XorRandShuffle

main.cpp

#include <cstdio>
#include <iostream>
#include "xorrand.h"

int main() {
  XorRand seed = 0;

  for (int i = 0; i != 64; ++i) {
    float f = XorRandShuffleFloat(seed);
    printf("X: %x, F: %f\n", seed, f);
  }

  enum {
    INIT = 0
  };

  seed = INIT;
  do {
    XorRand oldSeed = seed;
    XorRandShuffle(seed);
    if (oldSeed == seed) {
      printf("BREAK: %x\n", seed);
      break;
    }

    ++seed;
  } while (seed != INIT);

  seed = INIT;
  do {
    XorRand fseed = seed;
    float f = XorRandShuffleFloat(fseed);

    if (!(f >= 0.0f && f < 1.0f)) {
      printf("BREAK: %x, %x, %f\n", seed, fseed, f);
      break;
    }

    ++seed;
  } while (seed != INIT);

  int stat[128] = {};

  seed = INIT;
  do {
    XorRand old = seed;
    XorRandShuffle(old);
    ++stat[old % 128];

    ++seed;
  } while (seed != INIT);

  for (int i = 0; i != 128; ++i) {
    printf("stat[%i] == %i\n", i, stat[i]);
  }

  XorRand test = 0;
  seed = INIT;
  do {
    XorRand old = seed;
    XorRandShuffle(old);
    test ^= old;

    ++seed;
  } while (seed != INIT);
  printf("test = %x \n", test);
}

xorrand.h

#include <cstdint>

typedef uint32_t u32;
typedef u32 XorRand;

//https://en.wikipedia.org/wiki/Xorshift#Xorshift.2A
//http://www.jstatsoft.org/article/view/v008i14/xorshift.pdf page 4
void XorRandShuffle(XorRand & xorRand) {
  // this magic number helps to prevent collisions.
  xorRand ^= 0x926a0348;
  xorRand ^= xorRand << 13;
  xorRand ^= xorRand >> 17;
  xorRand ^= xorRand << 5;
}

float XorRandShuffleFloat(XorRand & xorRand) {
  XorRandShuffle(xorRand);

  union {
    XorRand u_x;
    float u_f;
  };

  // Manipulate the exponent and fraction of the floating point number
  // in a way, that we get a number from 1 (inlcusive) and 2 (exclusive).
  u_x = (xorRand & 0x7FFFFF) ^ 0x3F800000;

  return u_f - 1.0f;
}