# mt.pony

``````class MT is Random
"""
A Mersenne Twister. This is a non-cryptographic random number generator. This
should only be used for legacy applications that require a Mersenne Twister,
otherwise use Rand.
"""
embed _state: Array[U64]
var _index: USize

new create(x: U64 = 5489, y: U64 = 0) =>
"""
Create with the specified seed. Returned values are deterministic for a
given seed.
"""
_state = Array[U64](_n())
_index = _n()

var seed = x xor y

_state.push(seed)
var i: USize = 1

while i < _n() do
seed = ((seed xor (seed >> 62)) * 6364136223846793005) + i.u64()
_state.push(seed)
i = i + 1
end

fun ref next(): U64 =>
"""
A random integer in [0, 2^64)
"""
if _index >= _n() then
_populate()
end

try
var x = _state(_index)?
_index = _index + 1

x = x xor ((x >> 29) and 0x5555555555555555)
x = x xor ((x << 17) and 0x71d67fffeda60000)
x = x xor ((x << 37) and 0xfff7eee000000000)
x xor (x >> 43)
else
0
end

fun ref _populate() =>
"""
Repopulates the state array.
"""
try
_index = 0
var x = _state(0)?
var i: USize = 0

while i < _m() do
x = _lower(i, x)?
i = i + 1
end

x = _state(_m())?
i = _m()

while i < _n1() do
x = _upper(i, x)?
i = i + 1
end

_wrap()?
end

fun tag _n(): USize => 312
fun tag _m(): USize => 156
fun tag _n1(): USize => _n() - 1

fun tag _mask(x: U64, y: U64): U64 =>
(x and 0xffffffff80000000) or (y and 0x000000007fffffff)

fun tag _matrix(x: U64): U64 => (x and 1) * 0xb5026f5aa96619e9

fun tag _mix(x: U64, y: U64): U64 =>
(z >> 1) xor _matrix(z)

fun ref _lower(i: USize, x: U64): U64 ? =>
let y = _state(i + 1)?
_state(i)? = _state(i + _m())? xor _mix(x, y)
y

fun ref _upper(i: USize, x: U64): U64 ? =>
let y = _state(i + 1)?
_state(i)? = _state(i - _m())? xor _mix(x, y)
y

fun ref _wrap(): U64 ? =>
let x = _state(_n1())?
let y = _state(0)?
_state(_n1())? = _state(_m() - 1)? xor _mix(x, y)
y

``````