knuth_subtractive_random.vim

" Knuth Subtractive Random Number Generator

let knuth_random = { 'index': 0, 'state': repeat([0], 56) }

function! knuth_random.seed(seed) dict
  let l:seed = 161803398 - (a:seed < 0 ? -a:seed : a:seed)
  let self.state[55] = l:seed
  let [l:i, l:j, l:k] = [0, 0, 1]
  for l:i in range(1, 54)
    let l:j = (21 * l:i) % 55
    let self.state[l:j] = l:k
    let l:k = l:seed - l:k
    if l:k < 0
      let l:k = l:k + 2147483647
    endif
    let l:seed = self.state[l:j]
  endfor
  for l:j in range(1, 4)
    for l:i in range(1, 55)
      let self.state[l:i] = self.state[l:i] - self.state[1 + (l:i + 30) % 55]
      if self.state[l:i] < 0
        let self.state[l:i] = self.state[l:i] + 2147483647
      endif
    endfor
  endfor
  let self.index = 0
endfunction

function! knuth_random.next() dict
  let self.index = self.index + 1
  if self.index >= 56
    let self.index = 1
  endif
  let l:r = self.state[self.index]
  if self.index <= 34
    let l:r = l:r - self.state[self.index + 21]
  else
    let l:r = l:r - self.state[self.index - 34]
  endif
  if l:r < 0
    let l:r = l:r + 2147483647
  endif
  return l:r
endfunction