//! 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.
//!
//! Also derived `[[wiki-links]]` until they were removed (note 2897) — this is a
//! capture-and-recall surface, and a linking system is organization.

/// Extract every `#tag` name (without the leading `#`) from `body`.
pub fn extract_tags(body: &str) -> Vec<String> {
    let chars: Vec<char> = body.chars().collect();
    let mut out: Vec<String> = Vec::new();
    let mut i = 0;
    while i < chars.len() {
        if chars[i] == '#' {
            let boundary = i == 0 || (!is_tag_char(chars[i - 1]) && chars[i - 1] != '#');
            // A tag must start with a letter (so "#1" or a bare "#" is not a tag).
            if boundary && i + 1 < chars.len() && chars[i + 1].is_alphabetic() {
                let mut j = i + 1;
                while j < chars.len() && is_tag_char(chars[j]) {
                    j += 1;
                }
                let tag: String = chars[i + 1..j].iter().collect();
                push_unique(&mut out, &tag);
                i = j;
                continue;
            }
        }
        i += 1;
    }
    out
}

fn is_tag_char(c: char) -> bool {
    c.is_alphanumeric() || c == '_' || c == '-'
}

fn push_unique(out: &mut Vec<String>, candidate: &str) {
    if !out.iter().any(|x| x.eq_ignore_ascii_case(candidate)) {
        out.push(candidate.to_string());
    }
}

// ── 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}")
    }
}

/// The text of a line with its task marker removed, or the line as it was.
///
/// For naming a note: a list-only note is named by its first item, and calling one
/// "- [ ] milk" would be showing someone the storage instead of the note.
pub fn strip_marker(line: &str) -> &str {
    match parse_task_line(line) {
        Some(t) => t.text,
        None => line,
    }
}

/// 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.
///
/// `checked` is a parameter rather than always false because the two migrations that
/// fold existing rows into bodies have to carry the state those rows were in. A new
/// item from the UI passes false.
pub fn append_item(body: &str, text: &str, checked: bool) -> String {
    let line = render_task_line("", '-', checked, 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::*;

    #[test]
    fn tags_basic() {
        assert_eq!(
            extract_tags("a #todo and #Work-item_2 here"),
            vec!["todo", "Work-item_2"]
        );
    }

    #[test]
    fn tags_require_letter_start_and_boundary() {
        // "#1" (digit) and an in-word "#" (email-ish) are not tags.
        assert_eq!(extract_tags("#1 nope a#b no but #Yes"), vec!["Yes"]);
    }

    #[test]
    fn tags_dedupe_case_insensitive() {
        assert_eq!(extract_tags("#Home #home #HOME"), vec!["Home"]);
    }

    #[test]
    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", false), "a note\n\n- [ ] milk");
        // Nothing between consecutive items.
        let one = "a note\n\n- [ ] milk";
        assert_eq!(append_item(one, "eggs", false), format!("{one}\n- [ ] eggs"));
        // A list-only note starts at the first line.
        assert_eq!(append_item("", "milk", false), "- [ ] milk");
        assert_eq!(append_item("\n\n", "milk", false), "- [ ] milk");
        // Carries state, which is what the two migrations need of it.
        assert_eq!(append_item("", "done", true), "- [x] done");
    }

    #[test]
    fn strip_marker_names_a_list_only_note() {
        assert_eq!(strip_marker("- [x] milk"), "milk");
        assert_eq!(strip_marker("just prose"), "just prose");
    }

    #[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);
    }
}