PureXML/Sources/HTML/HTMLSerializer.swift

extension PureXML.HTML {
    /// Serializes a node tree as HTML (the libxml2 `HTMLtree.h` model): void
    /// elements are written without an end tag or self-closing slash, raw-text
    /// elements (`script`, `style`) keep their content unescaped, and text and
    /// attribute values are escaped per HTML.
    enum Serializer {
        static func serialize(_ node: PureXML.Model.Node) -> String {
            // Deferred-close work stack so a deeply-nested tree does not overflow
            // the stack (the same shape as the XML serializer and Canonicalizer);
            // children push reversed to emit in document order.
            var output = ""
            var stack: [HTMLSerializeStep] = [.node(node, rawTextParent: false)]
            while let step = stack.popLast() {
                switch step {
                case let .close(name):
                    output += "</\(name)>"
                case let .node(node, rawTextParent):
                    emit(node, rawTextParent: rawTextParent, into: &output, stack: &stack)
                }
            }
            return output
        }

        private static func emit(
            _ node: PureXML.Model.Node,
            rawTextParent: Bool,
            into output: inout String,
            stack: inout [HTMLSerializeStep],
        ) {
            switch node {
            case let .document(children):
                stack.append(contentsOf: children.reversed().map { HTMLSerializeStep.node($0, rawTextParent: false) })
            case let .element(element):
                let name = element.name.description
                output += "<" + name
                for attribute in element.attributes {
                    output += attributeText(attribute)
                }
                output += ">"
                // A void element has no end tag and no content (the html model);
                // anything nested is dropped, as the recursive form did.
                if Elements.void.contains(name.lowercased()) {
                    return
                }
                stack.append(HTMLSerializeStep.close(name))
                // A raw-text element (script, style) writes its own text children
                // verbatim; nested elements still serialize normally by their own
                // name, so the flag governs only this element's direct text.
                let childRawText = Elements.rawText.contains(name.lowercased())
                stack.append(contentsOf: element.children.reversed().map { HTMLSerializeStep.node($0, rawTextParent: childRawText) })
            case let .text(value), let .cdata(value):
                output += rawTextParent ? value : escapeText(value)
            case let .comment(value):
                output += "<!--\(PureXML.Emitting.Escaping.comment(value))-->"
            case let .processingInstruction(target, data):
                output += data.isEmpty ? "<?\(target)>" : "<?\(target) \(data)>"
            }
        }

        private static func attributeText(_ attribute: PureXML.Model.Attribute) -> String {
            let name = attribute.name.description
            if attribute.value.isEmpty {
                return " \(name)"
            }
            // Boolean attributes minimize when the value repeats the name
            // (CHECKED="CHECKED" serializes as CHECKED), the html form.
            if Elements.booleanAttributes.contains(name.lowercased()), attribute.value.lowercased() == name.lowercased() {
                return " \(name)"
            }
            let escaped = Elements.uriAttributes.contains(name.lowercased())
                ? escapeURI(attribute.value)
                : escapeAttribute(attribute.value)
            return " \(name)=\"\(escaped)\""
        }

        /// Escapes a URI-valued attribute value (XSLT 1.0 16.2, HTML 4.01 B.2.1,
        /// the libxml2 HTML output model): each non-ASCII or control character, and
        /// every space (leading ones included, where libxml2 skips them), is
        /// percent-escaped as the uppercase hex of its UTF-8 bytes, while `"` and
        /// `&` keep their entity form. A parse-then-serialize round-trip thus
        /// normalizes a space or non-ASCII byte in these attributes to `%HH`; this
        /// is idempotent, since a literal `%` is left as is.
        private static func escapeURI(_ value: String) -> String {
            var result = ""
            for scalar in value.unicodeScalars {
                if scalar.value > 0x20, scalar.value < 0x7F {
                    switch scalar {
                    case "\"": result += "&quot;"
                    case "&": result += "&amp;"
                    default: result.unicodeScalars.append(scalar)
                    }
                } else {
                    for byte in String(scalar).utf8 {
                        result += "%" + hexByte(byte)
                    }
                }
            }
            return result
        }

        /// A byte as two uppercase hex digits, without Foundation formatting.
        private static func hexByte(_ byte: UInt8) -> String {
            let digits = Array("0123456789ABCDEF")
            return String([digits[Int(byte >> 4)], digits[Int(byte & 0x0F)]])
        }

        private static func escapeText(_ value: String) -> String {
            var result = ""
            for scalar in value.unicodeScalars {
                switch scalar {
                case "&": result += "&amp;"
                case "<": result += "&lt;"
                case ">": result += "&gt;"
                default: result += latin1Entity(scalar) ?? String(Character(scalar))
                }
            }
            return result
        }

        private static func escapeAttribute(_ value: String) -> String {
            var result = ""
            var previousWasAmpersand = false
            for scalar in value.unicodeScalars {
                if previousWasAmpersand {
                    // The html method leaves & unescaped when a { follows
                    // (the &{...} convention, 16.2).
                    result += scalar == "{" ? "&" : "&amp;"
                    previousWasAmpersand = false
                }
                switch scalar {
                case "&":
                    previousWasAmpersand = true
                case "\"": result += "&quot;"
                // The html output method does not escape `<` (or `>`) in an
                // attribute value, unlike the xml method (XSLT 1.0 16.2).
                default: result += latin1Entity(scalar) ?? String(Character(scalar))
                }
            }
            if previousWasAmpersand {
                result += "&amp;"
            }
            return result
        }

        /// The HTML 4.01 Latin-1 named entities: U+00A0 through U+00FF map
        /// onto these names in order (the html output method's escapes).
        private static let latin1Names = [
            "nbsp", "iexcl", "cent", "pound", "curren", "yen", "brvbar", "sect",
            "uml", "copy", "ordf", "laquo", "not", "shy", "reg", "macr",
            "deg", "plusmn", "sup2", "sup3", "acute", "micro", "para", "middot",
            "cedil", "sup1", "ordm", "raquo", "frac14", "frac12", "frac34", "iquest",
            "Agrave", "Aacute", "Acirc", "Atilde", "Auml", "Aring", "AElig", "Ccedil",
            "Egrave", "Eacute", "Ecirc", "Euml", "Igrave", "Iacute", "Icirc", "Iuml",
            "ETH", "Ntilde", "Ograve", "Oacute", "Ocirc", "Otilde", "Ouml", "times",
            "Oslash", "Ugrave", "Uacute", "Ucirc", "Uuml", "Yacute", "THORN", "szlig",
            "agrave", "aacute", "acirc", "atilde", "auml", "aring", "aelig", "ccedil",
            "egrave", "eacute", "ecirc", "euml", "igrave", "iacute", "icirc", "iuml",
            "eth", "ntilde", "ograve", "oacute", "ocirc", "otilde", "ouml", "divide",
            "oslash", "ugrave", "uacute", "ucirc", "uuml", "yacute", "thorn", "yuml",
        ]

        private static func latin1Entity(_ scalar: Unicode.Scalar) -> String? {
            guard scalar.value >= 0xA0, scalar.value <= 0xFF else { return nil }
            return "&" + latin1Names[Int(scalar.value) - 0xA0] + ";"
        }
    }
}

public extension PureXML.HTML {
    /// Parses an HTML fragment into a ``PureXML/Model/Node`` document, leniently
    /// handling tag-soup input, void elements, optional end tags, and raw-text
    /// elements. Markup is taken as-is, with no implied `html`/`head`/`body`.
    static func parse(_ html: String) -> PureXML.Model.Node {
        var tokenizer = Tokenizer(html)
        return TreeBuilder.build(tokenizer.tokenize())
    }

    /// Parses a full HTML document by the HTML5 tree-construction insertion
    /// modes: the result always has an `html` root containing a `head` and a
    /// `body`, with head-only elements (`title`, `meta`, `link`, `style`,
    /// `script`, and the rest) routed into the head and flow content into the
    /// body, whether or not the source spelled those elements out.
    static func parseDocument(_ html: String) -> PureXML.Model.Node {
        var tokenizer = Tokenizer(html)
        return DocumentBuilder.build(tokenizer.tokenize())
    }

    /// Serializes a node tree as HTML.
    static func serialize(_ node: PureXML.Model.Node) -> String {
        Serializer.serialize(node)
    }
}

/// One unit of HTML serialization work: a node to emit (with whether its parent
/// is a raw-text element, so its own text is written unescaped) or a close tag
/// deferred until after the element's children. File-scoped so the serializer's
/// deferred-close work stack is not a third level of type nesting.
private enum HTMLSerializeStep {
    case node(PureXML.Model.Node, rawTextParent: Bool)
    case close(String)
}