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diff --git a/src/construct/thematic_break.rs b/src/construct/thematic_break.rs
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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
+//! setext headings.
+//! 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.
+//!
+//! ## 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)
+//!
+//! [html]: https://html.spec.whatwg.org/multipage/grouping-content.html#the-hr-element
+//!
+//! <!-- To do: link `flow` -->
+
+use crate::constant::THEMATIC_BREAK_MARKER_COUNT_MIN;
+use crate::tokenizer::{Code, State, StateFnResult, TokenType, Tokenizer};
+
+/// Start of a thematic break.
+///
+/// ```markdown
+/// |***
+/// ```
+pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
+    match code {
+        Code::Char(char) if char == '*' || char == '-' || char == '_' => {
+            tokenizer.enter(TokenType::ThematicBreak);
+            at_break(tokenizer, code, char, 0)
+        }
+        _ => (State::Nok, None),
+    }
+}
+
+/// After something but before something else.
+///
+/// ```markdown
+/// |***
+/// *| * *
+/// * |* *
+/// ```
+fn at_break(tokenizer: &mut Tokenizer, code: Code, marker: char, size: usize) -> StateFnResult {
+    match code {
+        Code::Char(char) if char == marker => {
+            tokenizer.enter(TokenType::ThematicBreakSequence);
+            sequence(tokenizer, code, marker, size)
+        }
+        Code::VirtualSpace | Code::Char('\t' | ' ') => {
+            tokenizer.enter(TokenType::ThematicBreakWhitespace);
+            whitespace(tokenizer, code, marker, size)
+        }
+        Code::None | Code::CarriageReturnLineFeed | Code::Char('\n' | '\r')
+            if size >= THEMATIC_BREAK_MARKER_COUNT_MIN =>
+        {
+            tokenizer.exit(TokenType::ThematicBreak);
+            (State::Ok, Some(vec![code]))
+        }
+        _ => (State::Nok, None),
+    }
+}
+
+/// In a sequence of markers.
+///
+/// ```markdown
+/// |***
+/// *|**
+/// **|*
+/// ```
+fn sequence(tokenizer: &mut Tokenizer, code: Code, marker: char, size: usize) -> StateFnResult {
+    match code {
+        Code::Char(char) if char == marker => {
+            tokenizer.consume(code);
+            (
+                State::Fn(Box::new(move |tokenizer, code| {
+                    sequence(tokenizer, code, marker, size + 1)
+                })),
+                None,
+            )
+        }
+        _ => {
+            tokenizer.exit(TokenType::ThematicBreakSequence);
+            at_break(tokenizer, code, marker, size)
+        }
+    }
+}
+
+/// In whitespace.
+///
+/// ```markdown
+/// * |* *
+/// * | * *
+/// ```
+fn whitespace(tokenizer: &mut Tokenizer, code: Code, marker: char, size: usize) -> StateFnResult {
+    match code {
+        Code::VirtualSpace | Code::Char('\t' | ' ') => {
+            tokenizer.consume(code);
+            (
+                State::Fn(Box::new(move |tokenizer, code| {
+                    whitespace(tokenizer, code, marker, size)
+                })),
+                None,
+            )
+        }
+        _ => {
+            tokenizer.exit(TokenType::ThematicBreakWhitespace);
+            at_break(tokenizer, code, marker, size)
+        }
+    }
+}