Linear Feedback Shift Register pseudorandom number generator in lua

lfsr.lua

-- LFSRs are good for games where you want a random number generator with a short period,
-- so that speed runners can device tactics to control the RNG's state.
-- Otherwise they're a bit of an obsolete piece of tech.
-- See: https://en.wikipedia.org/wiki/Linear-feedback_shift_register

-- LFSRs work by shifting all of the bits to the right by one, and using the shifted
-- out bit to XOR in at specific taps. Another way to put it is that they first check
-- if the state is odd, then divides by two, and if it was odd, Exclusive-Ors in a value.
-- The bit shifted out is suitably random. Want more bits, cycle the generator more.

-- Note: Uses bit library and not bit32, would need minor conversion between the two.
require 'bit'

LFSR = {
  value = 46080, -- taps 16 14 13 11
  state = 44257
}

LFSR.__index = LFSR

function LFSR:seed(state)
  self.state = bit.band(state, 65535)
  if self.state <= 0 then self.state = 1 end
end

function LFSR:randBits(n)
  n = n or 8
  local state, value, ret, b = self.state, self.value, 0
  repeat
    b = bit.band(state, 1)
    state = bit.bxor(bit.rshift(state, 1), bit.band(-b, value))
    ret, n = bit.lshift(ret, 1) + b, n - 1
  until n <= 0
  self.state = state
  return ret
end

--

function calculatePeriod(R)
  local start, iter = R.state, 0, 0
  repeat
    R:randBits(1)
    iter = iter + 1
    assert(iter < 10000000)
  until R.state == start
  return iter
end

function showSample(R)
  for i = 1, 16 do
    for j = 1, 12 do
      io.write(R:randBits(8), '\t')
    end
    io.write('\n')
  end
end

print("Period:", calculatePeriod(setmetatable({}, LFSR)))
showSample(setmetatable({}, LFSR))