jellytau/src-tauri/src/download/estimate.rs

//! How big a download is going to be when the server will not say.
//!
//! A direct copy answers with `Content-Length`, and the worker reports exact
//! progress from it. A transcode is produced as it is sent — chunked, with no
//! length — and the worker used to report `progress: 0.0` for its whole
//! duration: an empty bar and "0%" while the byte count climbed for an hour.
//! That is the case every film whose audio must be re-encoded lands in.
//!
//! The backend does know enough to estimate. It fetches the item to decide the
//! audio policy anyway, and that item carries the source's size and runtime;
//! the preset it chose fixes the bitrate. So the estimate is made where the
//! URL is, persisted on the row as its `file_size`, and used only as a
//! fallback: a real `Content-Length` always wins, and an estimated bar never
//! claims completion.
//!
//! TRACES: UR-071 | DR-290

use super::presets::download_preset;

/// Ticks per second in Jellyfin's runtime unit.
const TICKS_PER_SECOND: u64 = 10_000_000;

/// The progress bar never reports more than this from an estimate, so a source
/// that encodes a little larger than predicted shows 99% until the last byte
/// rather than 104% — completion is the worker's to announce.
pub const ESTIMATED_PROGRESS_CEILING: f64 = 0.99;

/// The size a download for `quality` is expected to produce, in bytes.
///
/// - A preset re-encodes both streams at fixed rates, so the size is rate ×
///   runtime. Jellyfin encodes to a target bitrate (`-b:v` with `-maxrate`), so
///   the average lands near the cap rather than well under it.
/// - `original` copies the picture and at most re-encodes the audio, so the
///   output is the source's size give or take the audio track — and when no
///   transcode is needed at all it is exactly the source's size.
///
/// `None` when the inputs needed are missing; the caller then has no total and
/// the bar is indeterminate, which is honest and was the status quo.
///
/// TRACES: UR-071 | DR-290 | UT-252
pub fn expected_download_bytes(
    quality: &str,
    runtime_ticks: Option<i64>,
    source_size: Option<i64>,
) -> Option<u64> {
    match download_preset(quality) {
        Some(preset) => {
            let seconds = u64::try_from(runtime_ticks?).ok()? / TICKS_PER_SECOND;
            (seconds > 0).then(|| preset.total_bit_rate() / 8 * seconds)
        }
        None => source_size
            .and_then(|s| u64::try_from(s).ok())
            .filter(|&s| s > 0),
    }
}

/// What the progress bar measures against.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct ProgressTotal {
    pub bytes: u64,
    /// The total is a prediction, not the server's word.
    pub estimated: bool,
}

/// The total to report progress against, given what the response said and what
/// was predicted before it was made. The server's `Content-Length` always
/// wins; the estimate fills in only when the server sent none.
///
/// TRACES: UR-071 | DR-290 | UT-252
pub fn progress_total(content_length: Option<u64>, expected: Option<u64>) -> Option<ProgressTotal> {
    match (
        content_length.filter(|&n| n > 0),
        expected.filter(|&n| n > 0),
    ) {
        (Some(bytes), _) => Some(ProgressTotal {
            bytes,
            estimated: false,
        }),
        (None, Some(bytes)) => Some(ProgressTotal {
            bytes,
            estimated: true,
        }),
        (None, None) => None,
    }
}

/// The fraction complete, in `0.0..=1.0`. An estimated total is capped at
/// [`ESTIMATED_PROGRESS_CEILING`] so a prediction that ran low never shows a
/// finished bar on a download still running.
///
/// TRACES: UR-071 | DR-290 | UT-252
pub fn progress_fraction(downloaded: u64, total: Option<ProgressTotal>) -> f64 {
    let Some(total) = total else { return 0.0 };
    let fraction = downloaded as f64 / total.bytes as f64;
    let ceiling = if total.estimated {
        ESTIMATED_PROGRESS_CEILING
    } else {
        1.0
    };
    fraction.clamp(0.0, ceiling)
}

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

    const HOUR_TICKS: i64 = 3600 * TICKS_PER_SECOND as i64;

    /// A transcode has no `Content-Length`, and this is the case that showed
    /// "0%" for its whole duration: with a prediction in hand the bar must move.
    ///
    /// TRACES: UR-071 | DR-290 | UT-252
    #[test]
    fn test_estimate_fills_in_when_the_server_sent_no_length() {
        let total = progress_total(None, Some(4_000));
        assert_eq!(
            total,
            Some(ProgressTotal {
                bytes: 4_000,
                estimated: true
            })
        );
        let fraction = progress_fraction(1_000, total);
        assert!((fraction - 0.25).abs() < 1e-9, "got {fraction}");
    }

    /// The server's own figure is never second-guessed by a prediction.
    #[test]
    fn test_content_length_wins_over_the_estimate() {
        let total = progress_total(Some(10_000), Some(4_000)).unwrap();
        assert_eq!(total.bytes, 10_000);
        assert!(!total.estimated);
        assert_eq!(progress_fraction(10_000, Some(total)), 1.0);
    }

    /// A prediction that ran low must not announce completion: that is the
    /// worker's to do when the last byte lands.
    #[test]
    fn test_estimated_progress_never_reaches_one() {
        let total = progress_total(None, Some(1_000));
        assert_eq!(progress_fraction(1_200, total), ESTIMATED_PROGRESS_CEILING);
        assert_eq!(progress_fraction(0, total), 0.0);
    }

    /// Nothing known → nothing claimed, and a zero length is "nothing known".
    #[test]
    fn test_no_total_means_no_progress_claim() {
        assert_eq!(progress_total(None, None), None);
        assert_eq!(progress_total(Some(0), Some(0)), None);
        assert_eq!(progress_fraction(500, None), 0.0);
    }

    /// A preset's size is its combined rate over the runtime — one hour of the
    /// medium preset (4 Mb/s + 256 kb/s) is about 1.9 GB.
    #[test]
    fn test_preset_estimate_is_rate_times_runtime() {
        let bytes = expected_download_bytes("medium", Some(HOUR_TICKS), Some(9_999)).unwrap();
        assert_eq!(bytes, (4_000_000 + 256_000) / 8 * 3600);
        // Without a runtime there is nothing to multiply.
        assert_eq!(expected_download_bytes("medium", None, Some(9_999)), None);
        assert_eq!(expected_download_bytes("medium", Some(0), None), None);
    }

    /// `original` copies the picture, so the source's size is the prediction —
    /// with or without the audio being re-encoded on the way.
    #[test]
    fn test_original_estimate_is_the_source_size() {
        assert_eq!(
            expected_download_bytes("original", Some(HOUR_TICKS), Some(3_000_000_000)),
            Some(3_000_000_000)
        );
        assert_eq!(
            expected_download_bytes("original", Some(HOUR_TICKS), None),
            None
        );
        assert_eq!(expected_download_bytes("original", None, Some(0)), None);
        // An unknown quality name is treated as original by the URL builder,
        // so it is here too.
        assert_eq!(expected_download_bytes("wat", None, Some(10)), Some(10));
    }
}