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tblcmp.c

/* tblcmp - table compression routines */

/*-
 * Copyright (c) 1990 The Regents of the University of California.
 * All rights reserved.
 *
 * This code is derived from software contributed to Berkeley by
 * Vern Paxson.
 * 
 * The United States Government has rights in this work pursuant
 * to contract no. DE-AC03-76SF00098 between the United States
 * Department of Energy and the University of California.
 *
 * Redistribution and use in source and binary forms with or without
 * modification are permitted provided that: (1) source distributions retain
 * this entire copyright notice and comment, and (2) distributions including
 * binaries display the following acknowledgement:  ``This product includes
 * software developed by the University of California, Berkeley and its
 * contributors'' in the documentation or other materials provided with the
 * distribution and in all advertising materials mentioning features or use
 * of this software.  Neither the name of the University nor the names of
 * its contributors may be used to endorse or promote products derived from
 * this software without specific prior written permission.
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
 * WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
 */

/* $Header: /home/daffy/u0/vern/flex/RCS/tblcmp.c,v 2.11 94/11/05 17:08:28 vern Exp $ */

#include "flexdef.h"


/* declarations for functions that have forward references */

void mkentry PROTO((register int*, int, int, int, int));
void mkprot PROTO((int[], int, int));
void mktemplate PROTO((int[], int, int));
void mv2front PROTO((int));
int tbldiff PROTO((int[], int, int[]));


/* bldtbl - build table entries for dfa state
 *
 * synopsis
 *   int state[numecs], statenum, totaltrans, comstate, comfreq;
 *   bldtbl( state, statenum, totaltrans, comstate, comfreq );
 *
 * State is the statenum'th dfa state.  It is indexed by equivalence class and
 * gives the number of the state to enter for a given equivalence class.
 * totaltrans is the total number of transitions out of the state.  Comstate
 * is that state which is the destination of the most transitions out of State.
 * Comfreq is how many transitions there are out of State to Comstate.
 *
 * A note on terminology:
 *    "protos" are transition tables which have a high probability of
 * either being redundant (a state processed later will have an identical
 * transition table) or nearly redundant (a state processed later will have
 * many of the same out-transitions).  A "most recently used" queue of
 * protos is kept around with the hope that most states will find a proto
 * which is similar enough to be usable, and therefore compacting the
 * output tables.
 *    "templates" are a special type of proto.  If a transition table is
 * homogeneous or nearly homogeneous (all transitions go to the same
 * destination) then the odds are good that future states will also go
 * to the same destination state on basically the same character set.
 * These homogeneous states are so common when dealing with large rule
 * sets that they merit special attention.  If the transition table were
 * simply made into a proto, then (typically) each subsequent, similar
 * state will differ from the proto for two out-transitions.  One of these
 * out-transitions will be that character on which the proto does not go
 * to the common destination, and one will be that character on which the
 * state does not go to the common destination.  Templates, on the other
 * hand, go to the common state on EVERY transition character, and therefore
 * cost only one difference.
 */

void bldtbl( state, statenum, totaltrans, comstate, comfreq )
int state[], statenum, totaltrans, comstate, comfreq;
      {
      int extptr, extrct[2][CSIZE + 1];
      int mindiff, minprot, i, d;

      /* If extptr is 0 then the first array of extrct holds the result
       * of the "best difference" to date, which is those transitions
       * which occur in "state" but not in the proto which, to date,
       * has the fewest differences between itself and "state".  If
       * extptr is 1 then the second array of extrct hold the best
       * difference.  The two arrays are toggled between so that the
       * best difference to date can be kept around and also a difference
       * just created by checking against a candidate "best" proto.
       */

      extptr = 0;

      /* If the state has too few out-transitions, don't bother trying to
       * compact its tables.
       */

      if ( (totaltrans * 100) < (numecs * PROTO_SIZE_PERCENTAGE) )
            mkentry( state, numecs, statenum, JAMSTATE, totaltrans );

      else
            {
            /* "checkcom" is true if we should only check "state" against
             * protos which have the same "comstate" value.
             */
            int checkcom =
                  comfreq * 100 > totaltrans * CHECK_COM_PERCENTAGE;

            minprot = firstprot;
            mindiff = totaltrans;

            if ( checkcom )
                  {
                  /* Find first proto which has the same "comstate". */
                  for ( i = firstprot; i != NIL; i = protnext[i] )
                        if ( protcomst[i] == comstate )
                              {
                              minprot = i;
                              mindiff = tbldiff( state, minprot,
                                          extrct[extptr] );
                              break;
                              }
                  }

            else
                  {
                  /* Since we've decided that the most common destination
                   * out of "state" does not occur with a high enough
                   * frequency, we set the "comstate" to zero, assuring
                   * that if this state is entered into the proto list,
                   * it will not be considered a template.
                   */
                  comstate = 0;

                  if ( firstprot != NIL )
                        {
                        minprot = firstprot;
                        mindiff = tbldiff( state, minprot,
                                    extrct[extptr] );
                        }
                  }

            /* We now have the first interesting proto in "minprot".  If
             * it matches within the tolerances set for the first proto,
             * we don't want to bother scanning the rest of the proto list
             * to see if we have any other reasonable matches.
             */

            if ( mindiff * 100 > totaltrans * FIRST_MATCH_DIFF_PERCENTAGE )
                  {
                  /* Not a good enough match.  Scan the rest of the
                   * protos.
                   */
                  for ( i = minprot; i != NIL; i = protnext[i] )
                        {
                        d = tbldiff( state, i, extrct[1 - extptr] );
                        if ( d < mindiff )
                              {
                              extptr = 1 - extptr;
                              mindiff = d;
                              minprot = i;
                              }
                        }
                  }

            /* Check if the proto we've decided on as our best bet is close
             * enough to the state we want to match to be usable.
             */

            if ( mindiff * 100 > totaltrans * ACCEPTABLE_DIFF_PERCENTAGE )
                  {
                  /* No good.  If the state is homogeneous enough,
                   * we make a template out of it.  Otherwise, we
                   * make a proto.
                   */

                  if ( comfreq * 100 >=
                       totaltrans * TEMPLATE_SAME_PERCENTAGE )
                        mktemplate( state, statenum, comstate );

                  else
                        {
                        mkprot( state, statenum, comstate );
                        mkentry( state, numecs, statenum,
                              JAMSTATE, totaltrans );
                        }
                  }

            else
                  { /* use the proto */
                  mkentry( extrct[extptr], numecs, statenum,
                        prottbl[minprot], mindiff );

                  /* If this state was sufficiently different from the
                   * proto we built it from, make it, too, a proto.
                   */

                  if ( mindiff * 100 >=
                       totaltrans * NEW_PROTO_DIFF_PERCENTAGE )
                        mkprot( state, statenum, comstate );

                  /* Since mkprot added a new proto to the proto queue,
                   * it's possible that "minprot" is no longer on the
                   * proto queue (if it happened to have been the last
                   * entry, it would have been bumped off).  If it's
                   * not there, then the new proto took its physical
                   * place (though logically the new proto is at the
                   * beginning of the queue), so in that case the
                   * following call will do nothing.
                   */

                  mv2front( minprot );
                  }
            }
      }


/* cmptmps - compress template table entries
 *
 * Template tables are compressed by using the 'template equivalence
 * classes', which are collections of transition character equivalence
 * classes which always appear together in templates - really meta-equivalence
 * classes.
 */

void cmptmps()
      {
      int tmpstorage[CSIZE + 1];
      register int *tmp = tmpstorage, i, j;
      int totaltrans, trans;

      peakpairs = numtemps * numecs + tblend;

      if ( usemecs )
            {
            /* Create equivalence classes based on data gathered on
             * template transitions.
             */
            nummecs = cre8ecs( tecfwd, tecbck, numecs );
            }

      else
            nummecs = numecs;

      while ( lastdfa + numtemps + 1 >= current_max_dfas )
            increase_max_dfas();

      /* Loop through each template. */

      for ( i = 1; i <= numtemps; ++i )
            {
            /* Number of non-jam transitions out of this template. */
            totaltrans = 0;

            for ( j = 1; j <= numecs; ++j )
                  {
                  trans = tnxt[numecs * i + j];

                  if ( usemecs )
                        {
                        /* The absolute value of tecbck is the
                         * meta-equivalence class of a given
                         * equivalence class, as set up by cre8ecs().
                         */
                        if ( tecbck[j] > 0 )
                              {
                              tmp[tecbck[j]] = trans;

                              if ( trans > 0 )
                                    ++totaltrans;
                              }
                        }

                  else
                        {
                        tmp[j] = trans;

                        if ( trans > 0 )
                              ++totaltrans;
                        }
                  }

            /* It is assumed (in a rather subtle way) in the skeleton
             * that if we're using meta-equivalence classes, the def[]
             * entry for all templates is the jam template, i.e.,
             * templates never default to other non-jam table entries
             * (e.g., another template)
             */

            /* Leave room for the jam-state after the last real state. */
            mkentry( tmp, nummecs, lastdfa + i + 1, JAMSTATE, totaltrans );
            }
      }



/* expand_nxt_chk - expand the next check arrays */

void expand_nxt_chk()
      {
      register int old_max = current_max_xpairs;

      current_max_xpairs += MAX_XPAIRS_INCREMENT;

      ++num_reallocs;

      nxt = reallocate_integer_array( nxt, current_max_xpairs );
      chk = reallocate_integer_array( chk, current_max_xpairs );

      zero_out( (char *) (chk + old_max),
            (size_t) (MAX_XPAIRS_INCREMENT * sizeof( int )) );
      }


/* find_table_space - finds a space in the table for a state to be placed
 *
 * synopsis
 *     int *state, numtrans, block_start;
 *     int find_table_space();
 *
 *     block_start = find_table_space( state, numtrans );
 *
 * State is the state to be added to the full speed transition table.
 * Numtrans is the number of out-transitions for the state.
 *
 * find_table_space() returns the position of the start of the first block (in
 * chk) able to accommodate the state
 *
 * In determining if a state will or will not fit, find_table_space() must take
 * into account the fact that an end-of-buffer state will be added at [0],
 * and an action number will be added in [-1].
 */

int find_table_space( state, numtrans )
int *state, numtrans;
      {
      /* Firstfree is the position of the first possible occurrence of two
       * consecutive unused records in the chk and nxt arrays.
       */
      register int i;
      register int *state_ptr, *chk_ptr;
      register int *ptr_to_last_entry_in_state;

      /* If there are too many out-transitions, put the state at the end of
       * nxt and chk.
       */
      if ( numtrans > MAX_XTIONS_FULL_INTERIOR_FIT )
            {
            /* If table is empty, return the first available spot in
             * chk/nxt, which should be 1.
             */
            if ( tblend < 2 )
                  return 1;

            /* Start searching for table space near the end of
             * chk/nxt arrays.
             */
            i = tblend - numecs;
            }

      else
            /* Start searching for table space from the beginning
             * (skipping only the elements which will definitely not
             * hold the new state).
             */
            i = firstfree;

      while ( 1 ) /* loops until a space is found */
            {
            while ( i + numecs >= current_max_xpairs )
                  expand_nxt_chk();

            /* Loops until space for end-of-buffer and action number
             * are found.
             */
            while ( 1 )
                  {
                  /* Check for action number space. */
                  if ( chk[i - 1] == 0 )
                        {
                        /* Check for end-of-buffer space. */
                        if ( chk[i] == 0 )
                              break;

                        else
                              /* Since i != 0, there is no use
                               * checking to see if (++i) - 1 == 0,
                               * because that's the same as i == 0,
                               * so we skip a space.
                               */
                              i += 2;
                        }

                  else
                        ++i;

                  while ( i + numecs >= current_max_xpairs )
                        expand_nxt_chk();
                  }

            /* If we started search from the beginning, store the new
             * firstfree for the next call of find_table_space().
             */
            if ( numtrans <= MAX_XTIONS_FULL_INTERIOR_FIT )
                  firstfree = i + 1;

            /* Check to see if all elements in chk (and therefore nxt)
             * that are needed for the new state have not yet been taken.
             */

            state_ptr = &state[1];
            ptr_to_last_entry_in_state = &chk[i + numecs + 1];

            for ( chk_ptr = &chk[i + 1];
                  chk_ptr != ptr_to_last_entry_in_state; ++chk_ptr )
                  if ( *(state_ptr++) != 0 && *chk_ptr != 0 )
                        break;

            if ( chk_ptr == ptr_to_last_entry_in_state )
                  return i;

            else
            ++i;
            }
      }


/* inittbl - initialize transition tables
 *
 * Initializes "firstfree" to be one beyond the end of the table.  Initializes
 * all "chk" entries to be zero.
 */
void inittbl()
      {
      register int i;

      zero_out( (char *) chk, (size_t) (current_max_xpairs * sizeof( int )) );

      tblend = 0;
      firstfree = tblend + 1;
      numtemps = 0;

      if ( usemecs )
            {
            /* Set up doubly-linked meta-equivalence classes; these
             * are sets of equivalence classes which all have identical
             * transitions out of TEMPLATES.
             */

            tecbck[1] = NIL;

            for ( i = 2; i <= numecs; ++i )
                  {
                  tecbck[i] = i - 1;
                  tecfwd[i - 1] = i;
                  }

            tecfwd[numecs] = NIL;
            }
      }


/* mkdeftbl - make the default, "jam" table entries */

void mkdeftbl()
      {
      int i;

      jamstate = lastdfa + 1;

      ++tblend; /* room for transition on end-of-buffer character */

      while ( tblend + numecs >= current_max_xpairs )
            expand_nxt_chk();

      /* Add in default end-of-buffer transition. */
      nxt[tblend] = end_of_buffer_state;
      chk[tblend] = jamstate;

      for ( i = 1; i <= numecs; ++i )
            {
            nxt[tblend + i] = 0;
            chk[tblend + i] = jamstate;
            }

      jambase = tblend;

      base[jamstate] = jambase;
      def[jamstate] = 0;

      tblend += numecs;
      ++numtemps;
      }


/* mkentry - create base/def and nxt/chk entries for transition array
 *
 * synopsis
 *   int state[numchars + 1], numchars, statenum, deflink, totaltrans;
 *   mkentry( state, numchars, statenum, deflink, totaltrans );
 *
 * "state" is a transition array "numchars" characters in size, "statenum"
 * is the offset to be used into the base/def tables, and "deflink" is the
 * entry to put in the "def" table entry.  If "deflink" is equal to
 * "JAMSTATE", then no attempt will be made to fit zero entries of "state"
 * (i.e., jam entries) into the table.  It is assumed that by linking to
 * "JAMSTATE" they will be taken care of.  In any case, entries in "state"
 * marking transitions to "SAME_TRANS" are treated as though they will be
 * taken care of by whereever "deflink" points.  "totaltrans" is the total
 * number of transitions out of the state.  If it is below a certain threshold,
 * the tables are searched for an interior spot that will accommodate the
 * state array.
 */

void mkentry( state, numchars, statenum, deflink, totaltrans )
register int *state;
int numchars, statenum, deflink, totaltrans;
      {
      register int minec, maxec, i, baseaddr;
      int tblbase, tbllast;

      if ( totaltrans == 0 )
            { /* there are no out-transitions */
            if ( deflink == JAMSTATE )
                  base[statenum] = JAMSTATE;
            else
                  base[statenum] = 0;

            def[statenum] = deflink;
            return;
            }

      for ( minec = 1; minec <= numchars; ++minec )
            {
            if ( state[minec] != SAME_TRANS )
                  if ( state[minec] != 0 || deflink != JAMSTATE )
                        break;
            }

      if ( totaltrans == 1 )
            {
            /* There's only one out-transition.  Save it for later to fill
             * in holes in the tables.
             */
            stack1( statenum, minec, state[minec], deflink );
            return;
            }

      for ( maxec = numchars; maxec > 0; --maxec )
            {
            if ( state[maxec] != SAME_TRANS )
                  if ( state[maxec] != 0 || deflink != JAMSTATE )
                        break;
            }

      /* Whether we try to fit the state table in the middle of the table
       * entries we have already generated, or if we just take the state
       * table at the end of the nxt/chk tables, we must make sure that we
       * have a valid base address (i.e., non-negative).  Note that
       * negative base addresses dangerous at run-time (because indexing
       * the nxt array with one and a low-valued character will access
       * memory before the start of the array.
       */

      /* Find the first transition of state that we need to worry about. */
      if ( totaltrans * 100 <= numchars * INTERIOR_FIT_PERCENTAGE )
            {
            /* Attempt to squeeze it into the middle of the tables. */
            baseaddr = firstfree;

            while ( baseaddr < minec )
                  {
                  /* Using baseaddr would result in a negative base
                   * address below; find the next free slot.
                   */
                  for ( ++baseaddr; chk[baseaddr] != 0; ++baseaddr )
                        ;
                  }

            while ( baseaddr + maxec - minec + 1 >= current_max_xpairs )
                  expand_nxt_chk();

            for ( i = minec; i <= maxec; ++i )
                  if ( state[i] != SAME_TRANS &&
                       (state[i] != 0 || deflink != JAMSTATE) &&
                       chk[baseaddr + i - minec] != 0 )
                        { /* baseaddr unsuitable - find another */
                        for ( ++baseaddr;
                              baseaddr < current_max_xpairs &&
                              chk[baseaddr] != 0; ++baseaddr )
                              ;

                        while ( baseaddr + maxec - minec + 1 >=
                              current_max_xpairs )
                              expand_nxt_chk();

                        /* Reset the loop counter so we'll start all
                         * over again next time it's incremented.
                         */

                        i = minec - 1;
                        }
            }

      else
            {
            /* Ensure that the base address we eventually generate is
             * non-negative.
             */
            baseaddr = MAX( tblend + 1, minec );
            }

      tblbase = baseaddr - minec;
      tbllast = tblbase + maxec;

      while ( tbllast + 1 >= current_max_xpairs )
            expand_nxt_chk();

      base[statenum] = tblbase;
      def[statenum] = deflink;

      for ( i = minec; i <= maxec; ++i )
            if ( state[i] != SAME_TRANS )
                  if ( state[i] != 0 || deflink != JAMSTATE )
                        {
                        nxt[tblbase + i] = state[i];
                        chk[tblbase + i] = statenum;
                        }

      if ( baseaddr == firstfree )
            /* Find next free slot in tables. */
            for ( ++firstfree; chk[firstfree] != 0; ++firstfree )
                  ;

      tblend = MAX( tblend, tbllast );
      }


/* mk1tbl - create table entries for a state (or state fragment) which
 *            has only one out-transition
 */

void mk1tbl( state, sym, onenxt, onedef )
int state, sym, onenxt, onedef;
      {
      if ( firstfree < sym )
            firstfree = sym;

      while ( chk[firstfree] != 0 )
            if ( ++firstfree >= current_max_xpairs )
                  expand_nxt_chk();

      base[state] = firstfree - sym;
      def[state] = onedef;
      chk[firstfree] = state;
      nxt[firstfree] = onenxt;

      if ( firstfree > tblend )
            {
            tblend = firstfree++;

            if ( firstfree >= current_max_xpairs )
                  expand_nxt_chk();
            }
      }


/* mkprot - create new proto entry */

void mkprot( state, statenum, comstate )
int state[], statenum, comstate;
      {
      int i, slot, tblbase;

      if ( ++numprots >= MSP || numecs * numprots >= PROT_SAVE_SIZE )
            {
            /* Gotta make room for the new proto by dropping last entry in
             * the queue.
             */
            slot = lastprot;
            lastprot = protprev[lastprot];
            protnext[lastprot] = NIL;
            }

      else
            slot = numprots;

      protnext[slot] = firstprot;

      if ( firstprot != NIL )
            protprev[firstprot] = slot;

      firstprot = slot;
      prottbl[slot] = statenum;
      protcomst[slot] = comstate;

      /* Copy state into save area so it can be compared with rapidly. */
      tblbase = numecs * (slot - 1);

      for ( i = 1; i <= numecs; ++i )
            protsave[tblbase + i] = state[i];
      }


/* mktemplate - create a template entry based on a state, and connect the state
 *              to it
 */

void mktemplate( state, statenum, comstate )
int state[], statenum, comstate;
      {
      int i, numdiff, tmpbase, tmp[CSIZE + 1];
      Char transset[CSIZE + 1];
      int tsptr;

      ++numtemps;

      tsptr = 0;

      /* Calculate where we will temporarily store the transition table
       * of the template in the tnxt[] array.  The final transition table
       * gets created by cmptmps().
       */

      tmpbase = numtemps * numecs;

      if ( tmpbase + numecs >= current_max_template_xpairs )
            {
            current_max_template_xpairs += MAX_TEMPLATE_XPAIRS_INCREMENT;

            ++num_reallocs;

            tnxt = reallocate_integer_array( tnxt,
                  current_max_template_xpairs );
            }

      for ( i = 1; i <= numecs; ++i )
            if ( state[i] == 0 )
                  tnxt[tmpbase + i] = 0;
            else
                  {
                  transset[tsptr++] = i;
                  tnxt[tmpbase + i] = comstate;
                  }

      if ( usemecs )
            mkeccl( transset, tsptr, tecfwd, tecbck, numecs, 0 );

      mkprot( tnxt + tmpbase, -numtemps, comstate );

      /* We rely on the fact that mkprot adds things to the beginning
       * of the proto queue.
       */

      numdiff = tbldiff( state, firstprot, tmp );
      mkentry( tmp, numecs, statenum, -numtemps, numdiff );
      }


/* mv2front - move proto queue element to front of queue */

void mv2front( qelm )
int qelm;
      {
      if ( firstprot != qelm )
            {
            if ( qelm == lastprot )
                  lastprot = protprev[lastprot];

            protnext[protprev[qelm]] = protnext[qelm];

            if ( protnext[qelm] != NIL )
                  protprev[protnext[qelm]] = protprev[qelm];

            protprev[qelm] = NIL;
            protnext[qelm] = firstprot;
            protprev[firstprot] = qelm;
            firstprot = qelm;
            }
      }


/* place_state - place a state into full speed transition table
 *
 * State is the statenum'th state.  It is indexed by equivalence class and
 * gives the number of the state to enter for a given equivalence class.
 * Transnum is the number of out-transitions for the state.
 */

void place_state( state, statenum, transnum )
int *state, statenum, transnum;
      {
      register int i;
      register int *state_ptr;
      int position = find_table_space( state, transnum );

      /* "base" is the table of start positions. */
      base[statenum] = position;

      /* Put in action number marker; this non-zero number makes sure that
       * find_table_space() knows that this position in chk/nxt is taken
       * and should not be used for another accepting number in another
       * state.
       */
      chk[position - 1] = 1;

      /* Put in end-of-buffer marker; this is for the same purposes as
       * above.
       */
      chk[position] = 1;

      /* Place the state into chk and nxt. */
      state_ptr = &state[1];

      for ( i = 1; i <= numecs; ++i, ++state_ptr )
            if ( *state_ptr != 0 )
                  {
                  chk[position + i] = i;
                  nxt[position + i] = *state_ptr;
                  }

      if ( position + numecs > tblend )
            tblend = position + numecs;
      }


/* stack1 - save states with only one out-transition to be processed later
 *
 * If there's room for another state on the "one-transition" stack, the
 * state is pushed onto it, to be processed later by mk1tbl.  If there's
 * no room, we process the sucker right now.
 */

void stack1( statenum, sym, nextstate, deflink )
int statenum, sym, nextstate, deflink;
      {
      if ( onesp >= ONE_STACK_SIZE - 1 )
            mk1tbl( statenum, sym, nextstate, deflink );

      else
            {
            ++onesp;
            onestate[onesp] = statenum;
            onesym[onesp] = sym;
            onenext[onesp] = nextstate;
            onedef[onesp] = deflink;
            }
      }


/* tbldiff - compute differences between two state tables
 *
 * "state" is the state array which is to be extracted from the pr'th
 * proto.  "pr" is both the number of the proto we are extracting from
 * and an index into the save area where we can find the proto's complete
 * state table.  Each entry in "state" which differs from the corresponding
 * entry of "pr" will appear in "ext".
 *
 * Entries which are the same in both "state" and "pr" will be marked
 * as transitions to "SAME_TRANS" in "ext".  The total number of differences
 * between "state" and "pr" is returned as function value.  Note that this
 * number is "numecs" minus the number of "SAME_TRANS" entries in "ext".
 */

int tbldiff( state, pr, ext )
int state[], pr, ext[];
      {
      register int i, *sp = state, *ep = ext, *protp;
      register int numdiff = 0;

      protp = &protsave[numecs * (pr - 1)];

      for ( i = numecs; i > 0; --i )
            {
            if ( *++protp == *++sp )
                  *++ep = SAME_TRANS;
            else
                  {
                  *++ep = *sp;
                  ++numdiff;
                  }
            }

      return numdiff;
      }

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