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/**
Copyright (c) Scott Gasch
Module Name:
nnuefeatures.c
Abstract:
Standalone reference implementation of the HalfKAv2_hm feature
index formula, used ONLY to cross-check against the Python
reference encoder (annotate_pgns/nnue_feature_spec.py) -- see
docs/NNUE.md's "Canonical HalfKAv2_hm feature-indexing spec"
section for the formulas this file implements, sourced verbatim
from Stockfish's src/nnue/features/half_ka_v2_hm.{h,cpp} (git tag
sf_16).
This is deliberately NOT wired into search/eval -- no accumulator,
no incremental maintenance, nothing performance-sensitive. It
exists purely so `dump nnuefeatures` can print the exact same
active-feature-index set the real C inference code produces, so a
Python/C cross-checker has a C-side ground truth to diff against.
The actual feature-index formula (NnueFeatureIndex) now lives in
chess.h, shared verbatim with the real MakeMove/UnmakeMove
accumulator maintenance (move.c) -- this file only walks the board
and calls it, so the two can never silently drift apart.
Author:
Scott Gasch ([email protected]) 15 Oct 2026
**/
#include "chess.h"
static int
CompareInts(const void *a, const void *b)
{
int x = *(const int *)a;
int y = *(const int *)b;
return (x > y) - (x < y);
}
//
// Fill pIndices (caller-allocated, at least 32 ints) with the sorted
// set of active HalfKAv2_hm feature indices for perspective
// uPerspective at position pos. Returns the count (<=32).
//
static int
ActiveFeatureIndices(POSITION *pos, ULONG uPerspective, int *pIndices)
{
int f, r;
COOR cKing = ILLEGAL_COOR;
int n = 0;
//
// Find this perspective's own king square.
//
for (f = 0; f < 8; f++)
{
for (r = 1; r <= 8; r++)
{
COOR c = FILE_RANK_TO_COOR(f, r);
PIECE p = pos->rgSquare[c].pPiece;
if (!IS_EMPTY(p) &&
(PIECE_TYPE(p) == KING) &&
(PIECE_COLOR(p) == uPerspective))
{
cKing = c;
}
}
}
ASSERT(cKing != ILLEGAL_COOR);
for (f = 0; f < 8; f++)
{
for (r = 1; r <= 8; r++)
{
COOR c = FILE_RANK_TO_COOR(f, r);
PIECE p = pos->rgSquare[c].pPiece;
if (IS_EMPTY(p))
{
continue;
}
ASSERT(n < 32);
pIndices[n++] = NnueFeatureIndex(uPerspective, cKing, c, p);
}
}
qsort(pIndices, n, sizeof(int), CompareInts);
return n;
}
void
DumpNnueFeatures(POSITION *pos)
/**
Routine description:
Prints the sorted set of active HalfKAv2_hm feature indices for
both perspectives at the current position, one line per
perspective, in a format trivial for a Python cross-checker to
parse:
NNUE_FEATURES white <idx> <idx> ...
NNUE_FEATURES black <idx> <idx> ...
Parameters:
POSITION *pos : position to dump
Return value:
void
**/
{
int rgIndices[32];
int n;
int i;
n = ActiveFeatureIndices(pos, WHITE, rgIndices);
Trace("NNUE_FEATURES white");
for (i = 0; i < n; i++)
{
Trace(" %d", rgIndices[i]);
}
Trace("\n");
n = ActiveFeatureIndices(pos, BLACK, rgIndices);
Trace("NNUE_FEATURES black");
for (i = 0; i < n; i++)
{
Trace(" %d", rgIndices[i]);
}
Trace("\n");
}
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