diff --git a/src/lib.rs b/src/lib.rs
index 64e2b17..301a8c3 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -2,14 +2,16 @@
 #![feature(trace_macros)]
 #![feature(auto_traits)]
 #![feature(negative_impls)]
+#![feature(strict_provenance)]
 
 pub mod cfg_match;
 pub mod innocent_linked_list;
 pub mod scratch;
 pub mod sendsync;
+pub mod thin_u128;
 pub mod unroll_int;
 pub mod unsized_clone;
 
 pub mod safe_extern {
     pub use pm::safe_extern;
-}
\ No newline at end of file
+}
diff --git a/src/thin_u128.rs b/src/thin_u128.rs
new file mode 100644
index 0000000..c49c4b9
--- /dev/null
+++ b/src/thin_u128.rs
@@ -0,0 +1,146 @@
+use std::{
+    fmt::{Debug, Display},
+    num::NonZeroUsize,
+    ptr::{self, NonNull},
+};
+
+/// thin.
+/// ```text
+/// 000000 ... 000000 0000000
+///                         ^-always 1 for niche
+///                        ^- tag, 1 for inline u62, 0 for box
+/// ```
+pub struct ThinU128(NonNull<u128>);
+
+enum Repr {
+    Inline(u128),
+    Boxed(NonNull<u128>),
+}
+
+const USIZE_TWO_BIT_LESS_MAX: u128 = (usize::MAX as u128) >> 2;
+
+const ALWAYS_ONE_NICHE: usize = 0b1;
+const TAG_MASK: usize = 0b10;
+
+impl ThinU128 {
+    pub fn new(int: u128) -> Self {
+        if int > USIZE_TWO_BIT_LESS_MAX {
+            let ptr = Box::into_raw(Box::new(int));
+            let repr = ptr.map_addr(|addr| addr | ALWAYS_ONE_NICHE);
+            unsafe { Self(NonNull::new_unchecked(repr)) }
+        } else {
+            let value = (int as usize) << 2;
+            let repr = value | TAG_MASK | ALWAYS_ONE_NICHE;
+            Self(NonNull::new(ptr::invalid_mut(repr)).unwrap())
+        }
+    }
+
+    fn is_inline(&self) -> bool {
+        (self.addr() & TAG_MASK) != 0
+    }
+
+    fn addr(&self) -> usize {
+        self.0.addr().get()
+    }
+
+    fn repr(&self) -> Repr {
+        if self.is_inline() {
+            let value = self.addr() >> 2;
+            Repr::Inline(value as u128)
+        } else {
+            let ptr = self.0.map_addr(|addr| unsafe {
+                NonZeroUsize::new_unchecked(addr.get() & !ALWAYS_ONE_NICHE)
+            });
+            Repr::Boxed(ptr)
+        }
+    }
+
+    pub fn value(&self) -> u128 {
+        match self.repr() {
+            Repr::Inline(value) => value,
+            Repr::Boxed(ptr) => unsafe { ptr.as_ptr().read() },
+        }
+    }
+}
+
+impl Drop for ThinU128 {
+    fn drop(&mut self) {
+        if let Repr::Boxed(ptr) = self.repr() {
+            unsafe {
+                drop(Box::from_raw(ptr.as_ptr()));
+            }
+        }
+    }
+}
+
+impl Debug for ThinU128 {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        Debug::fmt(&self.value(), f)
+    }
+}
+
+impl Display for ThinU128 {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        Display::fmt(&self.value(), f)
+    }
+}
+
+impl PartialEq for ThinU128 {
+    fn eq(&self, other: &Self) -> bool {
+        self.value().eq(&other.value())
+    }
+}
+
+impl Eq for ThinU128 {}
+
+impl PartialOrd for ThinU128 {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        self.value().partial_cmp(&other.value())
+    }
+}
+
+impl Ord for ThinU128 {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        self.value().cmp(&other.value())
+    }
+}
+
+impl Clone for ThinU128 {
+    fn clone(&self) -> Self {
+        Self::new(self.value())
+    }
+}
+
+unsafe impl Send for ThinU128 {}
+unsafe impl Sync for ThinU128 {}
+
+#[cfg(test)]
+mod tests {
+    use super::ThinU128;
+
+    fn roundtrip(a: u128) {
+        let thin = ThinU128::new(a);
+        assert_eq!(thin.value(), a);
+
+        let other = ThinU128::new(a);
+        assert_eq!(thin, other);
+        let dbg_a = format!("{a:?}{a}");
+        let dbg_thin = format!("{thin:?}{thin}");
+        assert_eq!(dbg_a, dbg_thin);
+    }
+
+    #[test]
+    fn small() {
+        roundtrip(0);
+        roundtrip(1);
+        roundtrip(100);
+        roundtrip((usize::MAX >> 2) as u128);
+    }
+
+    #[test]
+    fn big() {
+        roundtrip(((usize::MAX >> 2) as u128) + 1);
+        roundtrip(usize::MAX as u128);
+        roundtrip(u128::MAX);
+    }
+}