Problem Statement

Write a function that returns the longest common substring of two strings. Use it within a program that demonstrates sample output from the function, which will consist of the longest common substring between "thisisatest" and "testing123testing". Note that substrings are consecutive characters within a string.   This distinguishes them from subsequences, which is any sequence of characters within a string, even if there are extraneous characters in between them. Hence, the longest common subsequence between "thisisatest" and "testing123testing" is "tsitest", whereas the longest common substring is just "test".

Let's start with the solution:

program longest_common_substring;
    print(lcs("thisisatest", "testing123testing"));

    proc lcs(s1, s2);
        if #s1 < #s2 then [s1, s2] := [s2, s1]; end if;

        loop for l in [#s2, #s2-1..1] do
            loop for s in [1..#s2-l+1] do
                if (substr := s2(s..s+l)) in s1 then
                    return substr;
                end if;
            end loop;
        end loop;

        return "";
    end proc;
end program;