extension PureYAML.Parsing.LibYAML.Parser {
    /// Port of `yaml_parser_parse_block_sequence_entry`.
    mutating func parseBlockSequenceEntry(first: Bool) throws -> Event {
        if first {
            marks.append(peekToken().startMark)
            skipToken()
        }
        let token = peekToken()
        if case .blockEntry = token.type {
            let mark = token.endMark
            skipToken()
            let next = peekToken()
            if !isBlockEntry(next), !isBlockEnd(next) {
                states.append(.blockSequenceEntry)
                return try parseNode(block: true, indentlessSequence: false)
            }
            state = .blockSequenceEntry
            return processEmptyScalar(at: mark)
        }
        if case .blockEnd = token.type {
            state = states.removeLast()
            _ = marks.popLast()
            skipToken()
            return .sequenceEnd(mark: token.startMark)
        }
        throw ParserError(
            context: "while parsing a block collection",
            problem: "did not find expected '-' indicator",
            mark: marks.popLast() ?? token.startMark,
        )
    }

    /// Port of `yaml_parser_parse_indentless_sequence_entry`. The closing token
    /// belongs to the enclosing collection, so the SEQUENCE-END is emitted without
    /// consuming it.
    mutating func parseIndentlessSequenceEntry() throws -> Event {
        let token = peekToken()
        guard case .blockEntry = token.type else {
            state = states.removeLast()
            return .sequenceEnd(mark: token.startMark)
        }
        let mark = token.endMark
        skipToken()
        let next = peekToken()
        if !isBlockEntry(next), !isKey(next), !isValue(next), !isBlockEnd(next) {
            states.append(.indentlessSequenceEntry)
            return try parseNode(block: true, indentlessSequence: false)
        }
        state = .indentlessSequenceEntry
        return processEmptyScalar(at: mark)
    }

    /// Port of `yaml_parser_parse_block_mapping_key`.
    mutating func parseBlockMappingKey(first: Bool) throws -> Event {
        if first {
            marks.append(peekToken().startMark)
            skipToken()
        }
        let token = peekToken()
        if case .key = token.type {
            let mark = token.endMark
            skipToken()
            let next = peekToken()
            if !isKey(next), !isValue(next), !isBlockEnd(next) {
                states.append(.blockMappingValue)
                return try parseNode(block: true, indentlessSequence: true)
            }
            state = .blockMappingValue
            return processEmptyScalar(at: mark)
        }
        if case .blockEnd = token.type {
            state = states.removeLast()
            _ = marks.popLast()
            skipToken()
            return .mappingEnd(mark: token.startMark)
        }
        // YAML 1.2 permits an empty (null) key: a VALUE indicator with no
        // preceding key denotes a mapping entry whose key is empty. libyaml
        // rejects this here; instead emit the empty scalar key and leave the
        // VALUE token for `parseBlockMappingValue` to consume.
        if case .value = token.type {
            state = .blockMappingValue
            return processEmptyScalar(at: token.startMark)
        }
        throw ParserError(
            context: "while parsing a block mapping",
            problem: "did not find expected key",
            mark: marks.popLast() ?? token.startMark,
        )
    }

    /// Port of `yaml_parser_parse_block_mapping_value`.
    mutating func parseBlockMappingValue() throws -> Event {
        let token = peekToken()
        guard case .value = token.type else {
            state = .blockMappingKey
            return processEmptyScalar(at: token.startMark)
        }
        let mark = token.endMark
        skipToken()
        let next = peekToken()
        if !isKey(next), !isValue(next), !isBlockEnd(next) {
            states.append(.blockMappingKey)
            return try parseNode(block: true, indentlessSequence: true)
        }
        state = .blockMappingKey
        return processEmptyScalar(at: mark)
    }
}