//! Heading (atx) is a construct that occurs in the [flow] content type.
//!
//! They’re formed with the following BNF:
//!
//! ```bnf
//! heading_atx ::= 1*6'#' [ 1*space_or_tab text [ 1*space_or_tab 1*'#' ] ] *space_or_tab
//!
//! text ::= code - eol
//! space_or_tab ::= ' ' | '\t'
//! ```
//!
//! Headings in markdown relate to the `<h1>` through `<h6>` elements in HTML.
//! See [*§ 4.3.6 The `h1`, `h2`, `h3`, `h4`, `h5`, and `h6` elements* in the
//! HTML spec][html] for more info.
//!
//! `CommonMark` introduced the requirement on whitespace existing after the
//! opening sequence and before text.
//! In older markdown versions, this was not required, and headings would form
//! without it.
//!
//! In markdown, it is also possible to create headings with a
//! [heading (setext)][heading_setext] construct.
//! The benefit of setext headings is that their text can include line endings,
//! and by extensions also hard breaks (e.g., with
//! [hard break (escape)][hard_break_escape]).
//! However, their limit is that they cannot form `<h3>` through `<h6>`
//! headings.
//! Due to this limitation, it is recommended to use atx headings.
//!
//! > 🏛 **Background**: the word *setext* originates from a small markup
//! > language by Ian Feldman from 1991.
//! > See [*§ Setext* on Wikipedia][wiki-setext] for more info.
//! > The word *atx* originates from a tiny markup language by Aaron Swartz
//! > from 2002.
//! > See [*§ atx, the true structured text format* on `aaronsw.com`][atx] for
//! > more info.
//!
//! ## Tokens
//!
//! * [`HeadingAtx`][TokenType::HeadingAtx]
//! * [`HeadingAtxSequence`][TokenType::HeadingAtxSequence]
//! * [`HeadingAtxText`][TokenType::HeadingAtxText]
//! * [`HeadingAtxSpaceOrTab`][TokenType::HeadingAtxSpaceOrTab]
//!
//! ## References
//!
//! * [`heading-atx.js` in `micromark`](https://github.com/micromark/micromark/blob/main/packages/micromark-core-commonmark/dev/lib/heading-atx.js)
//! * [*§ 4.2 ATX headings* in `CommonMark`](https://spec.commonmark.org/0.30/#atx-headings)
//!
//! [flow]: crate::content::flow
//! [heading_setext]: crate::construct::heading_setext
//! [hard_break_escape]: crate::construct::hard_break_escape
//! [html]: https://html.spec.whatwg.org/multipage/sections.html#the-h1,-h2,-h3,-h4,-h5,-and-h6-elements
//! [wiki-setext]: https://en.wikipedia.org/wiki/Setext
//! [atx]: http://www.aaronsw.com/2002/atx/
use super::partial_space_or_tab::{
space_or_tab, space_or_tab_with_options, Options as SpaceOrTabOptions,
};
use crate::constant::HEADING_ATX_OPENING_FENCE_SIZE_MAX;
use crate::tokenizer::{Code, State, StateFnResult, TokenType, Tokenizer};
/// Start of a heading (atx).
///
/// ```markdown
/// |## alpha
/// ```
pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
tokenizer.enter(TokenType::HeadingAtx);
tokenizer.attempt_opt(space_or_tab(), before)(tokenizer, code)
}
/// Start of a heading (atx), after whitespace.
///
/// ```markdown
/// |## alpha
/// ```
pub fn before(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
if Code::Char('#') == code {
tokenizer.enter(TokenType::HeadingAtxSequence);
sequence_open(tokenizer, code, 0)
} else {
(State::Nok, None)
}
}
/// In the opening sequence.
///
/// ```markdown
/// #|# alpha
/// ```
fn sequence_open(tokenizer: &mut Tokenizer, code: Code, rank: usize) -> StateFnResult {
match code {
Code::None | Code::CarriageReturnLineFeed | Code::Char('\n' | '\r') if rank > 0 => {
tokenizer.exit(TokenType::HeadingAtxSequence);
at_break(tokenizer, code)
}
Code::Char('#') if rank < HEADING_ATX_OPENING_FENCE_SIZE_MAX => {
tokenizer.consume(code);
(
State::Fn(Box::new(move |tokenizer, code| {
sequence_open(tokenizer, code, rank + 1)
})),
None,
)
}
_ if rank > 0 => {
tokenizer.exit(TokenType::HeadingAtxSequence);
tokenizer.go(
space_or_tab_with_options(SpaceOrTabOptions {
kind: TokenType::HeadingAtxSpaceOrTab,
min: 1,
max: usize::MAX,
}),
at_break,
)(tokenizer, code)
}
_ => (State::Nok, None),
}
}
/// After something but before something else.
///
/// ```markdown
/// ## |alpha
/// ## alpha| bravo
/// ## alpha |bravo
/// ## alpha bravo|##
/// ## alpha bravo ##|
/// ```
fn at_break(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
match code {
Code::None | Code::CarriageReturnLineFeed | Code::Char('\n' | '\r') => {
tokenizer.exit(TokenType::HeadingAtx);
(State::Ok, Some(vec![code]))
}
Code::VirtualSpace | Code::Char('\t' | ' ') => tokenizer.go(
space_or_tab_with_options(SpaceOrTabOptions {
kind: TokenType::HeadingAtxSpaceOrTab,
min: 1,
max: usize::MAX,
}),
at_break,
)(tokenizer, code),
Code::Char('#') => {
tokenizer.enter(TokenType::HeadingAtxSequence);
further_sequence(tokenizer, code)
}
Code::Char(_) => {
tokenizer.enter(TokenType::HeadingAtxText);
tokenizer.enter(TokenType::ChunkText);
data(tokenizer, code)
}
}
}
/// In a further sequence (after whitespace).
/// Could be normal “visible” hashes in the heading or a final sequence.
///
/// ```markdown
/// ## alpha #|#
/// ```
fn further_sequence(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
if let Code::Char('#') = code {
tokenizer.consume(code);
(State::Fn(Box::new(further_sequence)), None)
} else {
tokenizer.exit(TokenType::HeadingAtxSequence);
at_break(tokenizer, code)
}
}
/// In text.
///
/// ```markdown
/// ## al|pha
/// ```
fn data(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
match code {
// Note: `#` for closing sequence must be preceded by whitespace, otherwise it’s just text.
Code::None | Code::CarriageReturnLineFeed | Code::Char('\t' | '\n' | '\r' | ' ') => {
tokenizer.exit(TokenType::ChunkText);
tokenizer.exit(TokenType::HeadingAtxText);
at_break(tokenizer, code)
}
_ => {
tokenizer.consume(code);
(State::Fn(Box::new(data)), None)
}
}
}
