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

    /// Port of `yaml_parser_scan_to_next_token`: skip a leading BOM, spaces and
    /// tabs, a comment, and a single line break, which re-allows a simple key in
    /// the block context. A tab is valid in-line separation, so it is skipped
    /// here; `tabbedSeparation` records when one preceded the token so a block
    /// indicator or implicit key (where the tab would be indentation) can reject.
    mutating func scanToNextToken() throws {
        crossedLineBreak = false
        flowLeadingTab = false
        while true {
            tabbedSeparation = false
            if reader.column == 0, reader.peek() == "\u{FEFF}" {
                reader.skip()
            }
            while reader.peek() == " " || Classify.isTab(reader.peek()) {
                if Classify.isTab(reader.peek()) {
                    noteLeadingTab()
                }
                reader.skip()
            }
            // YAML 1.2: a line containing only whitespace is blank even when that
            // whitespace includes tabs. libyaml leaves a leading tab in place and
            // later rejects it; instead consume the trailing blanks so the line
            // break below is reached and the blank line is skipped.
            if blankLineAhead() {
                while reader.peek() == " " || Classify.isTab(reader.peek()) {
                    reader.skip()
                }
            }
            if reader.peek() == "#" {
                try consumeComment()
            }
            if Classify.isBreak(reader.peek()) {
                reader.skipLine()
                crossedLineBreak = true
                if flowLevel == 0 {
                    simpleKeyAllowed = true
                }
            } else {
                break
            }
        }
    }

    /// Record what a tab in the leading whitespace implies. In block context with a
    /// simple key allowed it is potential indentation before an indicator. In flow
    /// context, below the continuation indent on a non-blank line it is indentation
    /// abuse (a tab is never indentation); at or above the indent it is separation.
    private mutating func noteLeadingTab() {
        if flowLevel == 0, simpleKeyAllowed {
            tabbedSeparation = true
        }
        if flowLevel > 0, crossedLineBreak, reader.column < indentEngine.indent + 1, !blankLineAhead() {
            flowLeadingTab = true
        }
    }

    /// Consume a comment to the end of the line. A comment must be preceded by
    /// whitespace or start the line; a `#` adjacent to the previous token (e.g.
    /// `"x"#`, `]#`, `,#`) is not a comment. The plain-scalar scanner consumes its
    /// own trailing blanks, so the character just before the `#` is the reliable
    /// signal. libyaml accepts the adjacent form.
    private mutating func consumeComment() throws {
        if reader.column > 0, let previous = reader.peek(-1), !Classify.isBlank(previous) {
            throw ScannerError(
                context: "while scanning for the next token",
                problem: "found a comment without a preceding space",
                mark: reader.mark,
            )
        }
        while !Classify.isBreakZ(reader.peek()) {
            reader.skip()
        }
    }

    /// Whether the rest of the current line, from the current position, is only
    /// spaces and tabs up to a line break or end of stream (a blank line).
    mutating func blankLineAhead() -> Bool {
        var offset = 0
        while let character = reader.peek(offset), character == " " || Classify.isTab(character) {
            offset += 1
        }
        return Classify.isBreakZ(reader.peek(offset))
    }
}