*
 *
 *
 * == Definition of visibility ==
 *
 * Two cells are in view of each other if there is any straight
 * line that meets both cells and that doesn't meet any opaque
 * cell in between, and if the cells are in LOS range of each
 * other.
 *
 * Here, to "meet" a cell means to intersect the interiour. In
 * particular, rays can pass between to diagonally adjacent
 * walls (as can the player).
 *
 * == Terminology ==
 *
 * A _ray_ is a line, specified by starting point (accx, accy)
 * and slope. A ray determines its _footprint_: the sequence of
 * cells whose interiour it meets.
 *
 * The footprint of a ray and any prefix is called a _cellray_.
 *
 * For the purposes of LOS calculation, only the footprints
 * are relevant, but rays are also used for shooting beams,
 * which may travel beyond LOS and which can be reflected.
 * See ray.cc.
 *
 * == Overview ==
 *
 * At first use, the LOS code makes some precomputations,
 * filling a list of all relevant rays in one quadrant,
 * and filling data structures that allow calculating LOS
 * in a quadrant without checking each ray.
 *
 * The code provides functions for filling LOS information
 * around a given center efficiently, and for querying rays
 * between two given cells.

// The LOS code now uses raycasting -- haranp

// The footprint of fullray[i] consists of fullray[i].length cells,
// whose coordinates are stored in ray_coords after the
//  coordinates of fullray[i-1].

// The footprint of ray=fullray[i] consists of ray.length cells,
// stored in ray_coords[ray.start..ray.length-1].

// 3D bit array indexed by x coord, y coord, cellray index.

// 3D bit array indexed by coordinate and cellray index.