package detail

import "salty/terrain/internal/manifest"

// MarginCells is the overlap a tile must carry for the particle pass, in detail cells.
//
// Rule 2 of the tiling plan says to size a margin by how far the pass can move material, and for droplets
// that is not simply the lifetime. Within one round a droplet travels at most its lifetime, plus one cell for
// the cut brush. Across rounds the error compounds: a droplet in round two reads heights the round-one
// droplets moved, so the cut edge's influence walks a lifetime further in with every round.
//
// Taking that literally would make the margin `rounds * lifetime`, which at the defaults is 640 cells against
// a 2500-cell tile. Measured instead, at lifetime 12 and 8 rounds (TestHowFarTheCutEdgeReachesIn), the worst
// difference between a tile and the same ground in one whole run falls off much faster than that:
//
//	cells in from the cut edge:   0     4     8    12    16    20    24    32    40
//	worst difference, metres:   7.97  2.53  0.72  0.49  0.44  0.18  0.03  0.00  0.00
//
// It is the first lifetime that carries almost all of it, and by three the error is gone - a droplet has to be
// unlucky in the same way several rounds running for it to keep propagating, and that stops happening. Three
// lifetimes plus the brush is the margin, which at the default lifetime of 40 is 122 detail cells, 244 m, or
// about five per cent of a 5 km tile on each side.
func MarginCells(cfg manifest.Particle) int {
	life := cfg.Lifetime
	if life < 1 {
		life = 1
	}
	return 3*life + 2
}