use cmp::Ordering;
use fmt;
use sys::{cvt, syscall};
use time::Duration;
use convert::TryInto;
use core::hash::{Hash, Hasher};

const NSEC_PER_SEC: u64 = 1_000_000_000;

#[derive(Copy, Clone)]
struct Timespec {
    t: syscall::TimeSpec,
}

impl Timespec {
    fn sub_timespec(&self, other: &Timespec) -> Result<Duration, Duration> {
        if self >= other {
            Ok(if self.t.tv_nsec >= other.t.tv_nsec {
                Duration::new((self.t.tv_sec - other.t.tv_sec) as u64,
                              (self.t.tv_nsec - other.t.tv_nsec) as u32)
            } else {
                Duration::new((self.t.tv_sec - 1 - other.t.tv_sec) as u64,
                              self.t.tv_nsec as u32 + (NSEC_PER_SEC as u32) -
                              other.t.tv_nsec as u32)
            })
        } else {
            match other.sub_timespec(self) {
                Ok(d) => Err(d),
                Err(d) => Ok(d),
            }
        }
    }

    fn checked_add_duration(&self, other: &Duration) -> Option<Timespec> {
        let mut secs = other
            .as_secs()
            .try_into() // <- target type would be `i64`
            .ok()
            .and_then(|secs| self.t.tv_sec.checked_add(secs))?;

        // Nano calculations can't overflow because nanos are <1B which fit
        // in a u32.
        let mut nsec = other.subsec_nanos() + self.t.tv_nsec as u32;
        if nsec >= NSEC_PER_SEC as u32 {
            nsec -= NSEC_PER_SEC as u32;
            secs = secs.checked_add(1)?;
        }
        Some(Timespec {
            t: syscall::TimeSpec {
                tv_sec: secs,
                tv_nsec: nsec as i32,
            },
        })
    }

    fn checked_sub_duration(&self, other: &Duration) -> Option<Timespec> {
        let mut secs = other
            .as_secs()
            .try_into() // <- target type would be `i64`
            .ok()
            .and_then(|secs| self.t.tv_sec.checked_sub(secs))?;

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

impl PartialEq for Timespec {
    fn eq(&self, other: &Timespec) -> bool {
        self.t.tv_sec == other.t.tv_sec && self.t.tv_nsec == other.t.tv_nsec
    }
}

impl Eq for Timespec {}

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

impl Ord for Timespec {
    fn cmp(&self, other: &Timespec) -> Ordering {
        let me = (self.t.tv_sec, self.t.tv_nsec);
        let other = (other.t.tv_sec, other.t.tv_nsec);
        me.cmp(&other)
    }
}

impl Hash for Timespec {
    fn hash<H : Hasher>(&self, state: &mut H) {
        self.t.tv_sec.hash(state);
        self.t.tv_nsec.hash(state);
    }
}

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

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

pub const UNIX_EPOCH: SystemTime = SystemTime {
    t: Timespec {
        t: syscall::TimeSpec {
            tv_sec: 0,
            tv_nsec: 0,
        },
    },
};

impl Instant {
    pub fn now() -> Instant {
        Instant { t: now(syscall::CLOCK_MONOTONIC) }
    }

    pub const fn zero() -> Instant {
        Instant { t: Timespec { t: syscall::TimeSpec { tv_sec: 0, tv_nsec: 0 } } }
    }

    pub fn actually_monotonic() -> bool {
        false
    }

    pub fn sub_instant(&self, other: &Instant) -> Duration {
        self.t.sub_timespec(&other.t).unwrap_or_else(|_| {
            panic!("specified instant was later than self")
        })
    }

    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.t.tv_sec)
         .field("tv_nsec", &self.t.t.tv_nsec)
         .finish()
    }
}

impl SystemTime {
    pub fn now() -> SystemTime {
        SystemTime { t: now(syscall::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<syscall::TimeSpec> for SystemTime {
    fn from(t: syscall::TimeSpec) -> SystemTime {
        SystemTime { t: Timespec { t } }
    }
}

impl fmt::Debug for SystemTime {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("SystemTime")
         .field("tv_sec", &self.t.t.tv_sec)
         .field("tv_nsec", &self.t.t.tv_nsec)
         .finish()
    }
}

pub type clock_t = usize;

fn now(clock: clock_t) -> Timespec {
    let mut t = Timespec {
        t: syscall::TimeSpec {
            tv_sec: 0,
            tv_nsec: 0,
        }
    };
    cvt(syscall::clock_gettime(clock, &mut t.t)).unwrap();
    t
}
