diff --git a/lib/factorial.rb b/lib/factorial.rb
index 6a4a408..091cc32 100644
--- a/lib/factorial.rb
+++ b/lib/factorial.rb
@@ -1,6 +1,15 @@
 # Computes factorial of the input number and returns it
-# Time complexity: ?
-# Space complexity: ?
+# Time complexity: O(n), where n is the value of the number
+# Space complexity: O(1)
 def factorial(number)
-  raise NotImplementedError
+  if !number
+    raise ArgumentError, "Number cannot be nil"
+  end
+
+  product = number > 1 ? number : 1
+  while number >= 2
+    product = product *= (number - 1)
+    number -= 1
+  end
+  return product
 end
diff --git a/specs/factorial_spec.rb b/specs/factorial_spec.rb
index de28055..287ce5c 100644
--- a/specs/factorial_spec.rb
+++ b/specs/factorial_spec.rb
@@ -1,6 +1,6 @@
-require 'minitest/autorun'
-require 'minitest/reporters'
-require_relative '../lib/factorial'
+require "minitest/autorun"
+require "minitest/reporters"
+require_relative "../lib/factorial"
 
 describe "factorial" do
   describe "basic tests" do
@@ -17,7 +17,7 @@
   describe "edge cases" do
     # if the parameter is an object, check for nil
     it "nil object is not an integer" do
-      proc {factorial(nil)}.must_raise ArgumentError
+      proc { factorial(nil) }.must_raise ArgumentError
     end
 
     it "factorial(0) = 1" do