/*
 * This file is licensed under the Affero General Public License (AGPL) version 3.
 *
 * Copyright (C) 2026 Element Creations Ltd
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as
 * published by the Free Software Foundation, either version 3 of the
 * License, or (at your option) any later version.
 *
 * See the GNU Affero General Public License for more details:
 * <https://www.gnu.org/licenses/agpl-3.0.html>.
 *
 */

//! A clock for the Rust side of Synapse.
//!
//! Rust code often needs the current time somewhere it cannot cheaply ask
//! Python for it, such as on a tokio worker without the GIL. Each
//! homeserver's [`RustRuntime`](crate::runtime::RustRuntime) owns a [`Clock`]
//! that can be read without the GIL.
//!
//! Synapse's unit tests run against a virtual reactor clock, where time only
//! moves when a test says so. The test reactor keeps this clock in step via
//! [`set_virtual_time_msec`]. See `ThreadedMemoryReactorClock` in
//! `tests/server.py`.

use std::{
    sync::atomic::{AtomicU64, Ordering},
    time::{Duration, SystemTime, UNIX_EPOCH},
};

use pyo3::pyfunction;

/// A sentinel value stored in [`VIRTUAL_MILLIS`] while the clock is
/// reading the real time. This value indicates that the clock should read the
/// real system time rather than a virtual time.
///
/// We choose a value that will never be used in tests, namely the maximum `u64`
/// value. This is 584 million years after the epoch.
const REAL_TIME_SENTINEL: u64 = u64::MAX;

/// The virtual time in milliseconds since the Unix epoch, or
/// [`REAL_TIME_SENTINEL`]. Only ever set by tests, via [`set_virtual_time_msec`].
///
/// We use an AtomicU64 (rather than e.g. `Option<..>`) so that reads are fast
/// and do not require locking.
static VIRTUAL_MILLIS: AtomicU64 = AtomicU64::new(REAL_TIME_SENTINEL);

/// Pin the clock to the given time, in milliseconds since the Unix epoch.
///
/// Only tests call this. Nothing unpins the clock, since a test reactor
/// and the runtimes attached to it live for a single test.
#[pyfunction]
pub fn set_virtual_time_msec(millis: u64) {
    if millis == REAL_TIME_SENTINEL {
        panic!("Cannot set virtual time to the sentinel value REAL_TIME_SENTINEL");
    }

    VIRTUAL_MILLIS.store(millis, Ordering::Relaxed);
}

/// The current time as the Rust side of a homeserver sees it.
///
/// Like `synapse.util.clock.Clock` on the Python side, this is a wall clock.
/// While these are two separate, independent implementations, they both read
/// the same system clock (on Linux both are
/// `clock_gettime(CLOCK_REALTIME)`).Code that needs a monotonic clock should
/// use [`std::time::Instant`] directly.
///
/// In future, this may be extended to support per-homeserver looping calls
/// (which is why this is not a global clock).
///
/// See [`crate::clock`] docs for more details.
pub struct Clock {}

impl Default for Clock {
    fn default() -> Self {
        Self::new()
    }
}

impl Clock {
    /// A clock reading the real time.
    pub fn new() -> Self {
        Clock {}
    }

    /// The current time.
    ///
    /// Reads the same system clock as `synapse.util.clock.Clock.time_msec()`
    /// on the Python side. See the module docs.
    pub fn now(&self) -> SystemTime {
        // Check if virtual time is set, otherwise fall back to real time.
        match VIRTUAL_MILLIS.load(Ordering::Relaxed) {
            // On Linux this is a vDSO call rather than a syscall, so it is
            // cheap enough to make on every read.
            REAL_TIME_SENTINEL => SystemTime::now(),
            virtual_millis => UNIX_EPOCH + Duration::from_millis(virtual_millis),
        }
    }

    /// [`Clock::now`] in milliseconds since the Unix epoch.
    ///
    /// Panics if the system clock is set before the Unix epoch, 1970. This was
    /// checked at startup, so this panic should only ever occur if the system
    /// clock is manually set before the Unix epoch during runtime.
    pub fn now_millis(&self) -> u64 {
        self.now()
            .duration_since(UNIX_EPOCH)
            .expect("system clock was set before the Unix epoch, 1970")
            .as_millis() as u64
    }
}

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

    #[test]
    fn virtual_time_overrides_the_real_clock() {
        // Any real wall clock reads later than this (2020-09-13).
        const PLAUSIBLE_WALL_CLOCK_MILLIS: u64 = 1_600_000_000_000;

        let clock = Clock::new();
        assert!(clock.now_millis() > PLAUSIBLE_WALL_CLOCK_MILLIS);

        set_virtual_time_msec(12345);
        assert_eq!(clock.now_millis(), 12345);
        assert_eq!(clock.now(), UNIX_EPOCH + Duration::from_millis(12345));

        // Zero is a legitimate virtual time. The memory reactor starts there.
        set_virtual_time_msec(0);
        assert_eq!(clock.now_millis(), 0);
    }
}