Use TreeLike to implement translate_pk

Remove recursive calls and use `TreeLike`'s post order iterator to
implement `translate_pk` for the `concrete::Policy`.

Note, no additional unit tests added for this code path, this is a
familiar code pattern now.

Author: tcharding

src/policy/concrete.rs

@@ -560,50 +560,36 @@ impl<Pk: MiniscriptKey> Policy<Pk> {
         T: Translator<Pk, Q, E>,
         Q: MiniscriptKey,
     {
-        self._translate_pk(t)
-    }
+        use Policy::*;
 
-    fn _translate_pk<Q, E, T>(&self, t: &mut T) -> Result<Policy<Q>, E>
-    where
-        T: Translator<Pk, Q, E>,
-        Q: MiniscriptKey,
-    {
-        match *self {
-            Policy::Unsatisfiable => Ok(Policy::Unsatisfiable),
-            Policy::Trivial => Ok(Policy::Trivial),
-            Policy::Key(ref pk) => t.pk(pk).map(Policy::Key),
-            Policy::Sha256(ref h) => t.sha256(h).map(Policy::Sha256),
-            Policy::Hash256(ref h) => t.hash256(h).map(Policy::Hash256),
-            Policy::Ripemd160(ref h) => t.ripemd160(h).map(Policy::Ripemd160),
-            Policy::Hash160(ref h) => t.hash160(h).map(Policy::Hash160),
-            Policy::Older(n) => Ok(Policy::Older(n)),
-            Policy::After(n) => Ok(Policy::After(n)),
-            Policy::Threshold(k, ref subs) => {
-                let new_subs: Result<Vec<Policy<Q>>, _> =
-                    subs.iter().map(|sub| sub._translate_pk(t)).collect();
-                new_subs
-                    .map(|ok| Policy::Threshold(k, ok.into_iter().map(|p| Arc::new(p)).collect()))
-            }
-            Policy::And(ref subs) => {
-                let new_subs = subs
-                    .iter()
-                    .map(|sub| sub._translate_pk(t))
-                    .collect::<Result<Vec<Policy<Q>>, E>>()?;
-                Ok(Policy::And(new_subs.into_iter().map(|p| Arc::new(p)).collect()))
-            }
-            Policy::Or(ref subs) => {
-                let new_subs = subs
+        let mut translated = vec![];
+        for data in self.post_order_iter() {
+            let child_n = |n| Arc::clone(&translated[data.child_indices[n]]);
+
+            let new_policy = match data.node {
+                Unsatisfiable => Unsatisfiable,
+                Trivial => Trivial,
+                Key(ref pk) => t.pk(pk).map(Key)?,
+                Sha256(ref h) => t.sha256(h).map(Sha256)?,
+                Hash256(ref h) => t.hash256(h).map(Hash256)?,
+                Ripemd160(ref h) => t.ripemd160(h).map(Ripemd160)?,
+                Hash160(ref h) => t.hash160(h).map(Hash160)?,
+                Older(ref n) => Older(*n),
+                After(ref n) => After(*n),
+                Threshold(ref k, ref subs) => Threshold(*k, (0..subs.len()).map(child_n).collect()),
+                And(ref subs) => And((0..subs.len()).map(child_n).collect()),
+                Or(ref subs) => Or(subs
                     .iter()
-                    .map(|(prob, sub)| Ok((*prob, sub._translate_pk(t)?)))
-                    .collect::<Result<Vec<(usize, Policy<Q>)>, E>>()?;
-                Ok(Policy::Or(
-                    new_subs
-                        .into_iter()
-                        .map(|(prob, sub)| (prob, Arc::new(sub)))
-                        .collect(),
-                ))
-            }
+                    .enumerate()
+                    .map(|(i, (prob, _))| (*prob, child_n(i)))
+                    .collect()),
+            };
+            translated.push(Arc::new(new_policy));
         }
+        // Unwrap is ok because we know we processed at least one node.
+        let root_node = translated.pop().unwrap();
+        // Unwrap is ok because we know `root_node` is the only strong reference.
+        Ok(Arc::try_unwrap(root_node).unwrap())
     }
 
     /// Translates `Concrete::Key(key)` to `Concrete::Unsatisfiable` when extracting `TapKey`.