//! Generating UUIDs from timestamps.
//!
//! Timestamps are used in a few UUID versions as a source of decentralized
//! uniqueness (as in versions 1 and 6), and as a way to enable sorting (as
//! in versions 6 and 7). Timestamps aren't encoded the same way by all UUID
//! versions so this module provides a single [`Timestamp`] type that can
//! convert between them.
//!
//! # Timestamp representations in UUIDs
//!
//! Versions 1 and 6 UUIDs use a bespoke timestamp that consists of the
//! number of 100ns ticks since `1582-10-15 00:00:00`, along with
//! a counter value to avoid duplicates.
//!
//! Version 7 UUIDs use a more standard timestamp that consists of the
//! number of millisecond ticks since the Unix epoch (`1970-01-01 00:00:00`).
//!
//! # References
//!
//! * [UUID Version 1 in RFC 9562](https://www.ietf.org/rfc/rfc9562.html#section-5.1)
//! * [UUID Version 7 in RFC 9562](https://www.ietf.org/rfc/rfc9562.html#section-5.7)
//! * [Timestamp Considerations in RFC 9562](https://www.ietf.org/rfc/rfc9562.html#section-6.1)

use core::cmp;

use crate::Uuid;

/// The number of 100 nanosecond ticks between the RFC 9562 epoch
/// (`1582-10-15 00:00:00`) and the Unix epoch (`1970-01-01 00:00:00`).
pub const UUID_TICKS_BETWEEN_EPOCHS: u64 = 0x01B2_1DD2_1381_4000;

/// A timestamp that can be encoded into a UUID.
///
/// This type abstracts the specific encoding, so versions 1, 6, and 7
/// UUIDs can both be supported through the same type, even
/// though they have a different representation of a timestamp.
///
/// # References
///
/// * [Timestamp Considerations in RFC 9562](https://www.ietf.org/rfc/rfc9562.html#section-6.1)
/// * [UUID Generator States in RFC 9562](https://www.ietf.org/rfc/rfc9562.html#section-6.3)
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub struct Timestamp {
    seconds: u64,
    subsec_nanos: u32,
    counter: u128,
    usable_counter_bits: u8,
}

impl Timestamp {
    /// Get a timestamp representing the current system time and up to a 128-bit counter.
    ///
    /// This method defers to the standard library's `SystemTime` type.
    #[cfg(feature = "std")]
    pub fn now(context: impl ClockSequence<Output = impl Into<u128>>) -> Self {
        let (seconds, subsec_nanos) = now();

        let (counter, seconds, subsec_nanos) =
            context.generate_timestamp_sequence(seconds, subsec_nanos);
        let counter = counter.into();
        let usable_counter_bits = context.usable_bits() as u8;

        Timestamp {
            seconds,
            subsec_nanos,
            counter,
            usable_counter_bits,
        }
    }

    /// Construct a `Timestamp` from the number of 100 nanosecond ticks since 00:00:00.00,
    /// 15 October 1582 (the date of Gregorian reform to the Christian calendar) and a 14-bit
    /// counter, as used in versions 1 and 6 UUIDs.
    ///
    /// # Overflow
    ///
    /// If conversion from RFC 9562 ticks to the internal timestamp format would overflow
    /// it will wrap.
    pub const fn from_gregorian_time(ticks: u64, counter: u16) -> Self {
        let (seconds, subsec_nanos) = Self::gregorian_to_unix(ticks);

        Timestamp {
            seconds,
            subsec_nanos,
            counter: counter as u128,
            usable_counter_bits: 14,
        }
    }

    /// Construct a `Timestamp` from a Unix timestamp and up to a 128-bit counter, as used in version 7 UUIDs.
    pub const fn from_unix_time(
        seconds: u64,
        subsec_nanos: u32,
        counter: u128,
        usable_counter_bits: u8,
    ) -> Self {
        Timestamp {
            seconds,
            subsec_nanos,
            counter,
            usable_counter_bits,
        }
    }

    /// Construct a `Timestamp` from a Unix timestamp and up to a 128-bit counter, as used in version 7 UUIDs.
    pub fn from_unix(
        context: impl ClockSequence<Output = impl Into<u128>>,
        seconds: u64,
        subsec_nanos: u32,
    ) -> Self {
        let (counter, seconds, subsec_nanos) =
            context.generate_timestamp_sequence(seconds, subsec_nanos);
        let counter = counter.into();
        let usable_counter_bits = context.usable_bits() as u8;

        Timestamp {
            seconds,
            subsec_nanos,
            counter,
            usable_counter_bits,
        }
    }

    /// Get the value of the timestamp as the number of 100 nanosecond ticks since 00:00:00.00,
    /// 15 October 1582 and a 14-bit counter, as used in versions 1 and 6 UUIDs.
    ///
    /// # Overflow
    ///
    /// If conversion from the internal timestamp format to ticks would overflow
    /// then it will wrap.
    ///
    /// If the internal counter is wider than 14 bits then it will be truncated to 14 bits.
    pub const fn to_gregorian(&self) -> (u64, u16) {
        (
            Self::unix_to_gregorian_ticks(self.seconds, self.subsec_nanos),
            (self.counter as u16) & 0x3FFF,
        )
    }

    // NOTE: This method is not public; the usable counter bits are lost in a version 7 UUID
    // so can't be reliably recovered.
    #[cfg(feature = "v7")]
    pub(crate) const fn counter(&self) -> (u128, u8) {
        (self.counter, self.usable_counter_bits)
    }

    /// Get the value of the timestamp as a Unix timestamp, as used in version 7 UUIDs.
    pub const fn to_unix(&self) -> (u64, u32) {
        (self.seconds, self.subsec_nanos)
    }

    const fn unix_to_gregorian_ticks(seconds: u64, nanos: u32) -> u64 {
        UUID_TICKS_BETWEEN_EPOCHS
            .wrapping_add(seconds.wrapping_mul(10_000_000))
            .wrapping_add(nanos as u64 / 100)
    }

    const fn gregorian_to_unix(ticks: u64) -> (u64, u32) {
        (
            ticks.wrapping_sub(UUID_TICKS_BETWEEN_EPOCHS) / 10_000_000,
            (ticks.wrapping_sub(UUID_TICKS_BETWEEN_EPOCHS) % 10_000_000) as u32 * 100,
        )
    }
}

#[doc(hidden)]
impl Timestamp {
    #[deprecated(
        since = "1.10.0",
        note = "use `Timestamp::from_gregorian(ticks, counter)`"
    )]
    pub const fn from_rfc4122(ticks: u64, counter: u16) -> Self {
        Timestamp::from_gregorian_time(ticks, counter)
    }

    #[deprecated(since = "1.10.0", note = "use `Timestamp::to_gregorian()`")]
    pub const fn to_rfc4122(&self) -> (u64, u16) {
        self.to_gregorian()
    }

    #[deprecated(
        since = "1.2.0",
        note = "`Timestamp::to_unix_nanos()` is deprecated and will be removed: use `Timestamp::to_unix()`"
    )]
    pub const fn to_unix_nanos(&self) -> u32 {
        panic!("`Timestamp::to_unix_nanos()` is deprecated and will be removed: use `Timestamp::to_unix()`")
    }

    #[deprecated(
        since = "1.23.0",
        note = "use `Timestamp::from_gregorian(ticks, counter)`"
    )]
    pub const fn from_gregorian(ticks: u64, counter: u16) -> Self {
        Timestamp::from_gregorian_time(ticks, counter)
    }
}

#[cfg(feature = "std")]
impl TryFrom<std::time::SystemTime> for Timestamp {
    type Error = crate::Error;

    /// Perform the conversion.
    ///
    /// This method will fail if the system time is earlier than the Unix Epoch.
    /// On some platforms it may panic instead.
    fn try_from(st: std::time::SystemTime) -> Result<Self, Self::Error> {
        let dur = st.duration_since(std::time::UNIX_EPOCH).map_err(|_| {
            crate::Error(crate::error::ErrorKind::InvalidSystemTime(
                "unable to convert the system tie into a Unix timestamp",
            ))
        })?;

        Ok(Self::from_unix_time(
            dur.as_secs(),
            dur.subsec_nanos(),
            0,
            0,
        ))
    }
}

#[cfg(feature = "std")]
impl From<Timestamp> for std::time::SystemTime {
    fn from(ts: Timestamp) -> Self {
        let (seconds, subsec_nanos) = ts.to_unix();

        Self::UNIX_EPOCH + std::time::Duration::new(seconds, subsec_nanos)
    }
}

pub(crate) const fn encode_gregorian_timestamp(
    ticks: u64,
    counter: u16,
    node_id: &[u8; 6],
) -> Uuid {
    let time_low = (ticks & 0xFFFF_FFFF) as u32;
    let time_mid = ((ticks >> 32) & 0xFFFF) as u16;
    let time_high_and_version = (((ticks >> 48) & 0x0FFF) as u16) | (1 << 12);

    let mut d4 = [0; 8];

    d4[0] = (((counter & 0x3F00) >> 8) as u8) | 0x80;
    d4[1] = (counter & 0xFF) as u8;
    d4[2] = node_id[0];
    d4[3] = node_id[1];
    d4[4] = node_id[2];
    d4[5] = node_id[3];
    d4[6] = node_id[4];
    d4[7] = node_id[5];

    Uuid::from_fields(time_low, time_mid, time_high_and_version, &d4)
}

pub(crate) const fn decode_gregorian_timestamp(uuid: &Uuid) -> (u64, u16) {
    let bytes = uuid.as_bytes();

    let ticks: u64 = ((bytes[6] & 0x0F) as u64) << 56
        | (bytes[7] as u64) << 48
        | (bytes[4] as u64) << 40
        | (bytes[5] as u64) << 32
        | (bytes[0] as u64) << 24
        | (bytes[1] as u64) << 16
        | (bytes[2] as u64) << 8
        | (bytes[3] as u64);

    let counter: u16 = ((bytes[8] & 0x3F) as u16) << 8 | (bytes[9] as u16);

    (ticks, counter)
}

pub(crate) const fn encode_sorted_gregorian_timestamp(
    ticks: u64,
    counter: u16,
    node_id: &[u8; 6],
) -> Uuid {
    let time_high = ((ticks >> 28) & 0xFFFF_FFFF) as u32;
    let time_mid = ((ticks >> 12) & 0xFFFF) as u16;
    let time_low_and_version = ((ticks & 0x0FFF) as u16) | (0x6 << 12);

    let mut d4 = [0; 8];

    d4[0] = (((counter & 0x3F00) >> 8) as u8) | 0x80;
    d4[1] = (counter & 0xFF) as u8;
    d4[2] = node_id[0];
    d4[3] = node_id[1];
    d4[4] = node_id[2];
    d4[5] = node_id[3];
    d4[6] = node_id[4];
    d4[7] = node_id[5];

    Uuid::from_fields(time_high, time_mid, time_low_and_version, &d4)
}

pub(crate) const fn decode_sorted_gregorian_timestamp(uuid: &Uuid) -> (u64, u16) {
    let bytes = uuid.as_bytes();

    let ticks: u64 = ((bytes[0]) as u64) << 52
        | (bytes[1] as u64) << 44
        | (bytes[2] as u64) << 36
        | (bytes[3] as u64) << 28
        | (bytes[4] as u64) << 20
        | (bytes[5] as u64) << 12
        | ((bytes[6] & 0xF) as u64) << 8
        | (bytes[7] as u64);

    let counter: u16 = ((bytes[8] & 0x3F) as u16) << 8 | (bytes[9] as u16);

    (ticks, counter)
}

pub(crate) const fn encode_unix_timestamp_millis(
    millis: u64,
    counter_random_bytes: &[u8; 10],
) -> Uuid {
    let millis_high = ((millis >> 16) & 0xFFFF_FFFF) as u32;
    let millis_low = (millis & 0xFFFF) as u16;

    let counter_random_version = (counter_random_bytes[1] as u16
        | ((counter_random_bytes[0] as u16) << 8) & 0x0FFF)
        | (0x7 << 12);

    let mut d4 = [0; 8];

    d4[0] = (counter_random_bytes[2] & 0x3F) | 0x80;
    d4[1] = counter_random_bytes[3];
    d4[2] = counter_random_bytes[4];
    d4[3] = counter_random_bytes[5];
    d4[4] = counter_random_bytes[6];
    d4[5] = counter_random_bytes[7];
    d4[6] = counter_random_bytes[8];
    d4[7] = counter_random_bytes[9];

    Uuid::from_fields(millis_high, millis_low, counter_random_version, &d4)
}

pub(crate) const fn decode_unix_timestamp_millis(uuid: &Uuid) -> u64 {
    let bytes = uuid.as_bytes();

    let millis: u64 = (bytes[0] as u64) << 40
        | (bytes[1] as u64) << 32
        | (bytes[2] as u64) << 24
        | (bytes[3] as u64) << 16
        | (bytes[4] as u64) << 8
        | (bytes[5] as u64);

    millis
}

#[cfg(all(
    feature = "std",
    feature = "js",
    all(target_arch = "wasm32", any(target_os = "unknown", target_os = "none"))
))]
fn now() -> (u64, u32) {
    use wasm_bindgen::prelude::*;

    #[wasm_bindgen]
    extern "C" {
        // NOTE: This signature works around https://bugzilla.mozilla.org/show_bug.cgi?id=1787770
        #[wasm_bindgen(js_namespace = Date, catch)]
        fn now() -> Result<f64, JsValue>;
    }

    let now = now().unwrap_throw();

    let secs = (now / 1_000.0) as u64;
    let nanos = ((now % 1_000.0) * 1_000_000.0) as u32;

    (secs, nanos)
}

#[cfg(all(
    feature = "std",
    not(miri),
    any(
        not(feature = "js"),
        not(all(target_arch = "wasm32", any(target_os = "unknown", target_os = "none")))
    )
))]
fn now() -> (u64, u32) {
    let dur = std::time::SystemTime::UNIX_EPOCH.elapsed().expect(
        "Getting elapsed time since UNIX_EPOCH. If this fails, we've somehow violated causality",
    );

    (dur.as_secs(), dur.subsec_nanos())
}

#[cfg(all(feature = "std", miri))]
fn now() -> (u64, u32) {
    use std::{sync::Mutex, time::Duration};

    static TS: Mutex<u64> = Mutex::new(0);

    let ts = Duration::from_nanos({
        let mut ts = TS.lock().unwrap();
        *ts += 1;
        *ts
    });

    (ts.as_secs(), ts.subsec_nanos())
}

/// A counter that can be used by versions 1 and 6 UUIDs to support
/// the uniqueness of timestamps.
///
/// # References
///
/// * [UUID Version 1 in RFC 9562](https://www.ietf.org/rfc/rfc9562.html#section-5.1)
/// * [UUID Version 6 in RFC 9562](https://www.ietf.org/rfc/rfc9562.html#section-5.6)
/// * [UUID Generator States in RFC 9562](https://www.ietf.org/rfc/rfc9562.html#section-6.3)
pub trait ClockSequence {
    /// The type of sequence returned by this counter.
    type Output;

    /// Get the next value in the sequence to feed into a timestamp.
    ///
    /// This method will be called each time a [`Timestamp`] is constructed.
    ///
    /// Any bits beyond [`ClockSequence::usable_bits`] in the output must be unset.
    fn generate_sequence(&self, seconds: u64, subsec_nanos: u32) -> Self::Output;

    /// Get the next value in the sequence, potentially also adjusting the timestamp.
    ///
    /// This method should be preferred over `generate_sequence`.
    ///
    /// Any bits beyond [`ClockSequence::usable_bits`] in the output must be unset.
    fn generate_timestamp_sequence(
        &self,
        seconds: u64,
        subsec_nanos: u32,
    ) -> (Self::Output, u64, u32) {
        (
            self.generate_sequence(seconds, subsec_nanos),
            seconds,
            subsec_nanos,
        )
    }

    /// The number of usable bits from the least significant bit in the result of [`ClockSequence::generate_sequence`]
    /// or [`ClockSequence::generate_timestamp_sequence`].
    ///
    /// The number of usable bits must not exceed 128.
    ///
    /// The number of usable bits is not expected to change between calls. An implementation of `ClockSequence` should
    /// always return the same value from this method.
    fn usable_bits(&self) -> usize
    where
        Self::Output: Sized,
    {
        cmp::min(128, core::mem::size_of::<Self::Output>() * 8)
    }
}

impl<T: ClockSequence + ?Sized> ClockSequence for &T {
    type Output = T::Output;

    fn generate_sequence(&self, seconds: u64, subsec_nanos: u32) -> Self::Output {
        (**self).generate_sequence(seconds, subsec_nanos)
    }

    fn generate_timestamp_sequence(
        &self,
        seconds: u64,
        subsec_nanos: u32,
    ) -> (Self::Output, u64, u32) {
        (**self).generate_timestamp_sequence(seconds, subsec_nanos)
    }

    fn usable_bits(&self) -> usize
    where
        Self::Output: Sized,
    {
        (**self).usable_bits()
    }
}

/// Default implementations for the [`ClockSequence`] trait.
pub mod context {
    use super::ClockSequence;

    #[cfg(any(feature = "v1", feature = "v6"))]
    mod v1_support {
        use super::*;

        #[cfg(all(feature = "std", feature = "rng"))]
        use crate::std::sync::LazyLock;

        use atomic::{Atomic, Ordering};

        #[cfg(all(feature = "std", feature = "rng"))]
        static CONTEXT: LazyLock<ContextV1> = LazyLock::new(ContextV1::new_random);

        #[cfg(all(feature = "std", feature = "rng"))]
        pub(crate) fn shared_context_v1() -> &'static ContextV1 {
            &*CONTEXT
        }

        /// An internally synchronized, wrapping counter that produces 14-bit values for version 1 and version 6 UUIDs.
        ///
        /// This type is:
        ///
        /// - **Non-reseeding:** The counter is not reseeded on each time interval (100ns).
        /// - **Non-adjusting:** The timestamp is not incremented when the counter wraps within a time interval (100ns).
        /// - **Thread-safe:** The underlying counter is atomic, so can be shared across threads.
        ///
        /// This type should be used when constructing versions 1 and 6 UUIDs.
        ///
        /// This type should not be used when constructing version 7 UUIDs. When used to
        /// construct a version 7 UUID, the 14-bit counter will be padded with random data.
        /// Counter overflows are more likely with a 14-bit counter than they are with a
        /// 42-bit counter when working at millisecond precision. This type doesn't attempt
        /// to adjust the timestamp on overflow.
        #[derive(Debug)]
        pub struct ContextV1 {
            count: Atomic<u16>,
        }

        impl ContextV1 {
            /// Construct a new context that's initialized with the given value.
            ///
            /// The starting value should be a random number, so that UUIDs from
            /// different systems with the same timestamps are less likely to collide.
            /// When the `rng` feature is enabled, prefer the [`ContextV1::new_random`] method.
            pub const fn new(count: u16) -> Self {
                Self {
                    count: Atomic::<u16>::new(count),
                }
            }

            /// Construct a new context that's initialized with a random value.
            #[cfg(feature = "rng")]
            pub fn new_random() -> Self {
                Self {
                    count: Atomic::<u16>::new(crate::rng::u16()),
                }
            }
        }

        impl ClockSequence for ContextV1 {
            type Output = u16;

            fn generate_sequence(&self, _seconds: u64, _nanos: u32) -> Self::Output {
                // RFC 9562 reserves 2 bits of the clock sequence so the actual
                // maximum value is smaller than `u16::MAX`. Since we unconditionally
                // increment the clock sequence we want to wrap once it becomes larger
                // than what we can represent in a "u14". Otherwise there'd be patches
                // where the clock sequence doesn't change regardless of the timestamp
                self.count.fetch_add(1, Ordering::AcqRel) & (u16::MAX >> 2)
            }

            fn usable_bits(&self) -> usize {
                14
            }
        }

        #[deprecated(since = "1.23.0", note = "renamed to `ContextV1`")]
        #[doc(hidden)]
        pub type Context = ContextV1;

        #[cfg(test)]
        mod tests {
            use crate::Timestamp;

            use super::*;

            #[test]
            fn context() {
                let seconds = 1_496_854_535;
                let subsec_nanos = 812_946_000;

                let context = ContextV1::new(u16::MAX >> 2);

                let ts = Timestamp::from_unix(&context, seconds, subsec_nanos);
                assert_eq!(16383, ts.counter);
                assert_eq!(14, ts.usable_counter_bits);

                let seconds = 1_496_854_536;

                let ts = Timestamp::from_unix(&context, seconds, subsec_nanos);
                assert_eq!(0, ts.counter);

                let seconds = 1_496_854_535;

                let ts = Timestamp::from_unix(&context, seconds, subsec_nanos);
                assert_eq!(1, ts.counter);
            }

            #[test]
            fn context_overflow() {
                let seconds = u64::MAX;
                let subsec_nanos = u32::MAX;

                let context = ContextV1::new(u16::MAX);

                // Ensure we don't panic
                Timestamp::from_unix(&context, seconds, subsec_nanos);
            }
        }
    }

    #[cfg(any(feature = "v1", feature = "v6"))]
    pub use v1_support::*;

    #[cfg(feature = "std")]
    mod std_support {
        use super::*;

        use core::panic::{AssertUnwindSafe, RefUnwindSafe};
        use std::{sync::Mutex, thread::LocalKey};

        /// A wrapper for a context that uses thread-local storage.
        pub struct ThreadLocalContext<C: 'static>(&'static LocalKey<C>);

        impl<C> std::fmt::Debug for ThreadLocalContext<C> {
            fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
                f.debug_struct("ThreadLocalContext").finish_non_exhaustive()
            }
        }

        impl<C: 'static> ThreadLocalContext<C> {
            /// Wrap a thread-local container with a context.
            pub const fn new(local_key: &'static LocalKey<C>) -> Self {
                ThreadLocalContext(local_key)
            }
        }

        impl<C: ClockSequence + 'static> ClockSequence for ThreadLocalContext<C> {
            type Output = C::Output;

            fn generate_sequence(&self, seconds: u64, subsec_nanos: u32) -> Self::Output {
                self.0
                    .with(|ctxt| ctxt.generate_sequence(seconds, subsec_nanos))
            }

            fn generate_timestamp_sequence(
                &self,
                seconds: u64,
                subsec_nanos: u32,
            ) -> (Self::Output, u64, u32) {
                self.0
                    .with(|ctxt| ctxt.generate_timestamp_sequence(seconds, subsec_nanos))
            }

            fn usable_bits(&self) -> usize {
                self.0.with(|ctxt| ctxt.usable_bits())
            }
        }

        impl<C: ClockSequence> ClockSequence for AssertUnwindSafe<C> {
            type Output = C::Output;

            fn generate_sequence(&self, seconds: u64, subsec_nanos: u32) -> Self::Output {
                self.0.generate_sequence(seconds, subsec_nanos)
            }

            fn generate_timestamp_sequence(
                &self,
                seconds: u64,
                subsec_nanos: u32,
            ) -> (Self::Output, u64, u32) {
                self.0.generate_timestamp_sequence(seconds, subsec_nanos)
            }

            fn usable_bits(&self) -> usize
            where
                Self::Output: Sized,
            {
                self.0.usable_bits()
            }
        }

        impl<C: ClockSequence + RefUnwindSafe> ClockSequence for Mutex<C> {
            type Output = C::Output;

            fn generate_sequence(&self, seconds: u64, subsec_nanos: u32) -> Self::Output {
                self.lock()
                    .unwrap_or_else(|err| err.into_inner())
                    .generate_sequence(seconds, subsec_nanos)
            }

            fn generate_timestamp_sequence(
                &self,
                seconds: u64,
                subsec_nanos: u32,
            ) -> (Self::Output, u64, u32) {
                self.lock()
                    .unwrap_or_else(|err| err.into_inner())
                    .generate_timestamp_sequence(seconds, subsec_nanos)
            }

            fn usable_bits(&self) -> usize
            where
                Self::Output: Sized,
            {
                self.lock()
                    .unwrap_or_else(|err| err.into_inner())
                    .usable_bits()
            }
        }
    }

    #[cfg(feature = "std")]
    pub use std_support::*;

    #[cfg(feature = "v7")]
    mod v7_support {
        use super::*;

        use core::{cell::Cell, cmp, panic::RefUnwindSafe};

        #[cfg(feature = "std")]
        static CONTEXT_V7: SharedContextV7 =
            SharedContextV7(std::sync::Mutex::new(ContextV7::new()));

        #[cfg(feature = "std")]
        pub(crate) fn shared_context_v7() -> &'static SharedContextV7 {
            &CONTEXT_V7
        }

        const USABLE_BITS: usize = 42;

        // Leave the most significant bit unset
        // This guarantees the counter has at least 2,199,023,255,552
        // values before it will overflow, which is exceptionally unlikely
        // even in the worst case
        const RESEED_MASK: u64 = u64::MAX >> 23;
        const MAX_COUNTER: u64 = u64::MAX >> 22;

        /// An unsynchronized, reseeding counter that produces 42-bit values for version 7 UUIDs.
        ///
        /// This type is:
        ///
        /// - **Reseeding:** The counter is reseeded on each time interval (1ms) with a random 41-bit value.
        ///   The 42nd bit is left unset so the counter can safely increment over the millisecond.
        /// - **Adjusting:** The timestamp is incremented when the counter wraps within a time interval (1ms).
        ///   All subsequent timestamps in that same interval will also be incremented until it changes and
        ///   the counter is reseeded.
        /// - **Non-thread-safe:** The underlying counter uses unsynchronized cells, so needs to be wrapped in
        ///   a mutex to share.
        ///
        /// The counter can use additional sub-millisecond precision from the timestamp to better
        /// synchronize UUID sorting in distributed systems. In these cases, the additional precision
        /// is masked into the left-most 12 bits of the counter. The counter is still reseeded on
        /// each new millisecond, and incremented within the millisecond. This behavior may change
        /// in the future. The only guarantee is monotonicity.
        ///
        /// This type can be used when constructing version 7 UUIDs. When used to construct a
        /// version 7 UUID, the 42-bit counter will be padded with random data. This type can
        /// be used to maintain ordering of UUIDs within the same millisecond.
        ///
        /// This type should not be used when constructing version 1 or version 6 UUIDs.
        /// When used to construct a version 1 or version 6 UUID, only the 14 least significant
        /// bits of the counter will be used.
        #[derive(Debug)]
        pub struct ContextV7 {
            timestamp: Cell<ReseedingTimestamp>,
            counter: Cell<Counter>,
            adjust: Adjust,
            precision: Precision,
        }

        impl RefUnwindSafe for ContextV7 {}

        impl ContextV7 {
            /// Construct a new context that will reseed its counter on the first
            /// non-zero timestamp it receives.
            pub const fn new() -> Self {
                ContextV7 {
                    timestamp: Cell::new(ReseedingTimestamp {
                        last_seed: 0,
                        seconds: 0,
                        subsec_nanos: 0,
                    }),
                    counter: Cell::new(Counter { value: 0 }),
                    adjust: Adjust { by_ns: 0 },
                    precision: Precision {
                        bits: 0,
                        mask: 0,
                        factor: 0,
                        shift: 0,
                    },
                }
            }

            /// Specify an amount to shift timestamps by to obfuscate their actual generation time.
            pub fn with_adjust_by_millis(mut self, millis: u32) -> Self {
                self.adjust = Adjust::by_millis(millis);
                self
            }

            /// Use the leftmost 12 bits of the counter for additional timestamp precision.
            ///
            /// This method can provide better sorting for distributed applications that generate frequent UUIDs
            /// by trading a small amount of entropy for better counter synchronization. Note that the counter
            /// will still be reseeded on millisecond boundaries, even though some of its storage will be
            /// dedicated to the timestamp.
            pub fn with_additional_precision(mut self) -> Self {
                self.precision = Precision::new(12);
                self
            }
        }

        impl ClockSequence for ContextV7 {
            type Output = u64;

            fn generate_sequence(&self, seconds: u64, subsec_nanos: u32) -> Self::Output {
                self.generate_timestamp_sequence(seconds, subsec_nanos).0
            }

            fn generate_timestamp_sequence(
                &self,
                seconds: u64,
                subsec_nanos: u32,
            ) -> (Self::Output, u64, u32) {
                let (seconds, subsec_nanos) = self.adjust.apply(seconds, subsec_nanos);

                let mut counter;
                let (mut timestamp, should_reseed) =
                    self.timestamp.get().advance(seconds, subsec_nanos);

                if should_reseed {
                    // If the observed system time has shifted forwards then regenerate the counter
                    counter = Counter::reseed(&self.precision, &timestamp);
                } else {
                    // If the observed system time has not shifted forwards then increment the counter

                    // If the incoming timestamp is earlier than the last observed one then
                    // use it instead. This may happen if the system clock jitters, or if the counter
                    // has wrapped and the timestamp is artificially incremented

                    counter = self.counter.get().increment(&self.precision, &timestamp);

                    // Unlikely: If the counter has overflowed its 42-bit storage then wrap it
                    // and increment the timestamp. Until the observed system time shifts past
                    // this incremented value, all timestamps will use it to maintain monotonicity
                    if counter.has_overflowed() {
                        // Increment the timestamp by 1 milli and reseed the counter
                        timestamp = timestamp.increment();
                        counter = Counter::reseed(&self.precision, &timestamp);
                    }
                };

                self.timestamp.set(timestamp);
                self.counter.set(counter);

                (counter.value, timestamp.seconds, timestamp.subsec_nanos)
            }

            fn usable_bits(&self) -> usize {
                USABLE_BITS
            }
        }

        /// A timestamp that keeps track of whether a reseed is necessary.
        #[derive(Debug, Default, Clone, Copy)]
        struct ReseedingTimestamp {
            last_seed: u64,
            seconds: u64,
            subsec_nanos: u32,
        }

        impl ReseedingTimestamp {
            #[inline]
            fn from_ts(seconds: u64, subsec_nanos: u32) -> Self {
                // Reseed when the millisecond advances
                let last_seed = seconds
                    .saturating_mul(1_000)
                    .saturating_add((subsec_nanos / 1_000_000) as u64);

                ReseedingTimestamp {
                    last_seed,
                    seconds,
                    subsec_nanos,
                }
            }

            /// Advance the timestamp to a new value, returning whether a reseed is necessary.
            #[inline]
            fn advance(&self, seconds: u64, subsec_nanos: u32) -> (Self, bool) {
                let incoming = ReseedingTimestamp::from_ts(seconds, subsec_nanos);

                if incoming.last_seed > self.last_seed {
                    // The incoming value is part of a new millisecond
                    (incoming, true)
                } else {
                    // The incoming value is part of the same or an earlier millisecond
                    // We may still have advanced the subsecond portion, so use the larger value
                    let mut value = *self;
                    value.subsec_nanos = cmp::max(self.subsec_nanos, subsec_nanos);

                    (value, false)
                }
            }

            /// Advance the timestamp by a millisecond.
            #[inline]
            fn increment(&self) -> Self {
                let (seconds, subsec_nanos) =
                    Adjust::by_millis(1).apply(self.seconds, self.subsec_nanos);

                ReseedingTimestamp::from_ts(seconds, subsec_nanos)
            }

            #[inline]
            fn submilli_nanos(&self) -> u32 {
                self.subsec_nanos % 1_000_000
            }
        }

        /// A counter that initializes to a safe random seed and tracks overflow.
        #[derive(Debug, Clone, Copy)]
        struct Counter {
            value: u64,
        }

        impl Counter {
            #[inline]
            fn reseed(precision: &Precision, timestamp: &ReseedingTimestamp) -> Self {
                Counter {
                    value: precision.apply(crate::rng::u64() & RESEED_MASK, timestamp),
                }
            }

            /// Advance the counter.
            #[inline]
            fn increment(&self, precision: &Precision, timestamp: &ReseedingTimestamp) -> Self {
                let mut counter = Counter {
                    value: precision.apply(self.value, timestamp),
                };

                // We unconditionally increment the counter even though the precision
                // may have set higher bits already. This could technically be avoided,
                // but the higher bits are a coarse approximation so we just avoid the
                // `if` branch and increment it either way

                // Guaranteed to never overflow u64
                counter.value += 1;

                counter
            }

            #[inline]
            fn has_overflowed(&self) -> bool {
                self.value > MAX_COUNTER
            }
        }

        /// A utility that adjusts an input timestamp by a given number of nanoseconds.
        #[derive(Debug)]
        struct Adjust {
            by_ns: u128,
        }

        impl Adjust {
            #[inline]
            fn by_millis(millis: u32) -> Self {
                Adjust {
                    by_ns: (millis as u128).saturating_mul(1_000_000),
                }
            }

            /// Apply the adjustment, returning the adjusted timestamp.
            #[inline]
            fn apply(&self, seconds: u64, subsec_nanos: u32) -> (u64, u32) {
                if self.by_ns == 0 {
                    // No shift applied
                    return (seconds, subsec_nanos);
                }

                let ts = (seconds as u128)
                    .saturating_mul(1_000_000_000)
                    .saturating_add(subsec_nanos as u128)
                    .saturating_add(self.by_ns);

                ((ts / 1_000_000_000) as u64, (ts % 1_000_000_000) as u32)
            }
        }

        /// A utility that overwrites some number of counter bits with additional timestamp precision.
        #[derive(Debug)]
        struct Precision {
            bits: usize,
            factor: u64,
            mask: u64,
            shift: u64,
        }

        impl Precision {
            fn new(bits: usize) -> Self {
                // The mask and shift are used to paste the sub-millisecond precision
                // into the most significant bits of the counter
                let mask = u64::MAX >> (64 - USABLE_BITS + bits);
                let shift = (USABLE_BITS - bits) as u64;

                // The factor reduces the size of the sub-millisecond precision to
                // fit into the specified number of bits
                let factor = (999_999 / u64::pow(2, bits as u32)) + 1;

                Precision {
                    bits,
                    factor,
                    mask,
                    shift,
                }
            }

            /// Apply additional precision from the given timestamp to the counter.
            #[inline]
            fn apply(&self, counter: u64, timestamp: &ReseedingTimestamp) -> u64 {
                if self.bits == 0 {
                    // No additional precision is being used
                    return counter;
                }

                let additional = timestamp.submilli_nanos() as u64 / self.factor;

                (counter & self.mask) | (additional << self.shift)
            }
        }

        #[cfg(feature = "std")]
        pub(crate) struct SharedContextV7(std::sync::Mutex<ContextV7>);

        #[cfg(feature = "std")]
        impl ClockSequence for SharedContextV7 {
            type Output = u64;

            fn generate_sequence(&self, seconds: u64, subsec_nanos: u32) -> Self::Output {
                self.0.generate_sequence(seconds, subsec_nanos)
            }

            fn generate_timestamp_sequence(
                &self,
                seconds: u64,
                subsec_nanos: u32,
            ) -> (Self::Output, u64, u32) {
                self.0.generate_timestamp_sequence(seconds, subsec_nanos)
            }

            fn usable_bits(&self) -> usize
            where
                Self::Output: Sized,
            {
                USABLE_BITS
            }
        }

        #[cfg(test)]
        mod tests {
            use core::time::Duration;

            use super::*;

            use crate::{Timestamp, Uuid};

            #[test]
            fn context() {
                let seconds = 1_496_854_535;
                let subsec_nanos = 812_946_000;

                let context = ContextV7::new();

                let ts1 = Timestamp::from_unix(&context, seconds, subsec_nanos);
                assert_eq!(42, ts1.usable_counter_bits);

                // Backwards second
                let seconds = 1_496_854_534;

                let ts2 = Timestamp::from_unix(&context, seconds, subsec_nanos);

                // The backwards time should be ignored
                // The counter should still increment
                assert_eq!(ts1.seconds, ts2.seconds);
                assert_eq!(ts1.subsec_nanos, ts2.subsec_nanos);
                assert_eq!(ts1.counter + 1, ts2.counter);

                // Forwards second
                let seconds = 1_496_854_536;

                let ts3 = Timestamp::from_unix(&context, seconds, subsec_nanos);

                // The counter should have reseeded
                assert_ne!(ts2.counter + 1, ts3.counter);
                assert_ne!(0, ts3.counter);
            }

            #[test]
            fn context_wrap() {
                let seconds = 1_496_854_535u64;
                let subsec_nanos = 812_946_000u32;

                // This context will wrap
                let context = ContextV7 {
                    timestamp: Cell::new(ReseedingTimestamp::from_ts(seconds, subsec_nanos)),
                    adjust: Adjust::by_millis(0),
                    precision: Precision {
                        bits: 0,
                        mask: 0,
                        factor: 0,
                        shift: 0,
                    },
                    counter: Cell::new(Counter {
                        value: u64::MAX >> 22,
                    }),
                };

                let ts = Timestamp::from_unix(&context, seconds, subsec_nanos);

                // The timestamp should be incremented by 1ms
                let expected_ts = Duration::new(seconds, subsec_nanos) + Duration::from_millis(1);
                assert_eq!(expected_ts.as_secs(), ts.seconds);
                assert_eq!(expected_ts.subsec_nanos(), ts.subsec_nanos);

                // The counter should have reseeded
                assert!(ts.counter < (u64::MAX >> 22) as u128);
                assert_ne!(0, ts.counter);
            }

            #[test]
            fn context_shift() {
                let seconds = 1_496_854_535;
                let subsec_nanos = 812_946_000;

                let context = ContextV7::new().with_adjust_by_millis(1);

                let ts = Timestamp::from_unix(&context, seconds, subsec_nanos);

                assert_eq!((1_496_854_535, 813_946_000), ts.to_unix());
            }

            #[test]
            fn context_additional_precision() {
                let seconds = 1_496_854_535;
                let subsec_nanos = 812_946_000;

                let context = ContextV7::new().with_additional_precision();

                let ts1 = Timestamp::from_unix(&context, seconds, subsec_nanos);

                // NOTE: Future changes in rounding may change this value slightly
                assert_eq!(3861, ts1.counter >> 30);

                assert!(ts1.counter < (u64::MAX >> 22) as u128);

                // Generate another timestamp; it should continue to sort
                let ts2 = Timestamp::from_unix(&context, seconds, subsec_nanos);

                assert!(Uuid::new_v7(ts2) > Uuid::new_v7(ts1));

                // Generate another timestamp with an extra nanosecond
                let subsec_nanos = subsec_nanos + 1;

                let ts3 = Timestamp::from_unix(&context, seconds, subsec_nanos);

                assert!(Uuid::new_v7(ts3) > Uuid::new_v7(ts2));
            }

            #[test]
            fn context_overflow() {
                let seconds = u64::MAX;
                let subsec_nanos = u32::MAX;

                // Ensure we don't panic
                for context in [
                    ContextV7::new(),
                    ContextV7::new().with_additional_precision(),
                    ContextV7::new().with_adjust_by_millis(u32::MAX),
                ] {
                    Timestamp::from_unix(&context, seconds, subsec_nanos);
                }
            }
        }
    }

    #[cfg(feature = "v7")]
    pub use v7_support::*;

    /// An empty counter that will always return the value `0`.
    ///
    /// This type can be used when constructing version 7 UUIDs. When used to
    /// construct a version 7 UUID, the entire counter segment of the UUID will be
    /// filled with a random value. This type does not maintain ordering of UUIDs
    /// within a millisecond but is efficient.
    ///
    /// This type should not be used when constructing version 1 or version 6 UUIDs.
    /// When used to construct a version 1 or version 6 UUID, the counter
    /// segment will remain zero.
    #[derive(Debug, Clone, Copy, Default)]
    pub struct NoContext;

    impl ClockSequence for NoContext {
        type Output = u16;

        fn generate_sequence(&self, _seconds: u64, _nanos: u32) -> Self::Output {
            0
        }

        fn usable_bits(&self) -> usize {
            0
        }
    }
}

#[cfg(all(test, any(feature = "v1", feature = "v6")))]
mod tests {
    use super::*;

    #[cfg(all(target_arch = "wasm32", any(target_os = "unknown", target_os = "none")))]
    use wasm_bindgen_test::*;

    #[test]
    #[cfg_attr(
        all(target_arch = "wasm32", any(target_os = "unknown", target_os = "none")),
        wasm_bindgen_test
    )]
    fn gregorian_unix_does_not_panic() {
        // Ensure timestamp conversions never panic
        Timestamp::unix_to_gregorian_ticks(u64::MAX, 0);
        Timestamp::unix_to_gregorian_ticks(0, u32::MAX);
        Timestamp::unix_to_gregorian_ticks(u64::MAX, u32::MAX);

        Timestamp::gregorian_to_unix(u64::MAX);
    }

    #[test]
    #[cfg_attr(
        all(target_arch = "wasm32", any(target_os = "unknown", target_os = "none")),
        wasm_bindgen_test
    )]
    fn to_gregorian_truncates_to_usable_bits() {
        let ts = Timestamp::from_gregorian_time(123, u16::MAX);

        assert_eq!((123, u16::MAX >> 2), ts.to_gregorian());
    }

    #[test]
    #[cfg_attr(
        all(target_arch = "wasm32", any(target_os = "unknown", target_os = "none")),
        wasm_bindgen_test
    )]
    fn clock_sequence_usable_bits() {
        struct MyContext;

        impl ClockSequence for MyContext {
            type Output = u16;

            fn generate_sequence(&self, _: u64, _: u32) -> Self::Output {
                0
            }
        }

        assert_eq!(16, MyContext.usable_bits());
    }

    #[cfg(all(test, feature = "std", not(miri)))]
    mod std_support {
        use super::*;

        use std::time::{Duration, SystemTime};

        // Components of an arbitrary timestamp with non-zero nanoseconds.
        const KNOWN_SECONDS: u64 = 1_501_520_400;
        const KNOWN_NANOS: u32 = 1_000;

        fn known_system_time() -> SystemTime {
            SystemTime::UNIX_EPOCH
                .checked_add(Duration::new(KNOWN_SECONDS, KNOWN_NANOS))
                .unwrap()
        }

        fn known_timestamp() -> Timestamp {
            Timestamp::from_unix_time(KNOWN_SECONDS, KNOWN_NANOS, 0, 0)
        }

        #[test]
        fn to_system_time() {
            let st: SystemTime = known_timestamp().into();

            assert_eq!(known_system_time(), st);
        }

        #[test]
        fn from_system_time() {
            let ts: Timestamp = known_system_time().try_into().unwrap();

            assert_eq!(known_timestamp(), ts);
        }

        #[test]
        fn from_system_time_before_epoch() {
            let before_epoch = match SystemTime::UNIX_EPOCH.checked_sub(Duration::from_nanos(1_000))
            {
                Some(st) => st,
                None => return,
            };

            Timestamp::try_from(before_epoch)
                .expect_err("Timestamp should not be created from before epoch");
        }
    }
}
