Avoid a Box, support custom heuristics functions

lib/src/adf.rs

@@ -1066,6 +1066,19 @@ mod test {
             stable.collect::<Vec<_>>(),
             vec![vec![Term(1), Term(0)], vec![Term(0), Term(1)]]
         );
+
+        let stable = adf.stable_nogood(Heuristic::Custom(&|_adf, interpr| {
+            for (idx, term) in interpr.iter().enumerate() {
+                if !term.is_truth_value() {
+                    return Some((Var(idx), Term::BOT));
+                }
+            }
+            None
+        }));
+        assert_eq!(
+            stable.collect::<Vec<_>>(),
+            vec![vec![Term(0), Term(1)], vec![Term(1), Term(0)]]
+        );
     }
 
     #[test]

lib/src/adf/heuristics.rs

@@ -14,21 +14,35 @@ pub(crate) fn heu_simple(_adf: &Adf, interpr: &[Term]) -> Option<(Var, Term)> {
     None
 }
 
-type RetVal = Option<(Var, Term)>;
+/// Return value for heuristics.
+pub type RetVal = Option<(Var, Term)>;
+/// Signature for heuristics functions.
+pub type HeuristicFn = dyn Fn(&Adf, &[Term]) -> RetVal;
 
 /// Enumeration of all currently implemented heuristics.
 /// It represents a public view on the crate-view implementations of heuristics.
-#[derive(Debug, Clone, Copy)]
+pub enum Heuristic<'a> {
-pub enum Heuristic {
     /// Implementation of a simple heuristic.
     /// This will just take the first not decided variable and maps it value to (`true`)[Term::TOP].   
     Simple,
+    /// Allow passing in an externally-defined custom heuristic.
+    Custom(&'a HeuristicFn),
 }
 
-impl Heuristic {
+impl std::fmt::Debug for Heuristic<'_> {
-    pub(crate) fn get_heuristic(&self) -> Box<dyn Fn(&Adf, &[Term]) -> RetVal> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
-            Heuristic::Simple => Box::new(heu_simple),
+            Self::Simple => write!(f, "Simple"),
+            Self::Custom(_) => f.debug_tuple("Custom function").finish(),
+        }
+    }
+}
+
+impl Heuristic<'_> {
+    pub(crate) fn get_heuristic(&self) -> &(dyn Fn(&Adf, &[Term]) -> RetVal + '_) {
+        match self {
+            Heuristic::Simple => &heu_simple,
+            Self::Custom(f) => f,
         }
     }
 }