Author: Ricardo Gibert
Date: 23:38:34 07/13/03
Go up one level in this thread
On July 14, 2003 at 02:00:39, Ricardo Gibert wrote:
>On July 14, 2003 at 00:56:12, Matthew White wrote:
>
>>Hi all,
>>
>>I was browsing through the code in KnightCap, and I noticed the following
>>function. Doesn't this seem like an incorrect definition of a pinned piece? I
>>thought that a pinned piece was when a lower valued piece was blocking the
>>attack on a higher valued piece (or the king). Any ideas?
>
>White: Re1
>Black: Be6, Pe7, Pf7
>
>The B is pinned, since if it is moved, the pawn on e7 drops. Note the B is of
>greater value than the P.
>
>If we remove the Pf7, then we have a skewer instead. The B must move and give up
>the P.
Here's a contrived example that is contrary to the 1st comment in the source
below:
White: Re3, Pf2, Pf3, Pd2
Black: Re6, Pe7, Pf7
The Re6 can't move off the e-file, since that would leave Pe7 undefended. The
Re6 also can't take the Re3, since that would repair the White pawn structure.
Therefore the Re6 is pinned even though it is a sliding piece of the same type
as Re3.
Note the Re6 is again of greater value than the Pe7.
>
>>
>>Matt
>>
>>static etype find_pins(Position *b)
>>{
>> int i, j, k;
>> uint32 pinner_mask;
>>
>> /* to find pins we loop over all pieces, looking for pieces
>> that are under attack by sliding pieces (but not if they
>> themselves are sliding pieces of the same type!). Then we
>> check to see if the sliding piece would attack another
>> piece, making it hung, if moved.
>>
>> This is a expensive procedure! */
>>
>> b->pinned_mask = 0;
>>
>> if (b->stage == MATING)
>> return 0;
>>
>> for (i=2;i<16;i++) {
>> if (!b->pieces[i].p) continue;
>>
>> pinner_mask = b->topieces[b->pieces[i].pos] &
>> b->sliding_mask & BLACK_MASK;
>>
>> while (pinner_mask) {
>> j = ff_one(pinner_mask);
>> pinner_mask &= ~(1<<j);
>>
>> /* If the pinned piece is attacking the pinner then
>> it isn't a pin! It might be a skewer tho.
>> */
>> if (b->topieces[b->pieces[j].pos] & (1<<i))
>> continue;
>>
>> /* look for a piece that this bit is pinned against */
>> for (k=0;k<8;k++) {
>> if (!b->pieces[k].p) continue;
>> if (k == i) continue;
>> if (!same_line(b->pieces[j].pos, b->pieces[i].pos,
>> b->pieces[k].pos)) continue;
>> if (!empty_line(b, b->pieces[i].pos,
>> b->pieces[k].pos))
>> continue;
>>
>> /* we have a likely pin. Now we need
>> to confirm that if the pinner could attack
>> the pinnedto piece then that piece
>> would be hung */
>> if (get_control(b,
>> b->topieces[b->pieces[k].pos] | (1<<j),
>> b->pieces[k].p,
>> b->pieces[k].pos) < 0) {
>> b->pinned_mask |= (1<<i);
>> if (debug)
>> lprintf(0,"w pinned %s -> %s -> %s\n",
>> posstr(b->pieces[j].pos),
>> posstr(b->pieces[i].pos),
>> posstr(b->pieces[k].pos)
>> );
>> }
>> }
>> }
>> }
>>
>>
>> for (i=18;i<32;i++) {
>> if (!b->pieces[i].p) continue;
>>
>> pinner_mask = b->topieces[b->pieces[i].pos] &
>> b->sliding_mask & WHITE_MASK;
>>
>> while (pinner_mask) {
>> j = ff_one(pinner_mask);
>> pinner_mask &= ~(1<<j);
>>
>> /* If the pinned piece is attacking the pinner then
>> it isn't a pin! It might be a skewer tho.
>> */
>> if (b->topieces[b->pieces[j].pos] & (1<<i))
>> continue;
>>
>> /* look for a piece that this bit is pinned against */
>> for (k=16;k<24;k++) {
>> if (!b->pieces[k].p) continue;
>> if (k == i) continue;
>> if (!same_line(b->pieces[j].pos, b->pieces[i].pos,
>> b->pieces[k].pos)) continue;
>> if (!empty_line(b, b->pieces[i].pos,
>> b->pieces[k].pos))
>> continue;
>>
>> /* we have a likely pin. Now we need
>> to confirm that if the pinner could attack
>> the pinnedto piece then that piece
>> would be hung */
>> if (get_control(b,
>> b->topieces[b->pieces[k].pos] | (1<<j),
>> b->pieces[k].p,
>> b->pieces[k].pos) > 0) {
>> b->pinned_mask |= (1<<i);
>> if (debug)
>> lprintf(0,"b pinned %s -> %s -> %s\n",
>> posstr(b->pieces[j].pos),
>> posstr(b->pieces[i].pos),
>> posstr(b->pieces[k].pos)
>> );
>> }
>> }
>> }
>> }
>>
>> return 0;
>>}
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