Add backtracking to the evaluator. This means we have to keep track of choicepoints which is implemented the easy but wasteful way for now.

I have also added a number which is used to differentiate variables from different application of the clauses.

1 files changed, 73 insertions, 22 deletions

diff --git a/eval.c b/eval.c
index e04963f..854d69c 100644
--- a/eval.c
+++ b/eval.c
@@ -6,10 +6,12 @@
 
 typedef struct Binding Binding;
 typedef struct Goal Goal;
+typedef struct Choicepoint Choicepoint;
 
 struct Binding
 {
 	Rune *name;
+	uvlong nr; /* Unique number for each clause. Every time a clause is used, it gets a new number. */
 	Term *value;
 	Binding *next;
 };
@@ -20,23 +22,40 @@ struct Goal
 	Goal *next;
 };
 
+struct Choicepoint
+{
+	Goal *goalstack;
+	Term *retryclause;
+	Choicepoint *next;
+};
+
 Goal *addgoals(Goal *, Term *);
 Term *findclause(Term *, Term *, Binding **);
 int unify(Term *, Term *, Binding **);
 int equalterms(Term *, Term *);
 void applybinding(Term *, Binding *);
+Goal *copygoals(Goal *);
+
+static uvlong clausenr;
 
 void
 evalquery(Term *database, Term *query)
 {
 	Goal *goals = addgoals(nil, query);
+	Choicepoint *choicestack = nil;
+
+	clausenr = 0;
 
 	while(goals != nil){
-		Term *goal = goals->goal;
-		goals = goals->next;
+		Term *dbstart;
+		Term *goal;
+
+		dbstart = database;
+Retry:
+		goal = goals->goal;
 
 		if(goal == nil){
-			print("Done with body\n");
+			goals = goals->next;
 			continue;
 		}
 
@@ -44,17 +63,20 @@ evalquery(Term *database, Term *query)
 
 		/* Find a clause where the head unifies with the goal */
 		Binding *bindings = nil;
-		Term *clause = findclause(database, goal, &bindings);
+		Term *clause = findclause(dbstart, goal, &bindings);
 		if(clause != nil){
-			print("Solving it using clause %S with new bindings: ", prettyprint(clause));
-			Binding *b;
-			for(b = bindings; b != nil; b = b->next){
-				print("%S = %S ", b->name, prettyprint(b->value));
+			if(clause->next != nil){
+				/* Add a choicepoint. Note we create a choicepoint every time, so there is room for improvement. */
+				Choicepoint *cp = malloc(sizeof(Choicepoint));
+				cp->goalstack = copygoals(goals);
+				cp->next = choicestack;
+				cp->retryclause = clause->next;
+				choicestack = cp;
 			}
-			print("\n");
+			goals = goals->next;
 			/* Apply bindings to all goals on the top of the stack, down to the "bodystart" goal */
 			Goal *g;
-			for(g = goals; g != nil && g->next != nil; g = g->next)
+			for(g = goals; g != nil && g->goal != nil; g = g->next)
 				applybinding(g->goal, bindings);
 			
 			/* Add clause body as goals, with bindings applied */
@@ -64,14 +86,26 @@ evalquery(Term *database, Term *query)
 				bodystart->next = goals;
 				goals = bodystart;
 
-				Term *subgoal = copyterm(clause->children->next);
+				Term *subgoal = copyterm(clause->children->next, nil);
 				applybinding(subgoal, bindings);
 				goals = addgoals(goals, subgoal);
 			}
 		}else{
-			print("No clause unifies with the goal, backtracking...(not really yet haha)\n");
+			if(choicestack == nil){
+				print("Fail\n");
+				return;
+			}
+			print("Backtracking...\n");
+			Choicepoint *cp = choicestack;
+			choicestack = cp->next;
+			/* freegoals(goals) */
+			goals = cp->goalstack;
+			dbstart = cp->retryclause;
+
+			goto Retry;
 		}
 	}
+	print("Success.\n");
 }
 
 Goal *
@@ -84,7 +118,6 @@ addgoals(Goal *goals, Term *t)
 		Goal *g = malloc(sizeof(Goal));
 		g->goal = t;
 		g->next = goals;
-		print("Added goal: %S\n", prettyprint(t));
 		goals = g;
 	}
 	return goals;
@@ -94,8 +127,11 @@ Term *
 findclause(Term *database, Term *goal, Binding **bindings)
 {
 	Term *clause;
-	for(clause = database; clause != nil; clause = clause->next){
-		Term *head;
+	Term *head;
+	for(; database != nil; database = database->next){
+		clausenr++;
+		clause = copyterm(database, &clausenr);
+		clause->next = database->next;
 		if(clause->tag == CompoundTerm && runestrcmp(clause->text, L":-") == 0 && clause->arity == 2)
 			head = clause->children;
 		else
@@ -116,8 +152,8 @@ unify(Term *a, Term *b, Binding **bindings)
 	Term *right;
 
 	*bindings = nil;
-	leftstack = copyterm(a);
-	rightstack = copyterm(b);
+	leftstack = copyterm(a, nil);
+	rightstack = copyterm(b, nil);
 
 	while(leftstack != nil && rightstack != nil){
 		left = leftstack;
@@ -125,7 +161,6 @@ unify(Term *a, Term *b, Binding **bindings)
 		right = rightstack;
 		rightstack = right->next;
 
-		print("Unifying:\n\t%S\n\t%S\n", prettyprint(left), prettyprint(right));
 		if(equalterms(left, right))
 			continue;
 		else if(left->tag == VariableTerm || right->tag == VariableTerm){
@@ -136,6 +171,7 @@ unify(Term *a, Term *b, Binding **bindings)
 			}
 			Binding *b = malloc(sizeof(Binding));
 			b->name = left->text;
+			b->nr = left->clausenr;
 			b->value = right;
 			b->next = *bindings;
 			*bindings = b;
@@ -151,12 +187,12 @@ unify(Term *a, Term *b, Binding **bindings)
 			Term *leftchild = left->children;
 			Term *rightchild = right->children;
 			while(leftchild != nil && rightchild != nil){
-				Term *t1 = copyterm(leftchild);
+				Term *t1 = copyterm(leftchild, nil);
 				t1->next = leftstack;
 				leftstack = t1;
 				leftchild = leftchild->next;
 
-				Term *t2 = copyterm(rightchild);
+				Term *t2 = copyterm(rightchild, nil);
 				t2->next = rightstack;
 				rightstack = t2;
 				rightchild = rightchild->next;
@@ -198,7 +234,7 @@ applybinding(Term *t, Binding *bindings)
 	if(t->tag == VariableTerm){
 		Binding *b;
 		for(b = bindings; b != nil; b = b->next){
-			if(runestrcmp(t->text, b->name) == 0){
+			if(runestrcmp(t->text, b->name) == 0 && t->clausenr == b->nr){
 				Term *next = t->next;
 				memcpy(t, b->value, sizeof(Term));
 				t->next = next;
@@ -210,4 +246,19 @@ applybinding(Term *t, Binding *bindings)
 		for(child = t->children; child != nil; child = child->next)
 			applybinding(child, bindings);
 	}
-}
\ No newline at end of file
+}
+
+Goal *
+copygoals(Goal *goals)
+{
+	if(goals != nil){
+		Goal *g = malloc(sizeof(Goal));
+		if(goals->goal)
+			g->goal = copyterm(goals->goal, nil);
+		else
+			g->goal = nil;
+		g->next = copygoals(goals->next);
+		return g;
+	}else
+		return nil;
+}