Monte Carlo: Moving a photon through tissue

Each photon is repeatedly allowed to HOP to a new position, DROP some weight due to absorption, SPIN into a new trajectory due to scattering, and CHECK whether the photon should die by ROULETTE.

/* HOP_DROP_SPIN_CHECK
   Propagate one photon until it dies as determined by ROULETTE.
*******/
do {

	/* WITHIN THIS D0-WHILE LOOP THE PHOTON HOPS, DROPS, SPINS, CHECKS */

} /* end STEP_CHECK_HOP_SPIN */
while (photon_status == ALIVE);

The following 4 sections describe the HOP, DROP, SPIN, and CHECK functions.