/*!
Defines an abstract syntax for regular expressions.
*/

use core::cmp::Ordering;

use alloc::{boxed::Box, string::String, vec, vec::Vec};

pub use crate::ast::visitor::{visit, Visitor};

pub mod parse;
pub mod print;
mod visitor;

/// An error that occurred while parsing a regular expression into an abstract
/// syntax tree.
///
/// Note that not all ASTs represents a valid regular expression. For example,
/// an AST is constructed without error for `\p{Quux}`, but `Quux` is not a
/// valid Unicode property name. That particular error is reported when
/// translating an AST to the high-level intermediate representation (`HIR`).
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub struct Error {
    /// The kind of error.
    kind: ErrorKind,
    /// The original pattern that the parser generated the error from. Every
    /// span in an error is a valid range into this string.
    pattern: String,
    /// The span of this error.
    span: Span,
}

impl Error {
    /// Return the type of this error.
    pub fn kind(&self) -> &ErrorKind {
        &self.kind
    }

    /// The original pattern string in which this error occurred.
    ///
    /// Every span reported by this error is reported in terms of this string.
    pub fn pattern(&self) -> &str {
        &self.pattern
    }

    /// Return the span at which this error occurred.
    pub fn span(&self) -> &Span {
        &self.span
    }

    /// Return an auxiliary span. This span exists only for some errors that
    /// benefit from being able to point to two locations in the original
    /// regular expression. For example, "duplicate" errors will have the
    /// main error position set to the duplicate occurrence while its
    /// auxiliary span will be set to the initial occurrence.
    pub fn auxiliary_span(&self) -> Option<&Span> {
        use self::ErrorKind::*;
        match self.kind {
            FlagDuplicate { ref original } => Some(original),
            FlagRepeatedNegation { ref original, .. } => Some(original),
            GroupNameDuplicate { ref original, .. } => Some(original),
            _ => None,
        }
    }
}

/// The type of an error that occurred while building an AST.
///
/// This error type is marked as `non_exhaustive`. This means that adding a
/// new variant is not considered a breaking change.
#[non_exhaustive]
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub enum ErrorKind {
    /// The capturing group limit was exceeded.
    ///
    /// Note that this represents a limit on the total number of capturing
    /// groups in a regex and not necessarily the number of nested capturing
    /// groups. That is, the nest limit can be low and it is still possible for
    /// this error to occur.
    CaptureLimitExceeded,
    /// An invalid escape sequence was found in a character class set.
    ClassEscapeInvalid,
    /// An invalid character class range was found. An invalid range is any
    /// range where the start is greater than the end.
    ClassRangeInvalid,
    /// An invalid range boundary was found in a character class. Range
    /// boundaries must be a single literal codepoint, but this error indicates
    /// that something else was found, such as a nested class.
    ClassRangeLiteral,
    /// An opening `[` was found with no corresponding closing `]`.
    ClassUnclosed,
    /// Note that this error variant is no longer used. Namely, a decimal
    /// number can only appear as a repetition quantifier. When the number
    /// in a repetition quantifier is empty, then it gets its own specialized
    /// error, `RepetitionCountDecimalEmpty`.
    DecimalEmpty,
    /// An invalid decimal number was given where one was expected.
    DecimalInvalid,
    /// A bracketed hex literal was empty.
    EscapeHexEmpty,
    /// A bracketed hex literal did not correspond to a Unicode scalar value.
    EscapeHexInvalid,
    /// An invalid hexadecimal digit was found.
    EscapeHexInvalidDigit,
    /// EOF was found before an escape sequence was completed.
    EscapeUnexpectedEof,
    /// An unrecognized escape sequence.
    EscapeUnrecognized,
    /// A dangling negation was used when setting flags, e.g., `i-`.
    FlagDanglingNegation,
    /// A flag was used twice, e.g., `i-i`.
    FlagDuplicate {
        /// The position of the original flag. The error position
        /// points to the duplicate flag.
        original: Span,
    },
    /// The negation operator was used twice, e.g., `-i-s`.
    FlagRepeatedNegation {
        /// The position of the original negation operator. The error position
        /// points to the duplicate negation operator.
        original: Span,
    },
    /// Expected a flag but got EOF, e.g., `(?`.
    FlagUnexpectedEof,
    /// Unrecognized flag, e.g., `a`.
    FlagUnrecognized,
    /// A duplicate capture name was found.
    GroupNameDuplicate {
        /// The position of the initial occurrence of the capture name. The
        /// error position itself points to the duplicate occurrence.
        original: Span,
    },
    /// A capture group name is empty, e.g., `(?P<>abc)`.
    GroupNameEmpty,
    /// An invalid character was seen for a capture group name. This includes
    /// errors where the first character is a digit (even though subsequent
    /// characters are allowed to be digits).
    GroupNameInvalid,
    /// A closing `>` could not be found for a capture group name.
    GroupNameUnexpectedEof,
    /// An unclosed group, e.g., `(ab`.
    ///
    /// The span of this error corresponds to the unclosed parenthesis.
    GroupUnclosed,
    /// An unopened group, e.g., `ab)`.
    GroupUnopened,
    /// The nest limit was exceeded. The limit stored here is the limit
    /// configured in the parser.
    NestLimitExceeded(u32),
    /// The range provided in a counted repetition operator is invalid. The
    /// range is invalid if the start is greater than the end.
    RepetitionCountInvalid,
    /// An opening `{` was not followed by a valid decimal value.
    /// For example, `x{}` or `x{]}` would fail.
    RepetitionCountDecimalEmpty,
    /// An opening `{` was found with no corresponding closing `}`.
    RepetitionCountUnclosed,
    /// A repetition operator was applied to a missing sub-expression. This
    /// occurs, for example, in the regex consisting of just a `*` or even
    /// `(?i)*`. It is, however, possible to create a repetition operating on
    /// an empty sub-expression. For example, `()*` is still considered valid.
    RepetitionMissing,
    /// The special word boundary syntax, `\b{something}`, was used, but
    /// either EOF without `}` was seen, or an invalid character in the
    /// braces was seen.
    SpecialWordBoundaryUnclosed,
    /// The special word boundary syntax, `\b{something}`, was used, but
    /// `something` was not recognized as a valid word boundary kind.
    SpecialWordBoundaryUnrecognized,
    /// The syntax `\b{` was observed, but afterwards the end of the pattern
    /// was observed without being able to tell whether it was meant to be a
    /// bounded repetition on the `\b` or the beginning of a special word
    /// boundary assertion.
    SpecialWordOrRepetitionUnexpectedEof,
    /// The Unicode class is not valid. This typically occurs when a `\p` is
    /// followed by something other than a `{`.
    UnicodeClassInvalid,
    /// When octal support is disabled, this error is produced when an octal
    /// escape is used. The octal escape is assumed to be an invocation of
    /// a backreference, which is the common case.
    UnsupportedBackreference,
    /// When syntax similar to PCRE's look-around is used, this error is
    /// returned. Some example syntaxes that are rejected include, but are
    /// not necessarily limited to, `(?=re)`, `(?!re)`, `(?<=re)` and
    /// `(?<!re)`. Note that all of these syntaxes are otherwise invalid; this
    /// error is used to improve the user experience.
    UnsupportedLookAround,
}

#[cfg(feature = "std")]
impl std::error::Error for Error {}

impl core::fmt::Display for Error {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        crate::error::Formatter::from(self).fmt(f)
    }
}

impl core::fmt::Display for ErrorKind {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        use self::ErrorKind::*;
        match *self {
            CaptureLimitExceeded => write!(
                f,
                "exceeded the maximum number of \
                 capturing groups ({})",
                u32::MAX
            ),
            ClassEscapeInvalid => {
                write!(f, "invalid escape sequence found in character class")
            }
            ClassRangeInvalid => write!(
                f,
                "invalid character class range, \
                 the start must be <= the end"
            ),
            ClassRangeLiteral => {
                write!(f, "invalid range boundary, must be a literal")
            }
            ClassUnclosed => write!(f, "unclosed character class"),
            DecimalEmpty => write!(f, "decimal literal empty"),
            DecimalInvalid => write!(f, "decimal literal invalid"),
            EscapeHexEmpty => write!(f, "hexadecimal literal empty"),
            EscapeHexInvalid => {
                write!(f, "hexadecimal literal is not a Unicode scalar value")
            }
            EscapeHexInvalidDigit => write!(f, "invalid hexadecimal digit"),
            EscapeUnexpectedEof => write!(
                f,
                "incomplete escape sequence, \
                 reached end of pattern prematurely"
            ),
            EscapeUnrecognized => write!(f, "unrecognized escape sequence"),
            FlagDanglingNegation => {
                write!(f, "dangling flag negation operator")
            }
            FlagDuplicate { .. } => write!(f, "duplicate flag"),
            FlagRepeatedNegation { .. } => {
                write!(f, "flag negation operator repeated")
            }
            FlagUnexpectedEof => {
                write!(f, "expected flag but got end of regex")
            }
            FlagUnrecognized => write!(f, "unrecognized flag"),
            GroupNameDuplicate { .. } => {
                write!(f, "duplicate capture group name")
            }
            GroupNameEmpty => write!(f, "empty capture group name"),
            GroupNameInvalid => write!(f, "invalid capture group character"),
            GroupNameUnexpectedEof => write!(f, "unclosed capture group name"),
            GroupUnclosed => write!(f, "unclosed group"),
            GroupUnopened => write!(f, "unopened group"),
            NestLimitExceeded(limit) => write!(
                f,
                "exceed the maximum number of \
                 nested parentheses/brackets ({})",
                limit
            ),
            RepetitionCountInvalid => write!(
                f,
                "invalid repetition count range, \
                 the start must be <= the end"
            ),
            RepetitionCountDecimalEmpty => {
                write!(f, "repetition quantifier expects a valid decimal")
            }
            RepetitionCountUnclosed => {
                write!(f, "unclosed counted repetition")
            }
            RepetitionMissing => {
                write!(f, "repetition operator missing expression")
            }
            SpecialWordBoundaryUnclosed => {
                write!(
                    f,
                    "special word boundary assertion is either \
                     unclosed or contains an invalid character",
                )
            }
            SpecialWordBoundaryUnrecognized => {
                write!(
                    f,
                    "unrecognized special word boundary assertion, \
                     valid choices are: start, end, start-half \
                     or end-half",
                )
            }
            SpecialWordOrRepetitionUnexpectedEof => {
                write!(
                    f,
                    "found either the beginning of a special word \
                     boundary or a bounded repetition on a \\b with \
                     an opening brace, but no closing brace",
                )
            }
            UnicodeClassInvalid => {
                write!(f, "invalid Unicode character class")
            }
            UnsupportedBackreference => {
                write!(f, "backreferences are not supported")
            }
            UnsupportedLookAround => write!(
                f,
                "look-around, including look-ahead and look-behind, \
                 is not supported"
            ),
        }
    }
}

/// Span represents the position information of a single AST item.
///
/// All span positions are absolute byte offsets that can be used on the
/// original regular expression that was parsed.
#[derive(Clone, Copy, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub struct Span {
    /// The start byte offset.
    pub start: Position,
    /// The end byte offset.
    pub end: Position,
}

impl core::fmt::Debug for Span {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        write!(f, "Span({:?}, {:?})", self.start, self.end)
    }
}

impl Ord for Span {
    fn cmp(&self, other: &Span) -> Ordering {
        (&self.start, &self.end).cmp(&(&other.start, &other.end))
    }
}

impl PartialOrd for Span {
    fn partial_cmp(&self, other: &Span) -> Option<Ordering> {
        Some(self.cmp(other))
    }
}

/// A single position in a regular expression.
///
/// A position encodes one half of a span, and include the byte offset, line
/// number and column number.
#[derive(Clone, Copy, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub struct Position {
    /// The absolute offset of this position, starting at `0` from the
    /// beginning of the regular expression pattern string.
    pub offset: usize,
    /// The line number, starting at `1`.
    pub line: usize,
    /// The approximate column number, starting at `1`.
    pub column: usize,
}

impl core::fmt::Debug for Position {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        write!(
            f,
            "Position(o: {:?}, l: {:?}, c: {:?})",
            self.offset, self.line, self.column
        )
    }
}

impl Ord for Position {
    fn cmp(&self, other: &Position) -> Ordering {
        self.offset.cmp(&other.offset)
    }
}

impl PartialOrd for Position {
    fn partial_cmp(&self, other: &Position) -> Option<Ordering> {
        Some(self.cmp(other))
    }
}

impl Span {
    /// Create a new span with the given positions.
    pub fn new(start: Position, end: Position) -> Span {
        Span { start, end }
    }

    /// Create a new span using the given position as the start and end.
    pub fn splat(pos: Position) -> Span {
        Span::new(pos, pos)
    }

    /// Create a new span by replacing the starting the position with the one
    /// given.
    pub fn with_start(self, pos: Position) -> Span {
        Span { start: pos, ..self }
    }

    /// Create a new span by replacing the ending the position with the one
    /// given.
    pub fn with_end(self, pos: Position) -> Span {
        Span { end: pos, ..self }
    }

    /// Returns true if and only if this span occurs on a single line.
    pub fn is_one_line(&self) -> bool {
        self.start.line == self.end.line
    }

    /// Returns true if and only if this span is empty. That is, it points to
    /// a single position in the concrete syntax of a regular expression.
    pub fn is_empty(&self) -> bool {
        self.start.offset == self.end.offset
    }
}

impl Position {
    /// Create a new position with the given information.
    ///
    /// `offset` is the absolute offset of the position, starting at `0` from
    /// the beginning of the regular expression pattern string.
    ///
    /// `line` is the line number, starting at `1`.
    ///
    /// `column` is the approximate column number, starting at `1`.
    pub fn new(offset: usize, line: usize, column: usize) -> Position {
        Position { offset, line, column }
    }
}

/// An abstract syntax tree for a singular expression along with comments
/// found.
///
/// Comments are not stored in the tree itself to avoid complexity. Each
/// comment contains a span of precisely where it occurred in the original
/// regular expression.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub struct WithComments {
    /// The actual ast.
    pub ast: Ast,
    /// All comments found in the original regular expression.
    pub comments: Vec<Comment>,
}

/// A comment from a regular expression with an associated span.
///
/// A regular expression can only contain comments when the `x` flag is
/// enabled.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub struct Comment {
    /// The span of this comment, including the beginning `#` and ending `\n`.
    pub span: Span,
    /// The comment text, starting with the first character following the `#`
    /// and ending with the last character preceding the `\n`.
    pub comment: String,
}

/// An abstract syntax tree for a single regular expression.
///
/// An `Ast`'s `fmt::Display` implementation uses constant stack space and heap
/// space proportional to the size of the `Ast`.
///
/// This type defines its own destructor that uses constant stack space and
/// heap space proportional to the size of the `Ast`.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub enum Ast {
    /// An empty regex that matches everything.
    Empty(Box<Span>),
    /// A set of flags, e.g., `(?is)`.
    Flags(Box<SetFlags>),
    /// A single character literal, which includes escape sequences.
    Literal(Box<Literal>),
    /// The "any character" class.
    Dot(Box<Span>),
    /// A single zero-width assertion.
    Assertion(Box<Assertion>),
    /// A single Unicode character class, e.g., `\pL` or `\p{Greek}`.
    ClassUnicode(Box<ClassUnicode>),
    /// A single perl character class, e.g., `\d` or `\W`.
    ClassPerl(Box<ClassPerl>),
    /// A single bracketed character class set, which may contain zero or more
    /// character ranges and/or zero or more nested classes. e.g.,
    /// `[a-zA-Z\pL]`.
    ClassBracketed(Box<ClassBracketed>),
    /// A repetition operator applied to an arbitrary regular expression.
    Repetition(Box<Repetition>),
    /// A grouped regular expression.
    Group(Box<Group>),
    /// An alternation of regular expressions.
    Alternation(Box<Alternation>),
    /// A concatenation of regular expressions.
    Concat(Box<Concat>),
}

impl Ast {
    /// Create an "empty" AST item.
    pub fn empty(span: Span) -> Ast {
        Ast::Empty(Box::new(span))
    }

    /// Create a "flags" AST item.
    pub fn flags(e: SetFlags) -> Ast {
        Ast::Flags(Box::new(e))
    }

    /// Create a "literal" AST item.
    pub fn literal(e: Literal) -> Ast {
        Ast::Literal(Box::new(e))
    }

    /// Create a "dot" AST item.
    pub fn dot(span: Span) -> Ast {
        Ast::Dot(Box::new(span))
    }

    /// Create a "assertion" AST item.
    pub fn assertion(e: Assertion) -> Ast {
        Ast::Assertion(Box::new(e))
    }

    /// Create a "Unicode class" AST item.
    pub fn class_unicode(e: ClassUnicode) -> Ast {
        Ast::ClassUnicode(Box::new(e))
    }

    /// Create a "Perl class" AST item.
    pub fn class_perl(e: ClassPerl) -> Ast {
        Ast::ClassPerl(Box::new(e))
    }

    /// Create a "bracketed class" AST item.
    pub fn class_bracketed(e: ClassBracketed) -> Ast {
        Ast::ClassBracketed(Box::new(e))
    }

    /// Create a "repetition" AST item.
    pub fn repetition(e: Repetition) -> Ast {
        Ast::Repetition(Box::new(e))
    }

    /// Create a "group" AST item.
    pub fn group(e: Group) -> Ast {
        Ast::Group(Box::new(e))
    }

    /// Create a "alternation" AST item.
    pub fn alternation(e: Alternation) -> Ast {
        Ast::Alternation(Box::new(e))
    }

    /// Create a "concat" AST item.
    pub fn concat(e: Concat) -> Ast {
        Ast::Concat(Box::new(e))
    }

    /// Return the span of this abstract syntax tree.
    pub fn span(&self) -> &Span {
        match *self {
            Ast::Empty(ref span) => span,
            Ast::Flags(ref x) => &x.span,
            Ast::Literal(ref x) => &x.span,
            Ast::Dot(ref span) => span,
            Ast::Assertion(ref x) => &x.span,
            Ast::ClassUnicode(ref x) => &x.span,
            Ast::ClassPerl(ref x) => &x.span,
            Ast::ClassBracketed(ref x) => &x.span,
            Ast::Repetition(ref x) => &x.span,
            Ast::Group(ref x) => &x.span,
            Ast::Alternation(ref x) => &x.span,
            Ast::Concat(ref x) => &x.span,
        }
    }

    /// Return true if and only if this Ast is empty.
    pub fn is_empty(&self) -> bool {
        match *self {
            Ast::Empty(_) => true,
            _ => false,
        }
    }

    /// Returns true if and only if this AST has any (including possibly empty)
    /// subexpressions.
    fn has_subexprs(&self) -> bool {
        match *self {
            Ast::Empty(_)
            | Ast::Flags(_)
            | Ast::Literal(_)
            | Ast::Dot(_)
            | Ast::Assertion(_)
            | Ast::ClassUnicode(_)
            | Ast::ClassPerl(_) => false,
            Ast::ClassBracketed(_)
            | Ast::Repetition(_)
            | Ast::Group(_)
            | Ast::Alternation(_)
            | Ast::Concat(_) => true,
        }
    }
}

/// Print a display representation of this Ast.
///
/// This does not preserve any of the original whitespace formatting that may
/// have originally been present in the concrete syntax from which this Ast
/// was generated.
///
/// This implementation uses constant stack space and heap space proportional
/// to the size of the `Ast`.
impl core::fmt::Display for Ast {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        use crate::ast::print::Printer;
        Printer::new().print(self, f)
    }
}

/// An alternation of regular expressions.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub struct Alternation {
    /// The span of this alternation.
    pub span: Span,
    /// The alternate regular expressions.
    pub asts: Vec<Ast>,
}

impl Alternation {
    /// Return this alternation as an AST.
    ///
    /// If this alternation contains zero ASTs, then `Ast::empty` is returned.
    /// If this alternation contains exactly 1 AST, then the corresponding AST
    /// is returned. Otherwise, `Ast::alternation` is returned.
    pub fn into_ast(mut self) -> Ast {
        match self.asts.len() {
            0 => Ast::empty(self.span),
            1 => self.asts.pop().unwrap(),
            _ => Ast::alternation(self),
        }
    }
}

/// A concatenation of regular expressions.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub struct Concat {
    /// The span of this concatenation.
    pub span: Span,
    /// The concatenation regular expressions.
    pub asts: Vec<Ast>,
}

impl Concat {
    /// Return this concatenation as an AST.
    ///
    /// If this alternation contains zero ASTs, then `Ast::empty` is returned.
    /// If this alternation contains exactly 1 AST, then the corresponding AST
    /// is returned. Otherwise, `Ast::concat` is returned.
    pub fn into_ast(mut self) -> Ast {
        match self.asts.len() {
            0 => Ast::empty(self.span),
            1 => self.asts.pop().unwrap(),
            _ => Ast::concat(self),
        }
    }
}

/// A single literal expression.
///
/// A literal corresponds to a single Unicode scalar value. Literals may be
/// represented in their literal form, e.g., `a` or in their escaped form,
/// e.g., `\x61`.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub struct Literal {
    /// The span of this literal.
    pub span: Span,
    /// The kind of this literal.
    pub kind: LiteralKind,
    /// The Unicode scalar value corresponding to this literal.
    pub c: char,
}

impl Literal {
    /// If this literal was written as a `\x` hex escape, then this returns
    /// the corresponding byte value. Otherwise, this returns `None`.
    pub fn byte(&self) -> Option<u8> {
        match self.kind {
            LiteralKind::HexFixed(HexLiteralKind::X) => {
                u8::try_from(self.c).ok()
            }
            _ => None,
        }
    }
}

/// The kind of a single literal expression.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub enum LiteralKind {
    /// The literal is written verbatim, e.g., `a` or `☃`.
    Verbatim,
    /// The literal is written as an escape because it is otherwise a special
    /// regex meta character, e.g., `\*` or `\[`.
    Meta,
    /// The literal is written as an escape despite the fact that the escape is
    /// unnecessary, e.g., `\%` or `\/`.
    Superfluous,
    /// The literal is written as an octal escape, e.g., `\141`.
    Octal,
    /// The literal is written as a hex code with a fixed number of digits
    /// depending on the type of the escape, e.g., `\x61` or `\u0061` or
    /// `\U00000061`.
    HexFixed(HexLiteralKind),
    /// The literal is written as a hex code with a bracketed number of
    /// digits. The only restriction is that the bracketed hex code must refer
    /// to a valid Unicode scalar value.
    HexBrace(HexLiteralKind),
    /// The literal is written as a specially recognized escape, e.g., `\f`
    /// or `\n`.
    Special(SpecialLiteralKind),
}

/// The type of a special literal.
///
/// A special literal is a special escape sequence recognized by the regex
/// parser, e.g., `\f` or `\n`.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub enum SpecialLiteralKind {
    /// Bell, spelled `\a` (`\x07`).
    Bell,
    /// Form feed, spelled `\f` (`\x0C`).
    FormFeed,
    /// Tab, spelled `\t` (`\x09`).
    Tab,
    /// Line feed, spelled `\n` (`\x0A`).
    LineFeed,
    /// Carriage return, spelled `\r` (`\x0D`).
    CarriageReturn,
    /// Vertical tab, spelled `\v` (`\x0B`).
    VerticalTab,
    /// Space, spelled `\ ` (`\x20`). Note that this can only appear when
    /// parsing in verbose mode.
    Space,
}

/// The type of a Unicode hex literal.
///
/// Note that all variants behave the same when used with brackets. They only
/// differ when used without brackets in the number of hex digits that must
/// follow.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub enum HexLiteralKind {
    /// A `\x` prefix. When used without brackets, this form is limited to
    /// two digits.
    X,
    /// A `\u` prefix. When used without brackets, this form is limited to
    /// four digits.
    UnicodeShort,
    /// A `\U` prefix. When used without brackets, this form is limited to
    /// eight digits.
    UnicodeLong,
}

impl HexLiteralKind {
    /// The number of digits that must be used with this literal form when
    /// used without brackets. When used with brackets, there is no
    /// restriction on the number of digits.
    pub fn digits(&self) -> u32 {
        match *self {
            HexLiteralKind::X => 2,
            HexLiteralKind::UnicodeShort => 4,
            HexLiteralKind::UnicodeLong => 8,
        }
    }
}

/// A Perl character class.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub struct ClassPerl {
    /// The span of this class.
    pub span: Span,
    /// The kind of Perl class.
    pub kind: ClassPerlKind,
    /// Whether the class is negated or not. e.g., `\d` is not negated but
    /// `\D` is.
    pub negated: bool,
}

/// The available Perl character classes.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub enum ClassPerlKind {
    /// Decimal numbers.
    Digit,
    /// Whitespace.
    Space,
    /// Word characters.
    Word,
}

/// An ASCII character class.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub struct ClassAscii {
    /// The span of this class.
    pub span: Span,
    /// The kind of ASCII class.
    pub kind: ClassAsciiKind,
    /// Whether the class is negated or not. e.g., `[[:alpha:]]` is not negated
    /// but `[[:^alpha:]]` is.
    pub negated: bool,
}

/// The available ASCII character classes.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub enum ClassAsciiKind {
    /// `[0-9A-Za-z]`
    Alnum,
    /// `[A-Za-z]`
    Alpha,
    /// `[\x00-\x7F]`
    Ascii,
    /// `[ \t]`
    Blank,
    /// `[\x00-\x1F\x7F]`
    Cntrl,
    /// `[0-9]`
    Digit,
    /// `[!-~]`
    Graph,
    /// `[a-z]`
    Lower,
    /// `[ -~]`
    Print,
    /// `[!-/:-@\[-`{-~]`
    Punct,
    /// `[\t\n\v\f\r ]`
    Space,
    /// `[A-Z]`
    Upper,
    /// `[0-9A-Za-z_]`
    Word,
    /// `[0-9A-Fa-f]`
    Xdigit,
}

impl ClassAsciiKind {
    /// Return the corresponding ClassAsciiKind variant for the given name.
    ///
    /// The name given should correspond to the lowercase version of the
    /// variant name. e.g., `cntrl` is the name for `ClassAsciiKind::Cntrl`.
    ///
    /// If no variant with the corresponding name exists, then `None` is
    /// returned.
    pub fn from_name(name: &str) -> Option<ClassAsciiKind> {
        use self::ClassAsciiKind::*;
        match name {
            "alnum" => Some(Alnum),
            "alpha" => Some(Alpha),
            "ascii" => Some(Ascii),
            "blank" => Some(Blank),
            "cntrl" => Some(Cntrl),
            "digit" => Some(Digit),
            "graph" => Some(Graph),
            "lower" => Some(Lower),
            "print" => Some(Print),
            "punct" => Some(Punct),
            "space" => Some(Space),
            "upper" => Some(Upper),
            "word" => Some(Word),
            "xdigit" => Some(Xdigit),
            _ => None,
        }
    }
}

/// A Unicode character class.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub struct ClassUnicode {
    /// The span of this class.
    pub span: Span,
    /// Whether this class is negated or not.
    ///
    /// Note: be careful when using this attribute. This specifically refers
    /// to whether the class is written as `\p` or `\P`, where the latter
    /// is `negated = true`. However, it also possible to write something like
    /// `\P{scx!=Katakana}` which is actually equivalent to
    /// `\p{scx=Katakana}` and is therefore not actually negated even though
    /// `negated = true` here. To test whether this class is truly negated
    /// or not, use the `is_negated` method.
    pub negated: bool,
    /// The kind of Unicode class.
    pub kind: ClassUnicodeKind,
}

impl ClassUnicode {
    /// Returns true if this class has been negated.
    ///
    /// Note that this takes the Unicode op into account, if it's present.
    /// e.g., `is_negated` for `\P{scx!=Katakana}` will return `false`.
    pub fn is_negated(&self) -> bool {
        match self.kind {
            ClassUnicodeKind::NamedValue {
                op: ClassUnicodeOpKind::NotEqual,
                ..
            } => !self.negated,
            _ => self.negated,
        }
    }
}

/// The available forms of Unicode character classes.
#[derive(Clone, Debug, Eq, PartialEq)]
pub enum ClassUnicodeKind {
    /// A one letter abbreviated class, e.g., `\pN`.
    OneLetter(char),
    /// A binary property, general category or script. The string may be
    /// empty.
    Named(String),
    /// A property name and an associated value.
    NamedValue {
        /// The type of Unicode op used to associate `name` with `value`.
        op: ClassUnicodeOpKind,
        /// The property name (which may be empty).
        name: String,
        /// The property value (which may be empty).
        value: String,
    },
}

#[cfg(feature = "arbitrary")]
impl arbitrary::Arbitrary<'_> for ClassUnicodeKind {
    fn arbitrary(
        u: &mut arbitrary::Unstructured,
    ) -> arbitrary::Result<ClassUnicodeKind> {
        #[cfg(any(
            feature = "unicode-age",
            feature = "unicode-bool",
            feature = "unicode-gencat",
            feature = "unicode-perl",
            feature = "unicode-script",
            feature = "unicode-segment",
        ))]
        {
            use alloc::string::ToString;

            use super::unicode_tables::{
                property_names::PROPERTY_NAMES,
                property_values::PROPERTY_VALUES,
            };

            match u.choose_index(3)? {
                0 => {
                    let all = PROPERTY_VALUES
                        .iter()
                        .flat_map(|e| e.1.iter())
                        .filter(|(name, _)| name.len() == 1)
                        .count();
                    let idx = u.choose_index(all)?;
                    let value = PROPERTY_VALUES
                        .iter()
                        .flat_map(|e| e.1.iter())
                        .take(idx + 1)
                        .last()
                        .unwrap()
                        .0
                        .chars()
                        .next()
                        .unwrap();
                    Ok(ClassUnicodeKind::OneLetter(value))
                }
                1 => {
                    let all = PROPERTY_VALUES
                        .iter()
                        .map(|e| e.1.len())
                        .sum::<usize>()
                        + PROPERTY_NAMES.len();
                    let idx = u.choose_index(all)?;
                    let name = PROPERTY_VALUES
                        .iter()
                        .flat_map(|e| e.1.iter())
                        .chain(PROPERTY_NAMES)
                        .map(|(_, e)| e)
                        .take(idx + 1)
                        .last()
                        .unwrap();
                    Ok(ClassUnicodeKind::Named(name.to_string()))
                }
                2 => {
                    let all = PROPERTY_VALUES
                        .iter()
                        .map(|e| e.1.len())
                        .sum::<usize>();
                    let idx = u.choose_index(all)?;
                    let (prop, value) = PROPERTY_VALUES
                        .iter()
                        .flat_map(|e| {
                            e.1.iter().map(|(_, value)| (e.0, value))
                        })
                        .take(idx + 1)
                        .last()
                        .unwrap();
                    Ok(ClassUnicodeKind::NamedValue {
                        op: u.arbitrary()?,
                        name: prop.to_string(),
                        value: value.to_string(),
                    })
                }
                _ => unreachable!("index chosen is impossible"),
            }
        }
        #[cfg(not(any(
            feature = "unicode-age",
            feature = "unicode-bool",
            feature = "unicode-gencat",
            feature = "unicode-perl",
            feature = "unicode-script",
            feature = "unicode-segment",
        )))]
        {
            match u.choose_index(3)? {
                0 => Ok(ClassUnicodeKind::OneLetter(u.arbitrary()?)),
                1 => Ok(ClassUnicodeKind::Named(u.arbitrary()?)),
                2 => Ok(ClassUnicodeKind::NamedValue {
                    op: u.arbitrary()?,
                    name: u.arbitrary()?,
                    value: u.arbitrary()?,
                }),
                _ => unreachable!("index chosen is impossible"),
            }
        }
    }

    fn size_hint(depth: usize) -> (usize, Option<usize>) {
        #[cfg(any(
            feature = "unicode-age",
            feature = "unicode-bool",
            feature = "unicode-gencat",
            feature = "unicode-perl",
            feature = "unicode-script",
            feature = "unicode-segment",
        ))]
        {
            arbitrary::size_hint::and_all(&[
                usize::size_hint(depth),
                usize::size_hint(depth),
                arbitrary::size_hint::or(
                    (0, Some(0)),
                    ClassUnicodeOpKind::size_hint(depth),
                ),
            ])
        }
        #[cfg(not(any(
            feature = "unicode-age",
            feature = "unicode-bool",
            feature = "unicode-gencat",
            feature = "unicode-perl",
            feature = "unicode-script",
            feature = "unicode-segment",
        )))]
        {
            arbitrary::size_hint::and(
                usize::size_hint(depth),
                arbitrary::size_hint::or_all(&[
                    char::size_hint(depth),
                    String::size_hint(depth),
                    arbitrary::size_hint::and_all(&[
                        String::size_hint(depth),
                        String::size_hint(depth),
                        ClassUnicodeOpKind::size_hint(depth),
                    ]),
                ]),
            )
        }
    }
}

/// The type of op used in a Unicode character class.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub enum ClassUnicodeOpKind {
    /// A property set to a specific value, e.g., `\p{scx=Katakana}`.
    Equal,
    /// A property set to a specific value using a colon, e.g.,
    /// `\p{scx:Katakana}`.
    Colon,
    /// A property that isn't a particular value, e.g., `\p{scx!=Katakana}`.
    NotEqual,
}

impl ClassUnicodeOpKind {
    /// Whether the op is an equality op or not.
    pub fn is_equal(&self) -> bool {
        match *self {
            ClassUnicodeOpKind::Equal | ClassUnicodeOpKind::Colon => true,
            _ => false,
        }
    }
}

/// A bracketed character class, e.g., `[a-z0-9]`.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub struct ClassBracketed {
    /// The span of this class.
    pub span: Span,
    /// Whether this class is negated or not. e.g., `[a]` is not negated but
    /// `[^a]` is.
    pub negated: bool,
    /// The type of this set. A set is either a normal union of things, e.g.,
    /// `[abc]` or a result of applying set operations, e.g., `[\pL--c]`.
    pub kind: ClassSet,
}

/// A character class set.
///
/// This type corresponds to the internal structure of a bracketed character
/// class. That is, every bracketed character is one of two types: a union of
/// items (literals, ranges, other bracketed classes) or a tree of binary set
/// operations.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub enum ClassSet {
    /// An item, which can be a single literal, range, nested character class
    /// or a union of items.
    Item(ClassSetItem),
    /// A single binary operation (i.e., &&, -- or ~~).
    BinaryOp(ClassSetBinaryOp),
}

impl ClassSet {
    /// Build a set from a union.
    pub fn union(ast: ClassSetUnion) -> ClassSet {
        ClassSet::Item(ClassSetItem::Union(ast))
    }

    /// Return the span of this character class set.
    pub fn span(&self) -> &Span {
        match *self {
            ClassSet::Item(ref x) => x.span(),
            ClassSet::BinaryOp(ref x) => &x.span,
        }
    }

    /// Return true if and only if this class set is empty.
    fn is_empty(&self) -> bool {
        match *self {
            ClassSet::Item(ClassSetItem::Empty(_)) => true,
            _ => false,
        }
    }
}

/// A single component of a character class set.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub enum ClassSetItem {
    /// An empty item.
    ///
    /// Note that a bracketed character class cannot contain a single empty
    /// item. Empty items can appear when using one of the binary operators.
    /// For example, `[&&]` is the intersection of two empty classes.
    Empty(Span),
    /// A single literal.
    Literal(Literal),
    /// A range between two literals.
    Range(ClassSetRange),
    /// An ASCII character class, e.g., `[:alnum:]` or `[:punct:]`.
    Ascii(ClassAscii),
    /// A Unicode character class, e.g., `\pL` or `\p{Greek}`.
    Unicode(ClassUnicode),
    /// A perl character class, e.g., `\d` or `\W`.
    Perl(ClassPerl),
    /// A bracketed character class set, which may contain zero or more
    /// character ranges and/or zero or more nested classes. e.g.,
    /// `[a-zA-Z\pL]`.
    Bracketed(Box<ClassBracketed>),
    /// A union of items.
    Union(ClassSetUnion),
}

impl ClassSetItem {
    /// Return the span of this character class set item.
    pub fn span(&self) -> &Span {
        match *self {
            ClassSetItem::Empty(ref span) => span,
            ClassSetItem::Literal(ref x) => &x.span,
            ClassSetItem::Range(ref x) => &x.span,
            ClassSetItem::Ascii(ref x) => &x.span,
            ClassSetItem::Perl(ref x) => &x.span,
            ClassSetItem::Unicode(ref x) => &x.span,
            ClassSetItem::Bracketed(ref x) => &x.span,
            ClassSetItem::Union(ref x) => &x.span,
        }
    }
}

/// A single character class range in a set.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub struct ClassSetRange {
    /// The span of this range.
    pub span: Span,
    /// The start of this range.
    pub start: Literal,
    /// The end of this range.
    pub end: Literal,
}

impl ClassSetRange {
    /// Returns true if and only if this character class range is valid.
    ///
    /// The only case where a range is invalid is if its start is greater than
    /// its end.
    pub fn is_valid(&self) -> bool {
        self.start.c <= self.end.c
    }
}

/// A union of items inside a character class set.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub struct ClassSetUnion {
    /// The span of the items in this operation. e.g., the `a-z0-9` in
    /// `[^a-z0-9]`
    pub span: Span,
    /// The sequence of items that make up this union.
    pub items: Vec<ClassSetItem>,
}

impl ClassSetUnion {
    /// Push a new item in this union.
    ///
    /// The ending position of this union's span is updated to the ending
    /// position of the span of the item given. If the union is empty, then
    /// the starting position of this union is set to the starting position
    /// of this item.
    ///
    /// In other words, if you only use this method to add items to a union
    /// and you set the spans on each item correctly, then you should never
    /// need to adjust the span of the union directly.
    pub fn push(&mut self, item: ClassSetItem) {
        if self.items.is_empty() {
            self.span.start = item.span().start;
        }
        self.span.end = item.span().end;
        self.items.push(item);
    }

    /// Return this union as a character class set item.
    ///
    /// If this union contains zero items, then an empty union is
    /// returned. If this concatenation contains exactly 1 item, then the
    /// corresponding item is returned. Otherwise, ClassSetItem::Union is
    /// returned.
    pub fn into_item(mut self) -> ClassSetItem {
        match self.items.len() {
            0 => ClassSetItem::Empty(self.span),
            1 => self.items.pop().unwrap(),
            _ => ClassSetItem::Union(self),
        }
    }
}

/// A Unicode character class set operation.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub struct ClassSetBinaryOp {
    /// The span of this operation. e.g., the `a-z--[h-p]` in `[a-z--h-p]`.
    pub span: Span,
    /// The type of this set operation.
    pub kind: ClassSetBinaryOpKind,
    /// The left hand side of the operation.
    pub lhs: Box<ClassSet>,
    /// The right hand side of the operation.
    pub rhs: Box<ClassSet>,
}

/// The type of a Unicode character class set operation.
///
/// Note that this doesn't explicitly represent union since there is no
/// explicit union operator. Concatenation inside a character class corresponds
/// to the union operation.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub enum ClassSetBinaryOpKind {
    /// The intersection of two sets, e.g., `\pN&&[a-z]`.
    Intersection,
    /// The difference of two sets, e.g., `\pN--[0-9]`.
    Difference,
    /// The symmetric difference of two sets. The symmetric difference is the
    /// set of elements belonging to one but not both sets.
    /// e.g., `[\pL~~[:ascii:]]`.
    SymmetricDifference,
}

/// A single zero-width assertion.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub struct Assertion {
    /// The span of this assertion.
    pub span: Span,
    /// The assertion kind, e.g., `\b` or `^`.
    pub kind: AssertionKind,
}

/// An assertion kind.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub enum AssertionKind {
    /// `^`
    StartLine,
    /// `$`
    EndLine,
    /// `\A`
    StartText,
    /// `\z`
    EndText,
    /// `\b`
    WordBoundary,
    /// `\B`
    NotWordBoundary,
    /// `\b{start}`
    WordBoundaryStart,
    /// `\b{end}`
    WordBoundaryEnd,
    /// `\<` (alias for `\b{start}`)
    WordBoundaryStartAngle,
    /// `\>` (alias for `\b{end}`)
    WordBoundaryEndAngle,
    /// `\b{start-half}`
    WordBoundaryStartHalf,
    /// `\b{end-half}`
    WordBoundaryEndHalf,
}

/// A repetition operation applied to a regular expression.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub struct Repetition {
    /// The span of this operation.
    pub span: Span,
    /// The actual operation.
    pub op: RepetitionOp,
    /// Whether this operation was applied greedily or not.
    pub greedy: bool,
    /// The regular expression under repetition.
    pub ast: Box<Ast>,
}

/// The repetition operator itself.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub struct RepetitionOp {
    /// The span of this operator. This includes things like `+`, `*?` and
    /// `{m,n}`.
    pub span: Span,
    /// The type of operation.
    pub kind: RepetitionKind,
}

/// The kind of a repetition operator.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub enum RepetitionKind {
    /// `?`
    ZeroOrOne,
    /// `*`
    ZeroOrMore,
    /// `+`
    OneOrMore,
    /// `{m,n}`
    Range(RepetitionRange),
}

/// A range repetition operator.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub enum RepetitionRange {
    /// `{m}`
    Exactly(u32),
    /// `{m,}`
    AtLeast(u32),
    /// `{m,n}`
    Bounded(u32, u32),
}

impl RepetitionRange {
    /// Returns true if and only if this repetition range is valid.
    ///
    /// The only case where a repetition range is invalid is if it is bounded
    /// and its start is greater than its end.
    pub fn is_valid(&self) -> bool {
        match *self {
            RepetitionRange::Bounded(s, e) if s > e => false,
            _ => true,
        }
    }
}

/// A grouped regular expression.
///
/// This includes both capturing and non-capturing groups. This does **not**
/// include flag-only groups like `(?is)`, but does contain any group that
/// contains a sub-expression, e.g., `(a)`, `(?P<name>a)`, `(?:a)` and
/// `(?is:a)`.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub struct Group {
    /// The span of this group.
    pub span: Span,
    /// The kind of this group.
    pub kind: GroupKind,
    /// The regular expression in this group.
    pub ast: Box<Ast>,
}

impl Group {
    /// If this group is non-capturing, then this returns the (possibly empty)
    /// set of flags. Otherwise, `None` is returned.
    pub fn flags(&self) -> Option<&Flags> {
        match self.kind {
            GroupKind::NonCapturing(ref flags) => Some(flags),
            _ => None,
        }
    }

    /// Returns true if and only if this group is capturing.
    pub fn is_capturing(&self) -> bool {
        match self.kind {
            GroupKind::CaptureIndex(_) | GroupKind::CaptureName { .. } => true,
            GroupKind::NonCapturing(_) => false,
        }
    }

    /// Returns the capture index of this group, if this is a capturing group.
    ///
    /// This returns a capture index precisely when `is_capturing` is `true`.
    pub fn capture_index(&self) -> Option<u32> {
        match self.kind {
            GroupKind::CaptureIndex(i) => Some(i),
            GroupKind::CaptureName { ref name, .. } => Some(name.index),
            GroupKind::NonCapturing(_) => None,
        }
    }
}

/// The kind of a group.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub enum GroupKind {
    /// `(a)`
    CaptureIndex(u32),
    /// `(?<name>a)` or `(?P<name>a)`
    CaptureName {
        /// True if the `?P<` syntax is used and false if the `?<` syntax is used.
        starts_with_p: bool,
        /// The capture name.
        name: CaptureName,
    },
    /// `(?:a)` and `(?i:a)`
    NonCapturing(Flags),
}

/// A capture name.
///
/// This corresponds to the name itself between the angle brackets in, e.g.,
/// `(?P<foo>expr)`.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct CaptureName {
    /// The span of this capture name.
    pub span: Span,
    /// The capture name.
    pub name: String,
    /// The capture index.
    pub index: u32,
}

#[cfg(feature = "arbitrary")]
impl arbitrary::Arbitrary<'_> for CaptureName {
    fn arbitrary(
        u: &mut arbitrary::Unstructured,
    ) -> arbitrary::Result<CaptureName> {
        let len = u.arbitrary_len::<char>()?;
        if len == 0 {
            return Err(arbitrary::Error::NotEnoughData);
        }
        let mut name: String = String::new();
        for _ in 0..len {
            let ch: char = u.arbitrary()?;
            let cp = u32::from(ch);
            let ascii_letter_offset = u8::try_from(cp % 26).unwrap();
            let ascii_letter = b'a' + ascii_letter_offset;
            name.push(char::from(ascii_letter));
        }
        Ok(CaptureName { span: u.arbitrary()?, name, index: u.arbitrary()? })
    }

    fn size_hint(depth: usize) -> (usize, Option<usize>) {
        arbitrary::size_hint::and_all(&[
            Span::size_hint(depth),
            usize::size_hint(depth),
            u32::size_hint(depth),
        ])
    }
}

/// A group of flags that is not applied to a particular regular expression.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub struct SetFlags {
    /// The span of these flags, including the grouping parentheses.
    pub span: Span,
    /// The actual sequence of flags.
    pub flags: Flags,
}

/// A group of flags.
///
/// This corresponds only to the sequence of flags themselves, e.g., `is-u`.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub struct Flags {
    /// The span of this group of flags.
    pub span: Span,
    /// A sequence of flag items. Each item is either a flag or a negation
    /// operator.
    pub items: Vec<FlagsItem>,
}

impl Flags {
    /// Add the given item to this sequence of flags.
    ///
    /// If the item was added successfully, then `None` is returned. If the
    /// given item is a duplicate, then `Some(i)` is returned, where
    /// `items[i].kind == item.kind`.
    pub fn add_item(&mut self, item: FlagsItem) -> Option<usize> {
        for (i, x) in self.items.iter().enumerate() {
            if x.kind == item.kind {
                return Some(i);
            }
        }
        self.items.push(item);
        None
    }

    /// Returns the state of the given flag in this set.
    ///
    /// If the given flag is in the set but is negated, then `Some(false)` is
    /// returned.
    ///
    /// If the given flag is in the set and is not negated, then `Some(true)`
    /// is returned.
    ///
    /// Otherwise, `None` is returned.
    pub fn flag_state(&self, flag: Flag) -> Option<bool> {
        let mut negated = false;
        for x in &self.items {
            match x.kind {
                FlagsItemKind::Negation => {
                    negated = true;
                }
                FlagsItemKind::Flag(ref xflag) if xflag == &flag => {
                    return Some(!negated);
                }
                _ => {}
            }
        }
        None
    }
}

/// A single item in a group of flags.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub struct FlagsItem {
    /// The span of this item.
    pub span: Span,
    /// The kind of this item.
    pub kind: FlagsItemKind,
}

/// The kind of an item in a group of flags.
#[derive(Clone, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub enum FlagsItemKind {
    /// A negation operator applied to all subsequent flags in the enclosing
    /// group.
    Negation,
    /// A single flag in a group.
    Flag(Flag),
}

impl FlagsItemKind {
    /// Returns true if and only if this item is a negation operator.
    pub fn is_negation(&self) -> bool {
        match *self {
            FlagsItemKind::Negation => true,
            _ => false,
        }
    }
}

/// A single flag.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
#[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
pub enum Flag {
    /// `i`
    CaseInsensitive,
    /// `m`
    MultiLine,
    /// `s`
    DotMatchesNewLine,
    /// `U`
    SwapGreed,
    /// `u`
    Unicode,
    /// `R`
    CRLF,
    /// `x`
    IgnoreWhitespace,
}

/// A custom `Drop` impl is used for `Ast` such that it uses constant stack
/// space but heap space proportional to the depth of the `Ast`.
impl Drop for Ast {
    fn drop(&mut self) {
        use core::mem;

        match *self {
            Ast::Empty(_)
            | Ast::Flags(_)
            | Ast::Literal(_)
            | Ast::Dot(_)
            | Ast::Assertion(_)
            | Ast::ClassUnicode(_)
            | Ast::ClassPerl(_)
            // Bracketed classes are recursive, they get their own Drop impl.
            | Ast::ClassBracketed(_) => return,
            Ast::Repetition(ref x) if !x.ast.has_subexprs() => return,
            Ast::Group(ref x) if !x.ast.has_subexprs() => return,
            Ast::Alternation(ref x) if x.asts.is_empty() => return,
            Ast::Concat(ref x) if x.asts.is_empty() => return,
            _ => {}
        }

        let empty_span = || Span::splat(Position::new(0, 0, 0));
        let empty_ast = || Ast::empty(empty_span());
        let mut stack = vec![mem::replace(self, empty_ast())];
        while let Some(mut ast) = stack.pop() {
            match ast {
                Ast::Empty(_)
                | Ast::Flags(_)
                | Ast::Literal(_)
                | Ast::Dot(_)
                | Ast::Assertion(_)
                | Ast::ClassUnicode(_)
                | Ast::ClassPerl(_)
                // Bracketed classes are recursive, so they get their own Drop
                // impl.
                | Ast::ClassBracketed(_) => {}
                Ast::Repetition(ref mut x) => {
                    stack.push(mem::replace(&mut x.ast, empty_ast()));
                }
                Ast::Group(ref mut x) => {
                    stack.push(mem::replace(&mut x.ast, empty_ast()));
                }
                Ast::Alternation(ref mut x) => {
                    stack.extend(x.asts.drain(..));
                }
                Ast::Concat(ref mut x) => {
                    stack.extend(x.asts.drain(..));
                }
            }
        }
    }
}

/// A custom `Drop` impl is used for `ClassSet` such that it uses constant
/// stack space but heap space proportional to the depth of the `ClassSet`.
impl Drop for ClassSet {
    fn drop(&mut self) {
        use core::mem;

        match *self {
            ClassSet::Item(ref item) => match *item {
                ClassSetItem::Empty(_)
                | ClassSetItem::Literal(_)
                | ClassSetItem::Range(_)
                | ClassSetItem::Ascii(_)
                | ClassSetItem::Unicode(_)
                | ClassSetItem::Perl(_) => return,
                ClassSetItem::Bracketed(ref x) => {
                    if x.kind.is_empty() {
                        return;
                    }
                }
                ClassSetItem::Union(ref x) => {
                    if x.items.is_empty() {
                        return;
                    }
                }
            },
            ClassSet::BinaryOp(ref op) => {
                if op.lhs.is_empty() && op.rhs.is_empty() {
                    return;
                }
            }
        }

        let empty_span = || Span::splat(Position::new(0, 0, 0));
        let empty_set = || ClassSet::Item(ClassSetItem::Empty(empty_span()));
        let mut stack = vec![mem::replace(self, empty_set())];
        while let Some(mut set) = stack.pop() {
            match set {
                ClassSet::Item(ref mut item) => match *item {
                    ClassSetItem::Empty(_)
                    | ClassSetItem::Literal(_)
                    | ClassSetItem::Range(_)
                    | ClassSetItem::Ascii(_)
                    | ClassSetItem::Unicode(_)
                    | ClassSetItem::Perl(_) => {}
                    ClassSetItem::Bracketed(ref mut x) => {
                        stack.push(mem::replace(&mut x.kind, empty_set()));
                    }
                    ClassSetItem::Union(ref mut x) => {
                        stack.extend(x.items.drain(..).map(ClassSet::Item));
                    }
                },
                ClassSet::BinaryOp(ref mut op) => {
                    stack.push(mem::replace(&mut op.lhs, empty_set()));
                    stack.push(mem::replace(&mut op.rhs, empty_set()));
                }
            }
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    // We use a thread with an explicit stack size to test that our destructor
    // for Ast can handle arbitrarily sized expressions in constant stack
    // space. In case we run on a platform without threads (WASM?), we limit
    // this test to Windows/Unix.
    #[test]
    #[cfg(any(unix, windows))]
    fn no_stack_overflow_on_drop() {
        use std::thread;

        let run = || {
            let span = || Span::splat(Position::new(0, 0, 0));
            let mut ast = Ast::empty(span());
            for i in 0..200 {
                ast = Ast::group(Group {
                    span: span(),
                    kind: GroupKind::CaptureIndex(i),
                    ast: Box::new(ast),
                });
            }
            assert!(!ast.is_empty());
        };

        // We run our test on a thread with a small stack size so we can
        // force the issue more easily.
        //
        // NOTE(2023-03-21): It turns out that some platforms (like FreeBSD)
        // will just barf with very small stack sizes. So we bump this up a bit
        // to give more room to breath. When I did this, I confirmed that if
        // I remove the custom `Drop` impl for `Ast`, then this test does
        // indeed still fail with a stack overflow. (At the time of writing, I
        // had to bump it all the way up to 32K before the test would pass even
        // without the custom `Drop` impl. So 16K seems like a safe number
        // here.)
        //
        // See: https://github.com/rust-lang/regex/issues/967
        thread::Builder::new()
            .stack_size(16 << 10)
            .spawn(run)
            .unwrap()
            .join()
            .unwrap();
    }

    // This tests that our `Ast` has a reasonable size. This isn't a hard rule
    // and it can be increased if given a good enough reason. But this test
    // exists because the size of `Ast` was at one point over 200 bytes on a
    // 64-bit target. Wow.
    #[test]
    fn ast_size() {
        let max = 2 * core::mem::size_of::<usize>();
        let size = core::mem::size_of::<Ast>();
        assert!(
            size <= max,
            "Ast size of {} bytes is bigger than suggested max {}",
            size,
            max
        );
    }
}