Access violation I cannot figure out

[shout]

This is code for a mersenne twister thing Ive been doing, and I have a slight problem. It runs correctly twice before resulting in a crash. Using the memory window of VS I can see that it is not overstepping its bounds. I am confused on how it runs twice but then decides that it is going to be problematic.

The problem line is:

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this->m_state[i] = (this->m_state[i + 396]) % 623 ^ (n >> 1);

And the entire thing:

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MersenneTwister::MersenneTwister(PULONG seed, DWORD length)
{
this->m_state = new ULONG[this->m_state_len];
memset(this->m_state, 0, this->m_state_len*sizeof(ULONG));
this->m_i = 0;

if (seed == NULL || length == 0)
{
//Now we seed by the return from GetTickCount
seed = new ULONG[1];
seed[0] = GetTickCount();

}

seed++;
for (ULONG i = 0; i < this->m_state_len && i < length; i++)
{
//It's redundant if the length is 1... but oh well.
if (i > length - 1) seed--;
this->m_state[i] = seed[i];
}

delete [] seed;

ULONG n = 0;
for (ULONG i = 1; i < 623; i++) {
this->m_state[i] = 69069 * m_state[i-1];
}
for (ULONG i = 1; i < 623; i++) {
n = (this->m_state[i] << 31) + (this->m_state[i-1] >> 1);
this->m_state[i] = (this->m_state[i + 397]) % 623 ^ (n >> 1);
//Do this if it's odd (?)
if (ISODD(this->m_state[i]))
this->m_state[i] ^= 0x9908B0DF;
}

n = (this->m_state[623] << 31) + (this->m_state[0] >> 1);
this->m_state[623] = (this->m_state[396]) ^ (n >> 1);
if (this->m_state[623] % 2 > 0)
this->m_state[623] ^= 0x9908B0DF;
}

[shout]

Weird.

When I do this is does not crash until the seventh time. Always the seventh time. It seems to a access violation accessing 'i' when it contains certain numbers after a certain number of iterations.

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for (ULONG i = 1; i < 441; i++) {
n = (this->m_state[i] << 31) + (this->m_state[i-1] >> 1);
this->m_state[i] = (this->m_state[i + 397]) % 623 ^ (n >> 1);
//Do this if it's odd (?)
if (ISODD(this->m_state[i]))
this->m_state[i] ^= 0x9908B0DF;
}

for (ULONG i = 442; i < 623; i++) {
n = (this->m_state[i] << 31) + (this->m_state[i-1] >> 1);

this->m_state[i] = (this->m_state[i + 397]) % 623 ^ (n >> 1);
//Do this if it's odd (?)
if (ISODD(this->m_state[i]))
this->m_state[i] ^= 0x9908B0DF;
}

[shout]

I figured it out.

When I was going at m_state, in this line of code,

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this->m_state[i] = (this->m_state[i + 397]) % 623 ^ (n >> 1);

I was suppost to be limiting the index of m_state, so it should of been

Code Select Expand


this->m_state[i] = (this->m_state[(i + 397) % 623]) ^ (n >> 1);