builtin: Add empty string verification suggested on PR #20540

[viniciusfdasilva]

Fulfilling the request of @spytheman on the merged PR #20540

// Check if a string is an octal value. Returns 'true' if it is, or 'false' if it is not
@[direct_array_access]
pub fn (str string) is_oct() bool {
	mut i := 0

	if str.len == 0 {
		return false
	}

	if str[i] == `0` {
		i++
	} else if str[i] == `-` || str[i] == `+` {
		i++

		if str[i] == `0` {
			i++
		} else {
			return false
		}
	} else {
		return false
	}

	if str[i] == `o` {
		i++
	} else {
		return false
	}

	if i == str.len {
		return false
	}

	for i < str.len {
		if str[i] < `0` || str[i] > `7` {
			return false
		}
		i++
	}

	return true
}

// Check if a string is an binary value. Returns 'true' if it is, or 'false' if it is not
@[direct_array_access]
pub fn (str string) is_bin() bool {
	mut i := 0

	if str.len == 0 {
		return false
	}

	if str[i] == `0` {
		i++
	} else if str[i] == `-` || str[i] == `+` {
		i++

		if str[i] == `0` {
			i++
		} else {
			return false
		}
	} else {
		return false
	}

	if str[i] == `b` {
		i++
	} else {
		return false
	}

	if i == str.len {
		return false
	}

	for i < str.len {
		if str[i] < `0` || str[i] > `1` {
			return false
		}
		i++
	}

	return true
}

// Check if a string is an hexadecimal value. Returns 'true' if it is, or 'false' if it is not
@[direct_array_access]
pub fn (str string) is_hex() bool {
	mut i := 0

	if str.len == 0 {
		return false
	}

	if str[i] == `0` {
		i++
	} else if str[i] == `-` || str[i] == `+` {
		i++

		if str[i] == `0` {
			i++
		} else {
			return false
		}
	} else {
		return false
	}

	if str[i] == `x` {
		i++
	} else {
		return false
	}

	if i == str.len {
		return false
	}

	for i < str.len {
		if (str[i] < `0` || str[i] > `9`) && ((str[i] < `a` || str[i] > `f`)
			&& (str[i] < `A` || str[i] > `F`)) {
			return false
		}
		i++
	}

	return true
}

// Check if a string is an integer value. Returns 'true' if it is, or 'false' if it is not
@[direct_array_access]
pub fn (str string) is_int() bool {
	mut i := 0

	if str.len == 0 {
		return false
	}

	if (str[i] != `-` && str[i] != `+`) && (!str[i].is_digit()) {
		return false
	} else {
		i++
	}

	if i == str.len && (!str[i - 1].is_digit()) {
		return false
	}

	for i < str.len {
		if str[i] < `0` || str[i] > `9` {
			return false
		}
		i++
	}

	return true
}

and add

assert ''.is_int() == false
assert ''.is_hex() == false
assert ''.is_bin() == false
assert ''.is_oct() == false

on string_test.v file

[JalonSolov]

v fmt -w . :-)

[viniciusfdasilva]

@JalonSolov Sorry! kkkkk Done! :)

[spytheman]

Good work.