Checkpoint

src/prolog/unify.kt

@@ -1,7 +1,83 @@
 package prolog
 
-import prolog.terms.Term
+import prolog.builtins.atomic
+import prolog.builtins.compound
+import prolog.builtins.variable
+import prolog.components.Goal
+import prolog.components.terms.Term
+import prolog.components.terms.Variable
+import prolog.components.terms.Structure
+import java.util.*
 
-fun unify(term1: Term, term2: Term): Result {
-    return Result(term1 == term2, emptyList())
+typealias Substitution = Map<Variable, Term>
+
+// Apply substitutions to a term
+fun applySubstitution(term: Term, substitution: Substitution): Term = when {
+    variable(term) -> (term as Variable).alias().map { applySubstitution(it, substitution) }.orElse(term)
+    atomic(term)   -> term
+    compound(term) -> {
+        val structure = term as Structure
+        Structure(structure.name, structure.arguments.map { applySubstitution(it, substitution) })
+    }
+    else -> term
+}
+
+// Check if a variable occurs in a term
+fun occurs(variable: Variable, term: Term): Boolean = when {
+    variable(term) -> term == variable
+    atomic(term) -> false
+    compound(term) -> {
+        val structure = term as Structure
+        structure.arguments.any { occurs(variable, it) }
+    }
+    else -> false
+}
+
+// Generate possible substitutions
+fun generateSubstitutions(term1: Term, term2: Term, substitution: Substitution): Sequence<Substitution> = sequence {
+    val t1 = applySubstitution(term1, substitution)
+    val t2 = applySubstitution(term2, substitution)
+
+    when {
+        t1 == t2      -> yield(substitution)
+        variable(t1) -> {
+            val variable = t1 as Variable
+            if (!occurs(variable, t2)) {
+                variable.bind(t2)
+                yield(substitution + (variable to t2))
+            }
+        }
+        variable(t2) -> {
+            val variable = t2 as Variable
+            if (!occurs(variable, t1)) {
+                variable.bind(t1)
+                yield(substitution + (variable to t1))
+            }
+        }
+        compound(t1) && compound(t2) -> {
+            val structure1 = t1 as Structure
+            val structure2 = t2 as Structure
+            if (structure1.functor == structure2.functor) {
+                val newSubstitution = structure1.arguments.zip(structure2.arguments).fold(substitution) { acc, (arg1, arg2) ->
+                    generateSubstitutions(arg1, arg2, acc).firstOrNull() ?: return@sequence
+                }
+                yield(newSubstitution)
+            }
+        }
+        else -> {}
+    }
+}
+
+// Unify two terms with backtracking and lazy evaluation
+fun unifyLazy(term1: Term, term2: Term, substitution: Substitution = emptyMap()): Sequence<Substitution> {
+    return generateSubstitutions(term1, term2, substitution)
+}
+
+fun unify(term1: Term, term2: Term): Optional<Substitution> {
+    val substitutions = unifyLazy(term1, term2).toList()
+    return if (substitutions.isEmpty()) {
+        Optional.empty()
+    } else {
+        Optional.of(substitutions.first())
+    }
 }