Longest common substring bug

Question

I have made this so far

function lcs(xstr, ystr)
        if xstr:len() == 0 or ystr:len() == 0 then
                return ""
        end
        x = xstr:sub(1,1)
        y = ystr:sub(1,1)
        xs = xstr:sub(2)
        ys = ystr:sub(2)
        if x == y then
                return x .. lcs(xs, ys)
        else
                l1 = lcs(xstr, ys)
                l2 = lcs(xs, ystr)
                if l1:len() > l2:len() then
                        return l1
                else
                        return l2
                end
        end
end

print(lcs("abcd", "bcd"))

unfortunately it prints just "d" and not "bcd" as expected. For me it looks like the line "l2 = lcs(xs, ystr)" hasn't be executed because if I add debug print at the beginning it prints that function hasn't been called wit arguments "bcd" and "bcd", but I'm sure that values are alright after beginning of else statement. I would appreciate any help.

Answer 1

Your xs variable is global

l1 = lcs(xstr, ys)
l2 = lcs(xs, ystr)

First line corrupts xs value used by second line.
Make all your temporary variables (x, y, xs, ys, l1, l2) local.