Add shuffle to math module

lib/pure/math.nim

@@ -284,6 +284,14 @@ proc random[T](a: openArray[T]): T =
   ## returns a random element from the openarray `a`.
   result = a[random(a.low..a.len)]
 
+proc shuffle[T](a: var openArray[T]) =
+  ## Shuffles the elements in `a`. Note that ``randomize`` needs to be called
+  ## to initialize the random number generator, otherwise ``shuffle`` always
+  ## shuffles in the same way.
+  for i in countdown(a.high, 0):
+    let j = random(i + 1)
+    swap(a[i], a[j])
+
 type
   RunningStat* = object                 ## an accumulator for statistical data
     n*: int                             ## number of pushed data