Inconsistency between yellow paper and Ethash wiki

[pgebal]

Hi,
Please take a look at eq. 230 in yellow paper and Ethash wiki implementation of get_cache_size function. If the function was implemented as in yellow paper we would have sz -= HASH_BYTES instead of sz -= 2 * HASH_BYTES. ethereumj and pyethereum implementations follow Ethash wiki. Which version is correct? Should I follow Ethash wiki or yellow paper when implementing Ethash?

[pirapira]

I'll compare both with implementations.

[pirapira]

Wait, the formula in YP doesn't even terminate.

[pirapira]

@pgebal does this look good? #519

[pirapira]

@pgebal I realized something. Whichever definition you choose, they result in the same cache size, because no even number is prime.

[pirapira]

Whenever (sz / HASH_BYTES) is an odd number at least three, the loop

    while not isprime(sz / HASH_BYTES):
        sz -= HASH_BYTES

runs even times. Say 119-> 118 (even, not a prime) -> 117 -> 116 (even, not a prime) -> 115.

So, the loop can be optimized, merging every two steps, into

    while not isprime(sz / HASH_BYTES):
        sz -= 2 * HASH_BYTES

[pgebal]

You are right. Thank you.

[pirapira]

I keep my pull requests open. Different formulations are confusing. Thank you for pointing them out.