/*
    Copyright 2017, Kelvin Ma (“taylorswift”), kelvin13ma@gmail.com

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

//Modified by izackp@gmail.com to include an api where I can read everything in pieces

#if os(Linux)
    import Glibc
#elseif os(OSX)
    import Darwin
#endif

final class EmptyStringError: Error, Sendable {
    init() { }
}

final class XmlError: Error, Sendable {
    
    let message: String
    let line:Int
    let column:Int
    
    init(_ message: String, _ line:Int, _ column:Int) {
        self.message = "\(line):\(column) \(message)"
        self.line = line
        self.column = column
    }
    
    static func failure<T>(_ message: String, _ line:Int, _ column:Int) -> Result<T, XmlError> {
        return .failure(XmlError(message, line, column))
    }
    
    var errorDescription: String? {
        get {
            return message
        }
    }
}

public enum XMLToken
{
    case data([Unicode.Scalar]),
         tag_start(String, [String: String]),
         tag_empty(String, [String: String]),
         tag_end(String),
         instruction(String, [Unicode.Scalar])
}

extension Unicode.Scalar
{
    // NameStartChar  ::=   ":" | [A-Z] | "_" | [a-z] | [#xC0-#xD6] | [#xD8-#xF6] | [#xF8-#x2FF]
    //                    | [#x370-#x37D]   | [#x37F-#x1FFF]  | [#x200C-#x200D] | [#x2070-#x218F]
    //                    | [#x2C00-#x2FEF] | [#x3001-#xD7FF] | [#xF900-#xFDCF] | [#xFDF0-#xFFFD]
    //                    | [#x10000-#xEFFFF]
    var is_xml_name_start:Bool
    {
        return "a" ... "z" ~= self || "A" ... "Z" ~= self // [a-z], [A-Z]
        || self == ":" || self == "_" // ":", "_"

        || "\u{C0}"   ... "\u{D6}"   ~= self || "\u{D8}"   ... "\u{F6}"   ~= self
        || "\u{F8}"   ... "\u{2FF}"  ~= self || "\u{370}"  ... "\u{37D}"  ~= self
        || "\u{37F}"  ... "\u{1FFF}" ~= self || "\u{200C}" ... "\u{200D}" ~= self
        || "\u{2070}" ... "\u{218F}" ~= self || "\u{2C00}" ... "\u{2FEF}" ~= self
        || "\u{3001}" ... "\u{D7FF}" ~= self || "\u{F900}" ... "\u{FDCF}" ~= self
        || "\u{FDF0}" ... "\u{FFFD}" ~= self || "\u{10000}" ... "\u{EFFFF}" ~= self
    }

    // NameChar       ::=   NameStartChar | "-" | "." | [0-9] | #xB7 | [#x0300-#x036F] | [#x203F-#x2040]
    var is_xml_name:Bool
    {
        return "a" ... "z" ~= self || "A" ... "Z" ~= self || "0" ... ":" ~= self
        || self == "_" || self == "-" || self == "." || self == "\u{B7}"

        || "\u{0300}" ... "\u{036F}" ~= self || "\u{203F}" ... "\u{2040}" ~= self

        || "\u{C0}"   ... "\u{D6}"   ~= self || "\u{D8}"   ... "\u{F6}"   ~= self
        || "\u{F8}"   ... "\u{2FF}"  ~= self || "\u{370}"  ... "\u{37D}"  ~= self
        || "\u{37F}"  ... "\u{1FFF}" ~= self || "\u{200C}" ... "\u{200D}" ~= self
        || "\u{2070}" ... "\u{218F}" ~= self || "\u{2C00}" ... "\u{2FEF}" ~= self
        || "\u{3001}" ... "\u{D7FF}" ~= self || "\u{F900}" ... "\u{FDCF}" ~= self
        || "\u{FDF0}" ... "\u{FFFD}" ~= self || "\u{10000}" ... "\u{EFFFF}" ~= self
    }

    // S       ::=       (#x20 | #x9 | #xD | #xA)+
    var is_xml_whitespace:Bool
    {
        return self == " " || self == "\u{9}" || self == "\u{D}" || self == "\u{A}"
    }
}

extension String
{
    init<C>(_ buffer:C) where C:Collection, C.Element == Unicode.Scalar
    {
        self.init(buffer.map(Character.init))
    }
}

enum State
{
    case data,
         begin_markup,
         slash1,
         name(Unicode.Scalar),
         attributes,
         no_attributes,
         label(Unicode.Scalar),
         space1,
         equals,
         space2,
         attribute_value,
         slash2,
         end_markup,

         exclam,
         hyphen1,
         comment,
         hyphen2,
         hyphen3,

         question1,
         pi_space,
         pi_data(Unicode.Scalar),
         question2
}

enum Markup
{
    case none,
         start,
         empty,
         end,
         comment,
         processing
}

struct Position
{
    var line:Int = 0,
        column:Int = 0

    @inline(__always)
    mutating
    func advance(_ u:Unicode.Scalar)
    {
        if u == "\n"
        {
            self.line   += 1
            self.column  = 0
        }
        else
        {
            self.column += 1
        }
    }
}

extension String.UnicodeScalarView.Iterator
{
    mutating
    func read_reference(position:inout Position) -> (after:Unicode.Scalar?, content:[Unicode.Scalar], error:String?)
    {
        enum ReferenceState
        {
        case initial,
             name,
             hashtag,
             x,
             decimal(UInt32),
             hex(UInt32)
        }

        let default_entities:[String: [Unicode.Scalar]] =
        ["amp": ["&"], "lt": ["<"], "gt": [">"], "apos": ["'"], "quot": ["\""]]

        var state:ReferenceState     = .initial,
            content:[Unicode.Scalar] = ["&"]


        @inline(__always)
        func _charref(_ u:Unicode.Scalar, scalar:UInt32) -> (after:Unicode.Scalar?, content:[Unicode.Scalar], error:String?)
        {
            guard scalar > 0
            else
            {
                return (u, content, "cannot reference null character '\\0'")
            }

            guard scalar <= 0xD7FF || 0xE000 ... 0xFFFD ~= scalar || 0x10000 ... 0x10FFFF ~= scalar
            else
            {
                return (u, content, "cannot reference illegal character '\\u{\(scalar)}'")
            }

            position.advance(u)
            return (self.next(), [Unicode.Scalar(scalar)!], nil)
        }

        while let u:Unicode.Scalar = self.next()
        {
            switch state
            {
            case .initial:
                if u == "#"
                {
                    state = .hashtag
                }
                else if u.is_xml_name_start
                {
                    state = .name
                }
                else
                {
                    return (u, content, "unescaped ampersand '&'")
                }

            case .name:
                if u == ";"
                {
                    content = default_entities[String(content.dropFirst())] ?? content
                    position.advance(u)
                    return (self.next(), content, nil)
                }
                else
                {
                    guard u.is_xml_name
                    else
                    {
                        return (u, content, "unexpected '\(u)' in entity reference")
                    }
                }

            case .hashtag:
                if "0" ... "9" ~= u
                {
                    state = .decimal(u.value - Unicode.Scalar("0").value)
                }
                else if u == "x"
                {
                    state = .x
                }
                else
                {
                    return (u, content, "unexpected '\(u)' in character reference")
                }

            case .decimal(let scalar):
                if "0" ... "9" ~= u
                {
                    state = .decimal(u.value - Unicode.Scalar("0").value + 10 * scalar)
                }
                else if u == ";"
                {
                    return _charref(u, scalar: scalar)
                }
                else
                {
                    return (u, content, "unexpected '\(u)' in character reference")
                }

            case .x:
                if "0" ... "9" ~= u
                {
                    state = .hex(u.value - Unicode.Scalar("0").value)
                }
                else if "a" ... "f" ~= u
                {
                    state = .hex(10 + u.value - Unicode.Scalar("a").value)
                }
                else if "A" ... "F" ~= u
                {
                    state = .hex(10 + u.value - Unicode.Scalar("A").value)
                }
                else
                {
                    return (u, content, "unexpected '\(u)' in character reference")
                }

            case .hex(let scalar):
                if "0" ... "9" ~= u
                {
                    state = .hex(u.value - Unicode.Scalar("0").value + scalar << 4)
                }
                else if "a" ... "f" ~= u
                {
                    state = .hex(10 + u.value - Unicode.Scalar("a").value + scalar << 4)
                }
                else if "A" ... "F" ~= u
                {
                    state = .hex(10 + u.value - Unicode.Scalar("A").value + scalar << 4)
                }
                else if u == ";"
                {
                    return _charref(u, scalar: scalar)
                }
                else
                {
                    return (u, content, "unexpected '\(u)' in character reference")
                }
            }

            position.advance(u)
            content.append(u)
        }

        return (nil, content, "unexpected EOF inside reference")
    }
}

public class XMLParser
{
    /*
    private static
    func posix_path(_ path:String) -> String
    {
        guard let first_char:Character = path.first
        else
        {
            return path
        }
        var expanded_path:String = path
        if first_char == "~"
        {
            if expanded_path.count == 1 || expanded_path[expanded_path.index(after: expanded_path.startIndex)] == "/"
            {
                expanded_path = String(cString: getenv("HOME")) + String(expanded_path.dropFirst())
            }
        }
        return expanded_path
    }

    private mutating
    func open_text_file(_ posix_path:String) -> String?
    {
        guard let f:UnsafeMutablePointer<FILE> = fopen(posix_path, "rb")
        else
        {
            self.handle_error("could not open file stream '\(posix_path)'", line: 0, column: 0)
            return nil
        }
        defer { fclose(f) }

        let fseek_status:CInt = fseek(f, 0, SEEK_END)
        guard fseek_status == 0
        else
        {
            self.handle_error("fseek() on file '\(posix_path)' failed with error code \(fseek_status)", line: 0, column: 0)
            return nil
        }

        let n:CLong = ftell(f)
        guard 0 ..< CLong.max ~= n
        else
        {
            self.handle_error("ftell() on file '\(posix_path)' returned too large file size (\(n) bytes)", line: 0, column: 0)
            return nil
        }
        rewind(f)

        let buffer:UnsafeMutablePointer<CChar> = UnsafeMutablePointer<CChar>.allocate(capacity: n + 1) // leave room for sentinel
        defer { buffer.deallocate(capacity: n + 1) }

        let n_read = fread(buffer, MemoryLayout<CChar>.size, n, f)
        guard n_read == n
        else
        {
            self.handle_error("fread() on file '\(posix_path)' read \(n_read) characters out of \(n)", line: 0, column: n_read)
            return nil
        }

        buffer[n] = 0 // cap with sentinel
        return String(cString: buffer)
    }*/
    
    init?(str:String) {
        iterator = str.unicodeScalars.makeIterator()
        position = Position()
    }
    
    
    var iterator:String.UnicodeScalarView.Iterator
    
    var state:State                                = .end_markup,
        markup_context:Markup                      = .none

    var name_buffer:[Unicode.Scalar]    = [],
        label_buffer:[Unicode.Scalar]   = [],
        attributes:[String: String]     = [:],
        string_delimiter:Unicode.Scalar = "\0"

    var position:Position
    //var u:Unicode.Scalar
    
    @inline(__always) func _emit_tag() -> XMLToken? {
        switch markup_context {
            case .none:
                return nil
            case .start:
                return .tag_start(String(name_buffer), attributes)
            case .empty:
                return .tag_empty(String(name_buffer), attributes)
            case .end:
                return .tag_end(String(name_buffer))
            case .comment:
                return nil
            case .processing:
                let copy = label_buffer
                label_buffer = []
                return .instruction(String(name_buffer), copy)
        }
    }
    
    var reuseLastCharacter:Unicode.Scalar? = nil
    
    public func readToken() throws -> XMLToken? {
        
        while true {
            
            let u:Unicode.Scalar
            if let next = reuseLastCharacter {
                u = next
                reuseLastCharacter = nil
            } else {
                guard let u_after:Unicode.Scalar = iterator.next() else {
                    switch state {
                        case .end_markup:
                            return nil
                            
                        case .data:
                            state = .end_markup
                            return nil
                        default:
                            throw XmlError("unexpected end of stream inside markup structure", position.line, position.column)
                    }
                }
                u = u_after
                position.advance(u)
            }
            
            
            fsm: switch state
            {
                case .end_markup:
                    markup_context = .none

                    name_buffer = []
                    attributes  = [:]

                    if u == "<"
                    {
                        state = .begin_markup
                    }
                    else
                    {
                        state = .data
                        reuseLastCharacter = u
                        continue
                    }
                case .data:
                    var u_current:Unicode.Scalar = u
                    var data_buffer:[Unicode.Scalar] = []

                    while u_current != "<"
                    {
                        let u_next:Unicode.Scalar?
                        if u_current == "&" {
                            let content:[Unicode.Scalar]
                            let error:String?
                            
                            (u_next, content, error) = iterator.read_reference(position: &position)
                            data_buffer.append(contentsOf: content)

                            position.advance(u_current)

                            if let error_message:String = error {
                                throw XmlError(error_message, position.line, position.column)
                            }
                        } else {
                            data_buffer.append(u_current)
                            position.advance(u_current)
                            u_next = iterator.next()
                        }

                        guard let u_after:Unicode.Scalar = u_next else {
                            return XMLToken.data(data_buffer)
                        }
                        u_current = u_after
                    }

                    state = .begin_markup
                    if (data_buffer.count > 0) {
                        return XMLToken.data(data_buffer)
                    }

                case .begin_markup:
                    markup_context = .start
                    
                    if u.is_xml_name_start {
                        state = .name(u)
                    } else if u == "/" {
                        state = .slash1
                    } else if u == "!" {
                        state = .exclam
                    } else if u == "?" {
                        state = .question1
                    } else {
                        throw XmlError("unexpected '\(u)' after left angle bracket '<'", position.line, position.column)
                    }

                case .slash1:
                    markup_context = .end
                    guard u.is_xml_name_start
                    else
                    {
                        throw XmlError("unexpected '\(u)' in end tag '\(String(name_buffer))'", position.line, position.column)
                    }

                    state = .name(u)

                case .name(let u_previous):
                    name_buffer.append(u_previous)
                    if u.is_xml_name
                    {
                        state = .name(u)
                        break
                    }

                    if markup_context == .start
                    {
                        if u.is_xml_whitespace
                        {
                            state = .attributes
                        }
                        else if u == "/"
                        {
                            state = .slash2
                        }
                        else if u == ">"
                        {
                            state = .end_markup
                        }
                        else
                        {
                            throw XmlError("unexpected '\(u)' in start tag '\(String(name_buffer))'", position.line, position.column)
                        }
                    }
                    else if markup_context == .end
                    {
                        if u.is_xml_whitespace
                        {
                            state = .no_attributes
                        }
                        else if u == ">"
                        {
                            state = .end_markup
                        }
                        else
                        {
                            throw XmlError("unexpected '\(u)' in end tag '\(String(name_buffer))'", position.line, position.column)
                        }
                    }
                    else if markup_context == .processing
                    {
                        if u.is_xml_whitespace
                        {
                            state = .pi_space
                        }
                        else if u == "?"
                        {
                            state = .question2
                        }
                        else
                        {
                            throw XmlError("unexpected '\(u)' in processing instruction '\(String(name_buffer))'", position.line, position.column)
                        }
                    }

                case .attributes:
                    if u.is_xml_name_start
                    {
                        state = .label(u)
                    }
                    else if u == "/"
                    {
                        state = .slash2
                    }
                    else if u == ">"
                    {
                        state = .end_markup
                    }
                    else
                    {
                        guard u.is_xml_whitespace
                        else
                        {
                            throw XmlError("unexpected '\(u)' in end tag '\(String(name_buffer))'", position.line, position.column)
                        }
                    }

                case .no_attributes:
                    if u == ">"
                    {
                        state = .end_markup
                    }
                    else
                    {
                        guard u.is_xml_whitespace
                        else
                        {
                            if u.is_xml_name_start
                            {
                                throw XmlError("end tag '\(String(name_buffer))' cannot contain attributes", position.line, position.column)
                            }
                            else
                            {
                                throw XmlError("unexpected '\(u)' in end tag '\(String(name_buffer))'", position.line, position.column)
                            }
                        }
                    }

                case .label(let u_previous):
                    label_buffer.append(u_previous)

                    if u.is_xml_name
                    {
                        state = .label(u)
                    }
                    else if u == "="
                    {
                        state = .equals
                    }
                    else if u == "/"
                    {
                        
                        let label_str:String = String(label_buffer)

                        guard attributes[label_str] == nil
                        else
                        {
                            throw XmlError("redefinition of attribute '\(label_str)'", position.line, position.column)
                        }

                        attributes[label_str] = ""
                        label_buffer = []
                        state = .slash2
                    }
                    else if u == ">"
                    {
                        let label_str:String = String(label_buffer)

                        guard attributes[label_str] == nil
                        else
                        {
                            throw XmlError("redefinition of attribute '\(label_str)'", position.line, position.column)
                        }

                        attributes[label_str] = ""
                        label_buffer = []
                        state = .end_markup
                    }
                    else
                    {
                        guard u.is_xml_whitespace
                        else
                        {
                            throw XmlError("unexpected '\(u)' in start tag '\(String(name_buffer))'", position.line, position.column)
                        }

                        state = .space1
                    }

                case .space1:
                    if u == "="
                    {
                        state = .equals
                    }
                    else if u == "/"
                    {
                        state = .slash2
                    }
                    else if u == ">"
                    {
                        state = .end_markup
                    }
                    else
                    {
                        if (!u.is_xml_whitespace) {
                            
                            let label_str:String = String(label_buffer)

                            guard attributes[label_str] == nil
                            else
                            {
                                throw XmlError("redefinition of attribute '\(label_str)'", position.line, position.column)
                            }

                            attributes[label_str] = ""
                            label_buffer = []
                            
                            reuseLastCharacter = u
                            state = .attributes
                            continue
                        }
                    }

                case .equals:
                    if u == "\"" || u == "'"
                    {
                        string_delimiter = u
                        state = .attribute_value
                    }
                    else
                    {
                        guard u.is_xml_whitespace
                        else
                        {
                            throw XmlError("unexpected '\(u)' in start tag '\(String(name_buffer))'", position.line, position.column)
                        }

                        state = .space2
                    }

                case .space2:
                    if u == "\"" || u == "'"
                    {
                        string_delimiter = u
                        state = .attribute_value
                    }
                    else
                    {
                        guard u.is_xml_whitespace
                        else
                        {
                            throw XmlError("unexpected '\(u)' in start tag '\(String(name_buffer))'", position.line, position.column)
                        }
                    }

                case .attribute_value:
                    var u_current:Unicode.Scalar      = u,
                        value_buffer:[Unicode.Scalar] = []

                    while u_current != string_delimiter
                    {
                        let u_next:Unicode.Scalar?
                        if u_current == "&"
                        {
                            let content:[Unicode.Scalar],
                                error:String?
                            (u_next, content, error) = iterator.read_reference(position: &position)
                            value_buffer.append(contentsOf: content)

                            position.advance(u_current)

                            if let error_message:String = error
                            {
                                throw XmlError(error_message, position.line, position.column)
                            }
                        }
                        else
                        {
                            value_buffer.append(u_current)
                            u_next = iterator.next()
                            position.advance(u_current)
                        }

                        guard let u_after:Unicode.Scalar = u_next
                        else
                        {
                            throw XmlError("unexpected end of stream inside of attribute", position.line, position.column)
                        }
                        u_current = u_after
                    }

                    string_delimiter = "\0"
                    let label_str:String = String(label_buffer)

                    guard attributes[label_str] == nil
                    else
                    {
                        throw XmlError("redefinition of attribute '\(label_str)'", position.line, position.column)
                    }

                    attributes[label_str] = String(value_buffer)
                    label_buffer = []
                    value_buffer = []

                    state = .attributes

                case .slash2:
                    markup_context = .empty
                    guard u == ">"
                    else
                    {
                        throw XmlError("unexpected '\(u)' in empty tag '\(String(name_buffer))'", position.line, position.column)
                    }

                    state = .end_markup

                case .exclam:
                    if u == "-" {
                        state = .hyphen1
                    } else {
                        state = .name(u)
                    }

                case .hyphen1:
                    guard u == "-"
                    else
                    {
                        throw XmlError("unexpected '\(u)' after '<!-'", position.line, position.column)
                    }

                    state = .comment

                case .comment:
                    markup_context = .comment
                    if u == "-"
                    {
                        state = .hyphen2
                    }

                case .hyphen2:
                    if u == "-"
                    {
                        state = .hyphen3
                    }
                    else
                    {
                        state = .comment
                    }

                case .hyphen3:
                    guard u == ">"
                    else
                    {
                        throw XmlError("unexpected double hyphen '--' inside comment body", position.line, position.column - 1)
                    }

                    state = .end_markup

                case .question1:
                    markup_context = .processing
                    guard u.is_xml_name_start
                    else
                    {
                        throw XmlError("unexpected '\(u)' after '<?'", position.line, position.column)
                    }

                    state = .name(u)

                case .pi_space:
                    if u == "?"
                    {
                        state = .question2
                    }
                    else if !u.is_xml_whitespace
                    {
                        state = .pi_data(u)
                    }

                case .pi_data(let u_previous):
                    label_buffer.append(u_previous)
                    if u == "?"
                    {
                        state = .question2
                    }
                    else
                    {
                        state = .pi_data(u)
                    }

                case .question2:
                    if u == ">"
                    {
                        state = .end_markup
                    }
                    else
                    {
                        label_buffer.append("?")
                        state = .pi_data(u)
                    }
            }
            
            if case .end_markup = state {
                return _emit_tag()
            }
        }
    }
}