knuth_subtractive_random.vim

" Knuth Subtractive Random Number Generator

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

function! s: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 = 1
endfunction

function! s:knuth_random.next() dict
  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
  let self.index = self.index + 1
  return l:r
endfunction

function! s:_sum(xs)
  let memo = 0
  for x in a:xs
    let memo += x
  endfor
  return memo
endfunction

function! s:rand_number()
  if s:knuth_random.index ==# 0
    if filereadable('/dev/urandom')
      let randoms = map(
            \ split(join(readfile('/dev/urandom', 'b', 4), '\n')[0:3], '\zs'),
            \ 'char2nr(v:val)')
    else
      let randoms = repeat([0], 4)
    endif
    if has('reltime')
      let [sec, usec] = reltime()
    else
      let [sec, usec] = [localtime(), 0]
    endif
    let randoms[0] = randoms[0] ^ sec
    let randoms[1] = randoms[1] ^ usec
    let randoms[2] = randoms[2] ^ getpid()
    let seed = s:_sum(randoms)
    call s:knuth_random.seed(seed)
  endif
  return s:knuth_random.next()
endfunction