use crate::fmt;
use crate::time::Duration;

const NSEC_PER_SEC: u64 = 1_000_000_000;
pub const UNIX_EPOCH: SystemTime = SystemTime { t: Timespec::zero() };
#[allow(dead_code)] // Used for pthread condvar timeouts
pub const TIMESPEC_MAX: libc::timespec =
    libc::timespec { tv_sec: <libc::time_t>::MAX, tv_nsec: 1_000_000_000 - 1 };

// This additional constant is only used when calling
// `libc::pthread_cond_timedwait`.
#[cfg(target_os = "nto")]
pub(super) const TIMESPEC_MAX_CAPPED: libc::timespec = libc::timespec {
    tv_sec: (u64::MAX / NSEC_PER_SEC) as i64,
    tv_nsec: (u64::MAX % NSEC_PER_SEC) as i64,
};

#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
#[repr(transparent)]
#[rustc_layout_scalar_valid_range_start(0)]
#[rustc_layout_scalar_valid_range_end(999_999_999)]
struct Nanoseconds(u32);

#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct SystemTime {
    pub(crate) t: Timespec,
}

#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub(crate) struct Timespec {
    tv_sec: i64,
    tv_nsec: Nanoseconds,
}

impl SystemTime {
    #[cfg_attr(any(target_os = "horizon", target_os = "hurd"), allow(unused))]
    pub fn new(tv_sec: i64, tv_nsec: i64) -> SystemTime {
        SystemTime { t: Timespec::new(tv_sec, tv_nsec) }
    }

    pub fn now() -> SystemTime {
        SystemTime { t: Timespec::now(libc::CLOCK_REALTIME) }
    }

    pub fn sub_time(&self, other: &SystemTime) -> Result<Duration, Duration> {
        self.t.sub_timespec(&other.t)
    }

    pub fn checked_add_duration(&self, other: &Duration) -> Option<SystemTime> {
        Some(SystemTime { t: self.t.checked_add_duration(other)? })
    }

    pub fn checked_sub_duration(&self, other: &Duration) -> Option<SystemTime> {
        Some(SystemTime { t: self.t.checked_sub_duration(other)? })
    }
}

impl From<libc::timespec> for SystemTime {
    fn from(t: libc::timespec) -> SystemTime {
        SystemTime { t: Timespec::from(t) }
    }
}

impl fmt::Debug for SystemTime {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("SystemTime")
            .field("tv_sec", &self.t.tv_sec)
            .field("tv_nsec", &self.t.tv_nsec.0)
            .finish()
    }
}

impl Timespec {
    pub const fn zero() -> Timespec {
        Timespec::new(0, 0)
    }

    const fn new(tv_sec: i64, tv_nsec: i64) -> Timespec {
        // On Apple OS, dates before epoch are represented differently than on other
        // Unix platforms: e.g. 1/10th of a second before epoch is represented as `seconds=-1`
        // and `nanoseconds=100_000_000` on other platforms, but is `seconds=0` and
        // `nanoseconds=-900_000_000` on Apple OS.
        //
        // To compensate, we first detect this special case by checking if both
        // seconds and nanoseconds are in range, and then correct the value for seconds
        // and nanoseconds to match the common unix representation.
        //
        // Please note that Apple OS nonetheless accepts the standard unix format when
        // setting file times, which makes this compensation round-trippable and generally
        // transparent.
        #[cfg(any(
            target_os = "macos",
            target_os = "ios",
            target_os = "tvos",
            target_os = "watchos"
        ))]
        let (tv_sec, tv_nsec) =
            if (tv_sec <= 0 && tv_sec > i64::MIN) && (tv_nsec < 0 && tv_nsec > -1_000_000_000) {
                (tv_sec - 1, tv_nsec + 1_000_000_000)
            } else {
                (tv_sec, tv_nsec)
            };
        assert!(tv_nsec >= 0 && tv_nsec < NSEC_PER_SEC as i64);
        // SAFETY: The assert above checks tv_nsec is within the valid range
        Timespec { tv_sec, tv_nsec: unsafe { Nanoseconds(tv_nsec as u32) } }
    }

    pub fn now(clock: libc::clockid_t) -> Timespec {
        use crate::mem::MaybeUninit;
        use crate::sys::cvt;

        // Try to use 64-bit time in preparation for Y2038.
        #[cfg(all(
            target_os = "linux",
            target_env = "gnu",
            target_pointer_width = "32",
            not(target_arch = "riscv32")
        ))]
        {
            use crate::sys::weak::weak;

            // __clock_gettime64 was added to 32-bit arches in glibc 2.34,
            // and it handles both vDSO calls and ENOSYS fallbacks itself.
            weak!(fn __clock_gettime64(libc::clockid_t, *mut __timespec64) -> libc::c_int);

            if let Some(clock_gettime64) = __clock_gettime64.get() {
                let mut t = MaybeUninit::uninit();
                cvt(unsafe { clock_gettime64(clock, t.as_mut_ptr()) }).unwrap();
                return Timespec::from(unsafe { t.assume_init() });
            }
        }

        let mut t = MaybeUninit::uninit();
        cvt(unsafe { libc::clock_gettime(clock, t.as_mut_ptr()) }).unwrap();
        Timespec::from(unsafe { t.assume_init() })
    }

    pub fn sub_timespec(&self, other: &Timespec) -> Result<Duration, Duration> {
        if self >= other {
            // NOTE(eddyb) two aspects of this `if`-`else` are required for LLVM
            // to optimize it into a branchless form (see also #75545):
            //
            // 1. `self.tv_sec - other.tv_sec` shows up as a common expression
            //    in both branches, i.e. the `else` must have its `- 1`
            //    subtraction after the common one, not interleaved with it
            //    (it used to be `self.tv_sec - 1 - other.tv_sec`)
            //
            // 2. the `Duration::new` call (or any other additional complexity)
            //    is outside of the `if`-`else`, not duplicated in both branches
            //
            // Ideally this code could be rearranged such that it more
            // directly expresses the lower-cost behavior we want from it.
            let (secs, nsec) = if self.tv_nsec.0 >= other.tv_nsec.0 {
                ((self.tv_sec - other.tv_sec) as u64, self.tv_nsec.0 - other.tv_nsec.0)
            } else {
                (
                    (self.tv_sec - other.tv_sec - 1) as u64,
                    self.tv_nsec.0 + (NSEC_PER_SEC as u32) - other.tv_nsec.0,
                )
            };

            Ok(Duration::new(secs, nsec))
        } else {
            match other.sub_timespec(self) {
                Ok(d) => Err(d),
                Err(d) => Ok(d),
            }
        }
    }

    pub fn checked_add_duration(&self, other: &Duration) -> Option<Timespec> {
        let mut secs = self.tv_sec.checked_add_unsigned(other.as_secs())?;

        // Nano calculations can't overflow because nanos are <1B which fit
        // in a u32.
        let mut nsec = other.subsec_nanos() + self.tv_nsec.0;
        if nsec >= NSEC_PER_SEC as u32 {
            nsec -= NSEC_PER_SEC as u32;
            secs = secs.checked_add(1)?;
        }
        Some(Timespec::new(secs, nsec.into()))
    }

    pub fn checked_sub_duration(&self, other: &Duration) -> Option<Timespec> {
        let mut secs = self.tv_sec.checked_sub_unsigned(other.as_secs())?;

        // Similar to above, nanos can't overflow.
        let mut nsec = self.tv_nsec.0 as i32 - other.subsec_nanos() as i32;
        if nsec < 0 {
            nsec += NSEC_PER_SEC as i32;
            secs = secs.checked_sub(1)?;
        }
        Some(Timespec::new(secs, nsec.into()))
    }

    #[allow(dead_code)]
    pub fn to_timespec(&self) -> Option<libc::timespec> {
        Some(libc::timespec {
            tv_sec: self.tv_sec.try_into().ok()?,
            tv_nsec: self.tv_nsec.0.try_into().ok()?,
        })
    }

    // On QNX Neutrino, the maximum timespec for e.g. pthread_cond_timedwait
    // is 2^64 nanoseconds
    #[cfg(target_os = "nto")]
    pub(super) fn to_timespec_capped(&self) -> Option<libc::timespec> {
        // Check if timeout in nanoseconds would fit into an u64
        if (self.tv_nsec.0 as u64)
            .checked_add((self.tv_sec as u64).checked_mul(NSEC_PER_SEC)?)
            .is_none()
        {
            return None;
        }
        self.to_timespec()
    }

    #[cfg(all(
        target_os = "linux",
        target_env = "gnu",
        target_pointer_width = "32",
        not(target_arch = "riscv32")
    ))]
    pub fn to_timespec64(&self) -> __timespec64 {
        __timespec64::new(self.tv_sec, self.tv_nsec.0 as _)
    }
}

impl From<libc::timespec> for Timespec {
    fn from(t: libc::timespec) -> Timespec {
        Timespec::new(t.tv_sec as i64, t.tv_nsec as i64)
    }
}

#[cfg(all(
    target_os = "linux",
    target_env = "gnu",
    target_pointer_width = "32",
    not(target_arch = "riscv32")
))]
#[repr(C)]
pub(crate) struct __timespec64 {
    pub(crate) tv_sec: i64,
    #[cfg(target_endian = "big")]
    _padding: i32,
    pub(crate) tv_nsec: i32,
    #[cfg(target_endian = "little")]
    _padding: i32,
}

#[cfg(all(
    target_os = "linux",
    target_env = "gnu",
    target_pointer_width = "32",
    not(target_arch = "riscv32")
))]
impl __timespec64 {
    pub(crate) fn new(tv_sec: i64, tv_nsec: i32) -> Self {
        Self { tv_sec, tv_nsec, _padding: 0 }
    }
}

#[cfg(all(
    target_os = "linux",
    target_env = "gnu",
    target_pointer_width = "32",
    not(target_arch = "riscv32")
))]
impl From<__timespec64> for Timespec {
    fn from(t: __timespec64) -> Timespec {
        Timespec::new(t.tv_sec, t.tv_nsec.into())
    }
}

#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct Instant {
    t: Timespec,
}

impl Instant {
    pub fn now() -> Instant {
        // https://www.manpagez.com/man/3/clock_gettime/
        //
        // CLOCK_UPTIME_RAW   clock that increments monotonically, in the same man-
        //                    ner as CLOCK_MONOTONIC_RAW, but that does not incre-
        //                    ment while the system is asleep.  The returned value
        //                    is identical to the result of mach_absolute_time()
        //                    after the appropriate mach_timebase conversion is
        //                    applied.
        //
        // Instant on macos was historically implemented using mach_absolute_time;
        // we preserve this value domain out of an abundance of caution.
        #[cfg(any(
            target_os = "macos",
            target_os = "ios",
            target_os = "watchos",
            target_os = "tvos"
        ))]
        const clock_id: libc::clockid_t = libc::CLOCK_UPTIME_RAW;
        #[cfg(not(any(
            target_os = "macos",
            target_os = "ios",
            target_os = "watchos",
            target_os = "tvos"
        )))]
        const clock_id: libc::clockid_t = libc::CLOCK_MONOTONIC;
        Instant { t: Timespec::now(clock_id) }
    }

    pub fn checked_sub_instant(&self, other: &Instant) -> Option<Duration> {
        self.t.sub_timespec(&other.t).ok()
    }

    pub fn checked_add_duration(&self, other: &Duration) -> Option<Instant> {
        Some(Instant { t: self.t.checked_add_duration(other)? })
    }

    pub fn checked_sub_duration(&self, other: &Duration) -> Option<Instant> {
        Some(Instant { t: self.t.checked_sub_duration(other)? })
    }
}

impl fmt::Debug for Instant {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Instant")
            .field("tv_sec", &self.t.tv_sec)
            .field("tv_nsec", &self.t.tv_nsec.0)
            .finish()
    }
}
