Question

I want to compare two sha1 hashes for equality. What could be the most efficient way to do this? Currently, I am trying using memcmp. Thanks.

Answer 1

Well, since you already know at compile-time how large the blocks are, you could do this:

#include <cstdint>

bool is_same_sha1(const char* p, const char* q)
{
    const std::uint32_t* a = (const std::uint32_t*)p;
    const std::uint32_t* b = (const std::uint32_t*)q;
    return a[0] == b[0] && a[1] == b[1] && a[2] == b[2]
        && a[3] == b[3] && a[4] == b[4];
}

But don’t take my advice blindly, you should measure any custom solution against the memcmp solution and only use it if it gives you a significant performance advantage. I wouldn’t even be surprised if memcmp was still faster because it did something extremely clever and dirty.