Best way to test CRC logic?

How can I verify two CRC implementations will generate the same checksums?

I'm looking for an exhaustive implementation evaluating me开发者_JAVA百科thodology specific to CRC.

You can separate the problem into edge cases and random samples.

Edge cases. There are two variables to the CRC input, number of bytes, and value of each byte. So create arrays of 0, 1, and MAX_BYTES, with values ranging from 0 to MAX_BYTE_VALUE. The edge case suite will be something you'll most likely want to keep within a JUnit suite.

Random samples. Using the ranges above, run CRC on randomly generated arrays of bytes in a loop. The longer you let the loop run, the more you exhaust the inputs. If you are low on computing power, consider deploying the test to EC2.

Create several unit tests with the same input that will compare the output of both implementations against each other.

One nice property of CRCs is that for a given set of parameters (polynomial, reflection, initial state, etc.) you will get a constant value when you recompute the CRC over the original dataset + the original CRC. These constants are documented for common CRCs but you can just blindly generate them using two different random data sets and check that they are the same:

implementation 1: crc(rand_data_1 + crc(rand_data_1)) -> constant_1
implementation 2: crc(rand_data_2 + crc(rand_data_2)) -> constant_2
assert constant_1 == constant_2

You can use the same method within an implementation to get a warm fuzzy feeling about its correctness. If your implementation works with arbitrary polynomials, you can have the unittest exhaustively check every possible polynomial using this method without needing to know what the constants are.

This technique is powerful but it would also be wise to add an independent test that verifies the result based on known input for the pathological case where your CRC implementations both produce bad results that happen to get by the constant equivalence check.

First, if it is a standard CRC implementation, you should be able to find known values somewhere on the net.

Second, you could generate some number of payloads and run the each CRC on the payloads and check that the CRC values match.

By writing a unit test for each which takes the same input and verify against the expected output.