//! Touch gestures straight from the kernel, replacing lisgd.
//!
//! libinput deliberately emits gesture events only for touchpads, never for
//! touchscreens, so libinput-gestures and friends cannot work here at all. We
//! read multitouch protocol B from the event device and synthesise swipes.
//!
//! Protocol B reports each contact in a numbered slot: `ABS_MT_SLOT` selects the
//! slot, `ABS_MT_TRACKING_ID` of -1 lifts it, and `ABS_MT_POSITION_X/Y` update
//! it. We record where each slot started and where it ended, then decide on the
//! release of the last contact.

use std::collections::HashMap;
use std::path::Path;

use anyhow::{bail, Context, Result};
use evdev::{AbsoluteAxisType, Device, InputEventKind};

use crate::config::{Direction, Touch};

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct Swipe {
    pub direction: Direction,
    /// Peak simultaneous contacts during the gesture.
    pub fingers: usize,
}

/// One contact's journey.
///
/// The start position is per axis and `Option`, NOT a `(0, 0)` sentinel. X and Y
/// arrive as SEPARATE events, so a contact's opening frame is `POSITION_X` then
/// `POSITION_Y`: capturing "the start" on the first of those records a Y of
/// zero, and every later comparison then measures from the top edge of the panel
/// rather than from the finger. That made `dy` enormous and positive, so every
/// swipe -- horizontal ones included -- classified as up-to-down.
#[derive(Debug, Default, Clone, Copy)]
struct Slot {
    start_x: Option<i32>,
    start_y: Option<i32>,
    last: (i32, i32),
    active: bool,
}

impl Slot {
    /// An axis that never reported a position contributes no displacement.
    fn track(&self) -> Track {
        (
            (
                self.start_x.unwrap_or(self.last.0),
                self.start_y.unwrap_or(self.last.1),
            ),
            self.last,
        )
    }
}

/// The parts of a multitouch protocol-B stream we care about. Kept separate from
/// evdev's own types so the slot bookkeeping is testable without a device --
/// which is precisely the code the above bug lived in, untested.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Touched {
    /// `ABS_MT_SLOT`: subsequent events apply to this slot.
    Slot(i32),
    /// `ABS_MT_TRACKING_ID`: a new contact when >= 0, a lift when -1.
    TrackingId(i32),
    X(i32),
    Y(i32),
}

/// Slot bookkeeping for multitouch protocol B.
#[derive(Debug, Default)]
pub struct SlotTracker {
    slots: HashMap<i32, Slot>,
    current: i32,
    peak: usize,
}

impl SlotTracker {
    pub fn feed(&mut self, ev: Touched) {
        match ev {
            Touched::Slot(n) => self.current = n,
            Touched::TrackingId(id) => {
                if id < 0 {
                    if let Some(s) = self.slots.get_mut(&self.current) {
                        s.active = false;
                    }
                } else {
                    self.slots.insert(
                        self.current,
                        Slot {
                            active: true,
                            ..Slot::default()
                        },
                    );
                    self.peak = self.peak.max(self.active());
                }
            }
            Touched::X(x) => {
                let s = self.slot();
                s.last.0 = x;
                s.start_x.get_or_insert(x);
            }
            Touched::Y(y) => {
                let s = self.slot();
                s.last.1 = y;
                s.start_y.get_or_insert(y);
            }
        }
    }

    fn slot(&mut self) -> &mut Slot {
        self.slots.entry(self.current).or_insert(Slot {
            active: true,
            ..Slot::default()
        })
    }

    fn active(&self) -> usize {
        self.slots.values().filter(|s| s.active).count()
    }

    /// A gesture is over when every contact seen has lifted.
    pub fn complete(&self) -> bool {
        !self.slots.is_empty() && self.active() == 0
    }

    pub fn tracks(&self) -> Vec<Track> {
        self.slots.values().map(Slot::track).collect()
    }

    /// Peak simultaneous contacts, which is what `touch.fingers` matches.
    pub fn peak(&self) -> usize {
        self.peak
    }

    pub fn reset(&mut self) {
        self.slots.clear();
        self.peak = 0;
    }
}

pub struct Touchpanel {
    device: Device,
    cfg: Touch,
}

/// Why a candidate gesture was not emitted. Only ever shown by `doctor`.
#[derive(Debug, Clone)]
pub enum Rejected {
    TooShort { travel: f64, threshold: u32 },
    OffAxis { degrees: f64, leniency: u32 },
    WrongFingerCount { saw: usize, want: Vec<usize> },
}

impl std::fmt::Display for Rejected {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::TooShort { travel, threshold } => write!(
                f,
                "travelled {travel:.0}px, needs {threshold}px (touch.threshold)"
            ),
            Self::OffAxis { degrees, leniency } => write!(
                f,
                "{degrees:.0}\u{b0} off axis, tolerance is {leniency}\u{b0} (touch.leniency)"
            ),
            Self::WrongFingerCount { saw, want } => write!(
                f,
                "saw {saw} contact(s), config accepts {want:?} -- add {saw} to touch.fingers"
            ),
        }
    }
}

#[derive(Debug, Clone)]
pub enum Event {
    Swipe(Swipe),
    /// A gesture was seen and discarded. Carries the direction it would have
    /// been, so `doctor` can say "that was a left swipe, but ...".
    Discarded(Direction, Rejected),
}

impl Touchpanel {
    pub fn open(cfg: &Touch) -> Result<Self> {
        let path = Path::new(&cfg.device);
        if !path.exists() {
            bail!(
                "{} does not exist.\n\
                 Touch devices move around when USB re-enumerates -- always use a \
                 /dev/input/by-id/ path, never eventN.\n\
                 Run `socktop-swipe doctor --list` to see what is present.",
                cfg.device
            );
        }

        let mut device = Device::open(path).with_context(|| {
            format!(
                "cannot open {}.\n\
                 Reading touch events needs access to the device. Either install the \
                 udev rule (packaging/70-socktop-swipe.rules) or add yourself to the \
                 'input' group and log out and back in.",
                cfg.device
            )
        })?;

        let has_mt = device
            .supported_absolute_axes()
            .is_some_and(|a| a.contains(AbsoluteAxisType::ABS_MT_POSITION_X));
        if !has_mt {
            bail!(
                "{} does not report multitouch positions -- it is probably not the \
                 touchscreen.\nRun `socktop-swipe doctor --list`.",
                cfg.device
            );
        }

        if cfg.grab {
            // Take the device exclusively so X never sees the touches. This is
            // what the v1 xorg.conf.d "Ignore" rule was faking, and it needs no
            // X restart or relogin.
            device.grab().with_context(|| {
                format!(
                    "cannot take exclusive control of {}.\n\
                     Something else may already hold it (another socktop-swipe?). \
                     Set touch.grab: false to share the device with X, but then also \
                     install the X ignore rule -- see the README.",
                    cfg.device
                )
            })?;
        }

        Ok(Self {
            device,
            cfg: cfg.clone(),
        })
    }

    /// Blocking gesture loop. Calls `on_event` for every completed gesture,
    /// including rejected ones, and stops when it returns `false`.
    pub fn run(&mut self, mut on_event: impl FnMut(Event) -> bool) -> Result<()> {
        let mut tracker = SlotTracker::default();

        loop {
            for ev in self.device.fetch_events().context("reading touch events")? {
                let touched = match ev.kind() {
                    InputEventKind::AbsAxis(AbsoluteAxisType::ABS_MT_SLOT) => {
                        Touched::Slot(ev.value())
                    }
                    InputEventKind::AbsAxis(AbsoluteAxisType::ABS_MT_TRACKING_ID) => {
                        Touched::TrackingId(ev.value())
                    }
                    InputEventKind::AbsAxis(AbsoluteAxisType::ABS_MT_POSITION_X) => {
                        Touched::X(ev.value())
                    }
                    InputEventKind::AbsAxis(AbsoluteAxisType::ABS_MT_POSITION_Y) => {
                        Touched::Y(ev.value())
                    }
                    _ => continue,
                };
                tracker.feed(touched);
            }

            // The gesture ends when the last contact lifts.
            if tracker.complete() {
                if let Some(ev) = classify(&self.cfg, &tracker.tracks(), tracker.peak()) {
                    if !on_event(ev) {
                        return Ok(());
                    }
                }
                tracker.reset();
            }
        }
    }
}

/// One contact's journey: where it landed and where it lifted.
pub type Track = ((i32, i32), (i32, i32));

/// Decide what a completed gesture was. Split out from the device so the rules
/// -- threshold, leniency, contact count -- can be tested without hardware.
pub fn classify(cfg: &Touch, tracks: &[Track], peak: usize) -> Option<Event> {
    // Average the contacts' travel: a "one finger" swipe that the panel reports
    // as two or three contacts is one motion counted repeatedly, so the mean is
    // the real displacement rather than a multiple of it.
    let n = tracks.len() as f64;
    if n == 0.0 {
        return None;
    }
    let (dx, dy) = tracks.iter().fold((0.0, 0.0), |(ax, ay), (start, last)| {
        (
            ax + (last.0 - start.0) as f64 / n,
            ay + (last.1 - start.1) as f64 / n,
        )
    });

    let travel = (dx * dx + dy * dy).sqrt();
    let horizontal = dx.abs() >= dy.abs();
    let direction = match (horizontal, dx > 0.0, dy > 0.0) {
        (true, true, _) => Direction::LR,
        (true, false, _) => Direction::RL,
        // Y grows downward on a touch panel, so a positive dy is a downward
        // swipe: up-to-down.
        (false, _, true) => Direction::UD,
        (false, _, false) => Direction::DU,
    };

    if travel < cfg.threshold as f64 {
        return Some(Event::Discarded(
            direction,
            Rejected::TooShort {
                travel,
                threshold: cfg.threshold,
            },
        ));
    }

    // Angle away from the dominant axis.
    let (along, across) = if horizontal {
        (dx.abs(), dy.abs())
    } else {
        (dy.abs(), dx.abs())
    };
    let degrees = across.atan2(along).to_degrees();
    if degrees > cfg.leniency as f64 {
        return Some(Event::Discarded(
            direction,
            Rejected::OffAxis {
                degrees,
                leniency: cfg.leniency,
            },
        ));
    }

    if !cfg.fingers.contains(&peak) {
        return Some(Event::Discarded(
            direction,
            Rejected::WrongFingerCount {
                saw: peak,
                want: cfg.fingers.clone(),
            },
        ));
    }

    Some(Event::Swipe(Swipe {
        direction,
        fingers: peak,
    }))
}

/// Candidate touchscreens, for `doctor --list` and the installer.
pub fn list_touchscreens() -> Vec<(String, String)> {
    let mut out = Vec::new();
    let by_id = Path::new("/dev/input/by-id");
    let entries = std::fs::read_dir(by_id).into_iter().flatten().flatten();
    for e in entries {
        let path = e.path();
        let Ok(dev) = Device::open(&path) else {
            continue;
        };
        let multitouch = dev
            .supported_absolute_axes()
            .is_some_and(|a| a.contains(AbsoluteAxisType::ABS_MT_POSITION_X));
        if multitouch {
            out.push((
                path.to_string_lossy().into_owned(),
                dev.name().unwrap_or("unnamed device").to_owned(),
            ));
        }
    }
    out.sort();
    out
}

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

    fn touch(threshold: u32, leniency: u32, fingers: Vec<usize>) -> Touch {
        Touch {
            device: "/dev/null".into(),
            width: 1280,
            height: 720,
            grab: false,
            threshold,
            leniency,
            fingers,
        }
    }

    /// One contact travelling from `from` by `(dx, dy)`.
    fn track(from: (i32, i32), dx: i32, dy: i32) -> Track {
        (from, (from.0 + dx, from.1 + dy))
    }

    fn swipe(cfg: &Touch, tracks: &[Track], peak: usize) -> Event {
        classify(cfg, tracks, peak).expect("a completed gesture should classify")
    }

    #[test]
    fn recognises_the_four_directions() {
        let cfg = touch(80, 30, vec![1]);
        let cases = [
            ((-200, 0), Direction::RL),
            ((200, 0), Direction::LR),
            // Y grows downward, so a negative dy is a swipe upward.
            ((0, -200), Direction::DU),
            ((0, 200), Direction::UD),
        ];
        for ((dx, dy), want) in cases {
            match swipe(&cfg, &[track((640, 360), dx, dy)], 1) {
                Event::Swipe(s) => assert_eq!(s.direction, want, "({dx},{dy})"),
                other => panic!("({dx},{dy}) should be a swipe, got {other:?}"),
            }
        }
    }

    #[test]
    fn ghost_contacts_do_not_multiply_the_travel() {
        // The ILITEK panel reports one physical finger as 2-3 contacts. Each
        // reports the same motion, so the average must equal one finger's
        // travel -- not the sum, which would make short drags look long.
        let cfg = touch(150, 30, vec![1, 2, 3]);
        let one = [track((900, 300), -100, 0)];
        let three = [
            track((900, 300), -100, 0),
            track((902, 305), -100, 0),
            track((898, 295), -100, 0),
        ];
        for tracks in [&one[..], &three[..]] {
            match classify(&cfg, tracks, tracks.len()) {
                Some(Event::Discarded(Direction::RL, Rejected::TooShort { travel, .. })) => {
                    assert!((travel - 100.0).abs() < 1.0, "travel was {travel}");
                }
                other => panic!("100px under a 150px threshold should be too short: {other:?}"),
            }
        }
    }

    #[test]
    fn short_drags_are_rejected_with_the_measurement() {
        let cfg = touch(80, 30, vec![1]);
        match swipe(&cfg, &[track((640, 360), -40, 0)], 1) {
            Event::Discarded(Direction::RL, Rejected::TooShort { travel, threshold }) => {
                assert_eq!(threshold, 80);
                assert!((travel - 40.0).abs() < 0.01);
            }
            other => panic!("40px should be too short: {other:?}"),
        }
    }

    #[test]
    fn diagonal_swipes_are_rejected_past_the_leniency() {
        let cfg = touch(80, 30, vec![1]);
        // 45 degrees: equal travel on both axes, well past a 30 degree tolerance.
        match swipe(&cfg, &[track((640, 360), -200, -200)], 1) {
            Event::Discarded(_, Rejected::OffAxis { degrees, leniency }) => {
                assert_eq!(leniency, 30);
                assert!((degrees - 45.0).abs() < 0.01, "was {degrees}");
            }
            other => panic!("a 45 degree drag should be off-axis: {other:?}"),
        }
        // 20 degrees off: within tolerance, still a left swipe.
        let dy = -(200.0 * 20f64.to_radians().tan()) as i32;
        match swipe(&cfg, &[track((640, 360), -200, dy)], 1) {
            Event::Swipe(s) => assert_eq!(s.direction, Direction::RL),
            other => panic!("20 degrees off axis should pass: {other:?}"),
        }
    }

    #[test]
    fn unconfigured_contact_counts_are_rejected_and_name_the_fix() {
        // The v1 failure that cost the most time: gestures detected perfectly,
        // nothing ever fires, because the panel reports 2 contacts and the
        // config accepts only 1.
        let cfg = touch(80, 30, vec![1]);
        match swipe(
            &cfg,
            &[track((900, 300), -200, 0), track((905, 305), -200, 0)],
            2,
        ) {
            Event::Discarded(Direction::RL, r @ Rejected::WrongFingerCount { saw, .. }) => {
                assert_eq!(saw, 2);
                assert!(r.to_string().contains("touch.fingers"), "{r}");
            }
            other => panic!("2 contacts against fingers:[1] should be rejected: {other:?}"),
        }
    }

    #[test]
    fn threshold_is_checked_before_the_contact_count() {
        // Otherwise a stray tap on a panel with ghost contacts reports the
        // finger-count problem, sending you to fix the wrong setting.
        let cfg = touch(80, 30, vec![1]);
        match swipe(
            &cfg,
            &[track((640, 360), -5, 0), track((641, 361), -5, 0)],
            2,
        ) {
            Event::Discarded(_, Rejected::TooShort { .. }) => {}
            other => panic!("expected the short-travel reason first: {other:?}"),
        }
    }

    // -- slot tracking: real protocol-B event streams -----------------------
    //
    // These exist because the classifier tests above build Track tuples by hand
    // and so never exercised the decoding. The bug that shipped to the rack
    // display lived exactly here: every swipe came out as up-to-down, and the
    // display got stuck on the bottom row because "down" from there is a no-op.

    /// One contact moving from `from` to `to`, reported the way the kernel does:
    /// tracking id, then X and Y as separate events, then a lift.
    fn contact(t: &mut SlotTracker, slot: i32, from: (i32, i32), to: (i32, i32), steps: i32) {
        t.feed(Touched::Slot(slot));
        t.feed(Touched::TrackingId(slot + 1));
        for i in 0..=steps {
            let x = from.0 + (to.0 - from.0) * i / steps;
            let y = from.1 + (to.1 - from.1) * i / steps;
            t.feed(Touched::Slot(slot));
            t.feed(Touched::X(x));
            t.feed(Touched::Y(y));
        }
    }

    fn lift(t: &mut SlotTracker, slot: i32) {
        t.feed(Touched::Slot(slot));
        t.feed(Touched::TrackingId(-1));
    }

    #[test]
    fn a_horizontal_swipe_low_on_the_panel_is_not_read_as_downward() {
        // THE REGRESSION. A right-to-left swipe at y=360 on a 720-tall panel.
        // With a (0,0) start sentinel, Y was captured as 0 and dy became +360,
        // dwarfing dx and classifying this as UD.
        let mut t = SlotTracker::default();
        contact(&mut t, 0, (900, 360), (700, 362), 10);
        lift(&mut t, 0);
        assert!(t.complete());

        match classify(&touch(80, 30, vec![1]), &t.tracks(), t.peak()) {
            Some(Event::Swipe(s)) => {
                assert_eq!(
                    s.direction,
                    Direction::RL,
                    "a left swipe must not read as down"
                );
                assert_eq!(s.fingers, 1);
            }
            other => panic!("expected an RL swipe, got {other:?}"),
        }
    }

    #[test]
    fn start_is_captured_per_axis() {
        // X arrives before Y, so a start captured on the first event alone would
        // record y=0 and report the finger travelling the height of the panel.
        let mut t = SlotTracker::default();
        t.feed(Touched::Slot(0));
        t.feed(Touched::TrackingId(1));
        t.feed(Touched::X(900));
        t.feed(Touched::Y(360));
        t.feed(Touched::X(700));
        t.feed(Touched::Y(360));
        lift(&mut t, 0);
        assert_eq!(t.tracks(), vec![((900, 360), (700, 360))]);
    }

    #[test]
    fn all_four_directions_survive_decoding() {
        let cfg = touch(80, 30, vec![1]);
        // Centre of a 1280x720 panel, 200px each way.
        for (to, want) in [
            ((440, 360), Direction::RL),
            ((840, 360), Direction::LR),
            ((640, 160), Direction::DU),
            ((640, 560), Direction::UD),
        ] {
            let mut t = SlotTracker::default();
            contact(&mut t, 0, (640, 360), to, 10);
            lift(&mut t, 0);
            match classify(&cfg, &t.tracks(), t.peak()) {
                Some(Event::Swipe(s)) => assert_eq!(s.direction, want, "moving to {to:?}"),
                other => panic!("moving to {to:?}: {other:?}"),
            }
        }
    }

    #[test]
    fn ghost_contacts_decode_as_one_swipe_of_the_right_length() {
        // The ILITEK panel reports one finger as two or three contacts.
        let mut t = SlotTracker::default();
        contact(&mut t, 0, (900, 300), (700, 300), 8);
        contact(&mut t, 1, (903, 305), (703, 305), 8);
        contact(&mut t, 2, (897, 295), (697, 295), 8);
        lift(&mut t, 0);
        lift(&mut t, 1);
        lift(&mut t, 2);

        assert_eq!(t.peak(), 3, "peak contacts drive the touch.fingers match");
        match classify(&touch(150, 30, vec![1, 2, 3]), &t.tracks(), t.peak()) {
            Some(Event::Swipe(s)) => {
                assert_eq!(s.direction, Direction::RL);
                assert_eq!(s.fingers, 3);
            }
            other => panic!("expected one RL swipe: {other:?}"),
        }
        // 200px of travel, not 600.
        match classify(&touch(250, 30, vec![1, 2, 3]), &t.tracks(), t.peak()) {
            Some(Event::Discarded(_, Rejected::TooShort { travel, .. })) => {
                assert!((travel - 200.0).abs() < 2.0, "travel was {travel}");
            }
            other => panic!("expected 200px of travel: {other:?}"),
        }
    }

    #[test]
    fn a_gesture_is_only_complete_once_every_contact_lifts() {
        let mut t = SlotTracker::default();
        assert!(!t.complete(), "nothing has been touched yet");
        contact(&mut t, 0, (900, 300), (800, 300), 4);
        assert!(!t.complete(), "still down");
        contact(&mut t, 1, (500, 300), (400, 300), 4);
        lift(&mut t, 0);
        assert!(!t.complete(), "one contact is still down");
        lift(&mut t, 1);
        assert!(t.complete());
    }

    #[test]
    fn reset_clears_the_previous_gesture() {
        let mut t = SlotTracker::default();
        contact(&mut t, 0, (900, 300), (700, 300), 4);
        lift(&mut t, 0);
        t.reset();
        assert!(!t.complete());
        assert_eq!(t.peak(), 0);
        assert!(t.tracks().is_empty());

        // A second swipe must measure from its own origin, not the first one's.
        contact(&mut t, 0, (300, 300), (500, 300), 4);
        lift(&mut t, 0);
        match classify(&touch(80, 30, vec![1]), &t.tracks(), t.peak()) {
            Some(Event::Swipe(s)) => assert_eq!(s.direction, Direction::LR),
            other => panic!("{other:?}"),
        }
    }

    #[test]
    fn a_gesture_with_no_contacts_is_not_a_gesture() {
        assert!(classify(&touch(80, 30, vec![1]), &[], 0).is_none());
    }
}