forked from osmocom/wireshark
69 lines
1.7 KiB
C
69 lines
1.7 KiB
C
/* crc5.c
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* CRC-5 routine
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*
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* 2019 Tomasz Mon <desowin@gmail.com>
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*
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* Wireshark - Network traffic analyzer
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* By Gerald Combs <gerald@wireshark.org>
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* Copyright 1998 Gerald Combs
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*
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* SPDX-License-Identifier: GPL-2.0-or-later
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*/
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#include "config.h"
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#include <glib.h>
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#include <wsutil/crc5.h>
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static guint8 crc5_usb_bits(guint32 v, int vl, guint8 ival)
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{
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/* This function is based on code posted by John Sullivan to Wireshark-dev
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* mailing list on Jul 21, 2019.
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*
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* "One of the properties of LFSRs is that a 1 bit in the input toggles a
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* completely predictable set of register bits *at any point in the
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* future*. This isn't often useful for most CRC caculations on variable
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* sized input, as the cost of working out which those bits are vastly
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* outweighs most other methods."
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*
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* In USB 2.0, the CRC5 is calculated on either 11 or 19 bits inputs,
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* and thus this approach is viable.
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*/
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guint8 rv = ival;
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static const guint8 bvals[19] = {
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0x1e, 0x15, 0x03, 0x06, 0x0c, 0x18, 0x19, 0x1b,
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0x1f, 0x17, 0x07, 0x0e, 0x1c, 0x11, 0x0b, 0x16,
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0x05, 0x0a, 0x14
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};
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for (int i = 0; i < vl; i++) {
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if (v & (1 << i)) {
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rv ^= bvals[19 - vl + i];
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}
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}
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return rv;
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}
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guint8 crc5_usb_11bit_input(guint16 input)
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{
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return crc5_usb_bits(input, 11, 0x02);
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}
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guint8 crc5_usb_19bit_input(guint32 input)
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{
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return crc5_usb_bits(input, 19, 0x1d);
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}
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/*
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* Editor modelines - https://www.wireshark.org/tools/modelines.html
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*
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* Local variables:
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* c-basic-offset: 4
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* tab-width: 8
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* indent-tabs-mode: nil
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* End:
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*
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* vi: set shiftwidth=4 tabstop=8 expandtab:
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* :indentSize=4:tabSize=8:noTabs=true:
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*/
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