Re: what kind of compressor is best for...

jules Gilbert wrote:

Let me pose this problem and see if you can help me,

First, let me described the data. You can build this small test env.

char buf[MEG];

void
build_buffer()
{
int r1 = random() & 255;
int r2 = random() & 255;
int rr = r1 & r2;

buf[ptr++] = rr;
}

Given that random() delivers samples from a uniform i.i.d. random source, then bits in r1 & r2 (binary-and) have indeed probabilities 1/4 and 3/4. As symbol probabilities are not exact powers of two, Huffman coding is not directly applicable. Instead, you need arithmetic coding here for optimal results. Non-adaptive coding is sufficient here as the source is stationary. Don't do bzip2, you're just adding overhead for a source as simple as this.

As a side remark, random() is most likely a pseudo-random generator based on linear congruence. In this case, using masks to extract the lower bits generates pretty poor generators with a period of at most 256 (in your case), and thus not a very random source. Rather, try to use the upper bits instead.

So long,
Thomas
.

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