Checkpoint

src/prolog/builtins/unification.kt

@@ -1,24 +1,24 @@
+/**
+ * Comparison and Unification of Terms
+ *
+ * [SWI Prolog Documentation](https://www.swi-prolog.org/pldoc/man?section=compare)
+ */
 package prolog.builtins
 
 import prolog.ast.terms.Atom
-import prolog.ast.terms.Structure
+import prolog.ast.terms.Operator
 import prolog.ast.terms.Term
-import prolog.ast.terms.Variable
+import prolog.logic.Substituted
+import prolog.logic.applySubstitution
+import prolog.logic.equivalent
 
-/**
- * True if Term1 is equivalent to Term2. A variable is only identical to a sharing variable.
- */
-fun equivalent(term1: Term, term2: Term): Boolean {
-    return when {
-        term1 is Variable && term2 is Variable -> term1 == term2
-        term1 is Variable -> term1.alias().isPresent && equivalent(term1.alias().get(), term2)
-        term2 is Variable -> term2.alias().isPresent && equivalent(term2.alias().get(), term1)
-        term1 is Atom && term2 is Atom -> term1.compareTo(term2) == 0
-        term1 is Structure && term2 is Structure -> term1.compareTo(term2) == 0
-        else -> false
+class Equivalent(private val term1: Term, private val term2: Term) : Operator(Atom("=="), term1, term2) {
+    override fun prove(subs: Substituted): Sequence<Substituted> = sequence {
+        val t1 = applySubstitution(term1, subs)
+        val t2 = applySubstitution(term2, subs)
+
+        if (equivalent(t1, t2)) {
+            yield(emptyMap())
+        }
     }
 }
-
-/**
- *
- */