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From: shaw@paralogics.UUCP (Guy Shaw)
Newsgroups: comp.lang.perl
Subject: Re: Even Parity Table Generation
Summary: Use reflected binary Gray codes
Keywords: reflected binary codes, gray codes, parity
Message-ID: <426@paralogics.UUCP>
Date: 12 Mar 91 11:56:21 GMT
References: <121817@uunet.UU.NET>
Organization: Paralogics;  Santa Monica, CA
Lines: 42


One non-recursive way to generate a parity table is to count through the
character set in (binary reflected) Gray code order.  Since the successor
of any Gray code number changes only one binary digit, the parity will
flip back and forth between even and odd.

#! /usr/local/bin/perl

# For purposes of demonstration,
# print out the first 32 Gray code numbers

print "bin gray\n";
print "--- ---\n";
for $i (0..31) {
    printf "%3x %3x\n", $i,$i^($i>>1);
    }
print "\n";

&MakeEvenParityTable;

# Generate full 256-entry even parity table,
# but just print the first 32

for $i (0..31) {
    printf "%3x %3x\n", $i,$even[$i];
    }
exit 0;

sub MakeEvenParityTable {
    local($bin,$gray,$parity);
    $even[0] = 0;
    $parity = 0;
    for $bin (1..255) {
        $gray = $bin ^ ($bin>>1);
        $parity = $parity ^ 128;
        $even[$gray] = $gray | $parity;
        }
    }
-- 
Guy Shaw
Paralogics
paralogics!shaw@uunet.uu.net  or  uunet!paralogics!shaw