func lComSubStr(w1, w2) {
    var (len, end) = (0, 0)
    var mat  = Array.Empty(w1.Length() + 1, () => Array.Empty(w2.Length() + 1, 0))
    var (i, j) = (0, 0)

    for sLett in w1 {
      for tLett in w2 {
        if tLett == sLett {
            let curLen = mat[i][j] + 1
            mat[i + 1][j + 1] = curLen
            if curLen > len {
                len = curLen
                end = i
            }
        }
        j += 1
      }
      j = 0
      i += 1
    }
    String(values: w1).Substring((end + 1) - len, len)
}

func comSubStr(w1, w2) {
  return String(lComSubStr(w1.Iterate().ToArray(), w2.Iterate().ToArray()))
}

comSubStr("thisisatest", "testing123testing") // "test"