path: root/lib/pure/mersenne.nim

#
#
#            Nim's Runtime Library
#        (c) Copyright 2015 Nim Contributors
#
#    See the file "copying.txt", included in this
#    distribution, for details about the copyright.
#

type
  MersenneTwister* = object
    mt: array[0..623, uint32]
    index: int

proc newMersenneTwister*(seed: uint32): MersenneTwister =
  result.index = 0
  result.mt[0] = seed
  for i in 1'u32 .. 623'u32:
    result.mt[i] = (0x6c078965'u32 * (result.mt[i-1] xor
                                      (result.mt[i-1] shr 30'u32)) + i)

proc generateNumbers(m: var MersenneTwister) =
  for i in 0..623:
    var y = (m.mt[i] and 0x80000000'u32) +
            (m.mt[(i+1) mod 624] and 0x7fffffff'u32)
    m.mt[i] = m.mt[(i+397) mod 624] xor uint32(y shr 1'u32)
    if (y mod 2'u32) != 0:
      m.mt[i] = m.mt[i] xor 0x9908b0df'u32

proc getNum*(m: var MersenneTwister): uint32 =
  ## Returns the next pseudo random number ranging from 0 to high(uint32)
  if m.index == 0:
    generateNumbers(m)
  result = m.mt[m.index]
  m.index = (m.index + 1) mod m.mt.len

  result = result xor (result shr 11'u32)
  result = result xor ((result shl 7'u32) and 0x9d2c5680'u32)
  result = result xor ((result shl 15'u32) and 0xefc60000'u32)
  result = result xor (result shr 18'u32)


runnableExamples:
  static:
    block:
      var rando: MersenneTwister = newMersenneTwister(uint32.high)  ## Must be "var".
      doAssert rando.getNum() != rando.getNum()  ## Pseudo random number. Works at compile-time.


# Test
when not defined(testing) and isMainModule:
  var mt = newMersenneTwister(2525)

  for i in 0..99:
    echo mt.getNum