fixup! Add better Policy

Author: gregoire-mullvad

src/policy/mod.rs

@@ -75,17 +75,20 @@ impl Policy {
             url: entity.1.clone(),
         })
     }
-
-    pub fn quorum(&self) -> Option<String> {
-        todo!()
-    }
 }
 
 // Quorum is an internal enum that represent possible quorum values, i.e. witnesses and groups.
 #[derive(Debug, Eq, PartialEq)]
 enum Quorum {
+    // A single witness, identified by its keyhash.
+    // Invariant: if Witness(h) is in Policy.quorums, then h must be in Policy.witnesses.
     Witness(Hash),
-    Group(usize, Vec<String>),
+
+    // A Group that requires at least k of its subquorums to pass.
+    // Invariant: k <= length(members)
+    // Invariant: if a group is in Policy.quorums, then all its members must be in
+    // Policy.quorums as well.
+    Group { k: usize, members: Vec<String> },
 }
 
 // Entity is a struct that is used internally to keep track of log/witness keys and URLs.
@@ -168,7 +171,7 @@ impl PolicyBuilder {
                 return Err(PolicyError::UnknownName(name.into()));
             }
         }
-        self.0.quorums.insert(name, Quorum::Group(k, members));
+        self.0.quorums.insert(name, Quorum::Group { k, members });
         Ok(())
     }
 
@@ -297,7 +300,10 @@ mod tests {
                 ),
                 (
                     "mygroup".into(),
-                    Quorum::Group(2, vec!["witness01".into(), "witness02".into()]),
+                    Quorum::Group {
+                        k: 2,
+                        members: vec!["witness01".into(), "witness02".into()],
+                    },
                 ),
             ]),
             quorum: Some("mygroup".into()),