phc-discussions - Re: [PHC] Combining sliding window and bit-reversal

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Message-ID: <CAOLP8p6cWONYXOiz3HcWKjbgPjrDY677n7gkwpEDpSCY4SRvbA@mail.gmail.com>
Date: Mon, 27 Jan 2014 23:50:58 -0500
From: Bill Cox <waywardgeek@...il.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Combining sliding window and bit-reversal

On Mon, Jan 27, 2014 at 7:17 PM, Gary W. Hvizdak
<gary.hvizdak@....rr.com> wrote:
> This looks like it's reversing at the byte level.  I had assumed the
> objective was to reverse the entire 64 bits?
>
> Gary

I should probably just paste more real code into emails when the
audience is this technically savvy.  Here's Catena's 64-bit reversal:

uint64_t reverse(uint64_t x, const uint8_t n)
{
  x = bswap_64(x);
  x = ((x & UINT64_C(0x0f0f0f0f0f0f0f0f)) << 4) |
    ((x & UINT64_C(0xf0f0f0f0f0f0f0f0)) >> 4);
  x = ((x & UINT64_C(0x3333333333333333)) << 2) |
    ((x & UINT64_C(0xcccccccccccccccc)) >> 2);
  x = ((x & UINT64_C(0x5555555555555555)) << 1) |
    ((x & UINT64_C(0xaaaaaaaaaaaaaaaa)) >> 1);
  return x >> (64 - n);
}

I've seen it before.  I had to implement something similar in an ASIC
technology mapper once.

Bill