def get_last_revision(revs, sample_rev):
    '''Returns last revision in branch defined by sample_rev
    '''
    def rev_digit_array():
        '''make array of rev rev_digit_array: "1.18.2.6" => ["1","18","2","6"]
        '''
        return sample_rev.split(".")

    def rev_branch_base():
        '''make rev_branch_base for branch: ["1","18","0","6"] => "1.18.6."
        '''
        return ".".join(rev_digit_array()[:-2] + [rev_digit_array()[-1], ""])

    def branch_base_length():
        '''Base length'''
        return len(rev_branch_base())

    def revision_tail(rev):
        '''Revisions tails (everything without rev_branch_base of branch)'''
        return rev[branch_base_length():]

    def is_rev_in_branch(rev):
        '''Check if revision is in the branch'''
        return rev.startswith(rev_branch_base()) and revision_tail(rev).isdigit()

    def tails_in_branch():
        '''List of tails of revisions in current branch'''
        return [int(revision_tail(rev)) for rev in revs if is_rev_in_branch(rev)]

    return "%s%d" % (rev_branch_base(), max(tails_in_branch()))


if __name__ == "__main__":
    revs = ["1.18", "1.19","1.20","1.21","2.50",
     "1.18.2.1", "1.18.2.2","1.18.2.3","1.18.2.6", "1.19.2.6",
     "1.18.2.1.3.1", "1.18.2.1.3.2","1.18.2.1.3.3","1.18.2.1.3.6","1.18.3.1.3.6"]
    assert get_last_revision(revs, "1.18.0.2") == "1.18.2.6"
    assert get_last_revision(revs, "1.18.2.1.0.3") == "1.18.2.1.3.6"
    print "OK"