Internal-key extraction done

The internal-key extraction method now selects the most-likely key (in the policy) as suggested by BIP341.
Also corresponding changes have been made to the tests.

Author: SarcasticNastik

src/policy/concrete.rs

@@ -27,8 +27,14 @@ use miniscript::limits::{HEIGHT_TIME_THRESHOLD, SEQUENCE_LOCKTIME_TYPE_FLAG};
 use miniscript::types::extra_props::TimeLockInfo;
 #[cfg(feature = "compiler")]
 use {
-    descriptor::TapTree, miniscript::ScriptContext, policy::compiler,
-    policy::compiler::CompilerError, std::sync::Arc, Descriptor, Miniscript, Tap,
+    descriptor::TapTree,
+    miniscript::ScriptContext,
+    policy::compiler::CompilerError,
+    policy::Concrete,
+    policy::{compiler, Liftable, Semantic},
+    std::collections::HashMap,
+    std::sync::Arc,
+    Descriptor, Miniscript, Tap,
 };
 use {Error, ForEach, ForEachKey, MiniscriptKey};
 
@@ -126,7 +132,30 @@ impl fmt::Display for PolicyError {
 }
 
 impl<Pk: MiniscriptKey> Policy<Pk> {
-    /// Single-Node compilation
+    /// Flatten the [`Policy`] tree structure into a Vector with corresponding leaf probability
+    #[cfg(feature = "compiler")]
+    fn to_tapleaf_prob_vec(&self, prob: f64) -> Vec<(f64, Policy<Pk>)> {
+        match *self {
+            Policy::Or(ref subs) => {
+                let total_odds: usize = subs.iter().map(|(ref k, _)| k).sum();
+                subs.iter()
+                    .map(|(k, ref policy)| {
+                        policy.to_tapleaf_prob_vec(prob * *k as f64 / total_odds as f64)
+                    })
+                    .flatten()
+                    .collect::<Vec<_>>()
+            }
+            Policy::Threshold(k, ref subs) if k == 1 => {
+                let total_odds = subs.len();
+                subs.iter()
+                    .map(|policy| policy.to_tapleaf_prob_vec(prob / total_odds as f64))
+                    .flatten()
+                    .collect::<Vec<_>>()
+            }
+            ref x => vec![(prob, x.clone())],
+        }
+    }
+
     #[cfg(feature = "compiler")]
     fn compile_leaf_taptree(&self) -> Result<TapTree<Pk>, Error> {
         let compilation = self.compile::<Tap>().unwrap();
@@ -135,17 +164,51 @@ impl<Pk: MiniscriptKey> Policy<Pk> {
 
     /// Extract the Taproot internal_key from policy tree.
     #[cfg(feature = "compiler")]
-    fn extract_key(&self, unspendable_key: Option<Pk>) -> Result<(Pk, &Policy<Pk>), Error> {
-        match unspendable_key {
-            Some(key) => Ok((key, self)),
-            None => Err(errstr("No internal key found")),
+    fn extract_key(self, unspendable_key: Option<Pk>) -> Result<(Pk, Policy<Pk>), Error> {
+        let mut internal_key: Option<Pk> = None;
+        {
+            let mut prob = 0.;
+            let semantic_policy = self.lift()?;
+            let concrete_keys = self.keys();
+            let key_prob_map: HashMap<_, _> = self
+                .to_tapleaf_prob_vec(1.0)
+                .into_iter()
+                .filter(|(_, ref pol)| match *pol {
+                    Concrete::Key(..) => true,
+                    _ => false,
+                })
+                .map(|(prob, key)| (key, prob))
+                .collect();
+
+            for key in concrete_keys.into_iter() {
+                if semantic_policy
+                    .clone()
+                    .satisfy_constraint(&Semantic::KeyHash(key.to_pubkeyhash()), true)
+                    == Semantic::Trivial
+                {
+                    match key_prob_map.get(&Concrete::Key(key.clone())) {
+                        Some(val) => {
+                            if *val > prob {
+                                prob = *val;
+                                internal_key = Some(key.clone());
+                            }
+                        }
+                        None => return Err(errstr("Key should have existed in the HashMap!")),
+                    }
+                }
+            }
+        }
+        match (internal_key, unspendable_key) {
+            (Some(ref key), _) => Ok((key.clone(), self.translate_unsatisfiable_pk(&key))),
+            (_, Some(key)) => Ok((key, self)),
+            _ => Err(errstr("No viable internal key found.")),
         }
     }
 
     /// Compile the [`Tr`] descriptor into optimized [`TapTree`] implementation
     #[cfg(feature = "compiler")]
     pub fn compile_tr(&self, unspendable_key: Option<Pk>) -> Result<Descriptor<Pk>, Error> {
-        let (internal_key, policy) = self.extract_key(unspendable_key)?;
+        let (internal_key, policy) = self.clone().extract_key(unspendable_key)?;
         let tree = Descriptor::new_tr(internal_key, Some(policy.compile_leaf_taptree()?))?;
         Ok(tree)
     }
@@ -250,6 +313,30 @@ impl<Pk: MiniscriptKey> Policy<Pk> {
         }
     }
 
+    /// Translate `Semantic::Key(key)` to `Semantic::Unsatisfiable` when extracting TapKey
+    pub fn translate_unsatisfiable_pk(self, key: &Pk) -> Policy<Pk> {
+        match self {
+            Policy::Key(ref k) if k.clone() == *key => Policy::Unsatisfiable,
+            Policy::And(subs) => Policy::And(
+                subs.into_iter()
+                    .map(|sub| sub.translate_unsatisfiable_pk(key))
+                    .collect::<Vec<_>>(),
+            ),
+            Policy::Or(subs) => Policy::Or(
+                subs.into_iter()
+                    .map(|(k, sub)| (k, sub.translate_unsatisfiable_pk(key)))
+                    .collect::<Vec<_>>(),
+            ),
+            Policy::Threshold(k, subs) => Policy::Threshold(
+                k,
+                subs.into_iter()
+                    .map(|sub| sub.translate_unsatisfiable_pk(key))
+                    .collect::<Vec<_>>(),
+            ),
+            x => x,
+        }
+    }
+
     /// Get all keys in the policy
     pub fn keys(&self) -> Vec<&Pk> {
         match *self {

src/policy/semantic.rs

@@ -189,7 +189,7 @@ impl<Pk: MiniscriptKey> Policy<Pk> {
     // policy.
     // Witness is currently encoded as policy. Only accepts leaf fragment and
     // a normalized policy
-    fn satisfy_constraint(self, witness: &Policy<Pk>, available: bool) -> Policy<Pk> {
+    pub(crate) fn satisfy_constraint(self, witness: &Policy<Pk>, available: bool) -> Policy<Pk> {
         debug_assert!(self.clone().normalized() == self.clone());
         match *witness {
             // only for internal purposes, safe to use unreachable!