1 files changed, 71 insertions, 0 deletions
diff --git a/src/access/mod.rs b/src/access/mod.rs
index e223756..e04be02 100644
--- a/src/access/mod.rs
+++ b/src/access/mod.rs
@@ -97,6 +97,41 @@ impl Capability {
             && (meta::is_protocol(&self.path) || !meta::is_protocol(path))
     }
 
+    /// What both `self` and `other` give, as one capability, if anything: at the path
+    /// both cover (the longer, a `*` of one taking the other's key there), with the
+    /// rights both give. Nothing where the paths part, no right is shared, or the path
+    /// is protocol data that one of them doesn't name (`covers`'s carve-out).
+    pub(crate) fn meet(&self, other: &Capability) -> Option<Capability> {
+        if !agree(&self.path, &other.path) {
+            return None;
+        }
+        let (longer, shorter) = if self.path.len() >= other.path.len() {
+            (&self.path, &other.path)
+        } else {
+            (&other.path, &self.path)
+        };
+        let path: Vec<String> = longer
+            .iter()
+            .enumerate()
+            .map(|(at, segment)| match shorter.get(at) {
+                Some(theirs) if segment == "*" => theirs.clone(),
+                _ => segment.clone(),
+            })
+            .collect();
+        if meta::is_protocol(&path)
+            && !(meta::is_protocol(&self.path) && meta::is_protocol(&other.path))
+        {
+            return None;
+        }
+        let rights: BTreeSet<String> = self.rights.intersection(&other.rights).cloned().collect();
+        (!rights.is_empty()).then(|| Capability {
+            path,
+            rights,
+            by: self.by.clone(),
+            since: self.since,
+        })
+    }
+
     /// Whether `inner` gives nothing it doesn't: at or below its path (a `*` of `inner`
     /// only under one of its own), with some of its rights. Of limits narrowing again.
     pub(crate) fn holds(&self, inner: &Capability) -> bool {
@@ -261,6 +296,42 @@ mod tests {
         assert!(!members.reaches(Right::Read, &path("_continuum/users")));
     }
 
+    /// What two capabilities both give: the deeper path, shared rights, protocol data
+    /// only where both name it.
+    #[test]
+    fn what_both_give() {
+        let both = cap("data/*/x", &["read", "write"])
+            .meet(&cap("data/eu", &["read"]))
+            .unwrap();
+        assert_eq!(
+            (both.path, both.rights),
+            (path("data/eu/x"), ["read".to_owned()].into())
+        );
+        assert!(
+            cap("data/us", &["read"])
+                .meet(&cap("data/eu", &["read"]))
+                .is_none()
+        );
+        assert!(
+            cap("data", &["write"])
+                .meet(&cap("data/eu", &["read"]))
+                .is_none()
+        );
+        // `[]` never covers protocol data; `_continuum` meets `_continuum/members`.
+        assert!(
+            cap("", &["read"])
+                .meet(&cap("_continuum/members", &["read"]))
+                .is_none()
+        );
+        assert_eq!(
+            cap("_continuum", &["read"])
+                .meet(&cap("_continuum/members", &["read"]))
+                .unwrap()
+                .path,
+            path("_continuum/members")
+        );
+    }
+
     /// `*` stands for any one key: in covering, in listings, and in narrowing.
     #[test]
     fn a_star_is_any_key() {