It looks like Furey wrote a random-number generator for Merc based on an algorithm from Knuth. When Alander wrote ROM, he said he noticed streakiness in the Furey-written RNG and reverted to the POSIX system function random() while seeding it using srandom() with a seed of a bitwise XOR of the time_t long and the program's PID. The relevant bits are below:

/*
 * I've gotten too many bad reports on OS-supplied random number generators.
 * This is the Mitchell-Moore algorithm from Knuth Volume II.
 * Best to leave the constants alone unless you've read Knuth.
 * -- Furey
 */

/* I noticed streaking with this random number generator, so I switched
   back to the system srandom call.  If this doesn't work for you,
   define OLD_RAND to use the old system -- Alander */

#if defined (OLD_RAND)
static int rgiState[2 + 55];
#endif

void init_mm ()
{
#if defined (OLD_RAND)
    int *piState;
    int iState;

    piState = &rgiState[2];

    piState[-2] = 55 - 55;
    piState[-1] = 55 - 24;

    piState[0] = ((int) current_time) & ((1 << 30) - 1);
    piState[1] = 1;
    for (iState = 2; iState < 55; iState++)
    {
        piState[iState] = (piState[iState - 1] + piState[iState - 2])
            & ((1 << 30) - 1);
    }
#else
    srandom (time (NULL) ^ getpid ());
#endif
    return;
}



long number_mm (void)
{
#if defined (OLD_RAND)
    int *piState;
    int iState1;
    int iState2;
    int iRand;

    piState = &rgiState[2];
    iState1 = piState[-2];
    iState2 = piState[-1];
    iRand = (piState[iState1] + piState[iState2]) & ((1 << 30) - 1);
    piState[iState1] = iRand;
    if (++iState1 == 55)
        iState1 = 0;
    if (++iState2 == 55)
        iState2 = 0;
    piState[-2] = iState1;
    piState[-1] = iState2;
    return iRand >> 6;
#else
    return random () >> 6;
#endif
}

/*
 * Generate a random number.
 */
int number_range (int from, int to)
{
    int power;
    int number;

    if (from == 0 && to == 0)
        return 0;

    if ((to = to - from + 1) <= 1)
        return from;

    for (power = 2; power < to; power <<= 1);

    while ((number = number_mm () & (power - 1)) >= to);

    return from + number;
}