Turn `Redactable` into a semilattice

Signed-off-by: Alexis Sellier <alexis@radicle.xyz>

radicle-cob/src/history/entry.rs

@@ -39,6 +39,7 @@ pub struct Entry {
     /// The identifier for this entry
     pub(super) id: EntryId,
     /// The content-address for this entry's author.
+    /// TODO: This shouldn't be here?
     pub(super) author: Option<Oid>,
     /// The content-address for the resource this entry lives under.
     pub(super) resource: Oid,

radicle-crdt/src/lib.rs

@@ -6,6 +6,7 @@ pub mod lwwmap;
 pub mod lwwreg;
 pub mod lwwset;
 pub mod ord;
+pub mod redactable;
 pub mod thread;
 
 #[cfg(test)]

radicle-crdt/src/redactable.rs

@@ -0,0 +1,60 @@
+use crate::Semilattice;
+
+/// An object that can be either present or removed.
+#[derive(Default, Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Redactable<T> {
+    /// When the object is present.
+    Present(T),
+    /// When the object has been removed.
+    #[default]
+    Redacted,
+}
+
+impl<T> From<Option<T>> for Redactable<T> {
+    fn from(option: Option<T>) -> Self {
+        match option {
+            Some(v) => Self::Present(v),
+            None => Self::Redacted,
+        }
+    }
+}
+
+impl<T: PartialOrd> Redactable<T> {
+    pub fn merge(&mut self, other: Self) {
+        match (&self, other) {
+            (Self::Redacted, _) => {}
+            (Self::Present(_), Self::Redacted) => {
+                *self = Self::Redacted;
+            }
+            (Self::Present(a), Self::Present(b)) => {
+                if &b > a {
+                    *self = Self::Present(b);
+                }
+            }
+        }
+    }
+}
+
+impl<T: PartialOrd> Semilattice for Redactable<T> {
+    fn join(mut self, other: Self) -> Self {
+        self.merge(other);
+        self
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use quickcheck_macros::quickcheck;
+
+    use super::*;
+    use crate::test;
+
+    #[quickcheck]
+    fn prop_invariants(a: Option<u8>, b: Option<u8>, c: Option<u8>) {
+        let a = Redactable::from(a);
+        let b = Redactable::from(b);
+        let c = Redactable::from(c);
+
+        test::assert_laws(&a, &b, &c);
+    }
+}

radicle-crdt/src/thread.rs

@@ -11,6 +11,7 @@ use radicle::hash;
 use crate::clock::LClock;
 use crate::lwwreg::LWWReg;
 use crate::lwwset::LWWSet;
+use crate::redactable::Redactable;
 
 /// Identifies a change.
 pub type ChangeId = radicle::hash::Digest;
@@ -63,16 +64,6 @@ pub struct Envelope {
     pub signature: Signature,
 }
 
-/// An object that can be either present or removed.
-#[derive(Default, Debug, Clone, Copy, PartialEq, Eq)]
-pub enum Redactable<T> {
-    /// When the object is present.
-    Present(T),
-    /// When the object has been removed.
-    #[default]
-    Redacted,
-}
-
 /// A comment on a discussion thread.
 #[derive(Default, Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub struct Comment {