162 lines
2.8 KiB
C
162 lines
2.8 KiB
C
#include <stdlib.h>
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#include <stdio.h>
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#include "terms.h"
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#include "termlists.h"
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#include "substitutions.h"
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#include "mgu.h"
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#include "memory.h"
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/*
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Most General Unifier
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Unification etc.
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New version yields a termlist with substituted variables, which can later be reset to NULL.
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*/
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//! Undo all substitutions in a list of variables.
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/**
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* The termlist should contain only variables.
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*/
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void
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termlistSubstReset (Termlist tl)
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{
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while (tl != NULL)
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{
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tl->term->subst = NULL;
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tl = tl->next;
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}
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}
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//! Most general unifier.
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/**
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* Try to determine the most general unifier of two terms.
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*@return Returns a list of variables, that were previously open, but are now closed
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* in such a way that the two terms unify. Returns \ref MGUFAIL if it is impossible.
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*/
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Termlist
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termMguTerm (Term t1, Term t2)
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{
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/* added for speed */
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t1 = deVar (t1);
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t2 = deVar (t2);
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if (t1 == t2)
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return NULL;
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#ifdef DEBUG
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void showSubst (Term t)
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{
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if (!DEBUGL (5))
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return;
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indent ();
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printf ("Substituting ");
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termPrint (t);
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printf (", typed ");
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termlistPrint (t->stype);
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printf ("->");
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termlistPrint (t->subst->stype);
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printf ("\n");
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}
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#endif
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if (!(hasTermVariable (t1) || hasTermVariable (t2)))
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{
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if (isTermEqual (t1, t2))
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{
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return NULL;
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}
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else
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{
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return MGUFAIL;
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}
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}
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/* symmetrical tests for single variable */
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if (isTermVariable (t1))
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{
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if (termOccurs (t2, t1))
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return MGUFAIL;
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else
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{
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t1->subst = t2;
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#ifdef DEBUG
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showSubst (t1);
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#endif
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return termlistAdd (NULL, t1);
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}
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}
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if (isTermVariable (t2))
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{
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if (termOccurs (t1, t2))
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return MGUFAIL;
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else
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{
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t2->subst = t1;
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#ifdef DEBUG
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showSubst (t2);
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#endif
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return termlistAdd (NULL, t2);
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}
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}
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/* left & right are compounds with variables */
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if (t1->type != t2->type)
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return MGUFAIL;
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/* identical compounds */
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/* encryption first */
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if (isTermEncrypt (t1))
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{
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Termlist tl1, tl2;
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tl1 = termMguTerm (t1->key, t2->key);
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if (tl1 == MGUFAIL)
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{
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return MGUFAIL;
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}
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else
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{
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tl2 = termMguTerm (t1->op, t2->op);
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if (tl2 == MGUFAIL)
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{
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termlistSubstReset (tl1);
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termlistDelete (tl1);
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return MGUFAIL;
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}
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else
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{
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return termlistConcat (tl1, tl2);
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}
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}
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}
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/* tupling second
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non-associative version ! TODO other version */
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if (isTermTuple (t1))
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{
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Termlist tl1, tl2;
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tl1 = termMguTerm (t1->op1, t2->op1);
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if (tl1 == MGUFAIL)
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{
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return MGUFAIL;
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}
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else
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{
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tl2 = termMguTerm (t1->op2, t2->op2);
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if (tl2 == MGUFAIL)
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{
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termlistSubstReset (tl1);
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termlistDelete (tl1);
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return MGUFAIL;
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}
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else
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{
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return termlistConcat (tl1, tl2);
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}
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}
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}
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return MGUFAIL;
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}
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