Use an approximation of the heightmap derivative to "erode" some layers of ground_structure (#1699)

Fixes #188

assets/cubyz/biomes/peak.zig.zon

@@ -12,6 +12,8 @@
 
 	.music = "cubyz:DarkTimes",
 
+	.soilCreep = 1.0,
+
 	.ground_structure = .{
 		"3 to 6 cubyz:snow",
 		"1 to 3 cubyz:permafrost",

assets/cubyz/biomes/tall_mountain/peak.zig.zon

@@ -8,6 +8,8 @@
 	.mountains = 30,
 	.chance = 0,
 
+	.soilCreep = 1.5,
+
 	.music = "cubyz:out_of_breath",
 
 	.parentBiomes = .{

src/server/terrain/biomes.zig

@@ -289,6 +289,8 @@ pub const Biome = struct { // MARK: Biome
 	caves: f32,
 	caveRadiusFactor: f32,
 	crystals: u32,
+	/// How much of the surface structure should be eroded depending on the slope.
+	soilCreep: f32,
 	stoneBlock: main.blocks.Block,
 	fogLower: f32,
 	fogHigher: f32,
@@ -334,6 +336,7 @@ pub const Biome = struct { // MARK: Biome
 			.caves = zon.get(f32, "caves", -0.375),
 			.caveRadiusFactor = @max(-2, @min(2, zon.get(f32, "caveRadiusFactor", 1))),
 			.crystals = zon.get(u32, "crystals", 0),
+			.soilCreep = zon.get(f32, "soilCreep", 0.5),
 			.minHeight = zon.get(i32, "minHeight", std.math.minInt(i32)),
 			.maxHeight = zon.get(i32, "maxHeight", std.math.maxInt(i32)),
 			.supportsRivers = zon.get(bool, "rivers", false),
@@ -462,11 +465,16 @@ pub const BlockStructure = struct { // MARK: BlockStructure
 		allocator.free(self.structure);
 	}
 
-	pub fn addSubTerranian(self: BlockStructure, chunk: *ServerChunk, startingDepth: i32, minDepth: i32, x: i32, y: i32, seed: *u64) i32 {
+	pub fn addSubTerranian(self: BlockStructure, chunk: *ServerChunk, startingDepth: i32, minDepth: i32, slope: i32, soilCreep: f32, x: i32, y: i32, seed: *u64) i32 {
 		var depth = startingDepth;
+		var remainingSkippedBlocks = @as(i32, @intFromFloat(@as(f32, @floatFromInt(slope))*soilCreep)) - 1;
 		for(self.structure) |blockStack| {
 			const total = blockStack.min + main.random.nextIntBounded(u32, seed, @as(u32, 1) + blockStack.max - blockStack.min);
 			for(0..total) |_| {
+				if(remainingSkippedBlocks > 0) {
+					remainingSkippedBlocks -= 1;
+					continue;
+				}
 				if(chunk.liesInChunk(x, y, depth)) {
 					chunk.updateBlockInGeneration(x, y, depth, blockStack.block);
 				}

src/server/terrain/chunkgen/TerrainGenerator.zig

@@ -73,11 +73,32 @@ pub fn generate(worldSeed: u64, chunk: *main.chunk.ServerChunk, caveMap: CaveMap
 				while(z >= zBiome) : (z -= chunk.super.pos.voxelSize) {
 					const mask = @as(u64, 1) << @intCast(z >> voxelSizeShift);
 					if(heightData & mask != 0) {
+						const cardinalDirections = [_]Vec3i{
+							Vec3i{1, 0, 0},
+							Vec3i{-1, 0, 0},
+							Vec3i{0, 1, 0},
+							Vec3i{0, -1, 0},
+						};
+
 						const surfaceBlock = caveMap.findTerrainChangeAbove(x, y, z) - chunk.super.pos.voxelSize;
+						var maxUp: i32 = 0;
+						var maxDown: i32 = 0;
+						for(cardinalDirections) |direction| {
+							if(caveMap.isSolid(x + direction[0], y + direction[1], z + direction[2])) {
+								const diff = caveMap.findTerrainChangeAbove(x + direction[0], y + direction[1], z + direction[2]) - chunk.super.pos.voxelSize - surfaceBlock;
+								maxUp = @max(maxUp, diff);
+							} else {
+								const diff = caveMap.findTerrainChangeBelow(x + direction[0], y + direction[1], z + direction[2]) - surfaceBlock;
+								maxDown = @max(maxDown, -diff);
+							}
+						}
+						const slope = @min(maxUp, maxDown);
+
+						const soilCreep: f32 = biome.soilCreep;
 						var bseed: u64 = random.initSeed3D(worldSeed, .{chunk.super.pos.wx + x, chunk.super.pos.wy + y, chunk.super.pos.wz + z});
 						const airBlockBelow = caveMap.findTerrainChangeBelow(x, y, z);
 						// Add the biomes surface structure:
-						z = @min(z + chunk.super.pos.voxelSize, biome.structure.addSubTerranian(chunk, surfaceBlock, @max(airBlockBelow, zBiome - 1), x, y, &bseed));
+						z = @min(z + chunk.super.pos.voxelSize, biome.structure.addSubTerranian(chunk, surfaceBlock, @max(airBlockBelow, zBiome - 1), slope, soilCreep, x, y, &bseed));
 						z -= chunk.super.pos.voxelSize;
 						if(z < zBiome) break;
 						if(z > airBlockBelow) {