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Subject: Re: Question for the MTD(f) experts

Author: Richard Pijl

Date: 01:26:10 04/14/04

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On April 14, 2004 at 03:49:06, Dann Corbit wrote:

>On April 14, 2004 at 03:46:08, Richard Pijl wrote:
>
>>On April 14, 2004 at 03:30:22, Dann Corbit wrote:
>>
>>>I decided to toss an MTD(f) search into TSCP, and I've got something wrong, but
>>>I can't quite see what it is.
>>
>>MTD(f) requires a transposition hashtable and a fail-soft search to function
>>properly. Did you add/change that too?
>
>With hash table, I get this behavior:
>post
>sd 99
>st 99
>go
>Depth   Eval    Time    Nodes   PV
>1       48      0       172
>2       0       20      2224    d2d3
>3       35      40      4858
>4       5       321     41505   e2e3
>5       35      711     93485
>6       13      5438    780563  d2d4
>7       30      11878   1638246
>8       18      81509   11563089        e2e4
>move e2e4
>

I guess you're referring to the empty PV lines?
That's normal, as TSCP doesn't store the PV on a fail high/low. As TSCP is
searching with an open window in the root, you will always have a PV. But with
MTD(f) you're searching with null-windows, so you won't have a PV.

Did you change stuff in search to deal with pv with fail high/lows?

In your code below, you're setting a move in the pv array, but that move is
basically random:

        if (f < beta)
            greatest = f;
        else {
            int             j;
            smallest = f; /* performs a cut-off */
            if (smallest >= greatest) {
                pv[ply][ply] = gen_dat[depth].m;
                for (j = ply + 1; j < pv_length[ply + 1]; ++j)
                    pv[ply][j] = pv[ply + 1][j];
                pv_length[ply] = pv_length[ply + 1];
            }
        }

gen_dat[depth].m is a generated move in the movelist, but it is not the
bestmove. It isn't even the first move in the move list as depth is increased on
every iteration.

bye,
Richard.


>>As the same position is searched again and again with a different null-window,
>>the transposition table will contain many of the positions to search with a
>>bound yielding many cuts.
>>
>>Richard.



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