extension PureXML.Emitting { /// The shared XML output escaping rules, used by both the tree serializer and /// the incremental writer so their output is identical. Element content /// escapes the markup characters; attribute values additionally escape the /// quote and the whitespace characters as numeric references (matching /// libxml2), so attributes survive attribute-value normalization. enum Escaping { static func text(_ value: String, asciiOnly: Bool = false, escapeCarriageReturn: Bool = false) -> String { // Fast path: most content has no character that needs escaping, so it // is returned unchanged rather than rebuilt character by character. if !textNeedsEscaping(value, asciiOnly: asciiOnly, escapeCarriageReturn: escapeCarriageReturn) { return value } var result = "" var runStart = value.startIndex // Find each character that needs escaping by scanning bytes (every // trigger is an ASCII marker, or in ASCII-only mode a non-ASCII lead // byte, both of which fall on a grapheme boundary), copy the verbatim // run before it in one append, and rewrite only that character. while let marker = value.utf8[runStart...].firstIndex(where: { textEscapableByte($0, asciiOnly: asciiOnly, escapeCarriageReturn: escapeCarriageReturn) }) { if runStart < marker { result += value[runStart ..< marker] } let character = value[marker] result += textEscape(character, asciiOnly: asciiOnly, escapeCarriageReturn: escapeCarriageReturn) ?? String(character) runStart = value.index(after: marker) } if runStart < value.endIndex { result += value[runStart...] } return result } /// Whether a byte can begin a character ``text`` must escape: an ASCII /// markup byte, a carriage return when asked, or any non-ASCII lead byte /// in ASCII-only mode (its character is reference-escaped whole). private static func textEscapableByte(_ byte: UInt8, asciiOnly: Bool, escapeCarriageReturn: Bool) -> Bool { switch byte { case 0x26, 0x3C, 0x3E: true case 0x0D where escapeCarriageReturn: true case 0x80... where asciiOnly: true default: false } } /// The escaped form of one content character, or nil when it is verbatim. private static func textEscape(_ character: Character, asciiOnly: Bool, escapeCarriageReturn: Bool) -> String? { switch character { case "&": "&" case "<": "<" case ">": ">" case "\r" where escapeCarriageReturn: " " default: asciiOnly ? plainIfNonASCII(character) : nil } } /// Whether ``text`` would change `value`: it escapes `&`, `<`, `>` always, /// a carriage return when asked, and (in ASCII-only mode) any non-ASCII /// byte. Scanned at the byte level, since every trigger is ASCII or, for /// ASCII-only mode, a non-ASCII lead byte. private static func textNeedsEscaping(_ value: String, asciiOnly: Bool, escapeCarriageReturn: Bool) -> Bool { for byte in value.utf8 { switch byte { case 0x26, 0x3C, 0x3E: return true case 0x0D where escapeCarriageReturn: return true case 0x80... where asciiOnly: return true default: continue } } return false } /// In ASCII-only mode, the numeric-reference escaping of a character that /// carries a non-ASCII scalar, or nil when the character is pure ASCII and /// so is copied verbatim with its run. private static func plainIfNonASCII(_ character: Character) -> String? { guard character.unicodeScalars.contains(where: { $0.value > 0x7F }) else { return nil } return plain(character, asciiOnly: true) } /// A character verbatim, or, in ASCII-only mode, each of its non-ASCII /// scalars as a numeric character reference so the output is pure ASCII. private static func plain(_ character: Character, asciiOnly: Bool) -> String { guard asciiOnly else { return String(character) } var result = "" for scalar in character.unicodeScalars { result += scalar.value > 0x7F ? "&#x\(String(scalar.value, radix: 16, uppercase: true));" : String(scalar) } return result } static func attribute(_ value: String, quote: Character = "\"", asciiOnly: Bool = false) -> String { // Fast path: a value with no character to escape is returned unchanged. let quoteByte: UInt8 = quote == "'" ? 0x27 : 0x22 if !attributeNeedsEscaping(value, quoteByte: quoteByte, asciiOnly: asciiOnly) { return value } var result = "" var runStart = value.startIndex while let marker = value.utf8[runStart...].firstIndex(where: { attributeEscapableByte($0, quoteByte: quoteByte, asciiOnly: asciiOnly) }) { if runStart < marker { result += value[runStart ..< marker] } let character = value[marker] result += attributeEscape(character, quote: quote, asciiOnly: asciiOnly) ?? String(character) runStart = value.index(after: marker) } if runStart < value.endIndex { result += value[runStart...] } return result } /// Whether a byte can begin a character ``attribute`` must escape: the /// quote, an ASCII markup or whitespace byte, or a non-ASCII lead byte in /// ASCII-only mode. private static func attributeEscapableByte(_ byte: UInt8, quoteByte: UInt8, asciiOnly: Bool) -> Bool { switch byte { case quoteByte, 0x26, 0x3C, 0x3E, 0x09, 0x0A, 0x0D: true case 0x80... where asciiOnly: true default: false } } /// The escaped form of one attribute-value character, or nil when it is /// copied verbatim. Split out so ``attribute`` stays a simple run loop. private static func attributeEscape(_ character: Character, quote: Character, asciiOnly: Bool) -> String? { if character == quote { return quote == "'" ? "'" : """ } switch character { case "&": return "&" case "<": return "<" case ">": return ">" case "\t": return " " case "\n": return " " case "\r": return " " default: return asciiOnly ? plainIfNonASCII(character) : nil } } /// Whether ``attribute`` would change `value`: it escapes the quote, the /// markup characters, the whitespace characters, and (in ASCII-only mode) /// any non-ASCII byte. Every trigger is ASCII bar the non-ASCII lead. private static func attributeNeedsEscaping(_ value: String, quoteByte: UInt8, asciiOnly: Bool) -> Bool { for byte in value.utf8 { switch byte { case quoteByte, 0x26, 0x3C, 0x3E, 0x09, 0x0A, 0x0D: return true case 0x80... where asciiOnly: return true default: continue } } return false } /// Makes `value` safe inside a comment: a comment may not contain `--` /// or end with `-`, so a space is inserted after each offending hyphen /// (the XSLT 1.0 16 recovery, so `after-` becomes `after- `). static func comment(_ value: String) -> String { // Fast path: a comment with no hyphen has no `--` and no trailing `-`, // so it is returned unchanged rather than rebuilt character by character. if !value.utf8.contains(0x2D) { return value } var result = "" var previousHyphen = false for character in value { if character == "-", previousHyphen { result.append(" ") } result.append(character) previousHyphen = character == "-" } if previousHyphen { result.append(" ") } return result } /// Makes `data` safe inside a processing instruction: PI data may not /// contain `?>`, so a space is inserted between an offending `?` and `>` /// (the XSLT 1.0 7.3 recovery, so `a?>b` becomes `a? >b`). static func processingInstruction(_ data: String) -> String { // Fast path: data with no '?' cannot contain `?>`, so it is returned // unchanged rather than rebuilt character by character. if !data.utf8.contains(0x3F) { return data } var result = "" var previousQuestion = false for character in data { if character == ">", previousQuestion { result.append(" ") } result.append(character) previousQuestion = character == "?" } return result } } }