Add vector.random_direction() (#14784)

Generate a random vector of unit length. Useful for many mods.

builtin/common/vector.lua

@@ -375,6 +375,18 @@ function vector.in_area(pos, min, max)
 		(pos.z >= min.z) and (pos.z <= max.z)
 end
 
+function vector.random_direction()
+	-- Generate a random direction of unit length, via rejection sampling
+	local x, y, z, l2
+	repeat -- expected less than two attempts on average (volume sphere vs. cube)
+		x, y, z = math.random() * 2 - 1, math.random() * 2 - 1, math.random() * 2 - 1
+        l2 = x*x + y*y + z*z
+	until l2 <= 1 and l2 >= 1e-6
+	-- normalize
+	local l = math.sqrt(l2)
+	return fast_new(x/l, y/l, z/l)
+end
+
 if rawget(_G, "core") and core.set_read_vector and core.set_push_vector then
 	local function read_vector(v)
 		return v.x, v.y, v.z