mapgen_rivers/init.lua

local modpath = minetest.get_modpath(minetest.get_current_modname()) .. '/'
local worldpath = minetest.get_worldpath() .. '/'
local load_map = dofile(modpath .. 'load.lua')
local geometry = dofile(modpath .. 'geometry.lua')

local function copy_if_needed(filename)
	local wfilename = worldpath..filename
	local wfile = io.open(wfilename, 'r')
	if wfile then
		wfile:close()
		return
	end
	local mfilename = modpath..filename
	local mfile = io.open(mfilename, 'r')
	local wfile = io.open(wfilename, 'w')
	wfile:write(mfile:read("*all"))
	mfile:close()
	wfile:close()
end

copy_if_needed('size')
local sfile = io.open(worldpath..'size')
local X = tonumber(sfile:read('*l'))
local Z = tonumber(sfile:read('*l'))

copy_if_needed('dem')
local dem = load_map(worldpath..'dem', 2, true, X*Z)
copy_if_needed('lakes')
local lakes = load_map(worldpath..'lakes', 2, true, X*Z)
copy_if_needed('bounds_x')
local bounds_x = load_map(worldpath..'bounds_x', 4, false, (X-1)*Z)
copy_if_needed('bounds_y')
local bounds_z = load_map(worldpath..'bounds_y', 4, false, X*(Z-1))

copy_if_needed('offset_x')
local offset_x = load_map(worldpath..'offset_x', 1, true, X*Z)
for k, v in ipairs(offset_x) do
	offset_x[k] = (v+0.5)/256
end

copy_if_needed('offset_y')
local offset_z = load_map(worldpath..'offset_y', 1, true, X*Z)
for k, v in ipairs(offset_z) do
	offset_z[k] = (v+0.5)/256
end


local function index(x, z)
	return z*X+x+1
end

local function get_point_location(x, z)
	local i = index(x, z)
	return x+offset_x[i], z+offset_z[i]
end

local function interp(v00, v01, v11, v10, xf, zf)
	local v0 = v01*xf + v00*(1-xf)
	local v1 = v11*xf + v10*(1-xf)
	return v1*zf + v0*(1-zf)
end

local data = {}

local blocksize = 12
local sea_level = 1
local min_catchment = 25
local max_catchment = 40000

local storage = minetest.get_mod_storage()
if storage:contains("blocksize") then
	blocksize = storage:get_int("blocksize")
else
	storage:set_int("blocksize", blocksize)
end
if storage:contains("sea_level") then
	sea_level = storage:get_int("sea_level")
else
	storage:set_int("sea_level", sea_level)
end
if storage:contains("min_catchment") then
	min_catchment = storage:get_float("min_catchment")
else
	storage:set_float("min_catchment", min_catchment)
end
if storage:contains("max_catchment") then
	max_catchment = storage:get_float("max_catchment")
else
	storage:set_float("max_catchment", max_catchment)
end

-- Width coefficients: coefficients solving
--   wfactor * min_catchment ^ wpower = 1/(2*blocksize)
--   wfactor * max_catchment ^ wpower = 1
local wpower = math.log(2*blocksize)/math.log(max_catchment/min_catchment)
local wfactor = 1 / max_catchment ^ wpower
local function river_width(flow)
	flow = math.abs(flow)
	if flow < min_catchment then
		return 0
	end

	return math.min(wfactor * flow ^ wpower, 1)
end

local function generate(minp, maxp, seed)
	local c_stone = minetest.get_content_id("default:stone")
	local c_dirt = minetest.get_content_id("default:dirt")
	local c_lawn = minetest.get_content_id("default:dirt_with_grass")
	local c_sand = minetest.get_content_id("default:sand")
	local c_water = minetest.get_content_id("default:water_source")
	local c_rwater = minetest.get_content_id("default:river_water_source")

	local vm, emin, emax = minetest.get_mapgen_object("voxelmanip")
	vm:get_data(data)

	local a = VoxelArea:new({MinEdge = emin, MaxEdge = emax})
	local ystride = a.ystride -- Tip : the ystride of a VoxelArea is the number to add to the array index to get the index of the position above. It's faster because it avoids to completely recalculate the index.
	local chulens = maxp.z - minp.z + 1

	local polygons = {}
	local xpmin, xpmax = math.max(math.floor(minp.x/blocksize - 0.5), 0), math.min(math.ceil(maxp.x/blocksize), X-2)
	local zpmin, zpmax = math.max(math.floor(minp.z/blocksize - 0.5), 0), math.min(math.ceil(maxp.z/blocksize), Z-2)

	for xp = xpmin, xpmax do
		for zp=zpmin, zpmax do
			local iA = index(xp, zp)
			local iB = index(xp+1, zp)
			local iC = index(xp+1, zp+1)
			local iD = index(xp, zp+1)
			local poly_x = {offset_x[iA]+xp, offset_x[iB]+xp+1, offset_x[iC]+xp+1, offset_x[iD]+xp}
			local poly_z = {offset_z[iA]+zp, offset_z[iB]+zp, offset_z[iC]+zp+1, offset_z[iD]+zp+1}
			local polygon = {x=poly_x, z=poly_z, i={iA, iB, iC, iD}}

			local bounds = {}
			local xmin = math.max(math.floor(blocksize*math.min(unpack(poly_x)))+1, minp.x)
			local xmax = math.min(math.floor(blocksize*math.max(unpack(poly_x))), maxp.x)
			for x=xmin, xmax do
				bounds[x] = {}
			end

			local i1 = 4
			for i2=1, 4 do -- Loop on 4 edges
				local x1, x2 = poly_x[i1], poly_x[i2]
				local lxmin = math.floor(blocksize*math.min(x1, x2))+1
				local lxmax = math.floor(blocksize*math.max(x1, x2))
				if lxmin <= lxmax then
					local z1, z2 = poly_z[i1], poly_z[i2]
					local a = (z1-z2) / (x1-x2)
					local b = blocksize*(z1 - a*x1)
					for x=math.max(lxmin, minp.x), math.min(lxmax, maxp.x) do
						table.insert(bounds[x], a*x+b)
					end
				end
				i1 = i2
			end
			for x=xmin, xmax do
				local xlist = bounds[x]
				table.sort(xlist)
				local c = math.floor(#xlist/2)
				for l=1, c do
					local zmin = math.max(math.floor(xlist[l*2-1])+1, minp.z)
					local zmax = math.min(math.floor(xlist[l*2]), maxp.z)
					local i = (x-minp.x) * chulens + (zmin-minp.z) + 1
					for z=zmin, zmax do
						polygons[i] = polygon
						i = i + 1
					end
				end
			end

			polygon.dem = {dem[iA], dem[iB], dem[iC], dem[iD]}
			polygon.lake = math.min(lakes[iA], lakes[iB], lakes[iC], lakes[iD])

			local river_west = river_width(bounds_z[iA])
			local river_north = river_width(bounds_x[iA-zp])
			local river_east = 1-river_width(bounds_z[iB])
			local river_south = 1-river_width(bounds_x[iD-zp-1])
			if river_west > river_east then
				local mean = (river_west + river_east) / 2
				river_west = mean
				river_east = mean
			end
			if river_north > river_south then
				local mean = (river_north + river_south) / 2
				river_north = mean
				river_south = mean
			end
			polygon.rivers = {river_west, river_north, river_east, river_south}

			local around = {0,0,0,0,0,0,0,0}
			if zp > 0 then
				around[1] = river_width(bounds_z[iA-X])
				around[2] = river_width(bounds_z[iB-X])
			end
			if xp < X-2 then
				around[3] = river_width(bounds_x[iB-zp])
				around[4] = river_width(bounds_x[iC-zp-1])
			end
			if zp < Z-2 then
				around[5] = river_width(bounds_z[iC])
				around[6] = river_width(bounds_z[iD])
			end
			if xp > 0 then
				around[7] = river_width(bounds_x[iD-zp-2])
				around[8] = river_width(bounds_x[iA-zp-1])
			end
			polygon.river_corners = {math.max(around[8], around[1]), math.max(around[2], around[3]), math.max(around[4], around[5]), math.max(around[6], around[7])}
		end
	end

	local i = 1
	for x = minp.x, maxp.x do
		for z = minp.z, maxp.z do
			local poly = polygons[i]
			if poly then
				local xf, zf = geometry.transform_quadri(poly.x, poly.z, x/blocksize, z/blocksize)
				local i00, i01, i11, i10 = unpack(poly.i)

				local is_river = false
				local r_west, r_north, r_east, r_south = unpack(poly.rivers)
				if xf >= r_east then
					is_river = true
					xf = 1
				elseif xf <= r_west then
					is_river = true
					xf = 0
				end
				if zf >= r_south then
					is_river = true
					zf = 1
				elseif zf <= r_north then
					is_river = true
					zf = 0
				end

				if not is_river then
					local c_NW, c_NE, c_SE, c_SW = unpack(poly.river_corners)
					if xf+zf <= c_NW then
						is_river = true
						xf, zf = 0, 0
					elseif 1-xf+zf <= c_NE then
						is_river = true
						xf, zf = 1, 0
					elseif 2-xf-zf <= c_SE then
						is_river = true
						xf, zf = 1, 1
					elseif xf+1-zf <= c_SW then
						is_river = true
						xf, zf = 0, 1
					end
				end

				if not is_river then
					xf = (xf-r_west) / (r_east-r_west)
					zf = (zf-r_north) / (r_south-r_north)
				end

				local vdem = poly.dem
				local terrain_height = math.floor(0.5+interp(
					vdem[1],
					vdem[2],
					vdem[3],
					vdem[4],
					xf, zf
				))

				local lake_height = math.floor(poly.lake)

				local is_lake = lake_height > terrain_height
				
				local ivm = a:index(x, minp.y-1, z)

				if terrain_height >= minp.y then
					for y=minp.y, math.min(maxp.y, terrain_height) do
						if y == terrain_height then
							if is_lake or y <= sea_level then
								data[ivm] = c_sand
							elseif is_river then
								data[ivm] = c_rwater
							else
								data[ivm] = c_lawn
							end
						else
							data[ivm] = c_stone
						end
						ivm = ivm + ystride
					end
				end

				if lake_height > sea_level then
					if is_lake and lake_height > minp.y then
						for y=math.max(minp.y, terrain_height+1), math.min(maxp.y, lake_height) do
							data[ivm] = c_rwater
							ivm = ivm + ystride
						end
					end
				else
					for y=math.max(minp.y, terrain_height+1), math.min(maxp.y, sea_level) do
						data[ivm] = c_water
						ivm = ivm + ystride
					end
				end
			end
			i = i + 1
		end
	end

	vm:set_data(data)
	minetest.generate_ores(vm, minp, maxp)
	vm:set_lighting({day = 0, night = 0})
	vm:calc_lighting()
	vm:update_liquids()
	vm:write_to_map()
end

minetest.register_on_generated(generate)