evansneath · January 27, 2013 22:29 · Jan 27, 2013
diff --git a/crc.py b/crc.py
@@ -0,0 +1,77 @@
+#!/usr/bin/env python3
+
+def crc(msg, div, code='000'):
+    """Cyclic Redundancy Check
+
+    Generates an error detecting code based on an inputted message
+    and divisor in the form of a polynomial representation.
+
+    Arguments:
+        msg: The input message of which to generate the output code.
+        div: The divisor in polynomial form. For example, if the polynomial
+            of x^3 + x + 1 is given, this should be represented as '1011' in
+            the div argument.
+        code: This is an option argument where a previously generated code may
+            be passed in. This can be used to check validity. If the inputted
+            code produces an outputted code of all zeros, then the message has
+            no errors.
+
+    Returns:
+        An error-detecting code generated by the message and the given divisor.
+    """
+    # Append the code to the message. If no code is given, default to '000'
+    msg = msg + code
+
+    # Convert msg and div into list form for easier handling
+    msg = list(msg)
+    div = list(div)
+
+    # Loop over every message bit (minus the appended code)
+    for i in range(len(msg)-len(code)):
+        # If that messsage bit is 1, perform modulo 2 multiplication
+        if msg[i] == '1':
+            for j in range(len(div)):
+                # Perform modulo 2 multiplication on each index of the divisor
+                msg[i+j] = str((int(msg[i+j])+int(div[j]))%2)
+
+    # Output the last error-checking code portion of the message generated
+    return ''.join(msg[-len(code):])
+
+
+# TEST 1 ####################################################################
+print('Test 1 ---------------------------')
+# Use a divisor that simulates: x^3 + x + 1
+div = '1011'
+msg = '11010011101100'
+
+print('Input message:', msg)
+print('Divisor:', div)
+
+# Enter the message and divisor to calculate the error-checking code
+code = crc(msg, div)
+
+print('Output code:', code)
+
+# Perform a test to check that the code, when run back through, returns an
+# output code of '000' proving that the function worked correctly
+print('Success:', crc(msg, div, code) == '000')
+
+
+# TEST 2 ####################################################################
+print('Test 2 ---------------------------')
+# Use a divisor that simulates: x^2 + 1
+div = '0101'
+msg = '00101111011101'
+
+print('Input message:', msg)
+print('Divisor:', div)
+
+# Enter the message and divisor to calculate the error-checking code
+code = crc(msg, div)
+
+print('Output code:', code)
+
+# Perform a test to check that the code, when run back through, returns an
+# output code of '000' proving that the function worked correctly
+print('Success:', crc(msg, div, code) == '000')
+
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