// F = logM is theoretically optimal. Mmax is our maximum needle size.
// if logM/Mmax is changed, be sure to also check the bt_off_t below
enum { logM = 32, Mmax = 1ull << logM, Mdflt = 65536, Mnil = Mmax - 1 };
enum { F = 16, Ftop = 1 << F, Fmask = Ftop - 1 };

//typedef uint32_t bt_off_t;        // must hold 0..Mnil
enum { BT_OFF_MAX = Mnil } __attribute__((packed)) typedef bt_off_t;