//! TRACES: FR-UI-4
//! Interaction documentation, extracted from the code that implements it.
//!
//! # Why this exists at all
//!
//! FR-UI-4 says a gesture with no visible counterpart is a feature only its
//! author knows about. The application has grown a real gesture vocabulary —
//! hold to select, arm a range and tap its end, pinch to resize the grid, drag
//! a selection onto a collection — and every one of them was documented only in
//! the comment beside the `TouchArea` that implements it. Excellent comments,
//! and unreachable by anyone who is not reading the source.
//!
//! Writing them out a second time, in a hand-kept help page, is the failure
//! this file is built to avoid. Two descriptions of one gesture drift, and the
//! one that drifts is always the prose: the code is exercised every time
//! somebody uses the application and the page is exercised never. A help screen
//! that confidently describes a double-tap the grid stopped honouring six
//! months ago is worse than no help screen.
//!
//! So the comment beside the implementation is the only copy. This scans it out
//! into two artefacts — `docs/gestures.md` for a reader, and a generated Rust
//! table the application itself displays — and CI fails if either has drifted
//! from the source. The same discipline, and the same gate, the requirements
//! matrix already runs under.
//!
//! # Why it lives in the traceability crate
//!
//! It is the same operation on the same input: walk the source tree, pull
//! structured tags out of comments, render, and fail if the committed artefact
//! no longer matches. Everything but the tag vocabulary is shared — the file
//! walk, the extension list, the exclusions — and a second crate would have had
//! to duplicate all of it to add one parser.
//!
//! # The tag
//!
//! ```text
//! // GESTURE: Select a range
//! //   where:   Library grid
//! //   touch:   "Select to…", then tap the last photograph
//! //   pointer: Shift-click
//! //   why:     A finger sweep runs out at the edge of the screen, and the
//! //            ranges that hurt are longer than a screenful.
//! ```
//!
//! Works in `.rs` and `.slint` alike, because both comment with `//` and the
//! gestures live in both — the arbitration in Rust, the affordance in Slint.
//!
//! **The tag opens its comment or it is not a tag.** A line that merely
//! mentions it — a sentence about the vocabulary, a worked example in a doc
//! comment like the one above — is describing the mechanism rather than
//! declaring a gesture, and position is the only thing that tells the two
//! apart.
//!
//! A block runs from that line to the first line that is not a comment, or to a
//! comment line that is empty after its marker. A line reading
//! `key: value` sets a field; any other line inside the block continues the
//! field before it, which is what lets `why` run to a sentence. An unrecognised
//! key is an error rather than a silently dropped line: `pointr:` should not
//! quietly cost a gesture its desktop half.
//!
//! # `manual:`, the section that shows it
//!
//! Optional. It names a heading of `docs/manual/README.md` by its anchor —
//! `manual: looking-closer`, with or without the `#` — and the help sheet then
//! offers "See it", opening the bundled manual there, where a picture shows
//! the move being made. The anchor must exist: [`check_manual`] fails the scan
//! when the manual has no such heading, so renaming a section cannot leave a
//! link on the sheet that opens the manual at the top.

use std::collections::BTreeMap;

use serde::Serialize;

/// One documented interaction.
#[derive(Debug, Clone, Serialize, PartialEq, Eq)]
pub struct Gesture {
    /// What it does, in the user's terms — "Select a range", not "arm ranging".
    pub title: String,
    /// The place it applies, and the heading it is grouped under.
    pub section: String,
    /// What a finger does. `None` where the gesture is pointer-only.
    pub touch: Option<String>,
    /// What a mouse or trackpad does.
    pub pointer: Option<String>,
    /// The keyboard route, where there is one.
    pub keys: Option<String>,
    /// Why it works this way. Shown in the document, not in the application:
    /// a user consulting the help sheet wants the gesture, and a reader of the
    /// document wants the argument.
    pub why: Option<String>,
    /// The manual section that shows it, as a heading anchor without `#`.
    pub manual: Option<String>,
    pub file: String,
    pub line: usize,
}

impl Gesture {
    /// Every way in, longest-lived first.
    ///
    /// Touch leads because that is the modality the gesture vocabulary was
    /// grown for: ctrl and shift already documented themselves by existing on
    /// every other application, and the hold-and-tap sequences did not.
    pub fn routes(&self) -> Vec<(&'static str, &str)> {
        let mut out = Vec::new();
        if let Some(t) = &self.touch {
            out.push(("Touch", t.as_str()));
        }
        if let Some(p) = &self.pointer {
            out.push(("Pointer", p.as_str()));
        }
        if let Some(k) = &self.keys {
            out.push(("Keyboard", k.as_str()));
        }
        out
    }
}

/// A key that may appear in a gesture block.
const KEYS: &[&str] = &["where", "touch", "pointer", "keys", "why", "manual"];

/// Something wrong with a tag, reported rather than dropped.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct GestureProblem {
    pub file: String,
    pub line: usize,
    pub what: String,
}

impl std::fmt::Display for GestureProblem {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{}:{}: {}", self.file, self.line, self.what)
    }
}

/// Extract every gesture block from one file's text.
///
/// Returns what parsed and what did not. Both, deliberately: a malformed tag
/// that merely vanished would leave a gesture undocumented and everything
/// looking fine, which is the failure mode this whole file exists to prevent.
pub fn extract_from_text(text: &str, path: &str) -> (Vec<Gesture>, Vec<GestureProblem>) {
    let lines: Vec<&str> = text.lines().collect();
    let mut out = Vec::new();
    let mut problems = Vec::new();

    let mut i = 0;
    while i < lines.len() {
        // **The tag must open its comment**, not merely appear somewhere in
        // one. Prose that mentions it — "generated from the `GESTURE:` comments
        // beside the code" — is talking *about* the vocabulary, not declaring a
        // member of it, and the two are only distinguishable by position. The
        // gate caught exactly that sentence in `dr-ui`'s own module list, which
        // is the right outcome for a typo and the wrong one for a description.
        //
        // This also excludes a tag inside a string literal, which has code in
        // front of it and so does not open a comment at all.
        let Some(body) = comment_body(lines[i]) else {
            i += 1;
            continue;
        };
        let Some(title) = body.trim_start().strip_prefix("GESTURE:") else {
            i += 1;
            continue;
        };

        let start = i;
        let title = title.trim().to_string();

        let mut fields: BTreeMap<String, String> = BTreeMap::new();
        let mut last: Option<String> = None;
        i += 1;
        while i < lines.len() {
            let Some(body) = comment_body(lines[i]) else {
                break;
            };
            if body.trim().is_empty() {
                break;
            }
            // The next gesture ends this one. Without this, two blocks written
            // back to back — which is how a screen's gestures naturally get
            // listed — had the first swallow the second's title as a
            // continuation of its last field, and the second vanished.
            if body.contains("GESTURE:") {
                break;
            }
            match split_key(body) {
                Some((key, value)) => {
                    if !KEYS.contains(&key.as_str()) {
                        problems.push(GestureProblem {
                            file: path.to_string(),
                            line: i + 1,
                            what: format!(
                                "unknown gesture key `{key}` (expected one of {})",
                                KEYS.join(", ")
                            ),
                        });
                    }
                    fields.insert(key.clone(), value.trim().to_string());
                    last = Some(key);
                }
                // A continuation of the field above it, joined with a single
                // space: the source wraps at the column the file wraps at, and
                // those line breaks are not part of the sentence.
                None => match &last {
                    Some(key) => {
                        let entry = fields.entry(key.clone()).or_default();
                        if !entry.is_empty() {
                            entry.push(' ');
                        }
                        entry.push_str(body.trim());
                    }
                    None => break,
                },
            }
            i += 1;
        }

        let take = |k: &str| fields.get(k).filter(|v| !v.is_empty()).cloned();
        let g = Gesture {
            title: title.clone(),
            section: take("where").unwrap_or_default(),
            touch: take("touch"),
            pointer: take("pointer"),
            keys: take("keys"),
            why: take("why"),
            manual: take("manual").map(|m| m.trim_start_matches('#').to_string()),
            file: path.to_string(),
            line: start + 1,
        };

        // Validated here rather than at the gate, so the problem names the line
        // that caused it.
        if g.title.is_empty() {
            problems.push(GestureProblem {
                file: path.to_string(),
                line: start + 1,
                what: "gesture has no title".into(),
            });
        }
        if g.section.is_empty() {
            problems.push(GestureProblem {
                file: path.to_string(),
                line: start + 1,
                what: format!("gesture `{}` has no `where:`", g.title),
            });
        }
        if g.routes().is_empty() {
            problems.push(GestureProblem {
                file: path.to_string(),
                line: start + 1,
                what: format!(
                    "gesture `{}` says how to do it in no modality — needs at least one of \
                     touch, pointer or keys",
                    g.title
                ),
            });
        }
        out.push(g);
    }

    (out, problems)
}

/// The text of a comment line, marker stripped. `None` for anything else.
///
/// `///` and `//!` are stripped to the same thing: a gesture block written as a
/// doc comment on the function that implements it is the natural place for one,
/// and it should not need a different tag.
fn comment_body(line: &str) -> Option<&str> {
    let t = line.trim_start();
    let rest = t.strip_prefix("//")?;
    Some(
        rest.strip_prefix('!')
            .or_else(|| rest.strip_prefix('/'))
            .unwrap_or(rest),
    )
}

/// Split `  key: value` into its halves.
///
/// The key must be a bare word against the left of the comment body, which is
/// what keeps a colon inside prose — "so: the run is resolved by the catalog" —
/// from being read as a new field. A continuation is indented past its key, and
/// the key it continues is a single word; requiring both is enough to tell them
/// apart without counting spaces.
fn split_key(body: &str) -> Option<(String, String)> {
    let (head, tail) = body.split_once(':')?;
    let key = head.trim();
    if key.is_empty() || !key.chars().all(|c| c.is_ascii_alphabetic()) {
        return None;
    }
    // A key sits at the start of the comment body, under a shallow indent. A
    // continuation line is pushed further right to line up under its value.
    let indent = body.len() - body.trim_start().len();
    if indent > 5 {
        return None;
    }
    Some((key.to_ascii_lowercase(), tail.to_string()))
}

/// Every `manual:` that names a section the manual does not have.
///
/// `anchors` is [`crate::manual::anchors`] of the README — the ids the bundled
/// page gives its headings — so this and the page cannot disagree about what
/// exists.
pub fn check_manual(gestures: &[Gesture], anchors: &[String]) -> Vec<GestureProblem> {
    gestures
        .iter()
        .filter_map(|g| {
            let m = g.manual.as_ref()?;
            (!anchors.iter().any(|a| a == m)).then(|| GestureProblem {
                file: g.file.clone(),
                line: g.line,
                what: format!(
                    "gesture `{}` names manual section `#{m}`, which docs/manual/README.md \
                     has no heading for",
                    g.title
                ),
            })
        })
        .collect()
}

/// Group gestures under their `where:`, keeping the order the sections were
/// first seen so the document reads in the order a user meets them.
pub fn by_section(gestures: &[Gesture]) -> Vec<(String, Vec<&Gesture>)> {
    let mut order: Vec<String> = Vec::new();
    let mut map: BTreeMap<String, Vec<&Gesture>> = BTreeMap::new();
    for g in gestures {
        if !map.contains_key(&g.section) {
            order.push(g.section.clone());
        }
        map.entry(g.section.clone()).or_default().push(g);
    }
    order
        .into_iter()
        .map(|s| {
            let v = map.remove(&s).unwrap_or_default();
            (s, v)
        })
        .collect()
}

// ── rendering ─────────────────────────────────────────────────────────────

/// The reader's artefact: `docs/gestures.md`.
///
/// Carries `why` where the application's sheet does not. Someone reading the
/// document is deciding whether a gesture is right; someone holding the tablet
/// has already decided and wants the move.
pub fn render_markdown(gestures: &[Gesture]) -> String {
    let mut m = String::new();
    m.push_str("# How the application is driven\n\n");
    m.push_str("<!-- GENERATED FILE — do not edit by hand. -->\n");
    m.push_str("<!-- Regenerate: cargo run -p traceability -- gestures -->\n\n");
    m.push_str(
        "Every entry here is extracted from the comment beside the code that implements it, \
         so this file cannot describe a gesture the application does not have. Add one by \
         writing a `GESTURE:` block next to the implementation; there is nowhere else to \
         write it.\n\n",
    );
    let places = by_section(gestures).len();
    m.push_str(&format!(
        "{} gesture{}, in {} place{}.\n",
        gestures.len(),
        if gestures.len() == 1 { "" } else { "s" },
        places,
        if places == 1 { "" } else { "s" },
    ));

    for (section, list) in by_section(gestures) {
        m.push_str(&format!("\n## {section}\n"));
        for g in list {
            m.push_str(&format!("\n### {}\n\n", g.title));
            for (modality, how) in g.routes() {
                let how = if modality == "Keyboard" {
                    crate::chord::render_keys(how, true)
                } else {
                    how.to_string()
                };
                m.push_str(&format!("- **{modality}** — {how}\n"));
            }
            if let Some(anchor) = &g.manual {
                m.push_str(&format!(
                    "- **See it** — [in the manual](manual/README.md#{anchor})\n"
                ));
            }
            if let Some(why) = &g.why {
                m.push_str(&format!("\n{why}\n"));
            }
            m.push_str(&format!("\n<sub>`{}:{}`</sub>\n", g.file, g.line));
        }
    }
    m
}

/// The application's artefact: a Rust table it displays.
///
/// Generated and *committed*, not built in a `build.rs`. Two reasons. A
/// generated file that only exists in `OUT_DIR` cannot be read in review, and
/// this one is user-facing text that should be reviewed like any other. And the
/// scanner walks the whole source tree, which is not something to do on every
/// incremental build of the crate being scanned.
///
/// `why` is deliberately not carried across. It is the argument for the design
/// and belongs in the document; putting it on a phone-sized help sheet would
/// bury the one line the user opened the sheet to read.
pub fn render_rust(gestures: &[Gesture]) -> String {
    let mut r = String::new();
    r.push_str("// @generated by `cargo run -p traceability -- gestures`. Do not edit.\n");
    r.push_str("//\n");
    r.push_str("// TRACES: FR-UI-4\n");
    r.push_str("//! The gesture vocabulary, as the application states it.\n");
    r.push_str("//!\n");
    r.push_str("//! Extracted from the tagged comments beside the code that implements each\n");
    r.push_str("//! one, so the help sheet cannot describe a gesture the application does not\n");
    r.push_str("//! have. Edit the comment beside the implementation, then regenerate with\n");
    r.push_str("//! `cargo run -p traceability -- gestures`.\n\n");

    r.push_str("/// One documented interaction, as drawn.\n");
    r.push_str("#[derive(Debug, Clone, Copy, PartialEq, Eq)]\n");
    r.push_str("pub struct Gesture {\n");
    r.push_str("    pub title: &'static str,\n");
    r.push_str("    /// The place it applies, and the heading it is grouped under.\n");
    r.push_str("    pub section: &'static str,\n");
    r.push_str("    /// Empty where the gesture has no counterpart in that modality.\n");
    r.push_str("    pub touch: &'static str,\n");
    r.push_str("    pub pointer: &'static str,\n");
    r.push_str("    pub keys: &'static str,\n");
    r.push_str("    /// The manual section that shows it, a heading anchor; empty for none.\n");
    r.push_str("    pub manual: &'static str,\n");
    r.push_str("}\n\n");

    r.push_str("/// Every gesture, in the order the sections were first seen in the source.\n");
    r.push_str("pub const GESTURES: &[Gesture] = &[\n");
    for (_, list) in by_section(gestures) {
        for g in list {
            r.push_str("    Gesture {\n");
            r.push_str(&format!("        title: {},\n", quote(&g.title)));
            r.push_str(&format!("        section: {},\n", quote(&g.section)));
            r.push_str(&format!(
                "        touch: {},\n",
                quote(g.touch.as_deref().unwrap_or(""))
            ));
            r.push_str(&format!(
                "        pointer: {},\n",
                quote(g.pointer.as_deref().unwrap_or(""))
            ));
            r.push_str(&format!(
                "        keys: {},\n",
                quote(&crate::chord::render_keys(
                    g.keys.as_deref().unwrap_or(""),
                    false
                ))
            ));
            r.push_str(&format!(
                "        manual: {},\n",
                quote(g.manual.as_deref().unwrap_or(""))
            ));
            r.push_str("    },\n");
        }
    }
    r.push_str("];\n");
    r
}

/// A Rust string literal for arbitrary gesture text.
///
/// The text is prose written by hand and routinely contains quotation marks —
/// `"Select to…", then tap the last photograph` — so escaping is not optional.
fn quote(s: &str) -> String {
    let mut out = String::with_capacity(s.len() + 2);
    out.push('"');
    for c in s.chars() {
        match c {
            '"' => out.push_str("\\\""),
            '\\' => out.push_str("\\\\"),
            _ => out.push(c),
        }
    }
    out.push('"');
    out
}

#[cfg(test)]
mod tests {
    use super::*;

    /// The shape every real tag is written in.
    const SAMPLE: &str = "\
// GESTURE: Select a range
//   where:   Library grid
//   touch:   \"Select to…\", then tap the last photograph
//   pointer: Shift-click
//   why:     A finger sweep runs out at the edge of the
//            screen.
property <bool> ranging: false;
";

    #[test]
    fn a_block_parses_every_field() {
        let (g, p) = extract_from_text(SAMPLE, "library.slint");
        assert!(p.is_empty(), "{p:?}");
        assert_eq!(g.len(), 1);
        assert_eq!(g[0].title, "Select a range");
        assert_eq!(g[0].section, "Library grid");
        assert_eq!(
            g[0].touch.as_deref(),
            Some("\"Select to…\", then tap the last photograph")
        );
        assert_eq!(g[0].pointer.as_deref(), Some("Shift-click"));
        assert_eq!(g[0].line, 1);
    }

    /// The source wraps where the file wraps; the sentence does not.
    #[test]
    fn a_continuation_joins_with_one_space() {
        let (g, _) = extract_from_text(SAMPLE, "x.slint");
        assert_eq!(
            g[0].why.as_deref(),
            Some("A finger sweep runs out at the edge of the screen.")
        );
    }

    /// A colon in prose is not a new field, or `why` could never contain one.
    #[test]
    fn a_colon_inside_a_continuation_is_not_a_key() {
        let text = "\
// GESTURE: Hold to select
//   where: Library grid
//   touch: Press and hold
//   why:   The platform convention: every gallery on the device
//          opens a selection this way.
";
        let (g, p) = extract_from_text(text, "x.rs");
        assert!(p.is_empty(), "{p:?}");
        assert_eq!(
            g[0].why.as_deref(),
            Some(
                "The platform convention: every gallery on the device opens a selection this way."
            )
        );
    }

    /// A typo must not quietly cost a gesture its desktop half.
    #[test]
    fn an_unknown_key_is_reported() {
        let text = "\
// GESTURE: Open a photograph
//   where: Library grid
//   touch: Tap it
//   pointr: Click it
";
        let (_, p) = extract_from_text(text, "x.rs");
        assert_eq!(p.len(), 1, "{p:?}");
        assert!(p[0].what.contains("pointr"), "{}", p[0].what);
    }

    /// A gesture nobody can perform is a documentation bug, not a gesture.
    #[test]
    fn a_gesture_with_no_modality_is_reported() {
        let text = "// GESTURE: Do a thing\n//   where: Somewhere\n";
        let (_, p) = extract_from_text(text, "x.rs");
        assert!(p.iter().any(|x| x.what.contains("no modality")), "{p:?}");
    }

    #[test]
    fn a_gesture_with_no_place_is_reported() {
        let text = "// GESTURE: Do a thing\n//   touch: Tap\n";
        let (_, p) = extract_from_text(text, "x.rs");
        assert!(p.iter().any(|x| x.what.contains("`where:`")), "{p:?}");
    }

    /// The tag inside this crate's own doc comments and test fixtures must not
    /// become a gesture in the user's help sheet.
    #[test]
    fn a_tag_inside_a_string_literal_is_not_a_gesture() {
        let text = "let s = \"GESTURE: not a real one\";\n";
        let (g, p) = extract_from_text(text, "x.rs");
        assert!(g.is_empty(), "{g:?}");
        assert!(p.is_empty());
    }

    /// Prose *about* the vocabulary is not a member of it. The gate caught this
    /// exact sentence in `dr-ui`'s module list, where a comment explaining
    /// where the generated table comes from named the tag in passing.
    #[test]
    fn a_sentence_that_mentions_the_tag_is_not_a_gesture() {
        let text = "\
// Generated from the `GESTURE:` comments beside the code that implements
// each one — see `tools/traceability`.
mod gesture_book;
";
        let (g, p) = extract_from_text(text, "lib.rs");
        assert!(g.is_empty(), "{g:?}");
        assert!(p.is_empty(), "{p:?}");
    }

    /// A blank comment line ends the block, so ordinary prose beneath a gesture
    /// is not swallowed into its `why`.
    #[test]
    fn a_blank_comment_line_ends_the_block() {
        let text = "\
// GESTURE: Tap to open
//   where: Library grid
//   touch: Tap a photograph
//
// This paragraph is about something else entirely.
";
        let (g, _) = extract_from_text(text, "x.rs");
        assert_eq!(g[0].touch.as_deref(), Some("Tap a photograph"));
        assert!(g[0].why.is_none(), "{:?}", g[0].why);
    }

    /// A doc comment is the natural place for a gesture on the function that
    /// implements it, and must not need a different tag.
    #[test]
    fn a_doc_comment_carries_a_gesture_too() {
        let text = "\
/// GESTURE: Pinch the grid
///   where: Library grid
///   touch: Pinch to resize the thumbnails
pub fn zoom() {}
";
        let (g, p) = extract_from_text(text, "x.rs");
        assert!(p.is_empty(), "{p:?}");
        assert_eq!(g[0].title, "Pinch the grid");
    }

    /// Two blocks back to back, which is how a screen's gestures get listed.
    #[test]
    fn one_block_does_not_swallow_the_next() {
        let text = "\
// GESTURE: First
//   where: Grid
//   touch: Tap
// GESTURE: Second
//   where: Grid
//   touch: Hold
";
        let (g, p) = extract_from_text(text, "x.rs");
        assert!(p.is_empty(), "{p:?}");
        assert_eq!(g.len(), 2, "{g:?}");
        assert_eq!(g[0].touch.as_deref(), Some("Tap"));
        assert_eq!(g[1].title, "Second");
    }

    /// `manual:` is read, with or without its `#`, and a section the manual
    /// does not have is a problem rather than a link to the top of the page.
    #[test]
    fn a_manual_anchor_must_name_a_section_the_manual_has() {
        let text = "\
// GESTURE: Zoom
//   where: Develop
//   touch: Pinch
//   manual: #looking-closer
// GESTURE: Pan
//   where: Develop
//   touch: Drag
//   manual: looking-further
";
        let (g, p) = extract_from_text(text, "x.slint");
        assert!(p.is_empty(), "{p:?}");
        assert_eq!(g[0].manual.as_deref(), Some("looking-closer"));
        let anchors = crate::manual::anchors("## Looking closer\n");
        let problems = check_manual(&g, &anchors);
        assert_eq!(problems.len(), 1, "{problems:?}");
        assert!(problems[0].what.contains("looking-further"));
        assert_eq!(problems[0].line, 5);
    }

    #[test]
    fn a_manual_anchor_is_linked_from_the_document_and_carried_in_the_table() {
        let text = "// GESTURE: Zoom\n//   where: Develop\n//   touch: Pinch\n//   manual: looking-closer\n";
        let (g, _) = extract_from_text(text, "x.slint");
        assert!(render_markdown(&g).contains("(manual/README.md#looking-closer)"));
        assert!(render_rust(&g).contains("manual: \"looking-closer\","));
    }

    #[test]
    fn sections_keep_the_order_they_were_first_seen() {
        let text = "\
// GESTURE: B
//   where: Second
//   touch: x
// GESTURE: A
//   where: First
//   touch: x
// GESTURE: C
//   where: Second
//   touch: x
";
        let (g, _) = extract_from_text(text, "x.rs");
        let grouped = by_section(&g);
        assert_eq!(grouped[0].0, "Second");
        assert_eq!(grouped[0].1.len(), 2);
        assert_eq!(grouped[1].0, "First");
    }
}