// Twister.cpp

#include "stdafx.h"
#include "Twister.h"

// Mersenne Twister MT19937 with some subtle changes.

const int Twister::StateCount=624;
DWORD     Twister::State[StateCount]; // Initially seeded using GetTickCount();
int       Twister::Index=Seed(); // Index to the State array.

int Twister::Seed(DWORD Seed/*=GetTickCount()*/) {
  DWORD* it=State;
  *it=Seed;
  for(int i=StateCount-1; i; --i) {
    DWORD Previous=*it++;
    *it=(1812433253*(Previous^(Previous>>30))+i);
  }
  return StateCount; // Only for the initialisation of Index, to get FillStates() get called during the next Get().
}

DWORD Twister::Get() { // The fundamental method (quickest return)
  if(Index>=StateCount) FillStates();
  DWORD d=State[Index++];
  d^=(d>>11); // Mersenne Twister MT19937 Coefficients:
  d^=(d<< 7) & 0x9D2C5680;
  d^=(d<<15) & 0xEFC60000;
  d^=(d>>18);
  return d;
}

void Twister::FillStates() {
  const int M=397;
  for(int i=StateCount-M; i--;) SetState(i, i+1, i+M);
  for(int i=StateCount-1; i--;) SetState(i, i+1, i+(M-StateCount));
  SetState(StateCount-1, 0, M-1);
  Index=0;
}

void Twister::SetState(int i, int j, int k) {
  DWORD y=(State[i] & 0x80000000)
        | (State[j] & 0x7FFFFFFF);
  State[i]=State[k]^(y>>1)^((y&1)*0x9908B0DF);
}