#!/usr/bin/perl -w
use strict;
#=================================
#
# ROHC CRC demo
#
# Carsten Bormann cabo@tzi.org 2001-08-02
#
# This little demo shows the four types of CRC in use in RFC 3095, 
# the robust header compression standard.  Note that the 32-bit CRC
# is identical to that used in PPP and Ethernet.  The 8-bit, 7-bit
# and 3-bit CRCs have been defined in an analogous way.
#
#=================================

#---------------------------------
#
# utility
#

sub dump_bytes($) {
    my $x = shift;
    my $i;
    for ($i = 0; $i < length($x); ) {
	printf("%02x ", ord(substr($x, $i, 1)));
	printf("\n") if (++$i % 16 == 0);
    }
    printf("\n") if ($i % 16 != 0);
}

#---------------------------------
#
# The CRC calculation algorithm.
# (NOTE: This is a simple and slow version.
#  A fast table-driven version can be derived easily.
#  Compare the PPP FCS code below...)
#

sub do_crc($$$) {
    my $nbits = shift;
    my $poly = shift;
    my $string = shift;

    my $crc = ($nbits == 32 ? 0xffffffff : (1 << $nbits) - 1);
    for (my $i = 0; $i < length($string); ++$i) {
	my $byte = ord(substr($string, $i, 1));
	for( my $b = 0; $b < 8; $b++ ) {
	    if (($crc & 1) ^ ($byte & 1)) {
		$crc >>= 1;
		$crc ^= $poly;
	    } else {
		$crc >>= 1;
	    }
	    $byte >>= 1;
	}
    }
    printf "%2d bits, ", $nbits;
    printf "CRC: %02x\n", $crc;
}


#---------------------------------
#
# Test harness
#

$/ = undef;
$_ = <>;		# read until EOF
my $string = "";	# extract all that looks hex:
s/([0-9a-fA-F][0-9a-fA-F])/$string .= chr(hex($1)), ""/eg;
dump_bytes($string);

#---------------------------------
#
# 32-bit segmentation CRC
# Note that the text implies this is complemented like for PPP
# (this differs from 8, 7, and 3-bit CRC)
# 
# 	C(x) = x^0 + x^1 + x^2 + x^4 + x^5 + x^7 + x^8 + x^10 + x^11 +
#              x^12 + x^16 + x^22 + x^23 + x^26 + x^32
#

do_crc(32, 0xedb88320, $string);

#---------------------------------
#
# 8-bit IR/IR-DYN CRC
# 
#      C(x) = x^0 + x^1 + x^2 + x^8
#

do_crc(8, 0xe0, $string);

#---------------------------------
#
# 7-bit FO/SO CRC
# 
#      C(x) = x^0 + x^1 + x^2 + x^3 + x^6 + x^7
#

do_crc(7, 0x79, $string);


#---------------------------------
#
# 3-bit FO/SO CRC
# 
#      C(x) = x^0 + x^1 + x^3
#

do_crc(3, 0x6, $string);


#---------------------------------
#
# This is it for ROHC, now just cross-check with RFC1662:
#

my @fcstab_32 = (
		 0x00000000, 0x77073096, 0xee0e612c, 0x990951ba,
		 0x076dc419, 0x706af48f, 0xe963a535, 0x9e6495a3,
		 0x0edb8832, 0x79dcb8a4, 0xe0d5e91e, 0x97d2d988,
		 0x09b64c2b, 0x7eb17cbd, 0xe7b82d07, 0x90bf1d91,
		 0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de,
		 0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7,
		 0x136c9856, 0x646ba8c0, 0xfd62f97a, 0x8a65c9ec,
		 0x14015c4f, 0x63066cd9, 0xfa0f3d63, 0x8d080df5,
		 0x3b6e20c8, 0x4c69105e, 0xd56041e4, 0xa2677172,
		 0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b,
		 0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940,
		 0x32d86ce3, 0x45df5c75, 0xdcd60dcf, 0xabd13d59,
		 0x26d930ac, 0x51de003a, 0xc8d75180, 0xbfd06116,
		 0x21b4f4b5, 0x56b3c423, 0xcfba9599, 0xb8bda50f,
		 0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924,
		 0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d,
		 0x76dc4190, 0x01db7106, 0x98d220bc, 0xefd5102a,
		 0x71b18589, 0x06b6b51f, 0x9fbfe4a5, 0xe8b8d433,
		 0x7807c9a2, 0x0f00f934, 0x9609a88e, 0xe10e9818,
		 0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01,
		 0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e,
		 0x6c0695ed, 0x1b01a57b, 0x8208f4c1, 0xf50fc457,
		 0x65b0d9c6, 0x12b7e950, 0x8bbeb8ea, 0xfcb9887c,
		 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3, 0xfbd44c65,
		 0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2,
		 0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb,
		 0x4369e96a, 0x346ed9fc, 0xad678846, 0xda60b8d0,
		 0x44042d73, 0x33031de5, 0xaa0a4c5f, 0xdd0d7cc9,
		 0x5005713c, 0x270241aa, 0xbe0b1010, 0xc90c2086,
		 0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
		 0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4,
		 0x59b33d17, 0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad,
		 0xedb88320, 0x9abfb3b6, 0x03b6e20c, 0x74b1d29a,
		 0xead54739, 0x9dd277af, 0x04db2615, 0x73dc1683,
		 0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8,
		 0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1,
		 0xf00f9344, 0x8708a3d2, 0x1e01f268, 0x6906c2fe,
		 0xf762575d, 0x806567cb, 0x196c3671, 0x6e6b06e7,
		 0xfed41b76, 0x89d32be0, 0x10da7a5a, 0x67dd4acc,
		 0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5,
		 0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252,
		 0xd1bb67f1, 0xa6bc5767, 0x3fb506dd, 0x48b2364b,
		 0xd80d2bda, 0xaf0a1b4c, 0x36034af6, 0x41047a60,
		 0xdf60efc3, 0xa867df55, 0x316e8eef, 0x4669be79,
		 0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236,
		 0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f,
		 0xc5ba3bbe, 0xb2bd0b28, 0x2bb45a92, 0x5cb36a04,
		 0xc2d7ffa7, 0xb5d0cf31, 0x2cd99e8b, 0x5bdeae1d,
		 0x9b64c2b0, 0xec63f226, 0x756aa39c, 0x026d930a,
		 0x9c0906a9, 0xeb0e363f, 0x72076785, 0x05005713,
		 0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38,
		 0x92d28e9b, 0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21,
		 0x86d3d2d4, 0xf1d4e242, 0x68ddb3f8, 0x1fda836e,
		 0x81be16cd, 0xf6b9265b, 0x6fb077e1, 0x18b74777,
		 0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c,
		 0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45,
		 0xa00ae278, 0xd70dd2ee, 0x4e048354, 0x3903b3c2,
		 0xa7672661, 0xd06016f7, 0x4969474d, 0x3e6e77db,
		 0xaed16a4a, 0xd9d65adc, 0x40df0b66, 0x37d83bf0,
		 0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
		 0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6,
		 0xbad03605, 0xcdd70693, 0x54de5729, 0x23d967bf,
		 0xb3667a2e, 0xc4614ab8, 0x5d681b02, 0x2a6f2b94,
		 0xb40bbe37, 0xc30c8ea1, 0x5a05df1b, 0x2d02ef8d
		 );

   #define PPPINITFCS32  0xffffffff   /* Initial FCS value */
   #define PPPGOODFCS32  0xdebb20e3   /* Good final FCS value */

my $fcs = 0xffffffff;		# PPPINITFCS32
for (my $i = 0; $i < length($string); ++$i) {
    my $byte = ord(substr($string, $i, 1));
    $fcs = ((($fcs) >> 8) ^ $fcstab_32[(($fcs) ^ $byte) & 0xff]);
}
printf "PPP FCS %x = ~%x\n", ~$fcs, $fcs;