PureYAML/Sources/Parsing/TokenEventParser.swift

extension PureYAML.Parsing {
    struct TokenEventParser {
        var cursor: TokenCursor
        let scalarParser: Parser
        var events: [Event]?
        var eventSink: ((Event) throws -> Void)?

        /// Current node recursion depth, tracked so deeply nested input fails
        /// with a diagnostic instead of overflowing the stack.
        var depth = 0

        /// Maximum node nesting the parser will follow before failing with
        /// ``ParseError/nestingTooDeep(line:column:)``.
        ///
        /// This bound is the single safety invariant for deeply nested input,
        /// and it is *necessary*, not merely convenient. A ``Model/Value`` is a
        /// recursive tree, so composing it, emitting it, comparing it, hashing
        /// it, and -- crucially -- deallocating it all recurse with the nesting
        /// depth. Swift tears down a recursive value type recursively, and that
        /// teardown cannot be made iterative cheaply, so the only way to keep
        /// every operation within the call stack is to bound the depth of the
        /// tree where it is built: here, while parsing.
        ///
        /// Because every parsing entry point routes through ``parseNode``, this
        /// one bound caps the depth of any parsed tree, which transitively keeps
        /// the parser, the event composers, the emitter, equality, hashing, and
        /// deallocation all well inside their limits. Measured on the main
        /// thread's 8 MB stack: the parser overflows near depth 225 and the
        /// tree operations near depth 50,000, while real-world specs nest below
        /// ~40. So 64 accepts every realistic document yet rejects pathological
        /// input -- for example a 100 KB `[[[...]]]` -- cleanly and early.
        ///
        /// The bound cannot simply be raised: above ~225 the recursive parser
        /// itself overflows before the guard fires (lifting it would require an
        /// iterative parser), and a tree past ~5,000 deep crashes on `deinit`
        /// regardless, so a higher limit would trade a parse-time failure for an
        /// un-fixable teardown crash. 64 is the largest value that is both safe
        /// here and sufficient for real input.
        ///
        /// Caveat: a tree built *without* the parser -- for example by encoding
        /// a pathologically deep `Codable` value -- bypasses this bound, and
        /// operations on such a tree are O(depth) in stack like any recursive
        /// value type. Keeping those shallow is the caller's responsibility, the
        /// same way the standard library cannot prevent unbounded recursion in
        /// user code.
        static let maxNestingDepth = 64

        init(
            tokens: [Token],
            scalarParser: Parser,
        ) {
            cursor = TokenCursor(tokens: tokens)
            self.scalarParser = scalarParser
            events = []
            eventSink = nil
        }

        mutating func parse() throws -> [Event] {
            try parseEvents()
            return events ?? []
        }

        init(
            tokens: [Token],
            scalarParser: Parser,
            eventSink: @escaping (Event) throws -> Void,
        ) {
            cursor = TokenCursor(tokens: tokens)
            self.scalarParser = scalarParser
            events = nil
            self.eventSink = eventSink
        }

        init(
            yaml: String,
            scalarParser: Parser,
            eventSink: @escaping (Event) throws -> Void,
        ) {
            cursor = TokenCursor(yaml: yaml)
            self.scalarParser = scalarParser
            events = nil
            self.eventSink = eventSink
        }

        mutating func parseStreaming() throws {
            try parseEvents()
        }

        mutating func parseEvents() throws {
            let streamStart = try expect("stream start") { kind in
                if case .streamStart = kind {
                    return true
                }
                return false
            }

            try emit(.streamStart(mark: streamStart.mark))
            var didParseDocument = false
            while cursor.current?.kind.isStreamEnd != true {
                try parseDocument()
                didParseDocument = true
            }
            if !didParseDocument {
                let mark = cursor.current?.mark ?? streamStart.endMark
                try emit(.documentStart(mark: mark))
                try appendEmptyDocument(at: mark)
                try emit(.documentEnd(mark: mark))
            }
            let streamEnd = try expect("stream end") { kind in
                if case .streamEnd = kind {
                    return true
                }
                return false
            }
            try emit(.streamEnd(mark: streamEnd.mark))
        }

        mutating func emit(_ event: Event) throws {
            if let eventSink {
                try eventSink(event)
            } else {
                events?.append(event)
            }
        }
    }
}

extension PureYAML.Parsing.TokenEventParser {
    mutating func parseDocument() throws {
        let startMark = try parseDocumentStartMark()
        try emit(.documentStart(mark: startMark))

        if cursor.current?.kind.isDocumentEnd == true {
            let end = try expect("document end") { $0.isDocumentEnd }
            try appendEmptyDocument(at: end.mark)
            try emit(.documentEnd(mark: end.endMark))
            return
        }

        if cursor.current?.kind.isDocumentStart == true || cursor.current?.kind.isStreamEnd == true {
            try appendEmptyDocument(at: cursor.current?.mark ?? startMark)
            try emit(.documentEnd(mark: cursor.current?.mark ?? startMark))
            return
        }

        var endMark = try parseDocumentRootNode().endMark
        while cursor.current?.kind.isDedent == true {
            let dedent = try expect("dedent") { $0.isDedent }
            endMark = dedent.endMark
        }
        if cursor.current?.kind.isDocumentEnd == true {
            let end = try expect("document end") { $0.isDocumentEnd }
            endMark = end.endMark
        }
        if let current = cursor.current, !canEndDocument(at: current) {
            throw PureYAML.Parsing.ParseError.unexpectedToken(
                expected: "document boundary or stream end",
                actual: current.kind.description,
                line: current.mark.line,
                column: current.mark.column,
            )
        }
        try emit(.documentEnd(mark: endMark))
    }

    mutating func parseDocumentRootNode() throws -> PureYAML.Parsing.NodeResult {
        guard cursor.current?.kind.isIndent == true else {
            return try parseNode()
        }
        _ = try expect("indent") { $0.isIndent }
        let result = try parseNode()
        if cursor.current?.kind.isDedent == true {
            _ = try expect("dedent") { $0.isDedent }
        }
        return result
    }

    mutating func parseDocumentStartMark() throws -> PureYAML.Parsing.Mark {
        guard cursor.current?.kind.isDocumentStart == true else {
            return cursor.current?.mark ?? cursor.previous?.endMark ?? .start
        }
        let start = try cursor.advance()
        return start?.mark ?? .start
    }

    mutating func appendEmptyDocument(at mark: PureYAML.Parsing.Mark) throws {
        try emit(.scalar(value: "", anchor: nil, tag: nil, style: .plain, mark: mark))
    }

    func canEndDocument(at token: PureYAML.Parsing.Token) -> Bool {
        token.kind.isDocumentStart || token.kind.isStreamEnd
    }

    mutating func parseNode() throws -> PureYAML.Parsing.NodeResult {
        let properties = try consumeProperties()
        return try parseNode(properties: properties)
    }

    /// Reject documents nested past ``maxNestingDepth`` with a diagnostic so a
    /// pathologically nested input fails cleanly instead of overflowing the
    /// recursive-descent stack.
    func enforceNestingLimit(properties: PureYAML.Parsing.NodeProperties) throws {
        guard depth > Self.maxNestingDepth else {
            return
        }
        let mark = properties.mark ?? cursor.current?.mark ?? cursor.previous?.endMark ?? .start
        throw PureYAML.Parsing.ParseError.nestingTooDeep(line: mark.line, column: mark.column)
    }

    mutating func parseNode(
        properties initialProperties: PureYAML.Parsing.NodeProperties,
    ) throws -> PureYAML.Parsing.NodeResult {
        depth += 1
        defer { depth -= 1 }
        try enforceNestingLimit(properties: initialProperties)
        var properties = initialProperties
        try properties.mergeUnset(from: consumeProperties())

        guard let token = cursor.current, !token.kind.isTerminator else {
            let mark = properties.mark ?? cursor.current?.mark ?? cursor.previous?.endMark ?? .start
            throw PureYAML.Parsing.ParseError.expectedNode(line: mark.line, column: mark.column)
        }

        switch token.kind {
        case let .alias(anchor):
            _ = try cursor.advance()
            let mark = properties.mark ?? token.mark
            try emit(.alias(anchor: anchor, mark: mark))
            return PureYAML.Parsing.NodeResult(kind: .alias, endMark: token.endMark)
        case .blockEntry:
            return try parseBlockSequence(properties: properties)
        case .blockScalarHeader:
            return try parseBlockScalar(properties: properties)
        case .flowMappingStart:
            return try parseFlowMapping(properties: properties)
        case .flowSequenceStart:
            return try parseFlowSequence(properties: properties)
        case .mappingKey:
            return try parseBlockMapping(
                properties: properties,
                includeIndentedContinuation: false,
            )
        case let .scalar(value, style):
            if try isCommentInterruptedScalar() {
                return try rejectCommentInterruptedScalar()
            }
            _ = try cursor.advance()
            let decoded = try decodeScalar(value, style: style, line: token.mark.line)
            let mark = properties.mark ?? token.mark
            try emit(.scalar(
                value: decoded,
                anchor: properties.anchor,
                tag: properties.tag,
                style: style,
                mark: mark,
            ))
            return PureYAML.Parsing.NodeResult(kind: .scalar, endMark: token.endMark)
        default:
            throw PureYAML.Parsing.ParseError.expectedNode(line: token.mark.line, column: token.mark.column)
        }
    }
}