@Override public void map( Chunk cs[] ) {
for( int col = 0; col<cs.length; col++ ) {
Chunk C = cs[col];
for( int row=0; row<C.len(); row++ ) {
if( C.at0(row) == _ds[col] || (C.isNA0(row) && Double.isNaN(_ds[col])) ) {
long r = C.start()+row;
if( r < _row ) { if( r > _prev ) _prev = r; }
else if( r > _row ) { if( r < _next ) _next = r; }
}
}
}