cl-structures: Add a simple potted tree structure, for future use in the module system

cl-structures/src/tree/tree_ref.rs

@@ -0,0 +1,70 @@
+//! An element in a [Tree](super::Tree)
+///
+/// Contains a niche, and as such, [`Option<TreeRef<T>>`] is free :D
+use std::{marker::PhantomData, num::NonZeroUsize};
+
+/// An element of in a [Tree](super::Tree).
+//? The index of the node is stored as a [NonZeroUsize] for space savings
+//? Making Refs T-specific helps the user keep track of which Refs belong to which trees.
+//? This isn't bulletproof, of course, but it'll keep Ref<Foo> from being used on Tree<Bar>
+pub struct Ref<T: ?Sized>(NonZeroUsize, PhantomData<T>);
+
+impl<T: ?Sized> Ref<T> {
+    /// Constructs a new [TreeRef] with the given index
+    pub fn new_unchecked(index: usize) -> Self {
+        // Safety: index cannot be zero because we use saturating addition on unsigned type.
+        Self(
+            unsafe { NonZeroUsize::new_unchecked(index.saturating_add(1)) },
+            PhantomData,
+        )
+    }
+}
+
+impl<T: ?Sized> From<Ref<T>> for usize {
+    fn from(value: Ref<T>) -> Self {
+        usize::from(value.0) - 1
+    }
+}
+
+/* --- implementations of derivable traits, because we don't need bounds here --- */
+
+impl<T: ?Sized> std::fmt::Debug for Ref<T> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_tuple("TreeRef").field(&self.0).finish()
+    }
+}
+
+impl<T: ?Sized> std::hash::Hash for Ref<T> {
+    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
+        self.0.hash(state);
+        self.1.hash(state);
+    }
+}
+
+impl<T: ?Sized> PartialEq for Ref<T> {
+    fn eq(&self, other: &Self) -> bool {
+        self.0 == other.0 && self.1 == other.1
+    }
+}
+
+impl<T: ?Sized> Eq for Ref<T> {}
+
+impl<T: ?Sized> PartialOrd for Ref<T> {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl<T: ?Sized> Ord for Ref<T> {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        self.0.cmp(&other.0)
+    }
+}
+
+impl<T: ?Sized> Clone for Ref<T> {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+impl<T: ?Sized> Copy for Ref<T> {}