Function verify

examples/sort/bench_byte_swap.cpp:107–116

// Correctness check: swap two distinct buffers, check that contents exchanged.
template <typename Fn>
void verify(const char * name, Fn fn, size_t width) {
    std::vector<unsigned char> a(width), b(width), a0(width), b0(width);
    std::mt19937 rng(0xBADBEEFu);
    for (size_t i = 0; i < width; i++) { a[i] = a0[i] = (unsigned char)(rng() & 0xFF); b[i] = b0[i] = (unsigned char)(rng() & 0xFF); }
    fn(a.data(), b.data(), width);
    if (memcmp(a.data(), b0.data(), width) != 0 || memcmp(b.data(), a0.data(), width) != 0) {
        std::fprintf(stderr, "FAIL: '%s' produced wrong swap at width=%zu\n", name, width);
        std::abort();
    }
}

template <typename Fn>
double time_op(Fn fn, size_t width, size_t pairs, int iters)