ref: 347e5bc533070a5e988d82e7588a4e905c7096f3
dir: pprolog/eval.c
#include <u.h>
#include <libc.h>
#include "dat.h"
#include "fns.h"
typedef struct Goal Goal;
typedef struct Choicepoint Choicepoint;
struct Goal
{
Term *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;
int
evalquery(Term *database, Term *query, Binding **resultbindings)
{
Goal *goals;
Choicepoint *choicestack = nil;
clausenr = 2; /* Start at two since 0 is for the facts in the database, and 1 is for queries */
/*
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);
while(goals->goal != nil){
Term *dbstart;
Term *goal;
dbstart = database;
Retry:
goal = goals->goal;
/* Find a clause where the head unifies with the goal */
Binding *bindings = nil;
Term *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;
choicestack = cp;
}
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->tag == CompoundTerm && clause->arity == 2 && runestrcmp(clause->text, L":-") == 0){
Term *subgoal = copyterm(clause->children->next, nil);
applybinding(subgoal, bindings);
goals = addgoals(goals, subgoal);
}
}else{
if(choicestack == nil)
return 0;
Choicepoint *cp = choicestack;
choicestack = cp->next;
/* freegoals(goals) */
goals = cp->goalstack;
dbstart = cp->retryclause;
goto Retry;
}
}
goals = goals->next;
unify(query, goals->goal, resultbindings);
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;
*bindings = nil;
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;
}