extension PureYAML.Parsing.LibYAML.Scanner {
    private typealias ScannerError = PureYAML.Parsing.LibYAML.ScannerError

    /// Reject a flow-collection continuation line that is not indented past the
    /// enclosing block context. YAML requires `s-flow-line-prefix` to be more
    /// indented than the parent block; libyaml does not enforce it, so multi-line
    /// flow content in column 0 (`[a,\nb]` under a key) wrongly parses.
    mutating func checkFlowContinuationIndent() throws {
        guard flowLevel > 0, crossedLineBreak, let next = reader.peek() else {
            return
        }
        // A closing `]`/`}` (or a `,` separator) may sit at any indentation; only
        // flow CONTENT must be indented past the parent block.
        if next == "]" || next == "}" || next == "," {
            return
        }
        // A tab in the leading whitespace is never valid indentation, even where
        // spaces of the same width would be (the tab inflates the column).
        if reader.column <= indentEngine.indent || flowLeadingTab {
            throw ScannerError(
                context: "while parsing a flow node",
                problem: "found insufficiently indented line in a flow collection",
                mark: reader.mark,
            )
        }
    }

    /// Whether the last emitted token bars a block sequence from opening on the
    /// same line: a mapping value indicator or a node property (anchor/tag). After
    /// another block entry a compact sequence is allowed, so that is not barred.
    func lastTokenForbidsInlineSequence() -> Bool {
        switch indentEngine.lastToken?.type {
        case .anchor, .tag:
            true
        case .value:
            lastValueWasInline
        default:
            false
        }
    }

    /// The error raised when a tab is used where indentation is required (before a
    /// block indicator or an implicit mapping key). A tab is valid separation but
    /// never indentation.
    func tabIndentationError() -> PureYAML.Parsing.LibYAML.ScannerError {
        ScannerError(
            context: "while scanning for the next token",
            problem: "found a tab character where an indentation space is expected",
            mark: reader.mark,
        )
    }

    /// Whether this flow level's pending simple key is a JSON (quoted) node, which
    /// lets an adjacent `:` open its value without a separating space.
    var flowKeyIsJSON: Bool {
        let key = simpleKeys[simpleKeys.count - 1]
        return key.possible && key.isJSON
    }
}