//! Progress and Cancel of an export running off the UI thread (Background Export), on the shared
//! [`vectorcraft_doc::cancel`] mechanism.
//!
//! The frontend makes a [`Progress`], runs the export inside [`scope`] on its worker and reads the
//! counts from the UI thread to draw them; its Cancel button calls [`Progress::cancel`]. Inside the
//! scope the encoders that work in steps (raster formats: one artboard at a time) report each step
//! through [`vectorcraft_doc::cancel::progress`] and stop at the next check once cancelled; the
//! renderer stops drawing and keeps no half-drawn pattern tile or effect raster. Encoders that
//! make one file in one go report nothing: the frontend shows them as busy and checks
//! [`Progress::cancelled`] itself before writing, so a cancelled export never writes a file.
//! Export for Screens ([`super::ScreenExport::write`]) reports one step per file instead, keeps
//! its encoders' reports out, and stops before the next file, so the files it wrote before the
//! cancel stay.

use std::rc::Rc;
use std::sync::Arc;
use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};

use vectorcraft_doc::cancel::{self, CancelToken, ProgressSink};

/// How far an export has got, shared between the worker and the UI thread.
#[derive(Debug, Default)]
pub struct Progress {
    token: CancelToken,
    done: AtomicU64,
    total: AtomicU64,
    /// The steps are files (Export for Screens), not artboards.
    files: bool,
    /// The last report came from a plug-in ([`vectorcraft_plugins::host::PROGRESS_STEP`]): its
    /// steps are its own, shown as a share of the work.
    share: AtomicBool,
}

impl Progress {
    pub fn new() -> Arc<Self> {
        Arc::new(Self::default())
    }

    /// A [`Progress`] whose steps are files: the export of [`super::ScreenExport::write`].
    pub fn of_files() -> Arc<Self> {
        Arc::new(Self { files: true, ..Self::default() })
    }

    /// Are the steps files rather than artboards?
    pub fn counts_files(&self) -> bool {
        self.files
    }

    /// Are the steps a plug-in's own, which only a share of the work done describes?
    pub fn counts_share(&self) -> bool {
        self.share.load(Ordering::Relaxed)
    }

    /// `(steps done, steps in all)`; `total` is 0 while the export can't say (one file made in one go).
    pub fn counts(&self) -> (u64, u64) {
        let total = self.total.load(Ordering::Relaxed);
        (self.done.load(Ordering::Relaxed).min(total), total)
    }

    /// The share of the export done, 0 to 1; `None` while the export can't say.
    pub fn fraction(&self) -> Option<f64> {
        let (done, total) = self.counts();
        (total > 0).then(|| done as f64 / total as f64)
    }

    /// Ask the export to stop: it writes nothing more, and a render under way stops drawing.
    pub fn cancel(&self) {
        self.token.cancel();
    }

    pub fn cancelled(&self) -> bool {
        self.token.is_cancelled()
    }
}

/// Run `f` with `progress`'s token in effect on this thread and the steps reported to it.
pub fn scope<R>(progress: &Arc<Progress>, f: impl FnOnce() -> R) -> R {
    let to = Arc::clone(progress);
    let sink: ProgressSink = Rc::new(move |done, total, what| {
        to.share.store(what == vectorcraft_plugins::host::PROGRESS_STEP, Ordering::Relaxed);
        to.total.store(total, Ordering::Relaxed);
        to.done.store(done, Ordering::Relaxed);
    });
    cancel::scope(&progress.token, Some(sink), f)
}

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

    #[test]
    fn a_raster_export_counts_its_artboards_and_stops_once_cancelled() {
        let mut doc = vectorcraft_doc::Document::new(100.0, 100.0);
        let second = doc.artboards[0].clone();
        doc.artboards.push(second);
        let p = Progress::new();
        assert_eq!(p.fraction(), None, "nothing reported yet");
        let png = scope(&p, || super::super::encode_all(&doc, "png", &serde_json::json!({"useArtboards": true})));
        assert!(png.is_ok(), "{:?}", png.err());
        assert_eq!(p.counts(), (1, 2), "the step under way when the last artboard started");
        p.cancel();
        let r = scope(&p, || super::super::encode_all(&doc, "png", &serde_json::json!({"useArtboards": true})));
        assert!(matches!(r, Err(crate::EngineError::Cancelled)), "{:?}", r.err());
        assert!(!cancel::is_cancelled(), "the scope ended");
    }

    /// A plug-in's reports count as a share of the work, not as artboards or files.
    #[test]
    fn a_plugin_s_reports_count_as_a_share_of_the_work() {
        let p = Progress::new();
        scope(&p, || cancel::progress(1, 4, "artboard"));
        assert_eq!((p.counts(), p.counts_share()), ((1, 4), false));
        scope(&p, || cancel::progress(3, 8, vectorcraft_plugins::host::PROGRESS_STEP));
        assert_eq!((p.counts(), p.counts_share()), ((3, 8), true));
    }
}