NDENTRYMASK
i = want & NDENTRYMASK;
if (fdp->fd_lomap[off] & (1U << (fd & NDENTRYMASK)))
fdp->fd_lomap[off] |= 1U << (fd & NDENTRYMASK);
fdp->fd_himap[off >> NDENTRYSHIFT] |= 1U << (off & NDENTRYMASK);
fdp->fd_himap[off >> NDENTRYSHIFT] &= ~(1U << (off & NDENTRYMASK));
fdp->fd_lomap[off] &= ~(1U << (fd & NDENTRYMASK));
new > off ? 0 : i & NDENTRYMASK,