How to evaluate the percentage of matches in the code?

Question

Text analysis

There is source code A before refactoring. There is A1 source code after refactoring, addition and rewriting.
Question: how to estimate what percentage of the code from A is included in A1?
Of course, the structure of the files does not match at all.

Answer 1

Compress A and A1 into 7z archives. Write down their size: S and S1. Then compress A and A1 together into one archive. Write down the SS size. Calculate the ratio D = 2 * (1 - ((S + S1) - SS) / (S + S1)).
I don't know how accurate it will show the percentage of repeated code, but it should be fun.