diff --git a/algo.lua b/algo.lua
index b95b43f..c05c73e 100644
--- a/algo.lua
+++ b/algo.lua
@@ -18,6 +18,12 @@ local algo = {}
 -- @treturn number clamped value.
 function algo.clamp(x, a, b) return min(max(x, a), b) end
 
+--- Sign function.
+--
+-- @number x value whose sign should be returned.
+-- @treturn number sign of x, -1 if negative, 1 if positive, 0 otherwise.
+function algo.sign(x) return x > 0 and 1 or x < 0 and -1 or 0 end
+
 --- Fast remove from array.
 --
 -- Replace 'array[i]' with last array's element and