{"abcd", -1, 1, "a", nil},
		{"abcd", 999, 999, "", nil},
		{"", 999, 999, "", nil},
		{"测试abc", 1, 1, "测", nil},
		{"测试abc", 5, 5, "c", nil},
	}

	for i, tc := range evalCases {
		res, err := evalSQLSubstring(tc.s, tc.startIdx, tc.length)
		if res != tc.resExpected || err != tc.errExpected {
			t.Errorf("Eval Case %d failed: %v %v", i, res, err)
		}
	}

	}
	return res, true
}

func dropRune(text string) (res string, ok bool) {
	r := []rune(text)
	if len(r) == 0 {
		return "", false
	}
	return string(r[1:]), true
}

func evalSQLSubstring(s string, startIdx, length int) (res string, err error) {
	rs := []rune(s)

	// According to s3 document, if startIdx < 1, it is set to 1.
	if startIdx < 1 {
		startIdx = 1
	}