crc16.py

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"""--------------------------------------------------------------------
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The software was written using Python programming language.  Your use 
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 \file   crc16.py

 \brief  This module contains a collection of functions for calculating
         a CRC-16.

 \Platform: Cross Platform
--------------------------------------------------------------------"""

"""
CRC16 defines
"""
CRC_ADJUSTMENT = 0xA001;
CRC_TABLE_SIZE = 256;
INITIAL_CRC = 0;

HIGH_BYTE_MASK  = 0xFF00;
LOW_BYTE_MASK   = 0x00FF;
MOVE_BYTE_SHIFT = 8;
LS_BIT          = 0x0001;
BITS_PER_BYTE   = 8;

crc_table= [0]*CRC_TABLE_SIZE;

"""
This creates the CRC table
"""
def generate_crc_table():

    for x in range(0,CRC_TABLE_SIZE):

        table_value = 0;
        k = x;

        for j in range(0,BITS_PER_BYTE):
            if (((table_value ^ k) & LS_BIT) == LS_BIT):
                table_value = (table_value >> 1) ^ CRC_ADJUSTMENT;
            else:
                table_value >>= 1;
            k >>= 1;

        crc_table[x] = table_value;

"""
This computes an updated CRC 16 given the current value of the CRC 16 and
a new data byte.
"""
def calculate_crc_16(old_crc,new_word):

    temp = old_crc ^ new_word;
    new_crc = (old_crc >> MOVE_BYTE_SHIFT) ^ crc_table[temp & LOW_BYTE_MASK];

    return new_crc;

"""
This computes the CRC for a buffer passed in
"""
def compute_buffer_crc(byte_buffer, bytes_in_buffer):
    crc_index = bytes_in_buffer - 2;
    new_crc = 0;

    """
    We'll loop through each word of the message and update
    the CRC.  Start with the value chosen for CRC initialization.
    """
    for x in range(0,crc_index):
        """
        Now we'll send each byte to the CRC calculation.
        """
        new_crc = calculate_crc_16(new_crc, byte_buffer[x]);


    byte_buffer[crc_index]   = int((new_crc & HIGH_BYTE_MASK) >> MOVE_BYTE_SHIFT);
    byte_buffer[crc_index+1] = int((new_crc & LOW_BYTE_MASK));
    
"""
This computes the CRC for a buffer passed in and checks it against
the one contained in the buffer
"""
def buffer_crc_is_valid(byte_buffer, bytes_in_buffer):
    
    crc_index = bytes_in_buffer - 2;
    new_crc = 0;
    success = False;

    """
    We'll loop through each word of the message and update
    the CRC.  Start with the value chosen for CRC initialization.
    """
    for x in range(0,crc_index):
        new_crc = calculate_crc_16(new_crc, byte_buffer[x]);

    """
    The new CRC is checked against that stored in the buffer.
    """
    received_crc = ((byte_buffer[crc_index] << MOVE_BYTE_SHIFT) & HIGH_BYTE_MASK);
    received_crc |= (byte_buffer[crc_index+1] & LOW_BYTE_MASK);
    
    if (received_crc == new_crc):
        success = True;
    else:
        success = False;

    return (success);