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Open pawns have no mechanical obstruction - an opponent pawn in front. They are at least half-free or even free passers. Thus, open pawns are a superset of passed pawns and half-free pawns like candidates but also weak pawns like the half-free straggler (backward pawn ) or half-free isolanis. Open pawns are vulnerable from opponent rook-attacks on half-open files.

Open Single Pawn

Working in the square centric world of the board, thus using a square index of one particular pawn, likely from bitboard traversal, to lookup pre-calculated pattern.

For a single pawn we need to access a lookup-table to get all squares on the same file in front of the pawn. If the intersection of the fronspan with the set of opponent pawns is empty, it is a open pawn.

U64 arrFrontSpans[2][64];
 
if ( (arrFrontSpans[white][sqOfWhitePawn] & pawnBB[black]) == 0 ) -> pawn open

arrFrontSpan[white][d4]
. . . 1 . . . .
. . . 1 . . . .
. . . 1 . . . .
. . . 1 . . . .
. . . p . . . .
. . . . . . . .
. . . . . . . .
. . . . . . . .

Open Pawns set-wise

Working in the bitboard centric world to determine pawn related pattern set-wise.

Open Pawns have no mechanical obstruction in front - an opponent pawn as member of the own frontfill or frontspan. A relative complement with the opponents frontspans is sufficient to determine open pawns:

U64 wOpenPawns(U64 wpawns, U64 bpawns) {
   return wpawns & ~bFrontSpan(bpawns);
}
 
U64 bOpenPawns(U64 bpawns, U64 wpawns) {
   return bpawns & ~wFrontSpan(wpawns);
}

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