From 6017510df08438e1456d6a4a551c85f31289f318 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ga=C3=ABl=20C?= <gael-de-sailly@netc.eu>
Date: Sun, 11 Feb 2024 08:40:48 +0100
Subject: [PATCH] Re-implement minetest.get_spawn_level

---
 init.lua  |  1 +
 spawn.lua | 77 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 78 insertions(+)
 create mode 100644 spawn.lua

diff --git a/init.lua b/init.lua
index c8c2886..df67d75 100644
--- a/init.lua
+++ b/init.lua
@@ -11,5 +11,6 @@ dofile(modpath .. 'gridmanager.lua')
 dofile(modpath .. 'polygons.lua')
 dofile(modpath .. 'heightmap.lua')
 dofile(modpath .. 'mapgen.lua')
+dofile(modpath .. 'spawn.lua')
 
 minetest.register_on_mods_loaded(mapgen_rivers.load_or_generate_grid)
diff --git a/spawn.lua b/spawn.lua
new file mode 100644
index 0000000..d9704ba
--- /dev/null
+++ b/spawn.lua
@@ -0,0 +1,77 @@
+local np_distort_x = mapgen_rivers.noise_params.distort_x
+local np_distort_z = mapgen_rivers.noise_params.distort_z
+local np_distort_amplitude = mapgen_rivers.noise_params.distort_amplitude
+
+local nobj_distort_x, nobj_distort_z, nobj_distort_amplitude
+
+local sea_level = mapgen_rivers.settings.sea_level
+local distort = mapgen_rivers.settings.distort
+
+-- Linear interpolation
+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 function estimate_spawn_level(pos, use_distort)
+	local x, z = pos.x, pos.z
+	if distort and use_distort then
+		nobj_distort_x = nobj_distort_x or minetest.get_perlin(np_distort_x)
+		nobj_distort_z = nobj_distort_z or minetest.get_perlin(np_distort_z)
+		nobj_distort_amplitude = nobj_distort_amplitude or minetest.get_perlin(np_distort_amplitude)
+
+		local amplitude = nobj_distort_amplitude:get_2d({x=pos.x, y=pos.z})
+		x = x + nobj_distort_x:get_3d(pos)*amplitude
+		z = z + nobj_distort_z:get_3d(pos)*amplitude
+	end
+
+	local terrain, lakes = mapgen_rivers.make_heightmaps(
+		{x=math.floor(x),   z=math.floor(z)  },
+		{x=math.floor(x)+1, z=math.floor(z)+1}
+	)
+
+	local ex, ez = x % 1, z % 1
+	--local h = terrain[1]*(1-ex)*(1-ez) + terrain[2]*ex*(1-ez) + terrain[3]*(1-ex)*ez + terrain[4]*ex*ez
+	local h = interp(terrain[1], terrain[2], terrain[4], terrain[3], ex, ez)
+	local lake = math.min(lakes[1], lakes[2], lakes[3], lakes[4])
+
+	if h < lake or h <= sea_level then
+		return false, h
+	end
+
+	return true, h
+end
+
+function minetest.get_spawn_level(x, z)
+	local pos = {x=x, z=z}
+	local suitable, y = estimate_spawn_level(pos, false)
+	if not suitable then
+		return
+	end
+
+	if not distort then
+		return math.floor(y) + 1
+	end
+
+	local low_bound = -math.huge
+	local high_bound = math.huge
+	local suitable_high = false
+
+	repeat
+		pos.y = math.max(math.min(math.floor(y+0.5), high_bound-1), low_bound+1)
+		suitable, y = estimate_spawn_level(pos, true)
+		if y > pos.y then
+			low_bound = pos.y
+		else
+			high_bound = pos.y
+			suitable_high = suitable
+		end
+	until high_bound - low_bound <= 1
+
+	if not suitable_high then
+		return
+	end
+
+	return high_bound
+end