Make paragraphs really fast

The approach that `micromark-js` takes is as follows: to parse a
paragraph, check whether each line starts with something else.
If it does, exit, otherwise continue.
That is slow, because our actual flow parser does similar things: the work was
being done twice.

To fix this, this commit introduces parsing each line of a paragraph separately.
And finally, when done with flow, combining adjacent paragraphs.

This same mechanism is reused for setext headings.

Additionally, this commit adds support for interrupting things (or not).
E.g., HTML (flow, complete) cannot interrupt paragraphs.
Definitions cannot interrupt paragraphs, and connect be interrupted either,
but they can follow each other.

10 files changed, 205 insertions, 297 deletions

diff --git a/src/construct/code_fenced.rs b/src/construct/code_fenced.rs
index d19cad0..f2d243a 100644
--- a/src/construct/code_fenced.rs
+++ b/src/construct/code_fenced.rs
@@ -179,7 +179,8 @@ struct Info {
 pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
     tokenizer.enter(TokenType::CodeFenced);
     tokenizer.enter(TokenType::CodeFencedFence);
-    tokenizer.attempt_opt(space_or_tab(), before_sequence_open)(tokenizer, code)
+    // To do: allow arbitrary when code (indented) is turned off.
+    tokenizer.go(space_or_tab_min_max(0, TAB_SIZE - 1), before_sequence_open)(tokenizer, code)
 }
 
 /// Inside the opening fence, after an optional prefix, before a sequence.
@@ -550,5 +551,7 @@ fn content_continue(tokenizer: &mut Tokenizer, code: Code, info: Info) -> StateF
 /// ```
 fn after(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
     tokenizer.exit(TokenType::CodeFenced);
+    // Feel free to interrupt.
+    tokenizer.interrupt = false;
     (State::Ok, Some(vec![code]))
 }
diff --git a/src/construct/code_indented.rs b/src/construct/code_indented.rs
index 99445b9..9bdfd71 100644
--- a/src/construct/code_indented.rs
+++ b/src/construct/code_indented.rs
@@ -59,8 +59,13 @@ use crate::tokenizer::{Code, State, StateFnResult, TokenType, Tokenizer};
 /// > filled line (that it has a non-whitespace character), because blank lines
 /// > are parsed already, so we never run into that.
 pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
-    tokenizer.enter(TokenType::CodeIndented);
-    tokenizer.go(space_or_tab_min_max(TAB_SIZE, TAB_SIZE), at_break)(tokenizer, code)
+    // Do not interrupt paragraphs.
+    if tokenizer.interrupt {
+        (State::Nok, None)
+    } else {
+        tokenizer.enter(TokenType::CodeIndented);
+        tokenizer.go(space_or_tab_min_max(TAB_SIZE, TAB_SIZE), at_break)(tokenizer, code)
+    }
 }
 
 /// At a break.
@@ -110,6 +115,8 @@ fn content(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
 /// ```
 fn after(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
     tokenizer.exit(TokenType::CodeIndented);
+    // Feel free to interrupt.
+    tokenizer.interrupt = false;
     (State::Ok, Some(vec![code]))
 }
 
diff --git a/src/construct/definition.rs b/src/construct/definition.rs
index f05064a..e1afd03 100644
--- a/src/construct/definition.rs
+++ b/src/construct/definition.rs
@@ -107,8 +107,19 @@ use crate::tokenizer::{Code, State, StateFnResult, TokenType, Tokenizer};
 /// |[a]: b "c"
 /// ```
 pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
-    tokenizer.enter(TokenType::Definition);
-    tokenizer.attempt_opt(space_or_tab(), before)(tokenizer, code)
+    let index = tokenizer.events.len();
+    let definition_before = index > 3
+        && tokenizer.events[index - 1].token_type == TokenType::LineEnding
+        && tokenizer.events[index - 3].token_type == TokenType::Definition;
+
+    // Do not interrupt paragraphs (but do follow definitions).
+    if tokenizer.interrupt && !definition_before {
+        (State::Nok, None)
+    } else {
+        tokenizer.enter(TokenType::Definition);
+        // Note: arbitrary whitespace allowed even if code (indented) is on.
+        tokenizer.attempt_opt(space_or_tab(), before)(tokenizer, code)
+    }
 }
 
 /// At the start of a definition, after whitespace.
@@ -218,6 +229,8 @@ fn after_whitespace(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
     match code {
         Code::None | Code::CarriageReturnLineFeed | Code::Char('\r' | '\n') => {
             tokenizer.exit(TokenType::Definition);
+            // You’d be interrupting.
+            tokenizer.interrupt = true;
             (State::Ok, Some(vec![code]))
         }
         _ => (State::Nok, None),
diff --git a/src/construct/heading_atx.rs b/src/construct/heading_atx.rs
index 2811894..3ce7052 100644
--- a/src/construct/heading_atx.rs
+++ b/src/construct/heading_atx.rs
@@ -54,8 +54,8 @@
 //! [wiki-setext]: https://en.wikipedia.org/wiki/Setext
 //! [atx]: http://www.aaronsw.com/2002/atx/
 
-use super::partial_space_or_tab::space_or_tab;
-use crate::constant::HEADING_ATX_OPENING_FENCE_SIZE_MAX;
+use super::partial_space_or_tab::{space_or_tab, space_or_tab_min_max};
+use crate::constant::{HEADING_ATX_OPENING_FENCE_SIZE_MAX, TAB_SIZE};
 use crate::tokenizer::{
     Code, ContentType, Event, EventType, State, StateFnResult, TokenType, Tokenizer,
 };
@@ -68,7 +68,8 @@ use crate::util::edit_map::EditMap;
 /// ```
 pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
     tokenizer.enter(TokenType::HeadingAtx);
-    tokenizer.attempt_opt(space_or_tab(), before)(tokenizer, code)
+    // To do: allow arbitrary when code (indented) is turned off.
+    tokenizer.go(space_or_tab_min_max(0, TAB_SIZE - 1), before)(tokenizer, code)
 }
 
 /// Start of a heading (atx), after whitespace.
@@ -127,6 +128,8 @@ fn at_break(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
         Code::None | Code::CarriageReturnLineFeed | Code::Char('\n' | '\r') => {
             tokenizer.exit(TokenType::HeadingAtx);
             tokenizer.register_resolver("heading_atx".to_string(), Box::new(resolve));
+            // Feel free to interrupt.
+            tokenizer.interrupt = false;
             (State::Ok, Some(vec![code]))
         }
         Code::VirtualSpace | Code::Char('\t' | ' ') => {
diff --git a/src/construct/heading_setext.rs b/src/construct/heading_setext.rs
index 03a2e55..df20aa7 100644
--- a/src/construct/heading_setext.rs
+++ b/src/construct/heading_setext.rs
@@ -58,10 +58,9 @@
 //! [atx]: http://www.aaronsw.com/2002/atx/
 
 use crate::constant::TAB_SIZE;
-use crate::construct::partial_space_or_tab::{space_or_tab, space_or_tab_with_options, Options};
-use crate::subtokenize::link;
-use crate::tokenizer::{Code, ContentType, State, StateFnResult, TokenType, Tokenizer};
-use crate::util::span::from_exit_event;
+use crate::construct::partial_space_or_tab::{space_or_tab, space_or_tab_min_max};
+use crate::tokenizer::{Code, Event, EventType, State, StateFnResult, TokenType, Tokenizer};
+use crate::util::edit_map::EditMap;
 
 /// Kind of underline.
 #[derive(Debug, Clone, PartialEq)]
@@ -109,150 +108,23 @@ impl Kind {
     }
 }
 
-/// Start of a heading (setext).
-///
-/// ```markdown
-/// |alpha
-/// ==
-/// ```
-pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
-    tokenizer.enter(TokenType::HeadingSetext);
-    tokenizer.attempt_opt(space_or_tab(), before)(tokenizer, code)
-}
-
-/// Start of a heading (setext), after whitespace.
-///
-/// ```markdown
-/// |alpha
-/// ==
-/// ```
-fn before(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
-    match code {
-        Code::None | Code::CarriageReturnLineFeed | Code::Char('\n' | '\r') => {
-            unreachable!("expected non-eol/eof");
-        }
-        _ => {
-            tokenizer.enter(TokenType::HeadingSetextText);
-            tokenizer.enter_with_content(TokenType::Data, Some(ContentType::Text));
-            text_inside(tokenizer, code)
-        }
-    }
-}
-
-/// Inside text.
-///
-/// ```markdown
-/// al|pha
-/// bra|vo
-/// ==
-/// ```
-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::Data);
-            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.
-    tokenizer.enter(TokenType::HeadingSetextText);
-    tokenizer.events.pop();
-    tokenizer.events.pop();
-
-    match code {
-        Code::CarriageReturnLineFeed | Code::Char('\n' | '\r') => {
-            tokenizer.enter_with_content(TokenType::LineEnding, Some(ContentType::Text));
-            let index = tokenizer.events.len() - 1;
-            link(&mut tokenizer.events, index);
-            tokenizer.consume(code);
-            tokenizer.exit(TokenType::LineEnding);
-
-            (
-                State::Fn(Box::new(tokenizer.attempt_opt(
-                    space_or_tab_with_options(Options {
-                        kind: TokenType::SpaceOrTab,
-                        min: 1,
-                        max: usize::MAX,
-                        content_type: Some(ContentType::Text),
-                        connect: true,
-                    }),
-                    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 {
-    match code {
-        // Blank lines not allowed.
-        Code::None | Code::CarriageReturnLineFeed | Code::Char('\n' | '\r') => (State::Nok, None),
-        _ => {
-            tokenizer.enter_with_content(TokenType::Data, Some(ContentType::Text));
-            let index = tokenizer.events.len() - 1;
-            link(&mut tokenizer.events, index);
-            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(
-                    tokenizer.attempt_opt(space_or_tab(), underline_sequence_start),
-                )),
-                None,
-            )
-        }
-        _ => unreachable!("expected eol"),
+pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
+    let index = tokenizer.events.len();
+    let paragraph_before = index > 3
+        && tokenizer.events[index - 1].token_type == TokenType::LineEnding
+        && tokenizer.events[index - 3].token_type == TokenType::Paragraph;
+
+    if paragraph_before {
+        // To do: allow arbitrary when code (indented) is turned off.
+        tokenizer.go(space_or_tab_min_max(0, TAB_SIZE - 1), before)(tokenizer, code)
+    } else {
+        (State::Nok, None)
     }
 }
 
@@ -262,26 +134,11 @@ fn underline_before(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
 /// 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::SpaceOrTab {
-            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);
-    }
-
+fn before(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
     match code {
         Code::Char(char) if char == '-' || char == '=' => {
             tokenizer.enter(TokenType::HeadingSetextUnderline);
-            underline_sequence_inside(tokenizer, code, Kind::from_char(char))
+            inside(tokenizer, code, Kind::from_char(char))
         }
         _ => (State::Nok, None),
     }
@@ -293,16 +150,13 @@ fn underline_sequence_start(tokenizer: &mut Tokenizer, code: Code) -> StateFnRes
 /// alpha
 /// =|=
 /// ```
-fn underline_sequence_inside(tokenizer: &mut Tokenizer, code: Code, kind: Kind) -> StateFnResult {
+fn inside(tokenizer: &mut Tokenizer, code: Code, kind: Kind) -> StateFnResult {
     match code {
         Code::Char(char) if char == kind.as_char() => {
             tokenizer.consume(code);
-            (
-                State::Fn(Box::new(move |t, c| underline_sequence_inside(t, c, kind))),
-                None,
-            )
+            (State::Fn(Box::new(move |t, c| inside(t, c, kind))), None)
         }
-        _ => tokenizer.attempt_opt(space_or_tab(), underline_after)(tokenizer, code),
+        _ => tokenizer.attempt_opt(space_or_tab(), after)(tokenizer, code),
     }
 }
 
@@ -312,12 +166,59 @@ fn underline_sequence_inside(tokenizer: &mut Tokenizer, code: Code, kind: Kind)
 /// alpha
 /// ==|
 /// ```
-fn underline_after(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
+fn after(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
     match code {
         Code::None | Code::CarriageReturnLineFeed | Code::Char('\n' | '\r') => {
             tokenizer.exit(TokenType::HeadingSetextUnderline);
+            // Feel free to interrupt.
+            tokenizer.interrupt = false;
+            tokenizer.register_resolver("heading_setext".to_string(), Box::new(resolve));
             (State::Ok, Some(vec![code]))
         }
         _ => (State::Nok, None),
     }
 }
+
+/// To do.
+pub fn resolve(tokenizer: &mut Tokenizer) -> Vec<Event> {
+    let mut edit_map = EditMap::new();
+    let mut index = 0;
+    let mut paragraph_enter: Option<usize> = None;
+    let mut paragraph_exit: Option<usize> = None;
+
+    while index < tokenizer.events.len() {
+        let event = &tokenizer.events[index];
+
+        // Find paragraphs.
+        if event.event_type == EventType::Enter {
+            if event.token_type == TokenType::Paragraph {
+                paragraph_enter = Some(index);
+            }
+        } else if event.token_type == TokenType::Paragraph {
+            paragraph_exit = Some(index);
+        }
+        // We know this is preceded by a paragraph.
+        // Otherwise we don’t parse.
+        else if event.token_type == TokenType::HeadingSetextUnderline {
+            let enter = paragraph_enter.take().unwrap();
+            let exit = paragraph_exit.take().unwrap();
+
+            // Change types of Enter:Paragraph, Exit:Paragraph.
+            tokenizer.events[enter].token_type = TokenType::HeadingSetextText;
+            tokenizer.events[exit].token_type = TokenType::HeadingSetextText;
+
+            // Add of Enter:HeadingSetext, Exit:HeadingSetext.
+            let mut heading_enter = tokenizer.events[enter].clone();
+            heading_enter.token_type = TokenType::HeadingSetext;
+            let mut heading_exit = tokenizer.events[index].clone();
+            heading_exit.token_type = TokenType::HeadingSetext;
+
+            edit_map.add(enter, 0, vec![heading_enter]);
+            edit_map.add(index + 1, 0, vec![heading_exit]);
+        }
+
+        index += 1;
+    }
+
+    edit_map.consume(&mut tokenizer.events)
+}
diff --git a/src/construct/html_flow.rs b/src/construct/html_flow.rs
index d0e0558..a1bddad 100644
--- a/src/construct/html_flow.rs
+++ b/src/construct/html_flow.rs
@@ -98,8 +98,10 @@
 //! [html_block_names]: crate::constant::HTML_BLOCK_NAMES
 //! [html-parsing]: https://html.spec.whatwg.org/multipage/parsing.html#parsing
 
-use crate::constant::{HTML_BLOCK_NAMES, HTML_RAW_NAMES, HTML_RAW_SIZE_MAX};
-use crate::construct::{blank_line::start as blank_line, partial_space_or_tab::space_or_tab};
+use crate::constant::{HTML_BLOCK_NAMES, HTML_RAW_NAMES, HTML_RAW_SIZE_MAX, TAB_SIZE};
+use crate::construct::{
+    blank_line::start as blank_line, partial_space_or_tab::space_or_tab_min_max,
+};
 use crate::tokenizer::{Code, State, StateFnResult, TokenType, Tokenizer};
 
 /// Kind of HTML (flow).
@@ -191,7 +193,8 @@ struct Info {
 pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
     tokenizer.enter(TokenType::HtmlFlow);
     tokenizer.enter(TokenType::HtmlFlowData);
-    tokenizer.attempt_opt(space_or_tab(), before)(tokenizer, code)
+    // To do: allow arbitrary when code (indented) is turned off.
+    tokenizer.go(space_or_tab_min_max(0, TAB_SIZE - 1), before)(tokenizer, code)
 }
 
 /// After optional whitespace, before `<`.
@@ -400,8 +403,10 @@ fn tag_name(tokenizer: &mut Tokenizer, code: Code, mut info: Info) -> StateFnRes
             } else {
                 info.kind = Kind::Complete;
 
-                // To do: do not support complete HTML when interrupting.
-                if info.start_tag {
+                // Do not support complete HTML when interrupting.
+                if tokenizer.interrupt {
+                    (State::Nok, None)
+                } else if info.start_tag {
                     complete_attribute_name_before(tokenizer, code, info)
                 } else {
                     complete_closing_tag_after(tokenizer, code, info)
@@ -784,6 +789,8 @@ fn html_continue_start(tokenizer: &mut Tokenizer, code: Code, info: Info) -> Sta
     match code {
         Code::None => {
             tokenizer.exit(TokenType::HtmlFlow);
+            // Feel free to interrupt.
+            tokenizer.interrupt = false;
             (State::Ok, Some(vec![code]))
         }
         // To do: do not allow lazy lines.
@@ -949,6 +956,8 @@ fn continuation_close(tokenizer: &mut Tokenizer, code: Code, info: Info) -> Stat
         Code::None | Code::CarriageReturnLineFeed | Code::Char('\n' | '\r') => {
             tokenizer.exit(TokenType::HtmlFlowData);
             tokenizer.exit(TokenType::HtmlFlow);
+            // Feel free to interrupt.
+            tokenizer.interrupt = false;
             (State::Ok, Some(vec![code]))
         }
         _ => {
diff --git a/src/construct/paragraph.rs b/src/construct/paragraph.rs
index fea7052..ae2f4de 100644
--- a/src/construct/paragraph.rs
+++ b/src/construct/paragraph.rs
@@ -32,14 +32,10 @@
 //! [code_text]: crate::construct::code_text
 //! [html]: https://html.spec.whatwg.org/multipage/grouping-content.html#the-p-element
 
-use crate::constant::TAB_SIZE;
-use crate::construct::{
-    blank_line::start as blank_line, code_fenced::start as code_fenced,
-    heading_atx::start as heading_atx, html_flow::start as html_flow,
-    partial_space_or_tab::space_or_tab_min_max, thematic_break::start as thematic_break,
+use crate::tokenizer::{
+    Code, ContentType, Event, EventType, State, StateFnResult, TokenType, Tokenizer,
 };
-use crate::subtokenize::link;
-use crate::tokenizer::{Code, ContentType, State, StateFnResult, TokenType, Tokenizer};
+use crate::util::edit_map::EditMap;
 
 /// Before a paragraph.
 ///
@@ -66,11 +62,14 @@ pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
 /// ```
 fn inside(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
     match code {
-        Code::None => end(tokenizer, code),
-        Code::CarriageReturnLineFeed | Code::Char('\n' | '\r') => tokenizer
-            .check(interrupt, |ok| {
-                Box::new(if ok { at_line_ending } else { end })
-            })(tokenizer, code),
+        Code::None | Code::CarriageReturnLineFeed | Code::Char('\n' | '\r') => {
+            tokenizer.exit(TokenType::Data);
+            tokenizer.exit(TokenType::Paragraph);
+            tokenizer.register_resolver_before("paragraph".to_string(), Box::new(resolve));
+            // You’d be interrupting.
+            tokenizer.interrupt = true;
+            (State::Ok, Some(vec![code]))
+        }
         _ => {
             tokenizer.consume(code);
             (State::Fn(Box::new(inside)), None)
@@ -78,90 +77,55 @@ fn inside(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
     }
 }
 
-/// At a line ending, not interrupting.
-///
-/// ```markdown
-/// alpha|
-/// bravo.
-/// ```
-fn at_line_ending(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
-    tokenizer.consume(code);
-    tokenizer.exit(TokenType::Data);
-    tokenizer.enter_with_content(TokenType::Data, Some(ContentType::Text));
-    let index = tokenizer.events.len() - 1;
-    link(&mut tokenizer.events, index);
-    (State::Fn(Box::new(inside)), None)
-}
+/// Merge “`Paragraph`”s, which currently span a single line, into actual
+/// `Paragraph`s that span multiple lines.
+pub fn resolve(tokenizer: &mut Tokenizer) -> Vec<Event> {
+    let mut edit_map = EditMap::new();
+    let len = tokenizer.events.len();
+    let mut index = 0;
 
-/// At a line ending, done.
-///
-/// ```markdown
-/// alpha|
-/// ***
-/// ```
-fn end(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
-    tokenizer.exit(TokenType::Data);
-    tokenizer.exit(TokenType::Paragraph);
-    (State::Ok, Some(vec![code]))
-}
+    while index < len {
+        let event = &tokenizer.events[index];
 
-/// Before a potential interruption.
-///
-/// ```markdown
-/// alpha|
-/// ***
-/// ```
-fn interrupt(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(interrupt_start)), None)
-        }
-        _ => unreachable!("expected eol"),
-    }
-}
+        if event.event_type == EventType::Enter && event.token_type == TokenType::Paragraph {
+            // Exit:Paragraph
+            let mut exit_index = index + 3;
+            // Enter:Paragraph
+            let mut enter_next_index = exit_index + 3;
 
-/// After a line ending.
-///
-/// ```markdown
-/// alpha
-/// |~~~js
-/// ~~~
-/// ```
-fn interrupt_start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
-    // To do: If code is disabled, indented lines are allowed to interrupt.
-    tokenizer.attempt(space_or_tab_min_max(TAB_SIZE, TAB_SIZE), |ok| {
-        Box::new(if ok { interrupt_indent } else { interrupt_cont })
-    })(tokenizer, code)
-}
+            // To do: assert that `LineEnding` between?
+            while enter_next_index < len
+                && tokenizer.events[enter_next_index].token_type == TokenType::Paragraph
+            {
+                // Remove Exit:Paragraph, Enter:LineEnding, Exit:LineEnding, Enter:Paragraph.
+                edit_map.add(exit_index, 4, vec![]);
+                println!("rm {:?} {:?}", exit_index, exit_index + 4);
 
-/// At an indent.
-///
-/// ```markdown
-/// alpha
-///     |
-/// ```
-fn interrupt_indent(_tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
-    (State::Ok, Some(vec![code]))
-}
+                // Add Exit:LineEnding position info to Exit:Data.
+                let line_ending_exit = &tokenizer.events[enter_next_index - 1];
+                let line_ending_point = line_ending_exit.point.clone();
+                let line_ending_index = line_ending_exit.index;
+                let data_exit = &mut tokenizer.events[exit_index - 1];
+                data_exit.point = line_ending_point;
+                data_exit.index = line_ending_index;
 
-/// Not at an indented line.
-///
-/// ```markdown
-/// alpha
-/// |<div>
-/// ```
-fn interrupt_cont(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
-    tokenizer.attempt_n(
-        vec![
-            Box::new(blank_line),
-            Box::new(code_fenced),
-            Box::new(html_flow),
-            Box::new(heading_atx),
-            Box::new(thematic_break),
-        ],
-        |ok| Box::new(move |_t, code| (if ok { State::Nok } else { State::Ok }, Some(vec![code]))),
-    )(tokenizer, code)
+                // Link Enter:Data on the previous line to Enter:Data on this line.
+                let data_enter_prev = &mut tokenizer.events[exit_index - 2];
+                data_enter_prev.next = Some(enter_next_index + 1);
+                let data_enter_next = &mut tokenizer.events[enter_next_index + 1];
+                data_enter_next.previous = Some(exit_index - 2);
+
+                // Potential next start.
+                exit_index = enter_next_index + 3;
+                enter_next_index = exit_index + 3;
+            }
+
+            // Move to `Exit:Paragraph`.
+            index = exit_index;
+        }
+
+        index += 1;
+    }
+
+    edit_map.consume(&mut tokenizer.events)
 }
diff --git a/src/construct/thematic_break.rs b/src/construct/thematic_break.rs
index 9978ee0..8d29157 100644
--- a/src/construct/thematic_break.rs
+++ b/src/construct/thematic_break.rs
@@ -49,8 +49,8 @@
 //!
 //! <!-- To do: link `lists` -->
 
-use super::partial_space_or_tab::space_or_tab;
-use crate::constant::THEMATIC_BREAK_MARKER_COUNT_MIN;
+use super::partial_space_or_tab::{space_or_tab, space_or_tab_min_max};
+use crate::constant::{TAB_SIZE, THEMATIC_BREAK_MARKER_COUNT_MIN};
 use crate::tokenizer::{Code, State, StateFnResult, TokenType, Tokenizer};
 
 /// Type of thematic break.
@@ -122,7 +122,8 @@ struct Info {
 /// ```
 pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
     tokenizer.enter(TokenType::ThematicBreak);
-    tokenizer.attempt_opt(space_or_tab(), before)(tokenizer, code)
+    // To do: allow arbitrary when code (indented) is turned off.
+    tokenizer.go(space_or_tab_min_max(0, TAB_SIZE - 1), before)(tokenizer, code)
 }
 
 /// Start of a thematic break, after whitespace.
@@ -157,6 +158,8 @@ fn at_break(tokenizer: &mut Tokenizer, code: Code, info: Info) -> StateFnResult
             if info.size >= THEMATIC_BREAK_MARKER_COUNT_MIN =>
         {
             tokenizer.exit(TokenType::ThematicBreak);
+            // Feel free to interrupt.
+            tokenizer.interrupt = false;
             (State::Ok, Some(vec![code]))
         }
         Code::Char(char) if char == info.kind.as_char() => {
diff --git a/src/content/flow.rs b/src/content/flow.rs
index 0d3ede0..3ff948d 100644
--- a/src/content/flow.rs
+++ b/src/content/flow.rs
@@ -92,26 +92,6 @@ fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
     }
 }
 
-/// After a blank line.
-///
-/// Move to `start` afterwards.
-///
-/// ```markdown
-/// ␠␠|
-/// ```
-fn blank_line_after(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
-    match code {
-        Code::None => (State::Ok, None),
-        Code::CarriageReturnLineFeed | Code::Char('\n' | '\r') => {
-            tokenizer.enter(TokenType::BlankLineEnding);
-            tokenizer.consume(code);
-            tokenizer.exit(TokenType::BlankLineEnding);
-            (State::Fn(Box::new(start)), None)
-        }
-        _ => unreachable!("expected eol/eof after blank line `{:?}`", code),
-    }
-}
-
 /// Before flow (initial).
 ///
 /// “Initial” flow means unprefixed flow, so right at the start of a line.
@@ -133,16 +113,38 @@ fn initial_before(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
                 Box::new(code_fenced),
                 Box::new(html_flow),
                 Box::new(heading_atx),
+                Box::new(heading_setext),
                 Box::new(thematic_break),
                 Box::new(definition),
-                Box::new(heading_setext),
             ],
             |ok| Box::new(if ok { after } else { before_paragraph }),
         )(tokenizer, code),
     }
 }
 
-/// After a flow construct.
+/// After a blank line.
+///
+/// Move to `start` afterwards.
+///
+/// ```markdown
+/// ␠␠|
+/// ```
+fn blank_line_after(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
+    match code {
+        Code::None => (State::Ok, None),
+        Code::CarriageReturnLineFeed | Code::Char('\n' | '\r') => {
+            tokenizer.enter(TokenType::BlankLineEnding);
+            tokenizer.consume(code);
+            tokenizer.exit(TokenType::BlankLineEnding);
+            // Feel free to interrupt.
+            tokenizer.interrupt = false;
+            (State::Fn(Box::new(start)), None)
+        }
+        _ => unreachable!("expected eol/eof after blank line `{:?}`", code),
+    }
+}
+
+/// After something.
 ///
 /// ```markdown
 /// ## alpha|
diff --git a/src/tokenizer.rs b/src/tokenizer.rs
index 817c1de..b70e706 100644
--- a/src/tokenizer.rs
+++ b/src/tokenizer.rs
@@ -1760,6 +1760,8 @@ pub struct Tokenizer<'a> {
     /// To do.
     pub label_start_list_loose: Vec<LabelStart>,
     /// To do.
+    pub interrupt: bool,
+    /// To do.
     pub media_list: Vec<Media>,
     /// To do.
     resolvers: Vec<Box<Resolver>>,
@@ -1783,6 +1785,7 @@ impl<'a> Tokenizer<'a> {
             label_start_stack: vec![],
             label_start_list_loose: vec![],
             media_list: vec![],
+            interrupt: false,
             resolvers: vec![],
             resolver_ids: vec![],
         }