function string.levenshteinDistance( s1, s2 )
    if string.utf8len(s1) == 0 then return string.utf8len(s2) end
    if string.utf8len(s2) == 0 then return string.utf8len(s1) end

    if string.utf8sub(s1, -1, -1) == string.utf8sub(s2, -1, -1) then
        return string.levenshteinDistance( string.utf8sub(s1, 1, -2), string.utf8sub(s2, 1, -2) )
    end

    local a = string.levenshteinDistance( string.utf8sub(s1, 1, -2), string.utf8sub(s2, 1, -2) )
    local b = string.levenshteinDistance( string.utf8sub(s1, 1, -1), string.utf8sub(s2, 1, -2) )
    local c = string.levenshteinDistance( string.utf8sub(s1, 1, -2), string.utf8sub(s2, 1, -1) )

    if a > b then return b + 1 end
    if a > c then return c + 1 end
    return a + 1
end