Add heading (setext)

1 files changed, 301 insertions, 0 deletions

diff --git a/src/construct/heading_setext.rs b/src/construct/heading_setext.rs
new file mode 100644
index 0000000..8cc4f6d
--- /dev/null
+++ b/src/construct/heading_setext.rs
@@ -0,0 +1,301 @@
+//! Heading (setext) is a construct that occurs in the [flow] content type.
+//!
+//! They’re formed with the following BNF:
+//!
+//! ```bnf
+//! heading_setext ::= line *(eol line) eol whitespace_optional (1*'-' | 1*'=') whitespace_optional
+//!
+//! whitespace ::= 1*space_or_tab
+//! whitespace_optional ::= [ whitespace ]
+//! line ::= code - eol
+//! eol ::= '\r' | '\r\n' | '\n'
+//! ```
+//!
+//! Heading (setext) in markdown relates to the `<h1>` and `<h2>` elements in
+//! HTML.
+//! See [*§ 4.3.6 The `h1`, `h2`, `h3`, `h4`, `h5`, and `h6` elements* in the
+//! HTML spec][html] for more info.
+//!
+//! In markdown, it is also possible to create headings with a
+//! [heading (atx)][heading_atx] 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.
+//!
+//! [Thematic breaks][thematic_break] formed with dashes (without whitespace)
+//! can also form heading (setext).
+//!
+//! > 🏛 **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.
+//!
+//! ## References
+//!
+//! *   [`setext-underline.js` in `micromark`](https://github.com/micromark/micromark/blob/main/packages/micromark-core-commonmark/dev/lib/setext-underline.js)
+//! *   [*§ 4.3 Setext headings* in `CommonMark`](https://spec.commonmark.org/0.30/#setext-headings)
+//!
+//! [flow]: crate::content::flow
+//! [heading_atx]: crate::construct::heading_atx
+//! [thematic_break]: crate::construct::thematic_break
+//! [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 crate::constant::TAB_SIZE;
+use crate::construct::partial_whitespace::start as whitespace;
+use crate::tokenizer::{Code, State, StateFnResult, TokenType, Tokenizer};
+use crate::util::span::from_exit_event;
+
+/// Kind of underline.
+#[derive(Debug, Clone, PartialEq)]
+pub enum Kind {
+    /// Grave accent (tick) code.
+    Dash,
+    /// Tilde code.
+    EqualsTo,
+}
+
+/// Start of a heading (setext).
+///
+/// ```markdown
+/// |alpha
+/// ==
+/// ```
+pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
+    match code {
+        Code::None | Code::CarriageReturnLineFeed | Code::Char('\n' | '\r') => {
+            unreachable!("expected non-eol/eof");
+        }
+        _ => {
+            tokenizer.enter(TokenType::HeadingSetext);
+            tokenizer.enter(TokenType::HeadingSetextText);
+            tokenizer.enter(TokenType::ChunkText);
+            text_inside(tokenizer, code)
+        }
+    }
+}
+
+/// Inside text.
+///
+/// ```markdown
+/// al|pha
+/// bra|vo
+/// ==
+/// ```
+pub fn text_inside(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
+    match code {
+        Code::None => (State::Nok, None),
+        Code::CarriageReturnLineFeed | Code::Char('\n' | '\r') => {
+            tokenizer.exit(TokenType::ChunkText);
+            tokenizer.exit(TokenType::HeadingSetextText);
+            tokenizer.attempt(underline_before, |ok| {
+                Box::new(if ok { after } else { text_continue })
+            })(tokenizer, code)
+        }
+        _ => {
+            tokenizer.consume(code);
+            (State::Fn(Box::new(text_inside)), None)
+        }
+    }
+}
+
+/// At a line ending, not at an underline.
+///
+/// ```markdown
+/// alpha
+/// |bravo
+/// ==
+/// ```
+fn text_continue(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
+    // Needed to connect the text.
+    // To do: does it work?
+    tokenizer.enter(TokenType::HeadingSetextText);
+    tokenizer.events.pop();
+    tokenizer.events.pop();
+
+    match code {
+        Code::CarriageReturnLineFeed | Code::Char('\n' | '\r') => {
+            let next = tokenizer.events.len();
+            let previous = next - 2;
+
+            tokenizer.enter(TokenType::LineEnding);
+            tokenizer.consume(code);
+            tokenizer.exit(TokenType::LineEnding);
+
+            tokenizer.events[previous].next = Some(next);
+            tokenizer.events[next].previous = Some(previous);
+
+            (
+                State::Fn(Box::new(tokenizer.attempt(
+                    |tokenizer, code| whitespace(tokenizer, code, TokenType::Whitespace),
+                    |_ok| Box::new(text_line_start),
+                ))),
+                None,
+            )
+        }
+        _ => unreachable!("expected eol"),
+    }
+}
+
+/// At a line ending after whitespace, not at an underline.
+///
+/// ```markdown
+/// alpha
+/// |bravo
+/// ==
+/// ```
+fn text_line_start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
+    let next = tokenizer.events.len() - 2;
+    let previous = next - 2;
+
+    // Link the whitespace, if it exists.
+    if tokenizer.events[next].token_type == TokenType::Whitespace {
+        tokenizer.events[previous].next = Some(next);
+        tokenizer.events[next].previous = Some(previous);
+    }
+
+    match code {
+        // Blank lines not allowed.
+        Code::None | Code::CarriageReturnLineFeed | Code::Char('\n' | '\r') => (State::Nok, None),
+        _ => {
+            let next = tokenizer.events.len();
+            let previous = next - 2;
+
+            tokenizer.enter(TokenType::ChunkText);
+
+            tokenizer.events[previous].next = Some(next);
+            tokenizer.events[next].previous = Some(previous);
+
+            text_inside(tokenizer, code)
+        }
+    }
+}
+
+/// After a heading (setext).
+///
+/// ```markdown
+/// alpha
+/// ==|
+/// ```
+fn after(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
+    tokenizer.exit(TokenType::HeadingSetext);
+    (State::Ok, Some(vec![code]))
+}
+
+/// At a line ending, presumably an underline.
+///
+/// ```markdown
+/// alpha|
+/// ==
+/// ```
+fn underline_before(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
+    match code {
+        Code::CarriageReturnLineFeed | Code::Char('\n' | '\r') => {
+            tokenizer.enter(TokenType::LineEnding);
+            tokenizer.consume(code);
+            tokenizer.exit(TokenType::LineEnding);
+            (State::Fn(Box::new(underline_start)), None)
+        }
+        _ => unreachable!("expected eol"),
+    }
+}
+
+/// After a line ending, presumably an underline.
+///
+/// ```markdown
+/// alpha
+/// |==
+/// ```
+fn underline_start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
+    tokenizer.attempt(
+        |tokenizer, code| whitespace(tokenizer, code, TokenType::Whitespace),
+        |_ok| Box::new(underline_sequence_start),
+    )(tokenizer, code)
+}
+
+/// After optional whitespace, presumably an underline.
+///
+/// ```markdown
+/// alpha
+/// |==
+/// ```
+fn underline_sequence_start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
+    let tail = tokenizer.events.last();
+    let mut prefix = 0;
+
+    if let Some(event) = tail {
+        if event.token_type == TokenType::Whitespace {
+            let span = from_exit_event(&tokenizer.events, tokenizer.events.len() - 1);
+            prefix = span.end_index - span.start_index;
+        }
+    }
+
+    // To do: 4+ should be okay if code (indented) is turned off!
+    if prefix >= TAB_SIZE {
+        return (State::Nok, None);
+    }
+
+    match code {
+        Code::Char(char) if char == '-' || char == '=' => {
+            let marker = if char == '-' {
+                Kind::Dash
+            } else {
+                Kind::EqualsTo
+            };
+            tokenizer.enter(TokenType::HeadingSetextUnderline);
+            underline_sequence_inside(tokenizer, code, marker)
+        }
+        _ => (State::Nok, None),
+    }
+}
+
+/// In an underline sequence.
+///
+/// ```markdown
+/// alpha
+/// =|=
+/// ```
+fn underline_sequence_inside(tokenizer: &mut Tokenizer, code: Code, kind: Kind) -> StateFnResult {
+    let marker = if kind == Kind::Dash { '-' } else { '=' };
+
+    match code {
+        Code::Char(char) if char == marker => {
+            tokenizer.consume(code);
+            (
+                State::Fn(Box::new(move |tokenizer, code| {
+                    underline_sequence_inside(tokenizer, code, kind)
+                })),
+                None,
+            )
+        }
+        Code::VirtualSpace | Code::Char('\t' | ' ') => tokenizer.attempt(
+            |tokenizer, code| whitespace(tokenizer, code, TokenType::Whitespace),
+            |_ok| Box::new(underline_after),
+        )(tokenizer, code),
+        _ => underline_after(tokenizer, code),
+    }
+}
+
+/// After an underline sequence, after optional whitespace.
+///
+/// ```markdown
+/// alpha
+/// ==|
+/// ```
+fn underline_after(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
+    match code {
+        Code::None | Code::CarriageReturnLineFeed | Code::Char('\n' | '\r') => {
+            tokenizer.exit(TokenType::HeadingSetextUnderline);
+            (State::Ok, Some(vec![code]))
+        }
+        _ => (State::Nok, None),
+    }
+}