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|
//! Thematic breaks, sometimes called horizontal rules, are a construct that
//! occurs in the [flow][] content type.
//!
//! They’re formed with the following BNF:
//!
//! ```bnf
//! ; Restriction: all markers must be identical.
//! ; Restriction: at least 3 markers must be used.
//! thematic_break ::= *space_or_tab 1*(1*marker *space_or_tab)
//!
//! space_or_tab ::= ' ' | '\t'
//! marker ::= '*' | '-' | '_'
//! ```
//!
//! Thematic breaks in markdown typically relate to the HTML element `<hr>`.
//! See [*§ 4.4.2 The `hr` element* in the HTML spec][html] for more info.
//!
//! It is recommended to use exactly three asterisks without whitespace when
//! writing markdown.
//! As using more than three markers has no effect other than wasting space,
//! it is recommended to use exactly three markers.
//! Thematic breaks formed with asterisks or dashes can interfere with lists
//! in if there is whitespace between them: `* * *` and `- - -`.
//! For these reasons, it is recommend to not use spaces or tabs between the
//! markers.
//! Thematic breaks formed with dashes (without whitespace) can also form
//! [heading (setext)][heading_setext].
//! As dashes and underscores frequently occur in natural language and URLs, it
//! is recommended to use asterisks for thematic breaks to distinguish from
//! such use.
//! Because asterisks can be used to form the most markdown constructs, using
//! them has the added benefit of making it easier to gloss over markdown: you
//! can look for asterisks to find syntax while not worrying about other
//! characters.
//!
//! ## Tokens
//!
//! * [`ThematicBreak`][TokenType::ThematicBreak]
//! * [`ThematicBreakSequence`][TokenType::ThematicBreakSequence]
//!
//! ## References
//!
//! * [`thematic-break.js` in `micromark`](https://github.com/micromark/micromark/blob/main/packages/micromark-core-commonmark/dev/lib/thematic-break.js)
//! * [*§ 4.1 Thematic breaks* in `CommonMark`](https://spec.commonmark.org/0.30/#thematic-breaks)
//!
//! [flow]: crate::content::flow
//! [heading_setext]: crate::construct::heading_setext
//! [html]: https://html.spec.whatwg.org/multipage/grouping-content.html#the-hr-element
//!
//! <!-- To do: link `lists` -->
use super::partial_space_or_tab::space_or_tab_opt;
use crate::constant::THEMATIC_BREAK_MARKER_COUNT_MIN;
use crate::tokenizer::{Code, State, StateFnResult, TokenType, Tokenizer};
/// Type of thematic break.
#[derive(Debug, PartialEq)]
enum Kind {
/// In a thematic break using asterisks (`*`).
///
/// ## Example
///
/// ```markdown
/// ***
/// ```
Asterisk,
/// In a thematic break using dashes (`-`).
///
/// ## Example
///
/// ```markdown
/// ---
/// ```
Dash,
/// In a thematic break using underscores (`_`).
///
/// ## Example
///
/// ```markdown
/// ___
/// ```
Underscore,
}
impl Kind {
/// Turn the kind into a [char].
fn as_char(&self) -> char {
match self {
Kind::Asterisk => '*',
Kind::Dash => '-',
Kind::Underscore => '_',
}
}
/// Turn a [char] into a kind.
///
/// ## Panics
///
/// Panics if `char` is not `*`, `_`, or `_`.
fn from_char(char: char) -> Kind {
match char {
'*' => Kind::Asterisk,
'-' => Kind::Dash,
'_' => Kind::Underscore,
_ => unreachable!("invalid char"),
}
}
}
/// State needed to parse thematic breaks.
#[derive(Debug)]
struct Info {
/// Kind of marker.
kind: Kind,
/// Number of markers.
size: usize,
}
/// Start of a thematic break.
///
/// ```markdown
/// |***
/// ```
pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
tokenizer.enter(TokenType::ThematicBreak);
tokenizer.go(space_or_tab_opt(), before)(tokenizer, code)
}
/// Start of a thematic break, after whitespace.
///
/// ```markdown
/// |***
/// ```
pub fn before(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
match code {
Code::Char(char) if char == '*' || char == '-' || char == '_' => at_break(
tokenizer,
code,
Info {
kind: Kind::from_char(char),
size: 0,
},
),
_ => (State::Nok, None),
}
}
/// After something but before something else.
///
/// ```markdown
/// |***
/// *| * *
/// * |* *
/// ```
fn at_break(tokenizer: &mut Tokenizer, code: Code, info: Info) -> StateFnResult {
match code {
Code::None | Code::CarriageReturnLineFeed | Code::Char('\n' | '\r')
if info.size >= THEMATIC_BREAK_MARKER_COUNT_MIN =>
{
tokenizer.exit(TokenType::ThematicBreak);
(State::Ok, Some(vec![code]))
}
Code::Char(char) if char == info.kind.as_char() => {
tokenizer.enter(TokenType::ThematicBreakSequence);
sequence(tokenizer, code, info)
}
_ => (State::Nok, None),
}
}
/// In a sequence of markers.
///
/// ```markdown
/// |***
/// *|**
/// **|*
/// ```
fn sequence(tokenizer: &mut Tokenizer, code: Code, mut info: Info) -> StateFnResult {
match code {
Code::Char(char) if char == info.kind.as_char() => {
tokenizer.consume(code);
info.size += 1;
(State::Fn(Box::new(|t, c| sequence(t, c, info))), None)
}
_ => {
tokenizer.exit(TokenType::ThematicBreakSequence);
tokenizer.go(space_or_tab_opt(), |t, c| at_break(t, c, info))(tokenizer, code)
}
}
}
|
