diff --git a/src/bin/day23.rs b/src/bin/day23.rs
new file mode 100644
index 0000000..98ef396
--- /dev/null
+++ b/src/bin/day23.rs
@@ -0,0 +1,217 @@
+use std::mem::replace;
+
+const INPUT: &str = "315679824";
+
+type Item = usize;
+
+const MARK_MISSING: usize = !0;
+
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+struct OptionItem(Item);
+
+impl OptionItem {
+    pub const NONE: Self = Self(MARK_MISSING);
+
+    fn item(self) -> Option<Item> {
+        if self.0 == MARK_MISSING {
+            None
+        } else {
+            Some(self.0)
+        }
+    }
+}
+
+impl From<Item> for OptionItem {
+    fn from(i: Item) -> Self {
+        assert_ne!(i, MARK_MISSING);
+        Self(i)
+    }
+}
+
+impl From<Option<Item>> for OptionItem {
+    fn from(i: Option<Item>) -> Self {
+        match i {
+            None => Self(MARK_MISSING),
+            Some(i) => Self::from(i),
+        }
+    }
+}
+
+#[derive(Clone, PartialEq, Eq, Debug)]
+struct Cups {
+    current: OptionItem,
+    last: OptionItem,
+    next_cup: Vec<OptionItem>,
+}
+
+impl Default for Cups {
+    fn default() -> Self {
+        Self {
+            current: OptionItem::NONE,
+            last: OptionItem::NONE,
+            next_cup: Vec::new(),
+        }
+    }
+}
+
+impl Cups {
+    fn last(&self) -> Option<Item> {
+        self.last.item()
+    }
+
+    fn insert_after(&mut self, prev: Item, value: Item) {
+        let next = replace(&mut self.next_cup[prev], value.into()).item().unwrap();
+        if value >= self.next_cup.len() {
+            self.next_cup.resize(value, OptionItem::NONE);
+            self.next_cup.push(next.into());
+            assert_eq!(self.next_cup[value], next.into());
+        } else {
+            assert_eq!(self.next_cup[value], OptionItem::NONE);
+            self.next_cup[value] = next.into();
+        }
+        if self.last.item().unwrap() == prev {
+            self.last = value.into();
+        }
+    }
+
+    fn extend_after<I>(&mut self, mut after: Item, iter: I)
+    where
+        I: IntoIterator<Item=Item>,
+    {
+        for item in iter {
+            self.insert_after(after, item);
+            after = item;
+        }
+    }
+
+    fn extend<I>(&mut self, iter: I)
+    where
+        I: IntoIterator<Item=Item>,
+    {
+        let mut iter = iter.into_iter();
+        if let Some(first) = iter.next() {
+            if let Some(prev) = self.last() {
+                self.insert_after(prev, first);
+            } else {
+                assert!(self.next_cup.is_empty());
+                self.next_cup.resize(first, OptionItem::NONE);
+                self.next_cup.push(first.into());
+                self.current = first.into();
+                self.last = first.into();
+            }
+            self.extend_after(first, iter);
+        }
+    }
+
+    fn iter_from(&self, start: Item) -> impl Iterator<Item=Item> + '_ {
+        let mut current = start;
+        let mut first = true;
+        std::iter::from_fn(move || {
+            if !first && current == start {
+                return None;
+            }
+            first = false;
+            let item = current;
+            current = self.next_cup[item].item().unwrap();
+            Some(item)
+        })
+    }
+
+    fn iter(&self) -> impl Iterator<Item=Item> + '_ {
+        self.iter_from(self.current.item().unwrap())
+    }
+
+    fn remove_after(&mut self, previous: Item) -> Item {
+        let item = self.next_cup[previous].item().unwrap();
+        let follow = self.next_cup[item].item().unwrap();
+        self.next_cup[item] = OptionItem::NONE;
+        self.next_cup[previous] = follow.into();
+        if item == self.current.item().unwrap() {
+            if item == previous {
+                // last item
+                self.current = OptionItem::NONE;
+                self.last = OptionItem::NONE;
+                self.next_cup.clear();
+            } else {
+                // if we removed the "current" item: move current pointer forward
+                self.current = follow.into();
+            }
+        } else if item == self.last.item().unwrap() {
+            // just removed the last item
+            self.last = previous.into();
+        }
+        item
+    }
+
+    fn drain_after(&mut self, previous: Item, mut len: usize) -> impl Iterator<Item=Item> + '_ {
+        std::iter::from_fn(move || {
+            if len == 0 {
+                return None;
+            }
+            len -= 1;
+            Some(self.remove_after(previous))
+        })
+    }
+
+    fn drain(&mut self, range: std::ops::Range<usize>) -> impl Iterator<Item=Item> + '_ {
+        let len = range.end - range.start;
+        let after = if len == 0 {
+            0 // don't care
+        } else if range.start > 0 {
+            self.iter().skip(range.start - 1).next().unwrap()
+        } else {
+            self.last().unwrap()
+        };
+        self.drain_after(after, len)
+    }
+
+    fn parse(cups: &str) -> Self {
+        let mut result = Cups::default();
+        result.extend(cups.bytes().map(|c| (c - b'0') as Item));
+        result
+    }
+
+    fn rotate_left(&mut self, items: usize) {
+        let next = self.iter().skip(items).next().unwrap();
+        self.current = next.into();
+    }
+
+    fn mix(&mut self) {
+        let max_entry = self.next_cup.len() - 1;
+        let pickup = self.drain(1..4).collect::<Vec<_>>();
+        let mut insert_after = self.current.item().unwrap();
+        loop {
+            insert_after = match insert_after.checked_sub(1) {
+                Some(n) => n,
+                None => max_entry,
+            };
+            if self.next_cup[insert_after] != OptionItem::NONE {
+                break;
+            }
+        }
+        self.extend_after(insert_after, pickup);
+        self.rotate_left(1);
+    }
+
+    fn rotate_to_one(&mut self) {
+        assert_ne!(self.next_cup[1], OptionItem::NONE);
+        self.current = 1.into();
+    }
+}
+
+fn main() {
+    let mut cups = Cups::parse(INPUT);
+    for _ in 0..100 {
+        cups.mix();
+    }
+    cups.rotate_to_one();
+    println!("Cups: {}", cups.iter().skip(1).map(|c| format!("{}", c)).collect::<Vec<_>>().join(""));
+
+    let mut cups = Cups::parse(INPUT);
+    cups.extend(10..=1000000);
+    for _ in 0..10000000 {
+        cups.mix()
+    }
+    cups.rotate_to_one();
+    println!("Cup product: {}", cups.drain(1..3).map(|c| c as u64).product::<u64>());
+}