" glibc-2.12.1: random_r.c

let s:RAND_MAX = 2147483647

function! Srand(seed)
  call s:srandom_r(a:seed, s:state)
endfunction

function! Rand()
  return s:random_r(s:state)
endfunction

" Linear congruential.
let s:TYPE_0 = 0
let s:BREAK_0 = 8
let s:DEG_0 = 0
let s:SEP_0 = 0

" x**7 + x**3 + 1.
let s:TYPE_1 = 1
let s:BREAK_1 = 32
let s:DEG_1 = 7
let s:SEP_1 = 3

" x**15 + x + 1.
let s:TYPE_2 = 2
let s:BREAK_2 = 64
let s:DEG_2 = 15
let s:SEP_2 = 1

" x**31 + x**3 + 1.
let s:TYPE_3 = 3
let s:BREAK_3 = 128
let s:DEG_3 = 31
let s:SEP_3 = 3

" x**63 + x + 1.
let s:TYPE_4 = 4
let s:BREAK_4 = 256
let s:DEG_4 = 63
let s:SEP_4 = 1

let s:MAX_TYPES = 5

let s:randtbl = [
      \ s:TYPE_3,
      \
      \ -1726662223, 379960547, 1735697613, 1040273694, 1313901226,
      \ 1627687941, -179304937, -2073333483, 1780058412, -1989503057,
      \ -615974602, 344556628, 939512070, -1249116260, 1507946756,
      \ -812545463, 154635395, 1388815473, -1926676823, 525320961,
      \ -1009028674, 968117788, -123449607, 1284210865, 435012392,
      \ -2017506339, -911064859, -370259173, 1132637927, 1398500161,
      \ -205601318,
      \ ]

let s:state = {
      \ 'tbl': s:randtbl,
      \ 'fptr': s:SEP_3 + 1,
      \ 'rptr': 1,
      \ 'state': 1,
      \ 'rand_type': s:TYPE_3,
      \ 'rand_deg': s:DEG_3,
      \ 'rand_sep': s:SEP_3,
      \ 'end_ptr': len(s:randtbl),
      \ }

function! s:srandom_r(seed, buf)
  let type = a:buf.rand_type

  let state = a:buf.state
  let seed = (a:seed == 0) ? 1 : a:seed
  let a:buf.tbl[state] = seed
  if type == s:TYPE_0
    return
  endif

  let dst = state
  let word = seed
  let kc = a:buf.rand_deg
  for i in range(kc - 1)
    let hi = word / 127773
    let lo = word % 127773
    let word = 16807 * lo - 2836 * hi
    if word < 0
      let word += 2147483647
    endif
    let dst += 1
    let a:buf.tbl[dst] = word
  endfor

  let a:buf.fptr = state + a:buf.rand_sep
  let a:buf.rptr = state
  let kc = kc * 10
  for i in range(kc)
    call s:random_r(a:buf)
  endfor
endfunction

function! s:random_r(buf)
  let state = a:buf.state

  if a:buf.rand_type == s:TYPE_0
    " val = ((state[0] * 1103515245) + 12345) & 0x7fffffff;
    let val = (a:buf.tbl[state] * 1103515245) + 12345
    if val < 0
      let val -= 0x80000000
    endif
    let a:buf.tbl[state] = val
    let result = val
  else
    let fptr = a:buf.fptr
    let rptr = a:buf.rptr
    let end_ptr = a:buf.end_ptr

    let a:buf.tbl[fptr] += a:buf.tbl[rptr]
    let val = a:buf.tbl[fptr]
    " result = (val >> 1) & 0x7fffffff
    if val < 0
      let result = ((val - 0x80000000) / 2) + 0x40000000
    else
      let result = val / 2
    endif
    let fptr += 1
    if fptr >= end_ptr
      let fptr = state
      let rptr += 1
    else
      let rptr += 1
      if rptr >= end_ptr
        let rptr = state
      endif
    endif
    let a:buf.fptr = fptr
    let a:buf.rptr = rptr
  endif

  return result
endfunction