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+#
+#
+#            Nimrod's Runtime Library
+#        (c) Copyright 2010 Andreas Rumpf
+#
+#    See the file "copying.txt", included in this
+#    distribution, for details about the copyright.
+#
+
+## This module implements some common generic algorithms.
+
+type
+  TSortOrder* = enum   ## sort order
+    Descending, Ascending 
+
+proc `*`*(x: int, order: TSortOrder): int {.inline.} = 
+  ## flips `x` if ``order == Descending``;
+  ## if ``order == Ascending`` then `x` is returned.
+  ## `x` is supposed to be the result of a comparator.
+  var y = order.ord - 1
+  result = (x xor y) - y
+
+proc reverse*[T](a: var openArray[T], first, last: int) =
+  ## reverses the array ``a[first..last]``.
+  var x = first
+  var y = last
+  while x < y:
+    swap(a[x], a[y])
+    dec(y)
+    inc(x)
+
+proc reverse*[T](a: var openArray[T]) =
+  ## reverses the array `a`.
+  reverse(a, 0, a.high)
+
+const
+  onlySafeCode = false
+
+proc merge[T](a, b: var openArray[T], lo, m, hi: int, 
+              cmp: proc (x, y: T): int, order: TSortOrder) =
+  template `<-` (a, b: expr) = 
+    when onlySafeCode:
+      a = b
+    else:
+      copyMem(addr(a), addr(b), sizeof(T))
+  # optimization: If max(left) <= min(right) there is nothing to do!
+  # 1 2 3 4  ## 5 6 7 8
+  # -> O(n) for sorted arrays.
+  # On random data this safes up to 40% of merge calls
+  if cmp(a[m], a[m+1]) * order <= 0: return
+  var j = lo
+  # copy a[j..m] into b:
+  assert j <= m
+  when onlySafeCode:
+    var bb = 0
+    while j <= m:
+      b[bb] = a[j]
+      inc(bb)
+      inc(j)
+  else:
+    CopyMem(addr(b[0]), addr(a[j]), sizeof(T)*(m-j+1))
+    j = m+1
+  var i = 0
+  var k = lo
+  # copy proper element back:
+  while k < j and j <= hi:
+    if cmp(b[i], a[j]) * order <= 0:
+      a[k] <- b[i]
+      inc(i)
+    else:
+      a[k] <- a[j]
+      inc(j)
+    inc(k)
+  # copy rest of b:
+  when onlySafeCode:
+    while k < j:
+      a[k] = b[i]
+      inc(k)
+      inc(i)
+  else:
+    if k < j: copyMem(addr(a[k]), addr(b[i]), sizeof(T)*(j-k))
+
+proc sort*[T](a: var openArray[T], 
+              cmp: proc (x, y: T): int = cmp,
+              order = TSortOrder.Ascending) =
+  ## Default Nimrod sort. The sorting is guaranteed to be stable and 
+  ## the worst case is guaranteed to be O(n log n).
+  ## The current implementation uses an iterative
+  ## mergesort to achieve this. It uses a temporary sequence of 
+  ## length ``a.len div 2``.
+  var n = a.len
+  var b: seq[T]
+  newSeq(b, n div 2)
+  var s = 1
+  while s < n:
+    var m = n-1-s
+    while m >= 0:
+      merge(a, b, max(m-s+1, 0), m, m+s, cmp, order)
+      dec(m, s*2)
+    s = s*2
+