static void longestCommonSequence(
  DContext *p,
  int iS1, int iE1,
  int iS2, int iE2,
  int *piSX, int *piEX,
  int *piSY, int *piEY
){
  double bestScore = -1e30;  /* Best score seen so far */
  int i, j;                  /* Loop counters */
  int iSX, iSY, iEX, iEY;    /* Current match */
  double score;              /* Current score */
  int skew;                  /* How lopsided is the match */
  int dist;                  /* Distance of match from center */
  int mid;                   /* Center of the span */
  int iSXb, iSYb, iEXb, iEYb;   /* Best match so far */
  int iSXp, iSYp, iEXp, iEYp;   /* Previous match */

  iSXb = iSXp = iS1;
  iEXb = iEXp = iS1;
  iSYb = iSYp = iS2;
  iEYb = iEYp = iS2;
  mid = (iE1 + iS1)/2;
  for(i=iS1; i<iE1; i++){
    int limit = 0;
    j = p->aTo[p->aFrom[i].h % p->nTo].iHash;
    while( j>0 
      && (j-1<iS2 || j>=iE2 || !same_dline(&p->aFrom[i], &p->aTo[j-1]))
    ){
      if( limit++ > 10 ){
        j = 0;
        break;
      }
      j = p->aTo[j-1].iNext;
    }
    if( j==0 ) continue;
    assert( i>=iSXb && i>=iSXp );
    if( i<iEXb && j>=iSYb && j<iEYb ) continue;
    if( i<iEXp && j>=iSYp && j<iEYp ) continue;
    iSX = i;
    iSY = j-1;
    while( iSX>iS1 && iSY>iS2 && same_dline(&p->aFrom[iSX-1],&p->aTo[iSY-1]) ){
      iSX--;
      iSY--;
    }
    iEX = i+1;
    iEY = j;
    while( iEX<iE1 && iEY<iE2 && same_dline(&p->aFrom[iEX],&p->aTo[iEY]) ){
      iEX++;
      iEY++;
    }
    skew = (iSX-iS1) - (iSY-iS2);
    if( skew<0 ) skew = -skew;
    dist = (iSX+iEX)/2 - mid;
    if( dist<0 ) dist = -dist;
    score = (iEX - iSX) - 0.05*skew - 0.05*dist;
    if( score>bestScore ){
      bestScore = score;
      iSXb = iSX;
      iSYb = iSY;
      iEXb = iEX;
      iEYb = iEY;
    }else{
      iSXp = iSX;
      iSYp = iSY;
      iEXp = iEX;
      iEYp = iEY;
    }
  }
  *piSX = iSXb;
  *piSY = iSYb;
  *piEX = iEXb;
  *piEY = iEYb;
  /* printf("LCS(%d..%d/%d..%d) = %d..%d/%d..%d\n", 
     iS1, iE1, iS2, iE2, *piSX, *piEX, *piSY, *piEY);  */
}

/*
** Do a single step in the difference.  Compute a sequence of
** copy/delete/insert steps that will convert lines iS1 through iE1-1 of

      appendTriple(p, iEX - iSX, 0, 0);
    }
    diff_step(p, iEX, iE1, iEY, iE2);
  }else{
    appendTriple(p, 0, iE1-iS1, iE2-iS2);
  }
}

/*
** Compute the differences between two files already loaded into
** the DContext structure.
*/
static void diff_all(DContext *p){
  int mnE, iS, iE1, iE2;

  /* Carve off the common header and footer */
  iE1 = p->nFrom;
  iE2 = p->nTo;
  while( iE1>0 && iE2>0 && same_dline(&p->aFrom[iE1-1], &p->aTo[iE2-1]) ){
    iE1--;
    iE2--;
  }
  mnE = iE1<iE2 ? iE1 : iE2;
  for(iS=0; iS<mnE && same_dline(&p->aFrom[iS],&p->aTo[iS]); iS++){}

  /* do the difference */
  if( iS>0 ){
    appendTriple(p, iS, 0, 0);
  }
  diff_step(p, iS, iE1, iS, iE2);
  if( iE1<p->nFrom ){
    appendTriple(p, p->nFrom - iE1, 0, 0);
  }

  /* Terminate the COPY/DELETE/INSERT triples with three zeros */
  expandEdit(p, p->nEdit+3);
  if( p->aEdit ){
    p->aEdit[p->nEdit++] = 0;
    p->aEdit[p->nEdit++] = 0;
    p->aEdit[p->nEdit++] = 0;
  }
}

/*
** Generate a report of the differences between files pA and pB.
** If pOut is not NULL then a unified diff is appended there.  It
** is assumed that pOut has already been initialized.  If pOut is
** NULL, then a pointer to an array of integers is returned.  
** The integers come in triples.  For each triple,

int *text_diff(
  Blob *pA_Blob,   /* FROM file */
  Blob *pB_Blob,   /* TO file */
  Blob *pOut,      /* Write unified diff here if not NULL */
  int nContext     /* Amount of context to unified diff */
){
  DContext c;

 
  /* Prepare the input files */
  memset(&c, 0, sizeof(c));
  c.aFrom = break_into_lines(blob_str(pA_Blob), &c.nFrom);
  c.aTo = break_into_lines(blob_str(pB_Blob), &c.nTo);
  if( c.aFrom==0 || c.aTo==0 ){
    free(c.aFrom);
    free(c.aTo);
    if( pOut ){
      blob_appendf(pOut, "cannot compute difference between binary files\n");
    }
    return 0;
  }











  /* Compute the difference */







  diff_all(&c);







  if( pOut ){
    /* Compute a context diff if requested */
    contextDiff(&c, pOut, nContext);
    free(c.aFrom);
    free(c.aTo);
    free(c.aEdit);
    return 0;
  }else{
    /* If a context diff is not requested, then return the
    ** array of COPY/DELETE/INSERT triples.

    */
    free(c.aFrom);
    free(c.aTo);
    return c.aEdit;
  }
}