Python port of the GLIBC rng

s-rand.py

#!/usr/bin/env python2
from ctypes import c_int, c_uint

# http://www.mscs.dal.ca/~selinger/random/


def srand(seed):
    srand.r = [0 for _ in range(34)]
    srand.r[0] = c_int(seed).value
    for i in range(1, 31):
        srand.r[i] = (16807 * srand.r[i - 1]) % 2147483647
    for i in range(31, 34):
        srand.r[i] = srand.r[i - 31]
    srand.k = 0
    for _ in range(34, 344):
        rand()


def rand():
    srand.r[srand.k] = srand.r[(srand.k - 31) % 34] + srand.r[(srand.k - 3) % 34]
    r = c_uint(srand.r[srand.k]).value >> 1
    srand.k = (srand.k + 1) % 34
    return r


if __name__ == '__main__':
    srand(1337)
    assert [rand() for _ in range(10)] == [
        292616681, 1638893262, 255706927, 995816787, 588263094, 1540293802, 343418821, 903681492,
        898530248, 1459533395
    ]

    srand(0xffffffff + 0xffffffff - 1)
    assert [rand() for _ in range(10)] == [
        853660264, 1568971201, 1203662233, 15207980, 1421679843, 1717493552, 811896681, 155106358,
        1156099704, 428649477
    ]

    srand(-1337)
    assert [rand() for _ in range(10)] == [
        1766598330, 413225925, 1792113474, 2120225281, 1445538174, 488114690, 1678701932,
        1108308242, 32946609, 1612248994
    ]

    srand(-0xffffffff - 0xffffffff)
    assert [rand() for _ in range(10)] == [
        1505335290, 1738766719, 190686788, 260874575, 747983061, 906156498, 1502820864, 142559277,
        1261608745, 1380759627
    ]

    # from ctypes import cdll
    # from ctypes.util import find_library
    # libc = cdll.LoadLibrary(find_library('c'))
    # libc.srand(1337)
    # print[libc.rand() for _ in range(10)]

test.c

#include <stdlib.h>
#include <stdio.h>


int main(int argc, char const *argv[])
{
    srand(1337);
    for (int i = 0; i < 10; ++i) {
        printf("%d\n", rand());
    }

    printf("\n");

    srand(0xffffffff + 0xffffffff - 1);
    for (int i = 0; i < 10; ++i) {
        printf("%d\n", rand());
    }

    printf("\n");

    srand(-1337);
    for (int i = 0; i < 10; ++i) {
        printf("%d\n", rand());
    }

    printf("\n");

    srand(-0xffffffff - 0xffffffff);
    for (int i = 0; i < 10; ++i) {
        printf("%d\n", rand());
    }

    return 0;
}

small correction: glibc srand algorithm breaks on seed==0, so it has following code in it:

/* We must make sure the seed is not 0.  Take arbitrarily 1 in this case.  */
if (seed == 0)
  seed = 1;

so srand(0) == srand(1)