How to pull a random value from two numbers based on a Mersenne Twister returned value

Question

How to pull a random value from two numbers based on a Mersenne Twister returned value

99 views Asked by Cameron Kilgore At 03 January 2017 at 12:04

I have implemented a Mersenne Twister in Python using the following example code and while it does work as intended, I am not clear on how to limit results returned to a range of integers. For example, if I wanted to use this mechanism to determine the value of a dice roll, I would (right now, in an incredibly inefficient manner) iterate through potential results from the MT until something falls within the set. There has to be a much more memory-efficient manner to get a value to fall within a set range, like 1-20.

Currently, the algorithm returns a random set of numbers that don't seem to peak anywhere close to the set range:

For testing purposes I implemented the current test logic (part of __main__) and so far it just runs infinitely.

#!/usr/bin/env python2
# -*- coding: utf-8 -*-
"""
Based on the pseudocode in https://en.wikipedia.org/wiki/Mersenne_Twister. Generates uniformly distributed 32-bit integers in the range [0, 232 − 1] with the MT19937 algorithm

Yaşar Arabacı <yasar11732 et gmail nokta com>
"""
# Create a length 624 list to store the state of the generator
MT = [0 for i in xrange(624)]
index = 0

# To get last 32 bits
bitmask_1 = (2 ** 32) - 1

# To get 32. bit
bitmask_2 = 2 ** 31

# To get last 31 bits
bitmask_3 = (2 ** 31) - 1

def initialize_generator(seed):
    "Initialize the generator from a seed"
    global MT
    global bitmask_1
    MT[0] = seed
    for i in xrange(1,624):
        MT[i] = ((1812433253 * MT[i-1]) ^ ((MT[i-1] >> 30) + i)) & bitmask_1


def extract_number():
    """
    Extract a tempered pseudorandom number based on the index-th value,
    calling generate_numbers() every 624 numbers
    """
    global index
    global MT
    if index == 0:
        generate_numbers()
    y = MT[index]
    y ^= y >> 11
    y ^= (y << 7) & 2636928640
    y ^= (y << 15) & 4022730752
    y ^= y >> 18

    index = (index + 1) % 624
    return y

def generate_numbers():
    "Generate an array of 624 untempered numbers"
    global MT
    for i in xrange(624):
        y = (MT[i] & bitmask_2) + (MT[(i + 1 ) % 624] & bitmask_3)
        MT[i] = MT[(i + 397) % 624] ^ (y >> 1)
        if y % 2 != 0:
            MT[i] ^= 2567483615

if __name__ == "__main__":
    from datetime import datetime
    now = datetime.now()
    solved = False
    initialize_generator(now.microsecond)
    #for i in xrange(10):
    #    "Print 10 random numbers as an example"
    while(solved != True):
        generated_number = extract_number()
        while(generated_number <= 20 and generated_number >= 1):
            print generated_number
            solved = True

Any advice on how to implement? It appears that it may not even get a chance to drop down to a number within the predefined set.

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How to pull a random value from two numbers based on a Mersenne Twister returned value

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