src/connectivity/lib/packet-formats/src/utils.rs

// Copyright 2020 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

//! Utilities useful when parsing and serializing wire formats.

use core::num::{NonZeroU32, NonZeroU64};
use core::time::Duration;

/// A thin wrapper over a [`Duration`] that guarantees that the underlying
/// `Duration` is non-zero.
#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Clone, Copy, Hash)]
pub struct NonZeroDuration(Duration);

impl NonZeroDuration {
    /// Creates a non-zero without checking the value.
    ///
    /// # Safety
    ///
    /// If `d` is zero, unsafe code which relies on the invariant that
    /// `NonZeroDuration` values are always zero may cause undefined behavior.
    pub const unsafe fn new_unchecked(d: Duration) -> NonZeroDuration {
        NonZeroDuration(d)
    }

    /// Creates a new `NonZeroDuration` from the specified number of whole
    /// seconds if that number is non-zero.
    pub const fn from_secs(secs: u64) -> Option<NonZeroDuration> {
        NonZeroDuration::new(Duration::from_secs(secs))
    }

    /// Creates a new `NonZeroDuration` from the specified non-zero number of
    /// whole seconds.
    pub const fn from_nonzero_secs(secs: NonZeroU64) -> NonZeroDuration {
        NonZeroDuration(Duration::from_secs(secs.get()))
    }

    /// Creates a new `NonZeroDuration` from the specified number of whole
    /// seconds and additional nanoseconds if the resulting duration is
    /// non-zero.
    ///
    /// If the number of nanoseconds is greater than 1 billion (the number of
    /// nanoseconds in a second), then it will carry over into the seconds
    /// provided.
    ///
    /// # Panics
    ///
    /// This constructor will panic if the carry from the nanoseconds overflows
    /// the seconds counter.
    pub const fn from_secs_nanos(secs: u64, nanos: u32) -> Option<NonZeroDuration> {
        NonZeroDuration::new(Duration::new(secs, nanos))
    }

    /// Creates a new `NonZeroDuration` from the specified non-zero number of
    /// whole seconds and additional nanoseconds.
    ///
    /// If the number of nanoseconds is greater than 1 billion (the number of
    /// nanoseconds in a second), then it will carry over into the seconds
    /// provided.
    ///
    /// # Panics
    ///
    /// This constructor will panic if the carry from the nanoseconds overflows
    /// the seconds counter.
    pub const fn from_nonzero_secs_nanos(secs: NonZeroU64, nanos: NonZeroU32) -> NonZeroDuration {
        NonZeroDuration(Duration::new(secs.get(), nanos.get()))
    }

    /// Creates a new `NonZeroDuration` from the specified number of
    /// milliseconds if that number is non-zero.
    pub const fn from_millis(millis: u64) -> Option<NonZeroDuration> {
        NonZeroDuration::new(Duration::from_millis(millis))
    }

    /// Creates a new `NonZeroDuration` from the specified non-zero number of
    /// milliseconds.
    pub const fn from_nonzero_millis(millis: NonZeroU64) -> NonZeroDuration {
        NonZeroDuration(Duration::from_millis(millis.get()))
    }

    /// Creates a new `NonZeroDuration` from the specified number of
    /// microseconds if that number is non-zero.
    pub const fn from_micros(micros: u64) -> Option<NonZeroDuration> {
        NonZeroDuration::new(Duration::from_micros(micros))
    }

    /// Creates a new `NonZeroDuration` from the specified non-zero number of
    /// microseconds.
    pub const fn from_nonzero_micros(micros: NonZeroU64) -> NonZeroDuration {
        NonZeroDuration(Duration::from_micros(micros.get()))
    }

    /// Creates a new `NonZeroDuration` from the specified number of nanoseconds
    /// if that number is non-zero.
    pub const fn from_nanos(nanos: u64) -> Option<NonZeroDuration> {
        NonZeroDuration::new(Duration::from_nanos(nanos))
    }

    /// Creates a new `NonZeroDuration` from the specified non-zero number of
    /// nanoseconds.
    pub const fn from_nonzero_nanos(nanos: NonZeroU64) -> NonZeroDuration {
        NonZeroDuration(Duration::from_nanos(nanos.get()))
    }

    /// Creates a non-zero if the given value is not zero.
    pub const fn new(d: Duration) -> Option<NonZeroDuration> {
        // Can't do this comparison as `d == Duration::from_secs(0)` because
        // equality checking is not const.
        if d.as_nanos() == 0 {
            return None;
        }

        Some(NonZeroDuration(d))
    }

    /// Returns the value as a [`Duration`].
    pub const fn get(self) -> Duration {
        self.0
    }
}

impl From<NonZeroDuration> for Duration {
    fn from(NonZeroDuration(d): NonZeroDuration) -> Duration {
        d
    }
}

impl core::ops::Add<Duration> for NonZeroDuration {
    type Output = Self;

    fn add(self, rhs: Duration) -> Self::Output {
        let Self(d) = self;
        Self(d + rhs)
    }
}

impl core::ops::Add<NonZeroDuration> for NonZeroDuration {
    type Output = Self;

    fn add(self, rhs: NonZeroDuration) -> Self::Output {
        self + rhs.get()
    }
}

impl core::ops::Mul<NonZeroU32> for NonZeroDuration {
    type Output = Self;

    fn mul(self, rhs: NonZeroU32) -> Self::Output {
        let Self(d) = self;
        Self(d * rhs.get())
    }
}

/// Rounds `x` up to the next multiple of 4 unless `x` is already a multiple of
/// 4.
pub(crate) fn round_to_next_multiple_of_four(x: usize) -> usize {
    (x + 3) & !3
}

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

    #[test]
    fn non_zero_duration() {
        // `NonZeroDuration` should not hold a zero duration.
        assert_eq!(NonZeroDuration::new(Duration::from_secs(0)), None);

        // `new_unchecked` should hold the duration as is.
        let d = Duration::from_secs(1);
        assert_eq!(unsafe { NonZeroDuration::new_unchecked(d) }, NonZeroDuration(d));

        // `NonZeroDuration` should hold a non-zero duration.
        let non_zero = NonZeroDuration::new(d);
        assert_eq!(non_zero, Some(NonZeroDuration(d)));

        let one_u64 = NonZeroU64::new(1).unwrap();
        let expect = NonZeroDuration(Duration::from_secs(1));
        assert_eq!(NonZeroDuration::from_secs(1), Some(expect));
        assert_eq!(NonZeroDuration::from_nonzero_secs(one_u64), expect);

        let expect = NonZeroDuration(Duration::new(1, 1));
        assert_eq!(NonZeroDuration::from_secs_nanos(1, 1), Some(expect));
        assert_eq!(
            NonZeroDuration::from_nonzero_secs_nanos(one_u64, NonZeroU32::new(1).unwrap()),
            expect
        );

        let expect = NonZeroDuration(Duration::from_millis(1));
        assert_eq!(NonZeroDuration::from_millis(1), Some(expect));
        assert_eq!(NonZeroDuration::from_nonzero_millis(one_u64), expect);

        let expect = NonZeroDuration(Duration::from_micros(1));
        assert_eq!(NonZeroDuration::from_micros(1), Some(expect));
        assert_eq!(NonZeroDuration::from_nonzero_micros(one_u64), expect);

        let expect = NonZeroDuration(Duration::from_nanos(1));
        assert_eq!(NonZeroDuration::from_nanos(1), Some(expect));
        assert_eq!(NonZeroDuration::from_nonzero_nanos(one_u64), expect);

        // `get` and `Into::into` should return the underlying duration.
        let non_zero = non_zero.unwrap();
        assert_eq!(d, non_zero.get());
        assert_eq!(d, non_zero.into());
    }

    #[test]
    fn test_next_multiple_of_four() {
        for x in 0usize..=(core::u16::MAX - 3) as usize {
            let y = round_to_next_multiple_of_four(x);
            assert_eq!(y % 4, 0);
            assert!(y >= x);
            if x % 4 == 0 {
                assert_eq!(x, y);
            } else {
                assert_eq!(x + (4 - x % 4), y);
            }
        }
    }

    #[test]
    fn add_duration() {
        let a = NonZeroDuration::new(Duration::from_secs(48291)).unwrap();
        let b = Duration::from_secs(195811);
        assert_eq!(Some(a + b), NonZeroDuration::new(a.get() + b));

        assert_eq!(a + Duration::from_secs(0), a);
    }

    #[test]
    fn add_nonzero_duration() {
        let a = NonZeroDuration::new(Duration::from_secs(48291)).unwrap();
        let b = NonZeroDuration::new(Duration::from_secs(195811)).unwrap();
        assert_eq!(Some(a + b), NonZeroDuration::new(a.get() + b.get()));
    }
}