use core::{
    cmp::Ordering,
    ops::{Div, Mul},
};

use crate::trezorhal::time;

const MILLIS_PER_SEC: u32 = 1000;
const MILLIS_PER_MINUTE: u32 = MILLIS_PER_SEC * 60;
const MILLIS_PER_HOUR: u32 = MILLIS_PER_MINUTE * 60;
const MILLIS_PER_DAY: u32 = MILLIS_PER_HOUR * 24;

#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Default)]
pub struct Duration {
    millis: u32,
}

impl Duration {
    pub const ZERO: Self = Self::from_millis(0);

    pub const fn from_millis(millis: u32) -> Self {
        Self { millis }
    }

    pub const fn from_secs(secs: u32) -> Self {
        // Check for potential overflow
        debug_assert!(secs < u32::MAX / MILLIS_PER_SEC);
        Self::from_millis(secs * MILLIS_PER_SEC)
    }

    pub const fn from_mins(mins: u32) -> Self {
        // Check for potential overflow
        debug_assert!(mins < u32::MAX / MILLIS_PER_MINUTE);
        Self::from_millis(mins * MILLIS_PER_MINUTE)
    }

    pub const fn from_hours(hours: u32) -> Self {
        // Check for potential overflow
        debug_assert!(hours < u32::MAX / MILLIS_PER_HOUR);
        Self::from_millis(hours * MILLIS_PER_HOUR)
    }
    pub const fn from_days(days: u32) -> Self {
        // Check for potential overflow
        debug_assert!(days < u32::MAX / MILLIS_PER_DAY);
        Self::from_millis(days * MILLIS_PER_DAY)
    }

    pub fn to_millis(self) -> u32 {
        self.millis
    }

    pub fn to_secs(self) -> u32 {
        self.millis / MILLIS_PER_SEC
    }
    pub fn to_mins(self) -> u32 {
        self.millis / MILLIS_PER_MINUTE
    }
    pub fn to_hours(self) -> u32 {
        self.millis / MILLIS_PER_HOUR
    }
    pub fn to_days(self) -> u32 {
        self.millis / MILLIS_PER_DAY
    }

    pub fn checked_add(self, rhs: Self) -> Option<Self> {
        self.millis.checked_add(rhs.millis).map(Self::from_millis)
    }

    pub fn checked_sub(self, rhs: Self) -> Option<Self> {
        self.millis.checked_sub(rhs.millis).map(Self::from_millis)
    }

    /// Returns a new Duration containing only the largest complete time unit
    /// (days, hours, minutes, or seconds)
    ///
    /// Examples:
    /// - 1 day, 3 hours → 1 day
    /// - 3 hours, 45 minutes → 3 hours
    /// - 59 seconds → 59 seconds
    pub fn crop_to_largest_unit(self) -> Self {
        if self.millis >= MILLIS_PER_DAY {
            Duration::from_days(self.to_days())
        } else if self.millis >= MILLIS_PER_HOUR {
            Duration::from_hours(self.to_hours())
        } else if self.millis >= MILLIS_PER_MINUTE {
            Duration::from_mins(self.to_mins())
        } else {
            Duration::from_secs(self.to_secs())
        }
    }

    /// Increment by one unit based on the current magnitude
    ///
    /// Examples:
    /// - 59s → 1m (moves to the next unit when crossing a boundary)
    /// - 1m → 2m
    /// - 23h → 1d
    ///
    /// Returns None if addition would overflow
    pub fn increment_unit(self) -> Option<Self> {
        let base = self.crop_to_largest_unit();

        let step = if base.millis < MILLIS_PER_MINUTE {
            Duration::from_secs(1)
        } else if base.millis < MILLIS_PER_HOUR {
            Duration::from_mins(1)
        } else if base.millis < MILLIS_PER_DAY {
            Duration::from_hours(1)
        } else {
            Duration::from_days(1)
        };

        base.checked_add(step)
    }

    /// Decrement by one unit based on the current magnitude
    ///
    /// Examples:
    /// - 1m → 59s (moves to the previous unit at boundaries)
    /// - 2m → 1m
    /// - 1h → 59m
    /// - 1d → 23h
    ///
    /// Returns None if subtraction would result in negative duration
    pub fn decrement_unit(self) -> Option<Self> {
        let base = self.crop_to_largest_unit();

        let step = if base.millis <= MILLIS_PER_MINUTE {
            Duration::from_secs(1)
        } else if base.millis <= MILLIS_PER_HOUR {
            Duration::from_mins(1)
        } else if base.millis <= MILLIS_PER_DAY {
            Duration::from_hours(1)
        } else {
            Duration::from_days(1)
        };

        base.checked_sub(step)
    }
}

impl Mul<f32> for Duration {
    // Multiplication by float is saturating -- in particular, casting from a float
    // to an int is saturating, value larger than INT_MAX casts to INT_MAX. So
    // this operation does not need to be checked.
    type Output = Self;

    fn mul(self, rhs: f32) -> Self::Output {
        Self::from_millis((self.millis as f32 * rhs) as u32)
    }
}

impl Div<u32> for Duration {
    // Division by integer cannot overflow so it does not need to be checked.
    type Output = Self;

    fn div(self, rhs: u32) -> Self::Output {
        Self::from_millis(self.millis / rhs)
    }
}

impl Div<Duration> for Duration {
    // Division by float results in float so it does not need to be checked.
    type Output = f32;

    fn div(self, rhs: Self) -> Self::Output {
        self.to_millis() as f32 / rhs.to_millis() as f32
    }
}

/* Instants can wrap around and we want them to be comparable even after
 * wrapping around. This works by setting a maximum allowable difference
 * between two Instants to half the range. In checked_add and checked_sub, we
 * make sure that the step from one Instant to another is at most
 * MAX_DIFFERENCE_IN_MILLIS. In the Ord implementation, if the difference is
 * more than MAX_DIFFERENCE_IN_MILLIS, we can assume that the smaller Instant
 * is actually wrapped around and so is in the future. */
const MAX_DIFFERENCE_IN_MILLIS: u32 = u32::MAX / 2;

#[derive(Copy, Clone, Debug, Eq, PartialEq)]
pub struct Instant {
    millis: u32,
}

impl Instant {
    pub fn now() -> Self {
        Self {
            millis: time::ticks_ms(),
        }
    }

    pub fn saturating_duration_since(self, earlier: Self) -> Duration {
        self.checked_duration_since(earlier)
            .unwrap_or(Duration::ZERO)
    }

    pub fn checked_duration_since(self, earlier: Self) -> Option<Duration> {
        if self >= earlier {
            Some(Duration::from_millis(
                self.millis.wrapping_sub(earlier.millis),
            ))
        } else {
            None
        }
    }

    pub fn checked_add(self, duration: Duration) -> Option<Self> {
        let add_millis = duration.to_millis();
        if add_millis <= MAX_DIFFERENCE_IN_MILLIS {
            Some(Self {
                millis: self.millis.wrapping_add(add_millis),
            })
        } else {
            None
        }
    }

    pub fn checked_sub(self, duration: Duration) -> Option<Self> {
        let sub_millis = duration.to_millis();
        if sub_millis <= MAX_DIFFERENCE_IN_MILLIS {
            Some(Self {
                millis: self.millis.wrapping_sub(sub_millis),
            })
        } else {
            None
        }
    }
}

impl PartialOrd for Instant {
    fn partial_cmp(&self, rhs: &Self) -> Option<Ordering> {
        Some(self.cmp(rhs))
    }
}

impl Ord for Instant {
    fn cmp(&self, rhs: &Self) -> Ordering {
        if self.millis == rhs.millis {
            Ordering::Equal
        } else {
            // If the difference is greater than MAX_DIFFERENCE_IN_MILLIS, we assume
            // that the larger Instant is in the past.
            // See explanation on MAX_DIFFERENCE_IN_MILLIS
            self.millis
                .wrapping_sub(rhs.millis)
                .cmp(&MAX_DIFFERENCE_IN_MILLIS)
                .reverse()
        }
    }
}

/// A stopwatch is a utility designed for measuring the amount of time
/// that elapses between its start and stop points. It can be used in various
/// situations - animation timing, event timing, testing and debugging.
#[derive(Clone)]
pub enum Stopwatch {
    Stopped(Duration),
    Running(Instant),
}

impl Default for Stopwatch {
    /// Returns a new stopped stopwatch by default.
    fn default() -> Self {
        Self::new_stopped()
    }
}

impl Stopwatch {
    /// Creates a new stopped stopwatch with duration of zero
    pub fn new_stopped() -> Self {
        Self::Stopped(Duration::ZERO)
    }

    /// Creates a new started stopwatch that starts
    /// at the current instant.
    pub fn new_started() -> Self {
        Self::Running(Instant::now())
    }

    /// Starts or restarts the stopwatch.
    ///
    /// If the stopwatch is already running, it restarts, setting
    /// the elapsed time to zero.
    pub fn start(&mut self) {
        *self = Self::Running(Instant::now());
    }

    /// Stops the stopwatch.
    ///
    /// When stopped, the `elapsed()` method will return the total
    /// duration for which the stopwatch was running.
    pub fn stop(&mut self) {
        *self = Self::Stopped(self.elapsed());
    }

    /// Returns the elapsed duration since the stopwatch was last started.
    ///
    /// If the stopwatch is running, it calculates the time from the last
    /// start instant to the current instant.
    pub fn elapsed(&self) -> Duration {
        match *self {
            Self::Stopped(duration) => duration,
            Self::Running(time) => unwrap!(Instant::now().checked_duration_since(time)),
        }
    }

    /// Returns `true` if the stopwatch is currently running.
    pub fn is_running(&self) -> bool {
        matches!(*self, Self::Running(_))
    }

    /// Checks if the stopwatch is running and whether the elapsed
    /// time since the last start is less than or equal to a specified limit.
    pub fn is_running_within(&self, limit: Duration) -> bool {
        match *self {
            Self::Stopped(_) => false,
            Self::Running(_) => self.elapsed() <= limit,
        }
    }
}

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

    #[test]
    fn instant_wraps_and_compares_correctly() {
        let milli = Duration { millis: 1 };
        let earlier = Instant { millis: u32::MAX };
        let later = earlier.checked_add(milli).unwrap();
        assert_eq!(later, Instant { millis: 0 });
        assert!(earlier < later);
    }

    #[test]
    fn stopwatch_builds_correctly() {
        let sw = Stopwatch::new_started();
        assert!(sw.is_running());

        let sw = Stopwatch::new_stopped();
        assert!(!sw.is_running());

        let sw: Stopwatch = Default::default();
        assert!(!sw.is_running());
    }

    fn wait(duration: Duration) {
        let origin = Instant::now();
        while Instant::now().checked_duration_since(origin).unwrap() < duration {}
    }

    #[test]
    fn stopwatch_starts_correcly() {
        let mut sw = Stopwatch::new_stopped();
        assert!(!sw.is_running());

        sw.start();
        assert!(sw.is_running());

        wait(Duration::from_millis(10));
        assert!(sw.elapsed() >= Duration::from_millis(10));
        assert!(!sw.is_running_within(Duration::from_millis(5)));
        assert!(sw.is_running_within(Duration::from_millis(10000)));
    }

    #[test]
    fn stopwatch_stops_correctly() {
        let mut sw = Stopwatch::new_started();
        assert!(sw.is_running());

        wait(Duration::from_millis(10));

        sw.stop();
        assert!(!sw.is_running());

        let elapsed = sw.elapsed();
        assert!(elapsed >= Duration::from_millis(10));

        wait(Duration::from_millis(10));
        assert!(sw.elapsed() == elapsed);
        assert!(!sw.is_running_within(Duration::from_millis(5)));
        assert!(!sw.is_running_within(Duration::from_millis(10000)));
    }

    #[test]
    fn test_crop_to_largest_unit() {
        assert_eq!(
            Duration::from_secs(59).crop_to_largest_unit(),
            Duration::from_secs(59)
        );
        assert_eq!(
            Duration::from_secs(60).crop_to_largest_unit(),
            Duration::from_mins(1)
        );
        assert_eq!(
            Duration::from_secs(61).crop_to_largest_unit(),
            Duration::from_mins(1)
        );
        assert_eq!(
            Duration::from_secs(3600).crop_to_largest_unit(),
            Duration::from_hours(1)
        );
        assert_eq!(
            Duration::from_secs(86399).crop_to_largest_unit(),
            Duration::from_hours(23)
        );
    }

    #[test]
    fn test_increment_decrement_unit() {
        // Increment
        assert_eq!(
            unwrap!(Duration::from_secs(59).increment_unit()),
            Duration::from_mins(1)
        );
        assert_eq!(
            unwrap!(Duration::from_mins(1).increment_unit()),
            Duration::from_mins(2)
        );
        assert_eq!(
            unwrap!(Duration::from_secs(61).increment_unit()),
            Duration::from_mins(2)
        );
        assert_eq!(
            unwrap!(Duration::from_days(3).increment_unit()),
            Duration::from_days(4)
        );

        // Decrement
        assert_eq!(
            unwrap!(Duration::from_mins(1).decrement_unit()),
            Duration::from_secs(59)
        );
        assert_eq!(
            unwrap!(Duration::from_secs(61).decrement_unit()),
            Duration::from_secs(59)
        );
        assert_eq!(
            unwrap!(Duration::from_mins(3).decrement_unit()),
            Duration::from_mins(2)
        );
        assert_eq!(
            unwrap!(Duration::from_hours(1).decrement_unit()),
            Duration::from_mins(59)
        );
    }
}