cryptoanarchy.wiki

Arise, you have nothing to lose but your barbed wire fences!

1994-07-03 - Re: Dr. Dobbs Dev. Update 1/5 July 94 & Schneier

Header Data

From: Eli Brandt <ebrandt@jarthur.cs.hmc.edu>
To: cypherpunks list <cypherpunks@toad.com>
Message Hash: 26a6d61372ecc50a234085581590e2ad65b3a4613691574ef06268fdf021b9df
Message ID: <9407031823.AA25826@toad.com>
Reply To: <4i5by0G00WBMA0jZF6@andrew.cmu.edu>
UTC Datetime: 1994-07-03 18:24:03 UTC
Raw Date: Sun, 3 Jul 94 11:24:03 PDT

Raw message

From: Eli Brandt <ebrandt@jarthur.cs.hmc.edu>
Date: Sun, 3 Jul 94 11:24:03 PDT
To: cypherpunks list <cypherpunks@toad.com>
Subject: Re: Dr. Dobbs Dev. Update 1/5 July 94 & Schneier
In-Reply-To: <4i5by0G00WBMA0jZF6@andrew.cmu.edu>
Message-ID: <9407031823.AA25826@toad.com>
MIME-Version: 1.0
Content-Type: text/plain


> Note that some compilers might not be smart enough to use logical shift
> ops and instead use expensive division ops.  Just to be safe...
> 
> int byte_ones(int a)
> {
>          a = (a & 0x55) + ((a & 0xAA) << 1);          // 0x55 == 01010101b
>          a = (a & 0x33) + ((a & 0xCC) << 2);          // 0x33 == 00110011b
>          a = (a & 0x0F) + ((a & 0xF0) << 4);          // 0x0F == 00001111b
>          return a;
> }

One advantage of writing it as division is that it's hard to accidentally
reverse, as above.  :-)  I was just trying to cut down on parens...

   Eli   ebrandt@hmc.edu

Thread