1 files changed, 86 insertions, 0 deletions
diff --git a/lib/pure/random.nim b/lib/pure/random.nim
new file mode 100644
index 000000000..5a4d45c38
--- /dev/null
+++ b/lib/pure/random.nim
@@ -0,0 +1,86 @@
+#
+#
+#            Nim's Runtime Library
+#        (c) Copyright 2016 Andreas Rumpf
+#
+#    See the file "copying.txt", included in this
+#    distribution, for details about the copyright.
+#
+
+## | Nim's standard random number generator. Based on
+## | http://xoroshiro.di.unimi.it/
+## | http://xoroshiro.di.unimi.it/xoroshiro128plus.c
+
+include "system/inclrtl"
+{.push debugger:off .} # the user does not want to trace a part
+                       # of the standard library!
+
+# XXX Expose RandomGenState
+type
+  RandomGenState = object
+    a0, a1: uint64
+
+# racy for multi-threading but good enough for now:
+var state = RandomGenState(
+  a0: 0x69B4C98CB8530805u64,
+  a1: 0xFED1DD3004688D67CAu64) # global for backwards compatibility
+
+proc rotl(x: uint64, k: uint64): uint64 =
+  result = (x shl k) or (x shr (64u64 - k))
+
+proc next(s: var RandomGenState): uint64 =
+  let s0 = s.a0
+  var s1 = s.a1
+  result = s0 + s1
+  s1 = s1 xor s0
+  s.a0 = rotl(s0, 55) xor s1 xor (s1 shl 14) # a, b
+  s.a1 = rotl(s1, 36) # c
+
+proc skipRandomNumbers(s: var RandomGenState) =
+  ## This is the jump function for the generator. It is equivalent
+  ## to 2^64 calls to next(); it can be used to generate 2^64
+  ## non-overlapping subsequences for parallel computations.
+  const helper = [0xbeac0467eba5facbu64, 0xd86b048b86aa9922u64]
+  var
+    s0 = 0u64
+    s1 = 0u64
+  for i in 0..high(helper):
+    for b in 0..< 64:
+      if (helper[i] and (1u64 shl uint64(b))) != 0:
+        s0 = s0 xor s.a0
+        s1 = s1 xor s.a1
+      discard next(s)
+  s.a0 = s0
+  s.a1 = s1
+
+proc random*(max: int): int {.benign.} =
+  ## Returns a random number in the range 0..max-1. The sequence of
+  ## random number is always the same, unless `randomize` is called
+  ## which initializes the random number generator with a "random"
+  ## number, i.e. a tickcount.
+  result = int(next(state) mod uint64(max))
+
+proc random*(max: float): float {.benign.} =
+  ## Returns a random number in the range 0..<max. The sequence of
+  ## random number is always the same, unless `randomize` is called
+  ## which initializes the random number generator with a "random"
+  ## number, i.e. a tickcount.
+  let x = next(state)
+  let u = (0x3FFu64 shl 52u64) or (x shr 12u64)
+  result = (cast[float](u) - 1.0) * max
+
+proc randomize*(seed: int) {.benign.} =
+  ## Initializes the random number generator with a specific seed.
+  state.a0 = uint64(seed shr 16)
+  state.a1 = uint64(seed and 0xffff)
+
+when not defined(nimscript):
+  import times
+
+  proc randomize*() {.benign.} =
+    ## Initializes the random number generator with a "random"
+    ## number, i.e. a tickcount. Note: Does nothing for the JavaScript target,
+    ## as JavaScript does not support this. Nor does it work for NimScript.
+    randomize(int times.getTime())
+
+{.pop.}