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use core::{fmt::Display, str::FromStr};

use crate::{Calendar, CalendarTime, OffsetTime, Sigil, TimeZone};

use super::InvalidSigilError;

/// A calendar that counts time like the current system clock
///
/// On most platforms this will be either POSIX accounting or UTC-SLS accounting, depending
/// on how NTP is set.
///
/// Note that POSIX time accounting leads to the clock being stalled for one second, which
/// means that during that second it will become impossible to know how much time has elapsed.
/// Unfortunately, this means that on such systems it is basically impossible to have precise
/// time accounting during leap seconds. As such, even `Time::now()` may return wrong results
/// at these times, in a way similar to if the system clock were not set correctly.
///
/// The best idea to improve on this would be to juggle between different clocks so that we
/// could use a fallback clock (like `CLOCK_MONOTONIC`) during a leap seconds. Contributions
/// are welcome.
#[derive(Clone, Copy, PartialEq, Eq)]
pub struct System;

/// Sigil for the system timezone
///
/// This is only exposed so that it is possible to write the `OffsetTime<UtcSigil>` struct.
#[derive(Clone, Copy, PartialEq, Eq)]
pub struct SystemSigil;

/// A time clock that ticks like the current system clock
///
/// On most platforms this will be either POSIX accounting or UTC-SLS accounting, depending
/// on how NTP is set.
///
/// Note that as this does not know of a calendar, its string functions will be giving out
/// raw numbers that may not be user-friendly.
pub type SystemTime = OffsetTime<SystemSigil>;

impl System {
    /// Retrieve the current date according to the system clock
    ///
    /// Note that this is infallible, as the system calendar is by definition always able to
    /// store a time returned by the system clock.
    ///
    /// However, it will panic if running on no-std and not having a known alternative
    /// implementation for time retrieval.
    pub fn now() -> SystemTime {
        Self::now_impl()
    }

    // TODO: split this into a separate feature so that later on when extern existential types come
    // in we can allow extern implementations of this?
    #[cfg(feature = "std")]
    fn now_impl() -> SystemTime {
        let duration = std::time::SystemTime::now()
            .duration_since(std::time::UNIX_EPOCH)
            .expect("Current time was before posix epoch");
        let pseudo_nanos = i128::try_from(duration.as_nanos())
            .expect("Overflow trying to retrieve the current time");
        // No extra nanoseconds for the `std`-based implementation because it'd be impossible to know
        let extra_nanos = 0;
        // TODO: Introduce an implementation of `Time::now()` based on other clocks for platforms that
        // can so this is not the best precision we could have.
        SystemTime::from_pseudo_nanos_since_posix_epoch(SystemSigil, pseudo_nanos, extra_nanos)
    }

    #[cfg(not(feature = "std"))]
    fn now_impl() -> SystemTime {
        panic!("Running on no-std with no known implementation of time-getting");
    }
}

impl TimeZone for System {
    type Sigil = SystemSigil;
}

impl Calendar for System {
    type Time = SystemTime;

    fn try_now(&self) -> crate::Result<Self::Time> {
        Ok(Self::now())
    }

    fn now(&self) -> Self::Time {
        Self::now()
    }

    fn write(&self, t: &crate::Time) -> crate::Result<Self::Time> {
        let t = crate::providers::SYSTEM.write(t)?;
        Ok(Self::Time::from_pseudo_nanos_since_posix_epoch(
            SystemSigil,
            t.as_pseudo_nanos_since_posix_epoch(),
            t.extra_nanos(),
        ))
    }
}

impl Sigil for SystemSigil {
    fn read(&self, t: &OffsetTime<Self>) -> crate::Result<crate::TimeResult> {
        OffsetTime::<crate::providers::SystemSigil>::from_pseudo_nanos_since_posix_epoch(
            crate::providers::SYSTEM_SIGIL,
            t.as_pseudo_nanos_since_posix_epoch(),
            t.extra_nanos(),
        )
        .read()
    }
}

impl Display for SystemSigil {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        f.write_str("Z")
    }
}

impl FromStr for SystemSigil {
    type Err = InvalidSigilError;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        match s {
            "Z" => Ok(SystemSigil),
            " UTC" => Ok(SystemSigil),
            _ => Err(InvalidSigilError),
        }
    }
}