scyther/src/mgu.c

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#include <stdlib.h>
#include <stdio.h>
#include "term.h"
#include "termlist.h"
#include "substitution.h"
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#include "mgu.h"
#include "memory.h"
#include "type.h"
#include "specialterm.h"
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/*
Most General Unifier
Unification etc.
New version yields a termlist with substituted variables, which can later be reset to NULL.
*/
//! Internal constant. If true, typed checking
/**
* Analoguous to switches.match
* 0 typed
* 1 basic typeflaws
* 2 all typeflaws
*/
static int mgu_match = 0;
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//! Set mgu mode (basically switches.match)
void
setMguMode (const int match)
{
mgu_match = match;
}
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void
showSubst (Term t)
{
#ifdef DEBUG
if (!DEBUGL (5))
return;
indent ();
printf ("Substituting ");
termPrint (t);
printf (", typed ");
termlistPrint (t->stype);
if (realTermLeaf (t->subst))
{
printf ("->");
termlistPrint (t->subst->stype);
}
else
{
printf (", composite term");
}
if (t->type != VARIABLE)
{
printf (" (bound roleconstant)");
}
printf ("\n");
#endif
}
//! See if this is preferred substitution
/**
* By default, ta->tb will map. Returning 0 (false) will swap them.
*/
int
preferSubstitutionOrder (Term ta, Term tb)
{
if (termlistLength (ta->stype) == 1 && inTermlist (ta->stype, TERM_Agent))
{
/**
* If the first one is an agent type, we prefer swapping.
*/
return 0;
}
// Per default, leave it as it is.
return 1;
}
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//! See if a substitution is valid
__inline__ int
goodsubst (Term tvar, Term tsubst)
{
Term tbuf;
int res;
tbuf = tvar->subst;
tvar->subst = tsubst;
res = checkTypeTerm (mgu_match, tvar);
tvar->subst = tbuf;
return res;
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}
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//! Undo all substitutions in a list of variables.
/**
* The termlist should contain only variables.
*/
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void
termlistSubstReset (Termlist tl)
{
while (tl != NULL)
{
tl->term->subst = NULL;
tl = tl->next;
}
}
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//! Most general unifier.
/**
* Try to determine the most general unifier of two terms.
* Resulting termlist must be termlistDelete'd.
*
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*@return Returns a list of variables, that were previously open, but are now closed
* in such a way that the two terms unify. Returns \ref MGUFAIL if it is impossible.
*/
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Termlist
termMguTerm (Term t1, Term t2)
{
/* added for speed */
t1 = deVar (t1);
t2 = deVar (t2);
if (t1 == t2)
return NULL;
if (!(hasTermVariable (t1) || hasTermVariable (t2)))
{
if (isTermEqual (t1, t2))
{
return NULL;
}
else
{
return MGUFAIL;
}
}
/*
* Distinguish a special case where both are unbound variables that will be
* connected, and I want to give one priority over the other for readability.
*
* Because t1 and t2 have been deVar'd means that if they are variables, they
* are also unbound.
*/
if (realTermVariable (t1) && realTermVariable (t2) && goodsubst (t1, t2))
{
/* Both are unbound variables. Decide.
*
* The plan: t1->subst will point to t2. But maybe we prefer the other
* way around?
*/
if (preferSubstitutionOrder (t2, t1))
{
Term t3;
// Swappy.
t3 = t1;
t1 = t2;
t2 = t3;
}
t1->subst = t2;
#ifdef DEBUG
showSubst (t1);
#endif
return termlistAdd (NULL, t1);
}
/* symmetrical tests for single variable.
*/
if (realTermVariable (t2))
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{
if (termSubTerm (t1, t2) || !goodsubst (t2, t1))
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return MGUFAIL;
else
{
t2->subst = t1;
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#ifdef DEBUG
showSubst (t2);
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#endif
return termlistAdd (NULL, t2);
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}
}
if (realTermVariable (t1))
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{
if (termSubTerm (t2, t1) || !goodsubst (t1, t2))
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return MGUFAIL;
else
{
t1->subst = t2;
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#ifdef DEBUG
showSubst (t1);
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#endif
return termlistAdd (NULL, t1);
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}
}
/* left & right are compounds with variables */
if (t1->type != t2->type)
return MGUFAIL;
/* identical compounds */
/* encryption first */
if (realTermEncrypt (t1))
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{
Termlist tl1, tl2;
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tl1 = termMguTerm (TermKey (t1), TermKey (t2));
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if (tl1 == MGUFAIL)
{
return MGUFAIL;
}
else
{
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tl2 = termMguTerm (TermOp (t1), TermOp (t2));
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if (tl2 == MGUFAIL)
{
termlistSubstReset (tl1);
termlistDelete (tl1);
return MGUFAIL;
}
else
{
return termlistConcat (tl1, tl2);
}
}
}
/* tupling second
non-associative version ! TODO other version */
if (isTermTuple (t1))
{
Termlist tl1, tl2;
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tl1 = termMguTerm (TermOp1 (t1), TermOp1 (t2));
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if (tl1 == MGUFAIL)
{
return MGUFAIL;
}
else
{
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tl2 = termMguTerm (TermOp2 (t1), TermOp2 (t2));
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if (tl2 == MGUFAIL)
{
termlistSubstReset (tl1);
termlistDelete (tl1);
return MGUFAIL;
}
else
{
return termlistConcat (tl1, tl2);
}
}
}
return MGUFAIL;
}
//! Most general interm unifiers of t1 interm t2
/**
* Try to determine the most general interm unifiers of two terms.
*@returns Nothing. Iteration gets termlist of substitutions.
*/
int
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termMguInTerm (Term t1, Term t2, int (*iterator) (Termlist))
{
Termlist tl;
int flag;
flag = 1;
t2 = deVar (t2);
if (t2 != NULL)
{
if (realTermTuple (t2))
{
// t2 is a tuple, consider interm options as well.
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flag = flag && termMguInTerm (t1, TermOp1 (t2), iterator);
flag = flag && termMguInTerm (t1, TermOp2 (t2), iterator);
}
// simple clause or combined
tl = termMguTerm (t1, t2);
if (tl != MGUFAIL)
{
// Iterate
flag = flag && iterator (tl);
// Reset variables
termlistSubstReset (tl);
// Remove list
termlistDelete (tl);
}
}
else
{
if (deVar (t1) != NULL)
{
flag = 0;
}
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}
return flag;
}
//! Most general subterm unifiers of smallterm subterm bigterm
/**
* Try to determine the most general subterm unifiers of two terms.
*@returns Nothing. Iteration gets termlist of subst, and list of keys needed
* to decrypt. This termlist does not need to be deleted, because it is handled
* by the mguSubTerm itself.
*/
int
termMguSubTerm (Term smallterm, Term bigterm,
int (*iterator) (Termlist, Termlist), Termlist inverses,
Termlist cryptlist)
{
int flag;
flag = 1;
smallterm = deVar (smallterm);
bigterm = deVar (bigterm);
if (bigterm != NULL)
{
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Termlist tl;
if (!realTermLeaf (bigterm))
{
if (realTermTuple (bigterm))
{
// 'simple' tuple
flag =
flag
&& termMguSubTerm (smallterm, TermOp1 (bigterm), iterator,
inverses, cryptlist);
flag = flag
&& termMguSubTerm (smallterm, TermOp2 (bigterm), iterator,
inverses, cryptlist);
}
else
{
// Must be encryption
Term keyneeded;
keyneeded = inverseKey (inverses, TermKey (bigterm));
// We can never produce the TERM_Hidden key, thus, this is not a valid iteration.
if (!isTermEqual (keyneeded, TERM_Hidden))
{
cryptlist = termlistAdd (cryptlist, bigterm); // Append, so the last encrypted term in the list is the most 'inner' one, and the first is the outer one.
// Recurse
flag =
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flag
&& termMguSubTerm (smallterm, TermOp (bigterm), iterator,
inverses, cryptlist);
cryptlist = termlistDelTerm (cryptlist);
}
termDelete (keyneeded);
}
}
// simple clause or combined
tl = termMguTerm (smallterm, bigterm);
if (tl != MGUFAIL)
{
// Iterate
flag = flag && iterator (tl, cryptlist);
// Reset variables
termlistSubstReset (tl);
// Remove list
termlistDelete (tl);
}
}
else
{
if (smallterm != NULL)
{
flag = 0;
}
}
return flag;
}