import Foundation

/// Decodes `ENCODING=QUOTED-PRINTABLE` values, as found in vCard 2.1 exports.
///
/// Older address books encode any non-ASCII character this way, so a contact
/// named `José` arrives as `Jos=C3=A9` — which a click-wheel iPod displays
/// literally rather than decoding.
public enum QuotedPrintable {
    /// Decodes `=XX` escapes and returns the bytes as text.
    ///
    /// `charset` is the value of the vCard `CHARSET` parameter when one was
    /// present. It is only a hint: whatever it claims, valid UTF-8 is decoded
    /// as UTF-8, since that is overwhelmingly what modern exports contain even
    /// when they label themselves otherwise.
    public static func decode(_ value: String, charset: String? = nil) -> String {
        var bytes: [UInt8] = []
        let source = Array(value.utf8)
        var index = 0

        while index < source.count {
            if source[index] == UInt8(ascii: "="), index + 2 < source.count,
               let high = hexDigit(source[index + 1]),
               let low = hexDigit(source[index + 2]) {
                bytes.append(high << 4 | low)
                index += 3
            } else if source[index] == UInt8(ascii: "="), index + 1 == source.count - 1 {
                // A trailing '=' is a soft line break that was never joined.
                index += 2
            } else {
                bytes.append(source[index])
                index += 1
            }
        }

        return decode(bytes: bytes, charset: charset)
    }

    static func decode(bytes: [UInt8], charset: String?) -> String {
        if let text = String(bytes: bytes, encoding: .utf8) {
            return text
        }
        if let charset, let encoding = encoding(named: charset),
           let text = String(bytes: bytes, encoding: encoding) {
            return text
        }
        if let text = String(bytes: bytes, encoding: .windowsCP1252) {
            return text
        }
        return String(decoding: bytes, as: UTF8.self)
    }

    private static func encoding(named charset: String) -> String.Encoding? {
        switch charset.uppercased() {
        case "UTF-8", "UTF8": return .utf8
        case "ISO-8859-1", "LATIN1", "ISO8859-1": return .isoLatin1
        case "WINDOWS-1252", "CP1252": return .windowsCP1252
        default: return nil
        }
    }

    private static func hexDigit(_ byte: UInt8) -> UInt8? {
        switch byte {
        case UInt8(ascii: "0")...UInt8(ascii: "9"): return byte - UInt8(ascii: "0")
        case UInt8(ascii: "A")...UInt8(ascii: "F"): return byte - UInt8(ascii: "A") + 10
        case UInt8(ascii: "a")...UInt8(ascii: "f"): return byte - UInt8(ascii: "a") + 10
        default: return nil
        }
    }
}