extension PureYAML.Parsing.TokenEventParser {
    mutating func parseBlockSequence(
        properties: PureYAML.Parsing.NodeProperties,
        allowMappingKeyTerminator: Bool = false,
    ) throws -> PureYAML.Parsing.NodeResult {
        let start = try expect("block sequence entry") { $0.isBlockEntry }
        try enforceBlockSequenceNotInlineAfterProperty(properties, firstEntry: start)
        // A compact nested sequence (`- - x`) carries no indent/dedent tokens, only block entries at
        // their columns, so the sibling-entry loop must stay at this sequence's own column: a later
        // entry at a smaller column belongs to an enclosing sequence and ends this one.
        let sequenceColumn = start.mark.column
        let startMark = properties.mark ?? start.mark
        var endMark = start.endMark
        try emit(.sequenceStart(
            anchor: properties.anchor,
            tag: properties.tag,
            style: .block,
            mark: startMark,
        ))

        try parseBlockSequenceItem(
            after: start,
            endMark: &endMark,
            allowMappingKeyTerminator: allowMappingKeyTerminator,
        )
        try consumeSequenceItemBoundaryDedent(sequenceIndentWidth: start.mark.column - 1)
        try consumeSequenceItemBoundaryIndent(sequenceIndentWidth: start.mark.column - 1)
        while cursor.current?.kind.isBlockEntry == true, cursor.current?.mark.column == sequenceColumn {
            let entry = try expect("block sequence entry") { $0.isBlockEntry }
            try parseBlockSequenceItem(
                after: entry,
                endMark: &endMark,
                allowMappingKeyTerminator: allowMappingKeyTerminator,
            )
            try consumeSequenceItemBoundaryDedent(sequenceIndentWidth: entry.mark.column - 1)
            try consumeSequenceItemBoundaryIndent(sequenceIndentWidth: entry.mark.column - 1)
        }
        if let current = cursor.current, current.kind.isMappingKey, !allowMappingKeyTerminator {
            throw PureYAML.Parsing.ParseError.mixedCollectionStyles(line: current.mark.line)
        }

        try emit(.sequenceEnd(mark: endMark))
        return PureYAML.Parsing.NodeResult(kind: .sequence, endMark: endMark)
    }

    /// A block collection must begin on the line after its node properties, so an anchor or tag on
    /// the same line as the sequence's first entry (`&a - x`) is malformed.
    func enforceBlockSequenceNotInlineAfterProperty(
        _ properties: PureYAML.Parsing.NodeProperties,
        firstEntry: PureYAML.Parsing.Token,
    ) throws {
        guard properties.anchor != nil || properties.tag != nil,
              let propertyMark = properties.mark, propertyMark.line == firstEntry.mark.line
        else {
            return
        }
        throw PureYAML.Parsing.ParseError.inlineBlockSequenceAfterProperty(
            line: propertyMark.line, column: propertyMark.column,
        )
    }

    mutating func consumeSequenceItemBoundaryDedent(sequenceIndentWidth: Int) throws {
        guard dedentWidth(cursor.current) == sequenceIndentWidth,
              try cursor.peek()?.kind.isBlockEntry == true
        else {
            return
        }
        _ = try expect("dedent") { $0.isDedent }
    }

    mutating func consumeSequenceItemBoundaryIndent(sequenceIndentWidth: Int) throws {
        guard indentWidth(cursor.current) == sequenceIndentWidth,
              try cursor.peek()?.kind.isBlockEntry == true
        else {
            return
        }
        _ = try expect("indent") { $0.isIndent }
    }

    mutating func parseBlockSequenceItem(
        after entry: PureYAML.Parsing.Token,
        endMark: inout PureYAML.Parsing.Mark,
        allowMappingKeyTerminator: Bool,
    ) throws {
        if hasNodeOnSameLine(after: entry) {
            let properties = try consumeProperties()
            if cursor.current?.kind.isIndent == true {
                _ = try expect("indent") { $0.isIndent }
                endMark = try parseNode(properties: properties).endMark
                _ = try expect("dedent") { $0.isDedent }
                return
            }
            if cursor.current?.kind.isMappingKey == true {
                endMark = try parseBlockMapping(
                    properties: properties,
                    includeIndentedContinuation: true,
                    minimumColumn: entry.mark.column + 1,
                ).endMark
            } else if cursor.current?.kind.isBlockEntry == true {
                endMark = try parseBlockSequence(
                    properties: properties,
                    allowMappingKeyTerminator: allowMappingKeyTerminator,
                ).endMark
            } else {
                endMark = try parseNode(properties: properties).endMark
            }
        } else if cursor.current?.kind.isIndent == true {
            _ = try expect("indent") { $0.isIndent }
            endMark = try parseNode().endMark
            _ = try expect("dedent") { $0.isDedent }
        } else {
            let mark = entry.endMark
            try emit(.scalar(value: "", anchor: nil, tag: nil, style: .plain, mark: mark))
            endMark = mark
        }
    }

    mutating func parseBlockMapping(
        properties: PureYAML.Parsing.NodeProperties,
        includeIndentedContinuation: Bool,
        minimumColumn: Int? = nil,
    ) throws -> PureYAML.Parsing.NodeResult {
        let startMark = properties.mark ?? cursor.current?.mark ?? .start
        let tokenColumn = cursor.current?.mark.column ?? startMark.column
        let contentColumn = min(startMark.column, tokenColumn)
        let boundaryColumn = minimumColumn ?? contentColumn
        var endMark = startMark
        // A property written inline before the first key (`&a key: v`) binds to that key, not to the
        // mapping: the spec requires a block collection's own properties to be followed by a line
        // break (`s-l+block-collection`), so a property on the key's line is the key's. This matters
        // for anchors, which must register with the key (early enough for an alias inside a value to
        // resolve) rather than with the mapping, which registers only at its end. A property on its
        // own line above the first key still binds to the mapping.
        let firstKeyProperties: PureYAML.Parsing.NodeProperties
        let mappingProperties: PureYAML.Parsing.NodeProperties
        if properties != .none, properties.mark?.line == cursor.current?.mark.line {
            firstKeyProperties = properties
            mappingProperties = .none
        } else {
            firstKeyProperties = .none
            mappingProperties = properties
        }
        try emit(.mappingStart(
            anchor: mappingProperties.anchor,
            tag: mappingProperties.tag,
            style: .block,
            mark: startMark,
        ))

        try parseBlockMappingPairs(
            boundaryColumn: boundaryColumn,
            includeIndentedContinuation: includeIndentedContinuation,
            firstKeyProperties: firstKeyProperties,
            endMark: &endMark,
        )
        if let current = cursor.current, !canEndBlockMapping(at: current, minimumColumn: boundaryColumn) {
            throw PureYAML.Parsing.ParseError.unexpectedToken(
                expected: "mapping key",
                actual: current.kind.description,
                line: current.mark.line,
                column: current.mark.column,
            )
        }
        try emit(.mappingEnd(mark: endMark))
        return PureYAML.Parsing.NodeResult(kind: .mapping, endMark: endMark)
    }

    mutating func parseBlockMappingPairs(
        boundaryColumn: Int,
        includeIndentedContinuation: Bool,
        firstKeyProperties: PureYAML.Parsing.NodeProperties = .none,
        endMark: inout PureYAML.Parsing.Mark,
    ) throws {
        var pendingKeyProperties = firstKeyProperties
        while canContinueBlockMapping(minimumColumn: boundaryColumn) {
            endMark = try parseMappingPairAndBoundaries(
                boundaryColumn: boundaryColumn,
                includeIndentedContinuation: includeIndentedContinuation,
                firstKeyProperties: pendingKeyProperties,
            ).endMark
            pendingKeyProperties = .none
        }
        while includeIndentedContinuation, cursor.current?.kind.isIndent == true {
            endMark = try parseIndentedBlockMappingPairs(boundaryColumn: boundaryColumn).endMark
        }
    }

    mutating func parseIndentedBlockMappingPairs(
        boundaryColumn: Int,
    ) throws -> PureYAML.Parsing.NodeResult {
        _ = try expect("indent") { $0.isIndent }
        guard cursor.current?.kind.isMappingKey == true else {
            throw unexpectedToken(expected: "mapping key")
        }
        var endMark = cursor.current?.mark ?? .start
        while canContinueBlockMapping(minimumColumn: boundaryColumn) {
            endMark = try parseMappingPairAndBoundaries(
                boundaryColumn: boundaryColumn,
                includeIndentedContinuation: true,
            ).endMark
        }
        if !canEndBlockMapping(at: cursor.current, minimumColumn: boundaryColumn) {
            _ = try expect("dedent") { $0.isDedent }
        }
        return PureYAML.Parsing.NodeResult(kind: .mapping, endMark: endMark)
    }

    mutating func parseMappingPairAndBoundaries(
        boundaryColumn: Int,
        includeIndentedContinuation: Bool,
        firstKeyProperties: PureYAML.Parsing.NodeProperties = .none,
    ) throws -> PureYAML.Parsing.NodeResult {
        let result = try parseMappingPair(firstKeyProperties: firstKeyProperties)
        try consumeMappingSiblingBoundaryDedents(minimumColumn: boundaryColumn)
        try consumeMappingSiblingBoundaryReindent(minimumColumn: boundaryColumn)
        if includeIndentedContinuation {
            try consumeDedents(atOrAbove: boundaryColumn)
            try consumeMappingSiblingBoundaryDedents(minimumColumn: boundaryColumn)
            try consumeMappingSiblingBoundaryReindent(minimumColumn: boundaryColumn)
        }
        return result
    }

    mutating func parseBlockScalar(
        properties: PureYAML.Parsing.NodeProperties,
    ) throws -> PureYAML.Parsing.NodeResult {
        let header = try expect("block scalar header") { kind in
            if case .blockScalarHeader = kind {
                return true
            }
            return false
        }
        var lines: [String] = []
        var endMark = header.endMark
        if cursor.current?.kind.isIndent == true {
            _ = try expect("indent") { $0.isIndent }
            while cursor.current != nil, cursor.current?.kind.isDedent != true {
                guard case let .scalar(value, style) = cursor.current?.kind else {
                    throw unexpectedToken(expected: "block scalar content")
                }
                let token = try cursor.advance() ?? header
                try lines.append(decodeScalar(value, style: style, line: token.mark.line))
                endMark = token.endMark
            }
            _ = try expect("dedent") { $0.isDedent }
        }

        let value = blockScalarValue(lines: lines, header: header.kind)
        let mark = properties.mark ?? header.mark
        try emit(.scalar(
            value: value,
            anchor: properties.anchor,
            tag: properties.tag,
            style: scalarStyle(for: header.kind),
            mark: mark,
        ))
        return PureYAML.Parsing.NodeResult(kind: .scalar, endMark: endMark)
    }

    func scalarStyle(for kind: PureYAML.Parsing.TokenKind) -> PureYAML.Parsing.ScalarStyle {
        switch kind {
        case .blockScalarHeader(.folded, _):
            .folded
        case .blockScalarHeader(.literal, _):
            .literal
        default:
            .plain
        }
    }

    func blockScalarValue(
        lines: [String],
        header: PureYAML.Parsing.TokenKind,
    ) -> String {
        let (style, chomping): (PureYAML.Parsing.BlockScalarStyle, PureYAML.Parsing.BlockScalarChomping)
        switch header {
        case let .blockScalarHeader(headerStyle, headerChomping):
            style = headerStyle
            chomping = headerChomping
        default:
            style = .literal
            chomping = .clip
        }

        // Each content line carries its own line break. The folded style folds a
        // single break between two non-empty lines into a space, while blank
        // lines stay as line breaks. Chomping then trims the trailing breaks.
        // For content with no blank lines (the only shape the scanner captures
        // today) this is identical to the previous join-and-append behavior.
        let body: String = switch style {
        case .literal:
            lines.map { $0 + "\n" }.joined()
        case .folded:
            foldedBlockBody(lines)
        }
        return chompedBlockScalar(body, chomping: chomping)
    }

    func foldedBlockBody(_ lines: [String]) -> String {
        guard let firstContent = lines.firstIndex(where: { !$0.isEmpty }) else {
            // The block is entirely blank lines: one break each, no folding.
            return String(repeating: "\n", count: lines.count)
        }
        // Leading blank lines each contribute a line break before the content.
        var result = String(repeating: "\n", count: firstContent) + lines[firstContent]
        var index = firstContent + 1
        while index < lines.count {
            if lines[index].isEmpty {
                // Between two non-more-indented lines a blank run of N folds to N line breaks: the
                // first break is the absorbed fold, the rest are literal. Next to a more-indented line
                // nothing folds, so the absorbed break survives too and the run yields N+1 breaks.
                let beforeMoreIndented = isMoreIndentedBlockLine(lines[index - 1])
                var blankCount = 0
                while index < lines.count, lines[index].isEmpty {
                    blankCount += 1
                    index += 1
                }
                let afterMoreIndented = index < lines.count && isMoreIndentedBlockLine(lines[index])
                let breaks = beforeMoreIndented || afterMoreIndented ? blankCount + 1 : blankCount
                result += String(repeating: "\n", count: breaks)
                if index < lines.count {
                    result += lines[index]
                    index += 1
                }
            } else if isMoreIndentedBlockLine(lines[index]) || isMoreIndentedBlockLine(lines[index - 1]) {
                // A line break adjacent to a more-indented line is kept literal,
                // so the more-indented line and its breaks survive folding.
                result += "\n" + lines[index]
                index += 1
            } else {
                result += " " + lines[index]
                index += 1
            }
        }
        return result + "\n"
    }

    func isMoreIndentedBlockLine(_ line: String) -> Bool {
        // In a block scalar the scanner strips content lines to the block's
        // indentation, so only a more-indented line retains leading whitespace.
        line.first == " " || line.first == "\t"
    }

    func chompedBlockScalar(
        _ body: String,
        chomping: PureYAML.Parsing.BlockScalarChomping,
    ) -> String {
        switch chomping {
        case .keep:
            return body
        case .strip:
            return blockScalarTrimmingTrailingNewlines(body)
        case .clip:
            let trimmed = blockScalarTrimmingTrailingNewlines(body)
            return trimmed.isEmpty ? "" : trimmed + "\n"
        }
    }

    func blockScalarTrimmingTrailingNewlines(_ value: String) -> String {
        var result = value
        while result.hasSuffix("\n") {
            result.removeLast()
        }
        return result
    }
}