Connascence of Algorithm

Connascence of Algorithm (CoA) occurs when two or more components must agree on using a particular algorithm.

Perhaps the most popular example of this kind of connascence is checksum for file verification; applying the algorithm remotely allows for a mathematical proof that the file has not been changed.

Unfortunately, if we have this kind of connascence in our code, the result is not useful. Let's see an example of a dumb checksum implementation that assumes we want to build it with a simple logic consisting of adding a character at the end of an integer so that the sum of the single numbers could be divided by 10:

AddChecksum("32143")    =>   "321437"
Check("321437")         =>   true
Check("321432")         =>   false

Let's see this snippet implementing it:

public string AddChecksum(string inputData){
    var sum = SumCharsOf(inputData);
    ...