Swift: DIY Base64 conversion (translated from Java on Wikipedia) - result compared to built-in Base64 encoding

Base64encode.swift

// a translation of the Java implementation provided here: https://en.wikipedia.org/wiki/Base64
let CODES = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/="
func base64Encode(u:[Int]) -> String  {
    var out = ""
    var b = 0
    var i = 0
    while i < u.count  {
        b = (u[i] & 0xFC) >> 2
        out.append(CODES[CODES.startIndex.advancedBy(b)])
        b = (u[i] & 0x03) << 4
        if (i + 1 < u.count)      {
            b |= (u[i + 1] & 0xF0) >> 4
            out.append(CODES[CODES.startIndex.advancedBy(b)])
            b = (u[i + 1] & 0x0F) << 2
            if (i + 2 < u.count)  {
                b |= (u[i + 2] & 0xC0) >> 6
                out.append(CODES[CODES.startIndex.advancedBy(b)])
                b = u[i + 2] & 0x3F
                out.append(CODES[CODES.startIndex.advancedBy(b)])
            } else  {
                out.append(CODES[CODES.startIndex.advancedBy(b)])
                out.appendContentsOf("=")
            }
        } else      {
            out.append(CODES[CODES.startIndex.advancedBy(b)])
            out.appendContentsOf("==")
        }
        i += 3
    }
    
    return out
}

extension String {
    func base64EncodedUTF8() -> String {
        let bytes = [UInt8](self.utf8).map{Int($0)}
        return base64Encode(bytes)
    }

}

let b64Str = "Help me encode using Base64!"
b64Str.base64EncodedUTF8() // SGVscCBtZSBlbmNvZGUgdXNpbmcgQmFzZTY0IQ==

// confirm that system encoding is the same as the homemade version
b64Str.dataUsingEncoding(NSUTF8StringEncoding)?.base64EncodedStringWithOptions([]) == b64Str.base64EncodedUTF8() // true

In the real world we'd probably simply write this extension:

extension String {
    func base64EncodedUTF8() -> String? {
        return self.dataUsingEncoding(NSUTF8StringEncoding)?.base64EncodedStringWithOptions([])
    }
}

and to decode, this:

extension String {
    func base64DecodeUTF8() -> String? {
        if let data = NSData(base64EncodedString: self, options: []) {
            return String(data: data, encoding: NSUTF8StringEncoding)
        }
        else {
            return nil
        }
    }
}