extension PureXML.Parsing.StreamingDecoder {
    /// Streams one character from a multi-byte CJK encoding.
    mutating func nextCJK(_ encoding: PureXML.Parsing.InputEncoding) -> Character? {
        switch encoding {
        case .shiftJIS: nextShiftJIS()
        case .eucJP: nextEUCJP()
        case .eucKR: nextEUCKR()
        case .gbk: nextGBK()
        case .gb18030: nextGB18030()
        case .big5: nextBig5()
        case .iso2022jp: nextISO2022JP()
        case .eucTW: nextEUCTW()
        default: nextUTF8()
        }
    }

    /// Streams one GBK character: ASCII, `0x80` (euro), or a lead+trail.
    private mutating func nextGBK() -> Character? {
        guard let lead = nextByte() else {
            finished = true
            return nil
        }
        if lead <= 0x7F {
            return Character(Unicode.Scalar(lead))
        }
        if lead == 0x80 {
            return "\u{20AC}"
        }
        guard (0x81 ... 0xFE).contains(lead) else { return Self.replacement }
        guard let trail = nextByte() else { finished = true
            return Self.replacement
        }
        return PureXML.Parsing.ByteDecoder.GBK.twoByteScalar(lead: lead, trail: trail).map(Character.init) ?? Self.replacement
    }

    /// Streams one GB18030 character: ASCII, `0x80` (euro), a four-byte
    /// sequence when the lead is followed by a digit, or a two-byte GBK pair.
    private mutating func nextGB18030() -> Character? {
        guard let lead = nextByte() else {
            finished = true
            return nil
        }
        if lead <= 0x7F {
            return Character(Unicode.Scalar(lead))
        }
        if lead == 0x80 {
            return "\u{20AC}"
        }
        guard (0x81 ... 0xFE).contains(lead) else { return Self.replacement }
        guard let second = nextByte() else { finished = true
            return Self.replacement
        }
        if (0x30 ... 0x39).contains(second) {
            guard let third = nextByte(), let fourth = nextByte() else { finished = true
                return Self.replacement
            }
            guard (0x81 ... 0xFE).contains(third), (0x30 ... 0x39).contains(fourth) else { return Self.replacement }
            let pointer = PureXML.Parsing.ByteDecoder.GB18030.fourBytePointer(lead, second, third, fourth)
            return PureXML.Parsing.ByteDecoder.GB18030.rangeScalar(pointer).map(Character.init) ?? Self.replacement
        }
        return PureXML.Parsing.ByteDecoder.GBK.twoByteScalar(lead: lead, trail: second).map(Character.init) ?? Self.replacement
    }

    /// Streams one Big5 character: ASCII, or a lead+trail through the index
    /// (a two-scalar combining sequence stays one grapheme, one `Character`).
    private mutating func nextBig5() -> Character? {
        guard let lead = nextByte() else {
            finished = true
            return nil
        }
        if lead <= 0x7F {
            return Character(Unicode.Scalar(lead))
        }
        guard (0x81 ... 0xFE).contains(lead) else { return Self.replacement }
        guard let trail = nextByte() else { finished = true
            return Self.replacement
        }
        guard let sequence = PureXML.Parsing.ByteDecoder.Big5.mapping(lead: lead, trail: trail) else {
            return Self.replacement
        }
        return Character(String(String.UnicodeScalarView(sequence)))
    }

    /// Streams one EUC-KR character: ASCII, or a lead+trail through CP949.
    private mutating func nextEUCKR() -> Character? {
        guard let lead = nextByte() else {
            finished = true
            return nil
        }
        if lead <= 0x7F {
            return Character(Unicode.Scalar(lead))
        }
        guard (0x81 ... 0xFE).contains(lead) else { return Self.replacement }
        guard let trail = nextByte() else { finished = true
            return Self.replacement
        }
        return PureXML.Parsing.ByteDecoder.EUCKR.twoByteScalar(lead: lead, trail: trail).map(Character.init) ?? Self.replacement
    }

    /// Streams one EUC-JP character: ASCII, `0x8E`+byte (half-width katakana),
    /// `0x8F`+two bytes (JIS X 0212), or a lead+byte through JIS X 0208.
    private mutating func nextEUCJP() -> Character? {
        guard let lead = nextByte() else {
            finished = true
            return nil
        }
        if lead <= 0x7F {
            return Character(Unicode.Scalar(lead))
        }
        if lead == 0x8E {
            guard let byte = nextByte() else { finished = true
                return Self.replacement
            }
            guard (0xA1 ... 0xDF).contains(byte) else { return Self.replacement }
            return Character(Unicode.Scalar(0xFF61 + UInt32(byte) - 0xA1) ?? "\u{FFFD}")
        }
        if lead == 0x8F {
            guard let mid = nextByte(), let trail = nextByte() else { finished = true
                return Self.replacement
            }
            return PureXML.Parsing.ByteDecoder.EUCJP.twoByteScalar(lead: mid, trail: trail, useJis0212: true).map(Character.init) ?? Self.replacement
        }
        guard let trail = nextByte() else { finished = true
            return Self.replacement
        }
        return PureXML.Parsing.ByteDecoder.EUCJP.twoByteScalar(lead: lead, trail: trail, useJis0212: false).map(Character.init) ?? Self.replacement
    }

    /// Streams one Shift-JIS character: a single byte (ASCII or half-width
    /// katakana) or a lead+trail two-byte sequence resolved through JIS X 0208.
    private mutating func nextShiftJIS() -> Character? {
        guard let lead = nextByte() else {
            finished = true
            return nil
        }
        if let single = PureXML.Parsing.ByteDecoder.ShiftJIS.singleByteScalar(lead) {
            return Character(single)
        }
        guard PureXML.Parsing.ByteDecoder.ShiftJIS.isLead(lead) else { return Self.replacement }
        guard let trail = nextByte() else {
            finished = true
            return Self.replacement
        }
        guard let scalar = PureXML.Parsing.ByteDecoder.ShiftJIS.twoByteScalar(lead: lead, trail: trail) else { return Self.replacement }
        return Character(scalar)
    }
}