local EvolutionModel = dofile(mapgen_rivers.modpath .. '/terrainlib_lua/erosion.lua')
local twist = dofile(mapgen_rivers.modpath .. '/terrainlib_lua/twist.lua')

local blocksize = mapgen_rivers.settings.blocksize
local tectonic_speed = mapgen_rivers.settings.tectonic_speed

local np_base = table.copy(mapgen_rivers.noise_params.base)
np_base.spread = vector.divide(np_base.spread, blocksize)

local evol_params = mapgen_rivers.settings.evol_params

local time = mapgen_rivers.settings.evol_time
local time_step = mapgen_rivers.settings.evol_time_step
local niter = math.ceil(time/time_step)
time_step = time / niter

local use_margin = mapgen_rivers.settings.margin
local margin_width = mapgen_rivers.settings.margin_width / blocksize
local margin_elev = mapgen_rivers.settings.margin_elev

local function margin(dem, width, elev)
	local X, Y = dem.X, dem.Y
	for i=1, width do
		local c1 = ((i-1)/width) ^ 0.5
		local c2 = (1-c1) * elev
		local index = (i-1)*X + 1
		for x=1, X do
			dem[index] = dem[index] * c1 + c2
			index = index + 1
		end
		index = i
		for y=1, Y do
			dem[index] = dem[index] * c1 + c2
			index = index + X
		end
		index = X*(Y-i) + 1
		for x=1, X do
			dem[index] = dem[index] * c1 + c2
			index = index + 1
		end
		index = X-i + 1
		for y=1, Y do
			dem[index] = dem[index] * c1 + c2
			index = index + X
		end
	end
end

function mapgen_rivers.pregenerate(grid)
	local size = grid.size

	if size.x * size.y > 4e6 then
		minetest.log("warning", "[mapgen_rivers] You are going to generate a very large grid (>4M nodes). If you experience problems, you should increase blocksize or reduce map size.")
	end

	local seed = tonumber(minetest.get_mapgen_setting("seed"))
	np_base.seed = (np_base.seed or 0) + seed

	local nobj_base = PerlinNoiseMap(np_base, {x=size.x, y=1, z=size.y})

	local dem = nobj_base:get_3d_map_flat({x=0, y=0, z=0})
	dem.X = size.x
	dem.Y = size.y

	if use_margin then
		margin(dem, margin_width, margin_elev)
	end

	local model = EvolutionModel(evol_params)
	model.dem = dem
	local ref_dem = model:define_isostasy(dem)

	local tectonic_step = tectonic_speed * time_step
	collectgarbage()
	for i=1, niter do
		minetest.log("info", "[mapgen_rivers] Iteration " .. i .. " of " .. niter)

		model:diffuse(time_step)
		model:flow()
		model:erode(time_step)
		if i < niter then
			if tectonic_step ~= 0 then
				nobj_base:get_3d_map_flat({x=0, y=tectonic_step*i, z=0}, ref_dem)
				if use_margin then
					margin(ref_dem, margin_width, margin_elev)
				end
			end
			model:isostasy()
		end

		collectgarbage()
	end
	model:flow()

	local mfloor = math.floor
	local mmin, mmax = math.min, math.max
	local offset_x, offset_y = twist(model.dirs, model.rivers, 5)
	for i=1, size.x*size.y do
		offset_x[i] = mmin(mmax(offset_x[i]*256, -128), 127)
		offset_y[i] = mmin(mmax(offset_y[i]*256, -128), 127)
	end

	grid.dem = model.dem
	grid.lakes = model.lakes
	grid.dirs = model.dirs
	grid.rivers = model.rivers
	grid.offset_x = offset_x
	grid.offset_y = offset_y

	grid.load_method = "full"
	grid.conv_applied = false
	collectgarbage()
end