path: root/eval.c

#include <u.h>
#include <libc.h>

#include "dat.h"
#include "fns.h"

Goal *addgoals(Goal *, Term *);
Term *findclause(Term *, Term *, Binding **);
int equalterms(Term *, Term *);
void applybinding(Term *, Binding *);
Goal *copygoals(Goal *);
Builtin findbuiltin(Term *);

static uvlong clausenr;

int
evalquery(Term *database, Term *query, Binding **resultbindings, Choicepoint **resultchoicestack)
{
	Goal *goals;
	Choicepoint *choicestack = *resultchoicestack;

	if(choicestack == nil){
	/*
		The goal stack has the original query at the very bottom, protected by a goal there the ->goal field is nil.
		This makes it so that we can continue until we hit the protective goal, at which point we have solved everything
		and to get the result we can unify the original query with the one at the bottom of the stack, to get the bindings
		applied.
	*/
		goals = malloc(sizeof(Goal));
		goals->goal = copyterm(query, nil);
		goals->next = nil;
		Goal *protector = malloc(sizeof(Goal));
		protector->goal = nil;
		protector->next = goals;
		goals = protector;

		/* Now add the actual goals */
		goals = addgoals(goals, query);

		clausenr = 2; /* Start at two since 0 is for the facts in the database, and 1 is for queries */
	}else{
		goto Backtrack;
	}

	while(goals->goal != nil){
		Term *dbstart;
		Term *goal;

		dbstart = database;
Retry:
		goal = goals->goal;

		if(debug)
			print("Working goal: %S\n", prettyprint(goal));

		Binding *bindings = nil;
		Term *clause = nil;

		/* Try to see if the goal can be solved using a builtin first */
		Builtin builtin = findbuiltin(goal);
		if(builtin != nil){
			int success = builtin(database, goal, &goals->next, &choicestack, &bindings);
			if(!success)
				goto Backtrack;
		}else{
			/* Find a clause where the head unifies with the goal */
			clause = findclause(dbstart, goal, &bindings);
			if(clause != nil){
				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;
					cp->id = clause->clausenr;
					choicestack = cp;
				}
			}else{
Backtrack:
				if(choicestack == nil)
					return 0;
				if(debug)
					print("Backtracking..\n");

				Choicepoint *cp = choicestack;
				choicestack = cp->next;
				/* freegoals(goals) */
				goals = cp->goalstack;
				dbstart = cp->retryclause;
				goto Retry;
			}
		}
		
		goals = goals->next;

		/* Apply bindings to all goals on the stack. */
		Goal *g;
		for(g = goals; g != nil; g = g->next){
			if(g->goal != nil)
				applybinding(g->goal, bindings);
		}

		/* Add clause body as goals, with bindings applied */
		if(clause != nil && clause->tag == CompoundTerm && clause->arity == 2 && runestrcmp(clause->text, L":-") == 0){
			Term *subgoal = copyterm(clause->children->next, nil);
			applybinding(subgoal, bindings);
			goals = addgoals(goals, subgoal);
		}
	}
	goals = goals->next;
	unify(query, goals->goal, resultbindings);
	*resultchoicestack = choicestack;
	return 1;
}

Goal *
addgoals(Goal *goals, Term *t)
{
	if(t->tag == CompoundTerm && runestrcmp(t->text, L",") == 0 && t->arity == 2){
		goals = addgoals(goals, t->children->next);
		goals = addgoals(goals, t->children);
	}else{
		Goal *g = malloc(sizeof(Goal));
		g->goal = t;
		g->next = goals;
		goals = g;
	}
	return goals;
}

Term *
findclause(Term *database, Term *goal, Binding **bindings)
{
	Term *clause;
	Term *head;
	for(; database != nil; database = database->next){
		clause = copyterm(database, &clausenr);
		clausenr++;
		clause->next = database->next;
		if(clause->tag == CompoundTerm && runestrcmp(clause->text, L":-") == 0 && clause->arity == 2)
			head = clause->children;
		else
			head = clause;

		if(unify(head, goal, bindings))
			return clause;
	}
	return nil;
}

int
unify(Term *a, Term *b, Binding **bindings)
{
	Term *leftstack;
	Term *rightstack;
	Term *left;
	Term *right;

	leftstack = copyterm(a, nil);
	rightstack = copyterm(b, nil);

	while(leftstack != nil && rightstack != nil){
		left = leftstack;
		leftstack = left->next;
		right = rightstack;
		rightstack = right->next;

		if(equalterms(left, right))
			continue;
		else if(left->tag == VariableTerm || right->tag == VariableTerm){
			if(right->tag == VariableTerm){
				Term *tmp = left;
				left = right;
				right = tmp;
			}

			if(runestrcmp(left->text, L"_") == 0)
				continue; /* _ doesn't introduce a new binding */

			Binding *b = malloc(sizeof(Binding));
			b->name = left->text;
			b->nr = left->clausenr;
			b->value = right;
			b->next = *bindings;
			*bindings = b;
			Term *t;
			for(t = leftstack; t != nil; t = t->next)
				applybinding(t, b);
			for(t = rightstack; t != nil; t = t->next)
				applybinding(t, b);
			Binding *tmpb;
			for(tmpb = *bindings; tmpb != nil; tmpb = tmpb->next)
				applybinding(tmpb->value, b);
		}else if(left->tag == CompoundTerm && right->tag == CompoundTerm && left->arity == right->arity && runestrcmp(left->text, right->text) == 0){
			Term *leftchild = left->children;
			Term *rightchild = right->children;
			while(leftchild != nil && rightchild != nil){
				Term *t1 = copyterm(leftchild, nil);
				t1->next = leftstack;
				leftstack = t1;
				leftchild = leftchild->next;

				Term *t2 = copyterm(rightchild, nil);
				t2->next = rightstack;
				rightstack = t2;
				rightchild = rightchild->next;
			}
		}else
			return 0; /* failure */
	}
	return 1;
}

int
equalterms(Term *a, Term *b)
{
	/* Check that two non-compound terms are identical */
	if(a->tag != b->tag)
		return 0;

	switch(a->tag){
	case AtomTerm:
	case StringTerm:
		return !runestrcmp(a->text, b->text);
	case VariableTerm:
		return (runestrcmp(a->text, b->text) == 0 && a->clausenr == b->clausenr);
	case NumberTerm:
		if(a->numbertype != b->numbertype)
			return 0;
		if(a->numbertype == NumberInt && a->ival == b->ival)
			return 1;
		if(a->numbertype == NumberFloat && a->dval == b->dval)
			return 1;
		return 0;	
	default:
		return 0;
	}
}

void
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 && t->clausenr == b->nr){
				Term *next = t->next;
				memcpy(t, b->value, sizeof(Term));
				t->next = next;
				return;
			}
		}
	}else if(t->tag == CompoundTerm){
		Term *child;
		for(child = t->children; child != nil; child = child->next)
			applybinding(child, bindings);
	}
}

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;
}