core: derive checklist items from the body, the way tags already are

First step of M304. Additive on its own — nothing calls this yet — so it can be
read and tested before anything depends on it.

A checklist is currently a TABLE, and a table can only ever render after the
body, because a row has no idea where in the note it belongs. That is why
"inline with the note" is not a styling problem: prose, three checkboxes, then
more prose is not expressible at all today.

derive.rs already owns "structure derived from body text" for #tags and says so
in its module doc. Task lines join it rather than opening a second home for the
same idea. The difference between the two is worth stating and now is: tags
MATERIALISE into label rows because the board queries by label; items
materialise into nothing, because nothing queries them. Their only readers are
the card, the editor, and display_title.

The grammar is fixed here because three languages will implement it —
derive.rs, notes/checklist.py, notes/markdown.ts — and any difference between
two of them is a checklist that changes shape when it syncs. `*` is accepted
since markdown.ts already takes it for a plain bullet, and a rule that allowed
`* item` but not `* [ ] item` would be one nobody could guess. `- [ ]` with
nothing after it parses as an empty item: that is what pressing Enter on a list
leaves behind, and refusing it would make a half-typed list stop being a list.
`- [X]` parses and normalises to lowercase on the first rewrite, so round trips
are stable.

append_item spaces its output exactly as import_export.py:_note_markdown does.
That is not cosmetic — the server migration will fold existing rows into bodies
with the same layout, so an export taken before it and one taken after have to
agree byte for byte.

A stale index is inert rather than fatal: the index comes from a UI that may be
a moment behind the store, and a late tap should do nothing rather than panic.

core/src/local/derive.rs

@@ -1,9 +1,18 @@
-//! Deriving `#tags` from a note's body — the local mirror of what the server computes
-//! on save. Pure string scanning (no regex dependency), kept in lockstep with the
-//! frontend's inline rules (see frontend notes/markdown.ts):
+//! Deriving structure from a note's body — the local mirror of what the server
+//! computes on save. Pure string scanning (no regex dependency), kept in lockstep
+//! with the frontend's inline rules (see frontend notes/markdown.ts):
 //!
 //! - `#tag`: `#` at a word boundary followed by tag characters (letter first).
 //!   On save these become labels attached with `via_tag = true`.
+//! - `- [ ] item`: a checklist item. The body IS the checklist (M304) — there is no
+//!   table of items beside it, so a list can sit between two paragraphs instead of
+//!   only after them.
+//!
+//! The two are the same idea at different strengths. Tags MATERIALISE into label
+//! rows, because the board queries by label. Items materialise into nothing,
+//! because nothing queries them: their only readers are the card, the editor and
+//! `display_title`. So `extract_items` is the whole storage layer for a checklist,
+//! and the rewriters below are how one is edited.
 //!
 //! Dedupes case-insensitively, preserving first-seen order.
 //!
@@ -45,6 +54,206 @@ fn push_unique(out: &mut Vec<String>, candidate: &str) {
     }
 }
 
+// ── checklist items ─────────────────────────────────────────────────────────
+//
+// The grammar, in one place, because three languages implement it (here,
+// `notes/checklist.py`, `notes/markdown.ts`) and a difference between any two of
+// them is a checklist that changes shape when it syncs:
+//
+//   optional indent, `-` or `*`, one-or-more spaces, `[ ]`/`[x]`/`[X]`,
+//   then either end-of-line or one-or-more spaces and the text.
+//
+// `*` is accepted because markdown.ts already accepts it for a plain bullet, and a
+// grammar that takes `* item` but not `* [ ] item` would be a rule with no reason
+// anyone could guess. `- [ ]` with nothing after it IS an item with empty text:
+// that is exactly what pressing Enter on a list leaves behind, and refusing to
+// parse it would make a half-typed list stop being a list.
+
+/// A checklist item, as found in the body. Its position in the returned vector is
+/// its identity — the same thing `position` meant when these were rows, and all the
+/// wire ever carried (`push.rs` sent text and checked, never an id).
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct DerivedItem {
+    pub text: String,
+    pub checked: bool,
+}
+
+/// One parsed task line, holding enough to put it back exactly as it was found.
+struct TaskLine<'a> {
+    indent: &'a str,
+    /// Preserved rather than normalised to `-`: rewriting someone's `*` bullets
+    /// because they ticked a box would be an edit they did not ask for.
+    bullet: char,
+    checked: bool,
+    text: &'a str,
+}
+
+fn parse_task_line(line: &str) -> Option<TaskLine<'_>> {
+    let indent_len = line.len() - line.trim_start().len();
+    let (indent, rest) = line.split_at(indent_len);
+
+    let bullet = rest.chars().next()?;
+    if bullet != '-' && bullet != '*' {
+        return None;
+    }
+    // At least one space after the bullet. `-[ ] x` is not a list item in any
+    // markdown either, so it stays prose here too.
+    let rest = &rest[bullet.len_utf8()..];
+    let gap = rest.len() - rest.trim_start_matches(' ').len();
+    if gap == 0 {
+        return None;
+    }
+    let rest = &rest[gap..];
+
+    let mut chars = rest.chars();
+    if chars.next()? != '[' {
+        return None;
+    }
+    let mark = chars.next()?;
+    if chars.next()? != ']' {
+        return None;
+    }
+    // Decided BEFORE the slice below, which is what guarantees `mark` is one byte
+    // and `[?]` is exactly three.
+    let checked = match mark {
+        ' ' => false,
+        'x' | 'X' => true,
+        _ => return None,
+    };
+    let rest = &rest[3..];
+
+    let text = if rest.is_empty() {
+        // "- [ ]" — an empty item, which is what an unfinished list line is.
+        rest
+    } else {
+        let gap = rest.len() - rest.trim_start_matches(' ').len();
+        // "- [ ]x" is prose: without the space this is not a marker, it is a
+        // sentence that happens to start with brackets.
+        if gap == 0 {
+            return None;
+        }
+        &rest[gap..]
+    };
+
+    Some(TaskLine {
+        indent,
+        bullet,
+        checked,
+        text,
+    })
+}
+
+fn render_task_line(indent: &str, bullet: char, checked: bool, text: &str) -> String {
+    // Always lowercase `x`, whatever was parsed: one canonical output is what makes
+    // a round trip stable, so `- [X]` normalises the first time it is touched and
+    // never again.
+    let mark = if checked { 'x' } else { ' ' };
+    if text.is_empty() {
+        format!("{indent}{bullet} [{mark}]")
+    } else {
+        format!("{indent}{bullet} [{mark}] {text}")
+    }
+}
+
+/// Every checklist item in `body`, in the order they appear.
+pub fn extract_items(body: &str) -> Vec<DerivedItem> {
+    let mut out = Vec::new();
+    for line in body.split('\n') {
+        if let Some(t) = parse_task_line(line) {
+            out.push(DerivedItem {
+                text: t.text.to_string(),
+                checked: t.checked,
+            });
+        }
+    }
+    out
+}
+
+/// Rewrite the `index`-th task line, or drop it when `f` returns None.
+///
+/// A body with fewer task lines than that is returned UNCHANGED rather than
+/// panicking: the index comes from a UI that may be a moment behind the store, and
+/// a stale tap should do nothing rather than take the app down.
+fn map_task_line<F>(body: &str, index: usize, f: F) -> String
+where
+    F: FnOnce(&TaskLine<'_>) -> Option<String>,
+{
+    let lines: Vec<&str> = body.split('\n').collect();
+    let mut target: Option<usize> = None;
+    let mut seen = 0usize;
+    for (n, line) in lines.iter().enumerate() {
+        if parse_task_line(line).is_some() {
+            if seen == index {
+                target = Some(n);
+                break;
+            }
+            seen += 1;
+        }
+    }
+    let target = match target {
+        Some(n) => n,
+        None => return body.to_string(),
+    };
+    let replacement = match parse_task_line(lines[target]) {
+        Some(parsed) => f(&parsed),
+        None => return body.to_string(),
+    };
+
+    let mut out: Vec<String> = Vec::with_capacity(lines.len());
+    for (n, line) in lines.iter().enumerate() {
+        if n != target {
+            out.push((*line).to_string());
+        } else if let Some(new_line) = &replacement {
+            out.push(new_line.clone());
+        }
+        // None at the target line drops it, which is `remove_item`.
+    }
+    out.join("\n")
+}
+
+/// Tick or untick the `index`-th item.
+pub fn set_item_checked(body: &str, index: usize, checked: bool) -> String {
+    map_task_line(body, index, |t| {
+        Some(render_task_line(t.indent, t.bullet, checked, t.text))
+    })
+}
+
+/// Replace the text of the `index`-th item, keeping its state and its bullet.
+pub fn set_item_text(body: &str, index: usize, text: &str) -> String {
+    map_task_line(body, index, |t| {
+        Some(render_task_line(t.indent, t.bullet, t.checked, text.trim()))
+    })
+}
+
+/// Delete the `index`-th item, line and all.
+pub fn remove_item(body: &str, index: usize) -> String {
+    map_task_line(body, index, |_| None)
+}
+
+/// Add an item at the end of the body.
+///
+/// Spaced exactly as `import_export.py:_note_markdown` writes a list — a blank line
+/// between prose and the list, and nothing between consecutive items. That is not
+/// cosmetic: the server migration folds existing rows into bodies using the same
+/// layout, so an export taken before the migration and one taken after have to
+/// agree byte for byte.
+pub fn append_item(body: &str, text: &str) -> String {
+    let line = render_task_line("", '-', false, text.trim());
+    let trimmed = body.trim_end_matches('\n');
+    if trimmed.trim().is_empty() {
+        return line;
+    }
+    let follows_a_list = trimmed
+        .split('\n')
+        .next_back()
+        .is_some_and(|l| parse_task_line(l).is_some());
+    if follows_a_list {
+        format!("{trimmed}\n{line}")
+    } else {
+        format!("{trimmed}\n\n{line}")
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -72,4 +281,128 @@ mod tests {
     fn empty_body() {
         assert!(extract_tags("").is_empty());
     }
+
+    // ── checklist items ─────────────────────────────────────────────────────
+
+    fn item(text: &str, checked: bool) -> DerivedItem {
+        DerivedItem {
+            text: text.to_string(),
+            checked,
+        }
+    }
+
+    #[test]
+    fn items_basic() {
+        let body = "shopping\n\n- [ ] milk\n- [x] eggs";
+        assert_eq!(extract_items(body), vec![item("milk", false), item("eggs", true)]);
+    }
+
+    #[test]
+    fn items_may_sit_between_paragraphs() {
+        // The whole reason the body owns the list: a table of rows could only ever
+        // render after the prose.
+        let body = "before\n- [ ] middle\nafter";
+        assert_eq!(extract_items(body), vec![item("middle", false)]);
+    }
+
+    #[test]
+    fn items_reject_near_misses() {
+        // Each of these is prose, and each has been someone's bug report somewhere.
+        for body in [
+            "-[ ] no space after the dash",
+            "- [] empty brackets",
+            "- [ ]no space after the brackets",
+            "- [y] not a mark",
+            "a [ ] mid sentence",
+            "[ ] no bullet at all",
+        ] {
+            assert!(extract_items(body).is_empty(), "should be prose: {body}");
+        }
+    }
+
+    #[test]
+    fn items_accept_star_bullets_and_indentation() {
+        // `*` because markdown.ts already takes it for a plain bullet.
+        let body = "* [ ] star\n    - [x] indented";
+        assert_eq!(extract_items(body), vec![item("star", false), item("indented", true)]);
+    }
+
+    #[test]
+    fn an_empty_item_is_still_an_item() {
+        // What pressing Enter on a list leaves behind.
+        assert_eq!(extract_items("- [ ]"), vec![item("", false)]);
+        assert_eq!(extract_items("- [ ] "), vec![item("", false)]);
+    }
+
+    #[test]
+    fn uppercase_x_parses_and_normalises_on_rewrite() {
+        assert_eq!(extract_items("- [X] done"), vec![item("done", true)]);
+        // Touching it once canonicalises it, and never again.
+        assert_eq!(set_item_checked("- [X] done", 0, true), "- [x] done");
+    }
+
+    #[test]
+    fn checking_preserves_indent_bullet_and_text() {
+        assert_eq!(set_item_checked("  * [ ] milk", 0, true), "  * [x] milk");
+        assert_eq!(set_item_checked("- [x] milk", 0, false), "- [ ] milk");
+    }
+
+    #[test]
+    fn checking_addresses_items_not_lines() {
+        let body = "note\n- [ ] a\nprose\n- [ ] b";
+        assert_eq!(set_item_checked(body, 1, true), "note\n- [ ] a\nprose\n- [x] b");
+    }
+
+    #[test]
+    fn set_text_keeps_state() {
+        assert_eq!(set_item_text("- [x] old", 0, "new"), "- [x] new");
+    }
+
+    #[test]
+    fn remove_takes_the_whole_line() {
+        let body = "keep\n- [ ] drop\n- [ ] stay";
+        assert_eq!(remove_item(body, 0), "keep\n- [ ] stay");
+    }
+
+    #[test]
+    fn append_spaces_like_the_exporter() {
+        // Prose then a blank line then the list — byte-for-byte what
+        // import_export.py:_note_markdown writes, which is what the server
+        // migration will fold existing rows into.
+        assert_eq!(append_item("a note", "milk"), "a note\n\n- [ ] milk");
+        // Nothing between consecutive items.
+        let one = "a note\n\n- [ ] milk";
+        assert_eq!(append_item(one, "eggs"), format!("{one}\n- [ ] eggs"));
+        // A list-only note starts at the first line.
+        assert_eq!(append_item("", "milk"), "- [ ] milk");
+        assert_eq!(append_item("\n\n", "milk"), "- [ ] milk");
+    }
+
+    #[test]
+    fn a_stale_index_does_nothing() {
+        // The index comes from a UI that may be a moment behind the store. A tap
+        // that arrives late should be inert, not fatal.
+        let body = "- [ ] only";
+        assert_eq!(set_item_checked(body, 7, true), body);
+        assert_eq!(remove_item(body, 7), body);
+        assert_eq!(set_item_text(body, 7, "x"), body);
+    }
+
+    #[test]
+    fn a_plain_body_is_returned_byte_identical() {
+        let body = "just prose\nwith two lines";
+        assert_eq!(set_item_checked(body, 0, true), body);
+        assert_eq!(set_item_text(body, 0, "x"), body);
+        assert_eq!(remove_item(body, 0), body);
+    }
+
+    #[test]
+    fn round_trip_is_stable() {
+        let body = "- [ ] a\n- [x] b\n- [ ] c";
+        let items = extract_items(body);
+        // Ticking and unticking returns the original bytes.
+        let touched = set_item_checked(&set_item_checked(body, 0, true), 0, false);
+        assert_eq!(touched, body);
+        assert_eq!(extract_items(&touched), items);
+    }
 }