }
+                    Err(e) => return Some(Err(e)),
+                }
+            }
+        }
+        Some(Ok(Round {
+            monkeys: self.monkeys.clone(),
+        }))
+    }
+}
+
+impl Round {
+    pub fn monkeys(&self) -> &[Monkey] {
+        &self.monkeys[..]
+    }
+}
diff --git a/day11/input b/day11/input
new file mode 100644
index 0000000..f7e9bf8
--- /dev/null
+++ b/day11/input
@@ -0,0 +1,56 @@
+Monkey 0:
+  Starting items: 98, 70, 75, 80, 84, 89, 55, 98
+  Operation: new = old * 2
+  Test: divisible by 11
+    If true: throw to monkey 1
+    If false: throw to monkey 4
+
+Monkey 1:
+  Starting items: 59
+  Operation: new = old * old
+  Test: divisible by 19
+    If true: throw to monkey 7
+    If false: throw to monkey 3
+
+Monkey 2:
+  Starting items: 77, 95, 54, 65, 89
+  Operation: new = old + 6
+  Test: divisible by 7
+    If true: throw to monkey 0
+    If false: throw to monkey 5
+
+Monkey 3:
+  Starting items: 71, 64, 75
+  Operation: new = old + 2
+  Test: divisible by 17
+    If true: throw to monkey 6
+    If false: throw to monkey 2
+
+Monkey 4:
+  Starting items: 74, 55, 87, 98
+  Operation: new = old * 11
+  Test: divisible by 3
+    If true: throw to monkey 1
+    If false: throw to monkey 7
+
+Monkey 5:
+  Starting items: 90, 98, 85, 52, 91, 60
+  Operation: new = old + 7
+  Test: divisible by 5
+    If true: throw to monkey 0
+    If false: throw to monkey 4
+
+Monkey 6:
+  Starting items: 99, 51
+  Operation: new = old + 1
+  Test: divisible by 13
+    If true: throw to monkey 5
+    If false: throw to monkey 2
+
+Monkey 7:
+  Starting items: 98, 94, 59, 76, 51, 65, 75
+  Operation: new = old + 5
+  Test: divisible by 2
+    If true: throw to monkey 3
+    If false: throw to monkey 6
+
diff --git a/day11/part1.rs b/day11/part1.rs
new file mode 100644
index 0000000..1b387d5
--- /dev/null
+++ b/day11/part1.rs
@@ -0,0 +1,36 @@
+// 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: ./part1 < input
+
+pub mod common;
+
+use common::{Game, Monkey};
+
+fn main() -> Result<(), &'static str> {
+    let game = Game::from_stdin(3)?;
+    let last_round = game.take(20).last().unwrap();
+    let mut item_counts: Vec<_> = last_round?
+        .monkeys()
+        .iter()
+        .map(Monkey::item_count)
+        .collect();
+    item_counts.sort();
+    println!(
+        "{}",
+        item_counts[item_counts.len() - 1] * item_counts[item_counts.len() - 2]
+    );
+    Ok(())
+}
diff --git a/day11/testinput b/day11/testinput
new file mode 100644
index 0000000..477d33b
--- /dev/null
+++ b/day11/testinput
@@ -0,0 +1,28 @@
+Monkey 0:
+  Starting items: 79, 98
+  Operation: new = old * 19
+  Test: divisible by 23
+    If true: throw to monkey 2
+    If false: throw to monkey 3
+
+Monkey 1:
+  Starting items: 54, 65, 75, 74
+  Operation: new = old + 6
+  Test: divisible by 19
+    If true: throw to monkey 2
+    If false: throw to monkey 0
+
+Monkey 2:
+  Starting items: 79, 60, 97
+  Operation: new = old * old
+  Test: divisible by 13
+    If true: throw to monkey 1
+    If false: throw to monkey 3
+
+Monkey 3:
+  Starting items: 74
+  Operation: new = old + 3
+  Test: divisible by 17
+    If true: throw to monkey 0
+    If false: throw to monkey 1
+