https://stackoverflow.com/a/79252310/1271826

StringProtocol+split.swift

extension StringProtocol {
    func split(into length: Int) -> [SubSequence] {
        let count = self.count

        return stride(from: 0, to: count, by: length).map { i in
            let start = index(startIndex, offsetBy: i)
            let end = index(startIndex, offsetBy: Swift.min(i + length, count))
            return self[start ..< end]
        }
    }
}

While I tried to make this generic, using StringProtocol, to support Substring, we could make this even a little more generic, to split any Collection type into SubSequences of a certain length. Admittedly, this is not immediately relevant to the question of “how do I split strings into substrings with a maximum length”, but just an academic observation:

extension Collection {
    func split(into length: Int) -> [SubSequence] {
        let count = self.count

        return stride(from: 0, to: count, by: length).map { i in
            let start = index(startIndex, offsetBy: i)
            let end = index(start, offsetBy: Swift.min(length, count - i))
            return self[start ..< end]
        }
    }
}

Or, technically, this might be more efficient:

extension Collection {
    func split(into length: Int) -> [SubSequence] {
        let count = self.count
        var subsequences: [SubSequence] = []
        
        var (subsequenceCount, remainder) = count.quotientAndRemainder(dividingBy: length)
        if remainder != 0 { subsequenceCount += 1}
        
        subsequences.reserveCapacity(subsequenceCount)
        
        var subsequenceStart = startIndex
        var startOffset = 0
        
        while startOffset < count {
            let subsequenceLength = Swift.min(startOffset + length, count) - startOffset
            let subsequenceEnd = index(subsequenceStart, offsetBy: subsequenceLength)
            subsequences.append(self[subsequenceStart ..< subsequenceEnd])
            startOffset += subsequenceLength
            subsequenceStart = subsequenceEnd
        }
        
        return subsequences
    }
}