** Conceptually, this object is as an encoding of the command-line options
** for the "fossil diff" command.  That is not a precise description, though,
** because not all diff operations are started from the command-line.  But
** the idea is sound.
**
** Information encoded by this object includes but is not limited to:
**
**    *   The desired output format (unified vs. side-by-side, 
**        TCL, JSON, HTML vs. plain-text).
**
**    *   Number of lines of context surrounding each difference block
**
**    *   Width of output columns for text side-by-side diffop          
*/
struct DiffConfig {
  u64 diffFlags;           /* Diff flags */
  int nContext;            /* Number of lines of context */
  int wColumn;             /* Column width in -y mode */
  u32 nFile;               /* Number of files diffed so far */
  const char *zDiffCmd;    /* External diff command to use instead of builtin */

/*
** This is an abstract superclass for an object that accepts difference
** lines and formats them for display.  Subclasses of this object format
** the diff output in different ways.
**
** To subclass, create an instance of the DiffBuilder object and fill
** in appropriate method implementations.
*/ 
typedef struct DiffBuilder DiffBuilder;
struct DiffBuilder {
  void (*xSkip)(DiffBuilder*, unsigned int, int);
  void (*xCommon)(DiffBuilder*,const DLine*);
  void (*xInsert)(DiffBuilder*,const DLine*);
  void (*xDelete)(DiffBuilder*,const DLine*);
  void (*xReplace)(DiffBuilder*,const DLine*,const DLine*);

    blob_append_char(p->pOut, ' ');
    blob_append_tcl_literal(p->pOut, pX->z + x, chng.a[i].iStart1 - x);
    x = chng.a[i].iStart1;
    blob_append_char(p->pOut, ' ');
    blob_append_tcl_literal(p->pOut, pX->z + x, chng.a[i].iLen1);
    x += chng.a[i].iLen1;
    blob_append_char(p->pOut, ' ');
    blob_append_tcl_literal(p->pOut, 
                         pY->z + chng.a[i].iStart2, chng.a[i].iLen2);
  }
  if( x<pX->n ){
    blob_append_char(p->pOut, ' ');
    blob_append_tcl_literal(p->pOut, pX->z + x, pX->n - x);
  }
  blob_append_char(p->pOut, '\n');

    }
    blob_append_json_literal(p->pOut, pX->z + x, chng.a[i].iStart1 - x);
    x = chng.a[i].iStart1;
    blob_append_char(p->pOut, ',');
    blob_append_json_literal(p->pOut, pX->z + x, chng.a[i].iLen1);
    x += chng.a[i].iLen1;
    blob_append_char(p->pOut, ',');
    blob_append_json_literal(p->pOut, 
                         pY->z + chng.a[i].iStart2, chng.a[i].iLen2);
  }
  blob_append_char(p->pOut, ',');
  blob_append_json_literal(p->pOut, pX->z + x, pX->n - x);
  blob_append(p->pOut, "],\n",3);
}
static void dfjsonEnd(DiffBuilder *p){

  /* "+" marks for the separator on inserted lines */
  for(i=0; i<p->nPending; i++) blob_append(&p->aCol[1], "+\n", 2);

  /* Text of the inserted lines */
  blob_append(&p->aCol[2], "<ins>", 5);
  blob_append_xfer(&p->aCol[2], &p->aCol[4]);
  blob_append(&p->aCol[2], "</ins>", 6);
  
  p->nPending = 0;
}
static void dfunifiedFinishRow(DiffBuilder *p){
  dfunifiedFinishDelete(p);
  dfunifiedFinishInsert(p);
  if( blob_size(&p->aCol[0])==0 ) return;
  blob_append(p->pOut, "</pre></td><td class=\"diffln difflnr\"><pre>\n", -1);

    aBig = aRight;
    nBig = nRight;
  }
  iDivBig = nBig/2;
  iDivSmall = nSmall/2;

  if( pCfg->diffFlags & DIFF_DEBUG ){
    fossil_print("  Divide at [%.*s]\n", 
                 aBig[iDivBig].n, aBig[iDivBig].z);
  }

  bestScore = 10000;
  for(i=0; i<nSmall; i++){
    score = match_dline(aBig+iDivBig, aSmall+i) + abs(i-nSmall/2)*2;
    if( score<bestScore ){