🚀 go-pugleaf

I have the following changes to the repetition code detection which I've
tested with a couple of test positions.  There are quite a number of 
places which have to be changed, so it can get a bit confusing.  I'll
try and simplify it by breaking up into different pieces.


1.  First change is to repetition() in search.c.  There is a line which
    goes like this.

    if (GameCnt > Game50 + 4)

    Change this to

    if (GameCnt > Game50 + 3)


2.  Next change is in search().  There is a portion near the top that 
    looks like the following.

    if ((ply <= Sdepth + 3) && rpthash[side][hashkey & 0xFF] > 0)
      {
       ..
       ..
       ..
    else
      *rpt = 0;


    Replace this with the following.

  if (rpthash[side][hashkey & 0xFF] > 0)
    {
      *rpt = repetition ();
      if (*rpt == 1)
        score = 0;
      else if (*rpt == 2)
        {
          bstline[ply] = 0;
          return (0);
        }
    }
  else
    *rpt = 0;


3.  The next change is in search(), slightly further down, where search
    extensions are being determined.  Replace the following..

    else
      {
        if (score >= alpha &&
            (InChk || (ply>1 && PawnThreat[ply - 1] ....
         ..
         ..
         ..
         depth= 1;
         ext++;
         Threat[ply]= threat= 1;
      }

    with the following.

  else
    {
      int tripple_check = 0;
      if (score >= alpha &&
          (InChk || (ply>1 && PawnThreat[ply - 1] && depth<DepthBeyond-4)
          || (hung[side] > 1 && !ext))) {
        threat2= 1;
        ext++;
        depth= 1;
      }
      else if (score <= beta &&
               ((ply<Sdepth+4 && ply>4 &&
                ChkFlag[ply-2] && ChkFlag[ply-4] &&
                (ChkFlag[ply-2] != ChkFlag[ply-4] ||
                (flag.threat && DOTHREAT && QueenCheck[ply-2])))
          ||
                (flag.threat && ply<DepthBeyond-DEPTHMARGIN && ply>6
                && ChkFlag[ply-2] && ChkFlag[ply-4] && ChkFlag[ply-6]
                &&  (tripple_check=1)
                && ((ply < Sdepth+4 ?
                  (ChkFlag[ply-2] != ChkFlag[ply-4] || ChkFlag[ply-2] !=
ChkFlag [ply-6])
                  : (ChkFlag[ply-2] != ChkFlag[ply-4] &&
                     ChkFlag[ply-2] != ChkFlag[ply-6] &&
                     ChkFlag[ply-4] != ChkFlag[ply-6]))
                || (DOTHREAT && QueenCheck[ply-2]
                && QueenCheck[ply-4] && QueenCheck[ply-6]
                && QueenCheck[ply-2] != QueenCheck[ply-6]))
                ))) {
          if (tripple_check && DepthBeyond < Sdepth+13+DEPTHMARGIN)
            {
              extdb += 2;
              DepthBeyond += 2;
            }
          depth= 1;
          ext++;
          Threat[ply]= threat= 1;
        }

Note:  The tripple_check idea comes from TomV.


4.  The last important change is made deep within search().
    Look for

    else if (rcnt == 1) node->score /= 2;

    and replace this with

    else if (rcnt == 1) node->score = 0;

    


The effects of the new changes can be tried against the following 2 test
positions where perpetual check is the theme.


White computer Black Human 1 eps 0 # To move Black correct f8f2
Castled White false Black false # ep 
TimeControl 0 Operator Time 0
White Clock 6000 Moves 1
Black Clock 6000 Moves 1

0 .....R.. 10 10 10 10 10 10 10 10
1 ....q.BK 10 10 10 10 10 10 10 10
2 ...P..PP 10 10 10 10 10 10 10 10
3 rP.....Q 10 10 10 10 10 10 10 10
4 .p.P.... 10 10 10 10 10 10 10 10
5 ...p..p. 10 10 10 10 10 10 10 10
6 ...bpp.p 10 10 10 10 10 10 10 10
7 ......k. 10 10 10 10 10 10 10 10
  abcdefgh

move  score depth  nodes  time flags capture color



White computer Black Human 1 eps 0 # To move White correct e1e8
Castled White false Black false # ep 
TimeControl 0 Operator Time 0
White Clock 6000 Moves 1
Black Clock 6000 Moves 1

0 RB.....K 10 10 10 10 10 10 10 10
1 .B.n..P. 10 10 10 10 10 10 10 10
2 P....N.P 10 10 10 10 10 10 10 10
3 NP...... 10 10 10 10 10 10 10 10
4 ........ 10 10 10 10 10 10 10 10
5 p.pq..pQ 10 10 10 10 10 10 10 10
6 bp.b.p.p 10 10 10 10 10 10 10 10
7 ....r.k. 10 10 10 10 10 10 10 10
  abcdefgh

move  score depth  nodes  time flags capture color



Kong Sian