diff --git a/day09/common.rs b/day09/common.rs
new file mode 100644
index 0000000..acaba7d
--- /dev/null
+++ b/day09/common.rs
@@ -0,0 +1,142 @@
+// 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;
+use std::str::FromStr;
+
+#[derive(Clone, Copy)]
+enum Direction {
+    Up,
+    Right,
+    Down,
+    Left,
+}
+
+#[derive(Debug, Clone, Copy)]
+pub struct RopeState {
+    head_pos: (isize, isize),
+    tail_pos: (isize, isize),
+}
+
+pub struct RopeStates {
+    direction: Direction,
+    count: isize,
+    state: RopeState,
+}
+
+impl FromStr for Direction {
+    type Err = &'static str;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        match s {
+            "U" => Ok(Self::Up),
+            "R" => Ok(Self::Right),
+            "D" => Ok(Self::Down),
+            "L" => Ok(Self::Left),
+            _ => Err("cannot parse direction"),
+        }
+    }
+}
+
+impl RopeState {
+    pub fn new() -> Self {
+        RopeState {
+            head_pos: (0, 0),
+            tail_pos: (0, 0),
+        }
+    }
+
+    pub fn head_pos(&self) -> (isize, isize) {
+        self.head_pos
+    }
+
+    pub fn tail_pos(&self) -> (isize, isize) {
+        self.tail_pos
+    }
+
+    fn adjust_tail(&mut self) {
+        let x_diff = self.head_pos.0.abs_diff(self.tail_pos.0);
+        let y_diff = self.head_pos.1.abs_diff(self.tail_pos.1);
+        let (move_x, move_y) = match (x_diff, y_diff) {
+            (2, 0) => (true, false),
+            (2, 1) => (true, true),
+            (0, 2) => (false, true),
+            (1, 2) => (true, true),
+            _ => (false, false),
+        };
+        if move_x {
+            match (self.head_pos.0, self.tail_pos.0) {
+                (h, t) if { h > t } => self.tail_pos.0 += 1,
+                (h, t) if { h < t } => self.tail_pos.0 -= 1,
+                _ => (),
+            }
+        }
+        if move_y {
+            match (self.head_pos.1, self.tail_pos.1) {
+                (h, t) if { h > t } => self.tail_pos.1 += 1,
+                (h, t) if { h < t } => self.tail_pos.1 -= 1,
+                _ => (),
+            }
+        }
+    }
+
+    fn move_head(&mut self, direction: Direction) -> Result<RopeState, &'static str> {
+        const ERROR: &'static str = "invalid move";
+        match direction {
+            Direction::Up => self.head_pos.1 = self.head_pos.1.checked_add(1).ok_or(ERROR)?,
+            Direction::Right => self.head_pos.0 = self.head_pos.0.checked_add(1).ok_or(ERROR)?,
+            Direction::Down => self.head_pos.1 = self.head_pos.1.checked_sub(1).ok_or(ERROR)?,
+            Direction::Left => self.head_pos.0 = self.head_pos.0.checked_sub(1).ok_or(ERROR)?,
+        }
+        self.adjust_tail();
+        Ok(*self)
+    }
+}
+
+impl Iterator for RopeStates {
+    type Item = Result<RopeState, &'static str>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.count == 0 {
+            let mut line = String::new();
+            match io::stdin().read_line(&mut line) {
+                Ok(_) if { line == "" || line == "\n" } => return None,
+                Ok(_) => {
+                    if let Some((left, right)) = line.trim().split_once(' ') {
+                        if let (Ok(direction), Ok(count)) = (left.parse(), right.parse()) {
+                            self.direction = direction;
+                            self.count = count;
+                        } else {
+                            return Some(Err("input file contains invalid line"));
+                        }
+                    } else {
+                        return Some(Err("input file contains invalid line"));
+                    }
+                }
+                Err(_) => assert!(false),
+            }
+        }
+        self.count -= 1;
+        Some(self.state.move_head(self.direction))
+    }
+}
+
+pub fn rope_states() -> RopeStates {
+    RopeStates {
+        direction: Direction::Up,
+        count: 0,
+        state: RopeState::new(),
+    }
+}
diff --git a/day09/input b/day09/input
new file mode 100644
index 0000000..1785c9e
--- /dev/null
+++ b/day09/input
@@ -0,0 +1,2001 @@
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+
diff --git a/day09/part1.rs b/day09/part1.rs
new file mode 100644
index 0000000..ec169bf
--- /dev/null
+++ b/day09/part1.rs
@@ -0,0 +1,27 @@
+// 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 std::collections::HashSet;
+
+fn main() -> Result<(), &'static str> {
+    let tail_positions: Result<HashSet<(isize, isize)>, _> =
+        common::rope_states().map(|s| Ok(s?.tail_pos())).collect();
+    println!("{}", tail_positions?.len());
+    Ok(())
+}
diff --git a/day09/testinput b/day09/testinput
new file mode 100644
index 0000000..9874df2
--- /dev/null
+++ b/day09/testinput
@@ -0,0 +1,8 @@
+R 4
+U 4
+L 3
+D 1
+R 4
+D 1
+L 5
+R 2