Fit Swift string in database VARCHAR(255) - swift

Question

I'm trying to get a valid substring of at most 255 UTF8 code units from a Swift string (the idea is to be able to store it an a database VARCHAR(255) field).
The standard way of getting a substring is this :
let string: String = "Hello world!"
let startIndex = string.startIndex
let endIndex = string.startIndex.advancedBy(255, limit: string.endIndex)
let databaseSubstring1 = string[startIndex..<endIndex]
But obviously that would give me a string of 255 characters that may require more than 255 bytes in UTF8 representation.
For UTF8 I can write this :
let utf8StartIndex = string.utf8.startIndex
let utf8EndIndex = utf8StartIndex.advancedBy(255, limit: string.utf8.endIndex)
let databaseSubstringUTF8View = name.utf8[utf8StartIndex..<utf8EndIndex]
let databaseSubstring2 = String(databaseSubstringUTF8View)
But I run the risk of having half a character at the end, which means my UTF8View would not be a valid UTF8 sequence.
And as expected databaseSubstring2 is an optional string because the initializer can fail (it is defined as public init?(_ utf8: String.UTF8View)).
So I need some way of stripping invalid UTF8 code points at the end, or – if possible – a builtin way of doing what I'm trying to do here.
EDIT
Turns out that databases understand characters, so I should not try to count UTF8 code units, but rather how many characters the database will count in my string (which will probably depend on the database).
According to #OOPer, MySQL counts characters as UTF-16 code units. I have come up with the following implementation :
private func databaseStringForString(string: String, maxLength: Int = 255) -> String
{
// Start by clipping to 255 characters
let startIndex = string.startIndex
let endIndex = startIndex.advancedBy(maxLength, limit: string.endIndex)
var string = string[startIndex..<endIndex]
// Remove characters from the end one by one until we have less than
// the maximum number of UTF-16 code units
while (string.utf16.count > maxLength) {
let startIndex = string.startIndex
let endIndex = string.endIndex.advancedBy(-1, limit: startIndex)
string = string[startIndex..<endIndex]
}
return string
}
The idea is to count UTF-16 code units, but remove characters from the end (that is what Swift think what a character is).
EDIT 2
Still according to #OOPer, Posgresql counts characters as unicode scalars, so this should probably work :
private func databaseStringForString(string: String, maxLength: Int = 255) -> String
{
// Start by clipping to 255 characters
let startIndex = string.startIndex
let endIndex = startIndex.advancedBy(maxLength, limit: string.endIndex)
var string = string[startIndex..<endIndex]
// Remove characters from the end one by one until we have less than
// the maximum number of Unicode Scalars
while (string.unicodeScalars.count > maxLength) {
let startIndex = string.startIndex
let endIndex = string.endIndex.advancedBy(-1, limit: startIndex)
string = string[startIndex..<endIndex]
}
return string
}

Answer 1

As I write in my comment, you may need your databaseStringForString(_:maxLength:) to truncate your string to match the length limit of your DBMS. PostgreSQL with utf8, MySQL with utf8mb4.
And I would write the same functionality as your EDIT 2:
func databaseStringForString(string: String, maxUnicodeScalarLength: Int = 255) -> String {
let start = string.startIndex
for index in start..<string.endIndex {
if string[start..<index.successor()].unicodeScalars.count > maxUnicodeScalarLength {
return string[start..<index]
}
}
return string
}
This may be less efficient, but a little bit shorter.
let s = "abc\u{1D122}\u{1F1EF}\u{1F1F5}" //->"abc𝄢🇯🇵"
let dbus = databaseStringForString(s, maxUnicodeScalarLength: 5) //->"abc𝄢"(=="abc\u{1D122}")
So, someone who works with MySQL with utf8(=utf8mb3) needs something like this:
func databaseStringForString(string: String, maxUTF16Length: Int = 255) -> String {
let start = string.startIndex
for index in start..<string.endIndex {
if string[start..<index.successor()].utf16.count > maxUTF16Length {
return string[start..<index]
}
}
return string
}
let dbu16 = databaseStringForString(s, maxUTF16Length: 4) //->"abc"