Merge remote-tracking branch 'origin/main' into add_cuc_time_impl

Reviewed-on: #2

CHANGELOG.md

@@ -24,6 +24,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 - Added `std::error::Error` implementation for all error enumerations if the `std` feature
   is enabled.
+- CUC timestamp implementation as specified in CCSDS 301.0-B-4 section 3.2.
 - ACII timestamps as specified in CCSDS 301.0-B-4 section 3.5.
 - Added MSRV in `Cargo.toml` with the `rust-version` field set to Rust 1.60.
 - `serde` `Serialize` and `Deserialize` added to all types.

src/time/cds.rs

@@ -1,4 +1,4 @@
-//! Module to generate or read CDS timestamps as specified in
+//! Module to generate or read CCSDS Day Segmented (CDS) timestamps as specified in
 //! [CCSDS 301.0-B-4](https://public.ccsds.org/Pubs/301x0b4e1.pdf) section 3.3 .
 //!
 //! The core data structure to do this is the [cds::TimeProvider] struct.

src/time/cuc.rs

@@ -1,15 +1,83 @@
+//! Module to generate or read CCSDS Unsegmented (CUC) timestamps as specified in
+//! [CCSDS 301.0-B-4](https://public.ccsds.org/Pubs/301x0b4e1.pdf) section 3.2 .
+//!
+//! The core data structure to do this is the [TimeProviderCcsdsEpoch] struct.
 use super::*;
 use core::fmt::Debug;
 
 const MIN_CUC_LEN: usize = 2;
+/// Maximum length if the preamble field is not extended.
+pub const MAX_CUC_LEN_SMALL_PREAMBLE: usize = 8;
+
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+pub enum FractionalResolution {
+    /// No fractional part, only second resolution
+    Seconds = 0,
+    /// 256 fractional parts, resulting in 1/255 ~= 4 ms fractional resolution
+    FourMs = 1,
+    /// 65535 fractional parts, resulting in 1/65535 ~= 15 us fractional resolution
+    FifteenUs = 2,
+    /// 16777215 fractional parts, resulting in 1/16777215 ~= 60 ns fractional resolution
+    SixtyNs = 3,
+}
+
+impl TryFrom<u8> for FractionalResolution {
+    type Error = ();
+
+    fn try_from(v: u8) -> Result<Self, Self::Error> {
+        match v {
+            0 => Ok(FractionalResolution::Seconds),
+            1 => Ok(FractionalResolution::FourMs),
+            2 => Ok(FractionalResolution::FifteenUs),
+            3 => Ok(FractionalResolution::SixtyNs),
+            _ => Err(()),
+        }
+    }
+}
+
+/// Please note that this function will panic if the fractional value is not smaller than
+/// the maximum number of fractions allowed for the particular resolution.
+/// (e.g. passing 270 when the resolution only allows 255 values).
+pub fn convert_fractional_part_to_ns(fractional_part: FractionalPart) -> u64 {
+    let div = fractional_res_to_div(fractional_part.0);
+    assert!(fractional_part.1 < div);
+    10_u64.pow(9) * fractional_part.1 as u64 / div as u64
+}
+
+pub const fn fractional_res_to_div(res: FractionalResolution) -> u32 {
+    2_u32.pow(8 * res as u32) - 1
+}
+
+/// Calculate the fractional part for a given resolution and subsecond nanoseconds.
+/// Please note that this function will panic if the passed nanoseconds exceeds 1 second
+/// as a nanosecond (10 to the power of 9). Furthermore, it will return [None] if the
+/// given resolution is [FractionalResolution::Seconds].
+pub fn fractional_part_from_subsec_ns(
+    res: FractionalResolution,
+    ns: u64,
+) -> Option<FractionalPart> {
+    if res == FractionalResolution::Seconds {
+        return None;
+    }
+    let sec_as_ns = 10_u64.pow(9);
+    if ns > sec_as_ns {
+        panic!("passed nanosecond value larger than 1 second");
+    }
+    // First determine the nanoseconds for the smallest segment given the resolution.
+    // Then divide by that to find out the fractional part. An integer division floors
+    // which is what we want here.
+    let fractional_part = ns / (sec_as_ns / fractional_res_to_div(res) as u64);
+    Some(FractionalPart(res, fractional_part as u32))
+}
 
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
 pub enum CucError {
     InvalidCounterWidth(u8),
-    InvalidFractionWidth(u8),
+    InvalidFractionResolution(FractionalResolution),
     InvalidCounter(u8, u64),
-    InvalidFractions(u8, u64),
+    InvalidFractions(FractionalResolution, u64),
 }
 
 impl Display for CucError {
@@ -18,14 +86,14 @@ impl Display for CucError {
             CucError::InvalidCounterWidth(w) => {
                 write!(f, "invalid cuc counter byte width {}", w)
             }
-            CucError::InvalidFractionWidth(w) => {
-                write!(f, "invalid cuc fractional part byte width {}", w)
+            CucError::InvalidFractionResolution(w) => {
+                write!(f, "invalid cuc fractional part byte width {:?}", w)
             }
             CucError::InvalidCounter(w, c) => {
                 write!(f, "invalid cuc counter {} for width {}", c, w)
             }
             CucError::InvalidFractions(w, c) => {
-                write!(f, "invalid cuc fractional part {} for width {}", c, w)
+                write!(f, "invalid cuc fractional part {} for width {:?}", c, w)
             }
         }
     }
@@ -37,18 +105,51 @@ impl Error for CucError {}
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
 pub struct WidthCounterPair(u8, u32);
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+pub struct FractionalPart(FractionalResolution, u32);
 
-/// This provider uses the CCSDS epoch. Furthermore the preamble field only has one byte,
-/// which allows a time code representation through the year 2094.
+/// This object is the abstraction for the CCSDS Unsegmented Time Code (CUC) using the CCSDS epoch
+/// and a small preamble field.
 ///
-/// More specifically, only having a preamble field of one byte limits the width of the counter
-/// type (generally seconds) to 4 bytes and the width of the fractions type to 3 bytes.
+/// It has the capability to generate and read timestamps as specified in the CCSDS 301.0-B-4
+/// section 3.2 . The preamble field only has one byte, which allows a time code representation
+/// through the year 2094. The time is represented as a simple binary counter starting from the
+/// fixed CCSDS epoch (1958-01-01 00:00:00). It is possible to provide subsecond accuracy using the
+/// fractional field with various available [resolutions][FractionalResolution].
+///
+/// Having a preamble field of one byte limits the width of the counter
+/// type (generally seconds) to 4 bytes and the width of the fractions type to 3 bytes. This limits
+/// the maximum time stamp size to [MAX_CUC_LEN_SMALL_PREAMBLE] (8 bytes).
+///
+/// # Example
+///
+/// ```
+/// use spacepackets::time::cuc::{FractionalResolution, TimeProviderCcsdsEpoch};
+/// use spacepackets::time::{TimeWriter, CcsdsTimeCodes, TimeReader, CcsdsTimeProvider};
+///
+/// // Highest fractional resolution
+/// let timestamp_now = TimeProviderCcsdsEpoch::from_now(FractionalResolution::SixtyNs).expect("creating cuc stamp failed");
+/// let mut raw_stamp = [0; 16];
+/// {
+///     let written = timestamp_now.write_to_bytes(&mut raw_stamp).expect("writing timestamp failed");
+///     assert_eq!((raw_stamp[0] >> 4) & 0b111, CcsdsTimeCodes::CucCcsdsEpoch as u8);
+///     // 1 byte preamble + 4 byte counter + 3 byte fractional part
+///     assert_eq!(written, 8);
+/// }
+/// {
+///     let read_result = TimeProviderCcsdsEpoch::from_bytes(&raw_stamp);
+///     assert!(read_result.is_ok());
+///     let stamp_deserialized = read_result.unwrap();
+///     assert_eq!(stamp_deserialized, timestamp_now);
+/// }
+/// ```
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
 pub struct TimeProviderCcsdsEpoch {
     pfield: u8,
     counter: WidthCounterPair,
-    fractions: Option<WidthCounterPair>,
+    fractions: Option<FractionalPart>,
 }
 
 #[inline]
@@ -60,24 +161,162 @@ pub fn pfield_len(pfield: u8) -> usize {
 }
 
 impl TimeProviderCcsdsEpoch {
-    fn build_p_field(counter_width: u8, fractions_width: Option<u8>) -> u8 {
+    /// Create a time provider with a four byte counter and no fractional part.
+    pub fn new(counter: u32) -> Self {
+        // These values are definitely valid, so it is okay to unwrap here.
+        Self::new_generic(WidthCounterPair(4, counter), None).unwrap()
+    }
+
+    /// Like [TimeProviderCcsdsEpoch::new] but allow to supply a fractional part as well.
+    pub fn new_with_fractions(counter: u32, fractions: FractionalPart) -> Result<Self, CucError> {
+        Self::new_generic(WidthCounterPair(4, counter), Some(fractions))
+    }
+
+    /// Fractions with a resolution of ~ 4 ms
+    pub fn new_with_coarse_fractions(counter: u32, subsec_fractions: u8) -> Self {
+        // These values are definitely valid, so it is okay to unwrap here.
+        Self::new_generic(
+            WidthCounterPair(4, counter),
+            Some(FractionalPart(
+                FractionalResolution::FourMs,
+                subsec_fractions as u32,
+            )),
+        )
+        .unwrap()
+    }
+
+    /// Fractions with a resolution of ~ 16 us
+    pub fn new_with_medium_fractions(counter: u32, subsec_fractions: u16) -> Self {
+        // These values are definitely valid, so it is okay to unwrap here.
+        Self::new_generic(
+            WidthCounterPair(4, counter),
+            Some(FractionalPart(
+                FractionalResolution::FifteenUs,
+                subsec_fractions as u32,
+            )),
+        )
+        .unwrap()
+    }
+
+    /// Fractions with a resolution of ~ 60 ns. The fractional part value is limited by the
+    /// 24 bits of the fractional field, so this function will fail with
+    /// [CucError::InvalidFractions] if the fractional value exceeds the value.
+    pub fn new_with_fine_fractions(counter: u32, subsec_fractions: u32) -> Result<Self, CucError> {
+        Self::new_generic(
+            WidthCounterPair(4, counter),
+            Some(FractionalPart(
+                FractionalResolution::SixtyNs,
+                subsec_fractions as u32,
+            )),
+        )
+    }
+
+    /// This function will return the current time as a CUC timestamp.
+    /// The counter width will always be set to 4 bytes because the normal CCSDS epoch will overflow
+    /// when using less than that.
+    #[cfg(feature = "std")]
+    pub fn from_now(fraction_resolution: FractionalResolution) -> Result<Self, StdTimestampError> {
+        let now = SystemTime::now().duration_since(SystemTime::UNIX_EPOCH)?;
+        let ccsds_epoch = unix_epoch_to_ccsds_epoch(now.as_secs());
+        if fraction_resolution == FractionalResolution::Seconds {
+            return Ok(Self::new(ccsds_epoch as u32));
+        }
+        let fractions =
+            fractional_part_from_subsec_ns(fraction_resolution, now.subsec_nanos() as u64);
+        Self::new_with_fractions(ccsds_epoch as u32, fractions.unwrap())
+            .map_err(|e| StdTimestampError::TimestampError(e.into()))
+    }
+
+    /// Updates the current time stamp from the current time. The fractional field width remains
+    /// the same and will be updated accordingly.
+    #[cfg(feature = "std")]
+    pub fn update_from_now(&mut self) -> Result<(), StdTimestampError> {
+        let now = SystemTime::now().duration_since(SystemTime::UNIX_EPOCH)?;
+        self.counter.1 = unix_epoch_to_ccsds_epoch(now.as_secs()) as u32;
+        if self.fractions.is_some() {
+            self.fractions = fractional_part_from_subsec_ns(
+                self.fractions.unwrap().0,
+                now.subsec_nanos() as u64,
+            );
+        }
+        Ok(())
+    }
+
+    pub fn new_u16_counter(counter: u16) -> Self {
+        // These values are definitely valid, so it is okay to unwrap here.
+        Self::new_generic(WidthCounterPair(2, counter as u32), None).unwrap()
+    }
+
+    pub fn width_counter_pair(&self) -> WidthCounterPair {
+        self.counter
+    }
+
+    pub fn width_fractions_pair(&self) -> Option<FractionalPart> {
+        self.fractions
+    }
+
+    pub fn set_fractions(&mut self, fractions: FractionalPart) -> Result<(), CucError> {
+        Self::verify_fractions_width(fractions.0)?;
+        Self::verify_fractions_value(fractions)?;
+        self.fractions = Some(fractions);
+        self.update_p_field_fractions();
+        Ok(())
+    }
+
+    /// Set a fractional resolution. Please note that this function will reset the fractional value
+    /// to 0 if the resolution changes.
+    pub fn set_fractional_resolution(&mut self, res: FractionalResolution) {
+        if res == FractionalResolution::Seconds {
+            self.fractions = None;
+        }
+        let mut update_fractions = true;
+        if let Some(existing_fractions) = self.fractions {
+            if existing_fractions.0 == res {
+                update_fractions = false;
+            }
+        };
+        if update_fractions {
+            self.fractions = Some(FractionalPart(res, 0));
+        }
+    }
+
+    pub fn new_generic(
+        counter: WidthCounterPair,
+        fractions: Option<FractionalPart>,
+    ) -> Result<Self, CucError> {
+        Self::verify_counter_width(counter.0)?;
+        if counter.1 > (2u64.pow(counter.0 as u32 * 8) - 1) as u32 {
+            return Err(CucError::InvalidCounter(counter.0, counter.1 as u64));
+        }
+        if let Some(fractions) = fractions {
+            Self::verify_fractions_width(fractions.0)?;
+            Self::verify_fractions_value(fractions)?;
+        }
+        Ok(Self {
+            pfield: Self::build_p_field(counter.0, fractions.map(|v| v.0)),
+            counter,
+            fractions,
+        })
+    }
+
+    fn build_p_field(counter_width: u8, fractions_width: Option<FractionalResolution>) -> u8 {
         let mut pfield = (CcsdsTimeCodes::CucCcsdsEpoch as u8) << 4;
         if !(1..=4).contains(&counter_width) {
             // Okay to panic here, this function is private and all input values should
             // have been sanitized
             panic!("invalid counter width {} for cuc timestamp", counter_width);
         }
-        pfield |= (counter_width - 1) << 3;
+        pfield |= (counter_width - 1) << 2;
         if let Some(fractions_width) = fractions_width {
-            if !(1..=3).contains(&fractions_width) {
+            if !(1..=3).contains(&(fractions_width as u8)) {
                 // Okay to panic here, this function is private and all input values should
                 // have been sanitized
                 panic!(
-                    "invalid fractions width {} for cuc timestamp",
+                    "invalid fractions width {:?} for cuc timestamp",
                     fractions_width
                 );
             }
-            pfield |= fractions_width;
+            pfield |= fractions_width as u8;
         }
         pfield
     }
@@ -85,14 +324,10 @@ impl TimeProviderCcsdsEpoch {
     fn update_p_field_fractions(&mut self) {
         self.pfield &= !(0b11);
         if let Some(fractions) = self.fractions {
-            self.pfield |= fractions.0;
+            self.pfield |= fractions.0 as u8;
         }
     }
 
-    pub fn len_packed(&self) -> usize {
-        Self::len_packed_from_pfield(self.pfield)
-    }
-
     #[inline]
     pub fn len_cntr_from_pfield(pfield: u8) -> u8 {
         ((pfield >> 2) & 0b11) + 1
@@ -127,8 +362,7 @@ impl TimeProviderCcsdsEpoch {
         base_len
     }
 
-    /// Verifies the raw width parameter and returns the actual length, which is the raw
-    /// value plus 1.
+    /// Verifies the raw width parameter.
     fn verify_counter_width(width: u8) -> Result<(), CucError> {
         if width == 0 || width > 4 {
             return Err(CucError::InvalidCounterWidth(width));
@@ -136,14 +370,14 @@ impl TimeProviderCcsdsEpoch {
         Ok(())
     }
 
-    fn verify_fractions_width(width: u8) -> Result<(), CucError> {
-        if width > 3 {
-            return Err(CucError::InvalidFractionWidth(width));
+    fn verify_fractions_width(width: FractionalResolution) -> Result<(), CucError> {
+        if width as u8 > 3 {
+            return Err(CucError::InvalidFractionResolution(width));
         }
         Ok(())
     }
 
-    fn verify_fractions_value(val: WidthCounterPair) -> Result<(), CucError> {
+    fn verify_fractions_value(val: FractionalPart) -> Result<(), CucError> {
         if val.1 > 2u32.pow((val.0 as u32) * 8) - 1 {
             return Err(CucError::InvalidFractions(val.0, val.1 as u64));
         }
@@ -151,44 +385,6 @@ impl TimeProviderCcsdsEpoch {
     }
 }
 
-impl TimeProviderCcsdsEpoch {
-    pub fn new_default(counter: u32) -> Self {
-        // These values are definitely valid, so it is okay to unwrap here.
-        Self::new(WidthCounterPair(4, counter), None).unwrap()
-    }
-    pub fn new_u16_counter(counter: u16) -> Self {
-        // These values are definitely valid, so it is okay to unwrap here.
-        Self::new(WidthCounterPair(2, counter as u32), None).unwrap()
-    }
-
-    pub fn set_fractions(&mut self, fractions: WidthCounterPair) -> Result<(), CucError> {
-        Self::verify_fractions_width(fractions.0)?;
-        Self::verify_fractions_value(fractions)?;
-        self.fractions = Some(fractions);
-        self.update_p_field_fractions();
-        Ok(())
-    }
-
-    pub fn new(
-        counter: WidthCounterPair,
-        fractions: Option<WidthCounterPair>,
-    ) -> Result<Self, CucError> {
-        Self::verify_counter_width(counter.0)?;
-        if counter.1 > 2u32.pow(counter.0 as u32 * 8) - 1 {
-            return Err(CucError::InvalidCounter(counter.0, counter.1 as u64));
-        }
-        if let Some(fractions) = fractions {
-            Self::verify_fractions_width(fractions.0)?;
-            Self::verify_fractions_value(fractions)?;
-        }
-        Ok(Self {
-            pfield: Self::build_p_field(counter.0, fractions.map(|v| v.0)),
-            counter,
-            fractions,
-        })
-    }
-}
-
 impl TimeReader for TimeProviderCcsdsEpoch {
     fn from_bytes(buf: &[u8]) -> Result<Self, TimestampError>
     where
@@ -202,6 +398,22 @@ impl TimeReader for TimeProviderCcsdsEpoch {
                 }),
             ));
         }
+        match ccsds_time_code_from_p_field(buf[0]) {
+            Ok(code) => {
+                if code != CcsdsTimeCodes::CucCcsdsEpoch {
+                    return Err(TimestampError::InvalidTimeCode(
+                        CcsdsTimeCodes::CucCcsdsEpoch,
+                        code as u8,
+                    ));
+                }
+            }
+            Err(raw) => {
+                return Err(TimestampError::InvalidTimeCode(
+                    CcsdsTimeCodes::CucCcsdsEpoch,
+                    raw,
+                ))
+            }
+        }
         let (cntr_len, fractions_len, total_len) =
             Self::len_components_and_total_from_pfield(buf[0]);
         if buf.len() < total_len {
@@ -228,10 +440,15 @@ impl TimeReader for TimeProviderCcsdsEpoch {
         let mut fractions = None;
         if fractions_len > 0 {
             match fractions_len {
-                1 => fractions = Some(WidthCounterPair(fractions_len, buf[current_idx] as u32)),
+                1 => {
+                    fractions = Some(FractionalPart(
+                        fractions_len.try_into().unwrap(),
+                        buf[current_idx] as u32,
+                    ))
+                }
                 2 => {
-                    fractions = Some(WidthCounterPair(
-                        fractions_len,
+                    fractions = Some(FractionalPart(
+                        fractions_len.try_into().unwrap(),
                         u16::from_be_bytes(buf[current_idx..current_idx + 2].try_into().unwrap())
                             as u32,
                     ))
@@ -239,15 +456,15 @@ impl TimeReader for TimeProviderCcsdsEpoch {
                 3 => {
                     let mut tmp_buf: [u8; 4] = [0; 4];
                     tmp_buf[1..4].copy_from_slice(&buf[current_idx..current_idx + 3]);
-                    fractions = Some(WidthCounterPair(
-                        fractions_len,
+                    fractions = Some(FractionalPart(
+                        fractions_len.try_into().unwrap(),
                         u32::from_be_bytes(tmp_buf) as u32,
                     ))
                 }
                 _ => panic!("unreachable match arm"),
             }
         }
-        let provider = Self::new(WidthCounterPair(cntr_len, counter), fractions)?;
+        let provider = Self::new_generic(WidthCounterPair(cntr_len, counter), fractions)?;
         Ok(provider)
     }
 }
@@ -255,11 +472,11 @@ impl TimeReader for TimeProviderCcsdsEpoch {
 impl TimeWriter for TimeProviderCcsdsEpoch {
     fn write_to_bytes(&self, bytes: &mut [u8]) -> Result<usize, TimestampError> {
         // Cross check the sizes of the counters against byte widths in the ctor
-        if bytes.len() < self.len_packed() {
+        if bytes.len() < self.len_as_bytes() {
             return Err(TimestampError::ByteConversionError(
                 ByteConversionError::ToSliceTooSmall(SizeMissmatch {
                     found: bytes.len(),
-                    expected: self.len_packed(),
+                    expected: self.len_as_bytes(),
                 }),
             ));
         }
@@ -286,10 +503,10 @@ impl TimeWriter for TimeProviderCcsdsEpoch {
         current_idx += self.counter.0 as usize;
         if let Some(fractions) = self.fractions {
             match fractions.0 {
-                1 => bytes[current_idx] = fractions.1 as u8,
-                2 => bytes[current_idx..current_idx + 2]
+                FractionalResolution::FourMs => bytes[current_idx] = fractions.1 as u8,
+                FractionalResolution::FifteenUs => bytes[current_idx..current_idx + 2]
                     .copy_from_slice(&(fractions.1 as u16).to_be_bytes()),
-                3 => bytes[current_idx..current_idx + 3]
+                FractionalResolution::SixtyNs => bytes[current_idx..current_idx + 3]
                     .copy_from_slice(&fractions.1.to_be_bytes()[1..4]),
                 // Should also never happen
                 _ => panic!("invalid fractions value"),
@@ -299,3 +516,402 @@ impl TimeWriter for TimeProviderCcsdsEpoch {
         Ok(current_idx)
     }
 }
+
+impl CcsdsTimeProvider for TimeProviderCcsdsEpoch {
+    fn len_as_bytes(&self) -> usize {
+        Self::len_packed_from_pfield(self.pfield)
+    }
+
+    fn p_field(&self) -> (usize, [u8; 2]) {
+        (1, [self.pfield, 0])
+    }
+
+    fn ccdsd_time_code(&self) -> CcsdsTimeCodes {
+        CcsdsTimeCodes::CucCcsdsEpoch
+    }
+
+    /// Please note that this function only works as intended if the time counter resolution
+    /// is one second.
+    fn unix_seconds(&self) -> i64 {
+        ccsds_epoch_to_unix_epoch(self.counter.1 as u64) as i64
+    }
+
+    fn date_time(&self) -> Option<DateTime<Utc>> {
+        let unix_seconds = self.unix_seconds();
+        let ns = if let Some(fractional_part) = self.fractions {
+            convert_fractional_part_to_ns(fractional_part)
+        } else {
+            0
+        };
+        if let LocalResult::Single(res) = Utc.timestamp_opt(unix_seconds, ns as u32) {
+            return Some(res);
+        }
+        None
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use chrono::{Datelike, Timelike};
+    #[allow(unused_imports)]
+    use std::println;
+
+    #[test]
+    fn test_basic_zero_epoch() {
+        let zero_cuc = TimeProviderCcsdsEpoch::new(0);
+        assert_eq!(zero_cuc.len_as_bytes(), 5);
+        assert_eq!(zero_cuc.ccdsd_time_code(), CcsdsTimeCodes::CucCcsdsEpoch);
+        let counter = zero_cuc.width_counter_pair();
+        assert_eq!(counter.0, 4);
+        assert_eq!(counter.1, 0);
+        let fractions = zero_cuc.width_fractions_pair();
+        assert!(fractions.is_none());
+        let dt = zero_cuc.date_time();
+        assert!(dt.is_some());
+        let dt = dt.unwrap();
+        assert_eq!(dt.year(), 1958);
+        assert_eq!(dt.month(), 1);
+        assert_eq!(dt.day(), 1);
+        assert_eq!(dt.hour(), 0);
+        assert_eq!(dt.minute(), 0);
+        assert_eq!(dt.second(), 0);
+    }
+
+    #[test]
+    fn test_write_no_fractions() {
+        let mut buf: [u8; 16] = [0; 16];
+        let zero_cuc = TimeProviderCcsdsEpoch::new_generic(WidthCounterPair(4, 0x20102030), None);
+        assert!(zero_cuc.is_ok());
+        let zero_cuc = zero_cuc.unwrap();
+        let res = zero_cuc.write_to_bytes(&mut buf);
+        assert!(res.is_ok());
+        assert_eq!(zero_cuc.len_as_bytes(), 5);
+        assert_eq!(pfield_len(buf[0]), 1);
+        let written = res.unwrap();
+        assert_eq!(written, 5);
+        assert_eq!((buf[0] >> 7) & 0b1, 0);
+        let time_code = ccsds_time_code_from_p_field(buf[0]);
+        assert!(time_code.is_ok());
+        assert_eq!(time_code.unwrap(), CcsdsTimeCodes::CucCcsdsEpoch);
+        assert_eq!((buf[0] >> 2) & 0b11, 0b11);
+        assert_eq!(buf[0] & 0b11, 0);
+        let raw_counter = u32::from_be_bytes(buf[1..5].try_into().unwrap());
+        assert_eq!(raw_counter, 0x20102030);
+        assert_eq!(buf[5], 0);
+    }
+
+    #[test]
+    fn test_datetime_now() {
+        let now = Utc::now();
+        let cuc_now = TimeProviderCcsdsEpoch::from_now(FractionalResolution::SixtyNs);
+        assert!(cuc_now.is_ok());
+        let cuc_now = cuc_now.unwrap();
+        let dt_opt = cuc_now.date_time();
+        assert!(dt_opt.is_some());
+        let dt = dt_opt.unwrap();
+        let diff = dt - now;
+        assert!(diff.num_milliseconds() < 1000);
+        println!("datetime from cuc: {}", dt);
+        println!("datetime now: {}", now);
+    }
+
+    #[test]
+    fn test_read_no_fractions() {
+        let mut buf: [u8; 16] = [0; 16];
+        let zero_cuc =
+            TimeProviderCcsdsEpoch::new_generic(WidthCounterPair(4, 0x20102030), None).unwrap();
+        zero_cuc.write_to_bytes(&mut buf).unwrap();
+        let cuc_read_back =
+            TimeProviderCcsdsEpoch::from_bytes(&buf).expect("reading cuc timestamp failed");
+        assert_eq!(cuc_read_back, zero_cuc);
+        assert_eq!(cuc_read_back.width_counter_pair().1, 0x20102030);
+        assert_eq!(cuc_read_back.width_fractions_pair(), None);
+    }
+
+    #[test]
+    fn invalid_read_len() {
+        let mut buf: [u8; 16] = [0; 16];
+        for i in 0..2 {
+            let res = TimeProviderCcsdsEpoch::from_bytes(&buf[0..i]);
+            assert!(res.is_err());
+            let err = res.unwrap_err();
+            if let TimestampError::ByteConversionError(ByteConversionError::FromSliceTooSmall(e)) =
+                err
+            {
+                assert_eq!(e.found, i);
+                assert_eq!(e.expected, 2);
+            }
+        }
+        let large_stamp = TimeProviderCcsdsEpoch::new_with_fine_fractions(22, 300).unwrap();
+        large_stamp.write_to_bytes(&mut buf).unwrap();
+        for i in 2..large_stamp.len_as_bytes() - 1 {
+            let res = TimeProviderCcsdsEpoch::from_bytes(&buf[0..i]);
+            assert!(res.is_err());
+            let err = res.unwrap_err();
+            if let TimestampError::ByteConversionError(ByteConversionError::FromSliceTooSmall(e)) =
+                err
+            {
+                assert_eq!(e.found, i);
+                assert_eq!(e.expected, large_stamp.len_as_bytes());
+            }
+        }
+    }
+
+    #[test]
+    fn write_and_read_tiny_stamp() {
+        let mut buf = [0; 2];
+        let cuc = TimeProviderCcsdsEpoch::new_generic(WidthCounterPair(1, 200), None);
+        assert!(cuc.is_ok());
+        let cuc = cuc.unwrap();
+        assert_eq!(cuc.len_as_bytes(), 2);
+        let res = cuc.write_to_bytes(&mut buf);
+        assert!(res.is_ok());
+        let written = res.unwrap();
+        assert_eq!(written, 2);
+        assert_eq!(buf[1], 200);
+        let cuc_read_back = TimeProviderCcsdsEpoch::from_bytes(&buf);
+        assert!(cuc_read_back.is_ok());
+        let cuc_read_back = cuc_read_back.unwrap();
+        assert_eq!(cuc_read_back, cuc);
+    }
+
+    #[test]
+    fn write_slightly_larger_stamp() {
+        let mut buf = [0; 4];
+        let cuc = TimeProviderCcsdsEpoch::new_generic(WidthCounterPair(2, 40000), None);
+        assert!(cuc.is_ok());
+        let cuc = cuc.unwrap();
+        assert_eq!(cuc.len_as_bytes(), 3);
+        let res = cuc.write_to_bytes(&mut buf);
+        assert!(res.is_ok());
+        let written = res.unwrap();
+        assert_eq!(written, 3);
+        assert_eq!(u16::from_be_bytes(buf[1..3].try_into().unwrap()), 40000);
+        let cuc_read_back = TimeProviderCcsdsEpoch::from_bytes(&buf);
+        assert!(cuc_read_back.is_ok());
+        let cuc_read_back = cuc_read_back.unwrap();
+        assert_eq!(cuc_read_back, cuc);
+    }
+
+    #[test]
+    fn invalid_buf_len_for_read() {}
+    #[test]
+    fn write_read_three_byte_cntr_stamp() {
+        let mut buf = [0; 4];
+        let cuc = TimeProviderCcsdsEpoch::new_generic(WidthCounterPair(3, 2_u32.pow(24) - 2), None);
+        assert!(cuc.is_ok());
+        let cuc = cuc.unwrap();
+        assert_eq!(cuc.len_as_bytes(), 4);
+        let res = cuc.write_to_bytes(&mut buf);
+        assert!(res.is_ok());
+        let written = res.unwrap();
+        assert_eq!(written, 4);
+        let mut temp_buf = [0; 4];
+        temp_buf[1..4].copy_from_slice(&buf[1..4]);
+        assert_eq!(u32::from_be_bytes(temp_buf), 2_u32.pow(24) - 2);
+        let cuc_read_back = TimeProviderCcsdsEpoch::from_bytes(&buf);
+        assert!(cuc_read_back.is_ok());
+        let cuc_read_back = cuc_read_back.unwrap();
+        assert_eq!(cuc_read_back, cuc);
+    }
+
+    #[test]
+    fn test_write_invalid_buf() {
+        let mut buf: [u8; 16] = [0; 16];
+        let res = TimeProviderCcsdsEpoch::new_with_fine_fractions(0, 0);
+        let cuc = res.unwrap();
+        for i in 0..cuc.len_as_bytes() - 1 {
+            let err = cuc.write_to_bytes(&mut buf[0..i]);
+            assert!(err.is_err());
+            let err = err.unwrap_err();
+            if let TimestampError::ByteConversionError(ByteConversionError::ToSliceTooSmall(e)) =
+                err
+            {
+                assert_eq!(e.expected, cuc.len_as_bytes());
+                assert_eq!(e.found, i);
+            } else {
+                panic!("unexpected error: {}", err);
+            }
+        }
+    }
+    #[test]
+    fn invalid_ccsds_stamp_type() {
+        let mut buf: [u8; 16] = [0; 16];
+        buf[0] |= (CcsdsTimeCodes::CucAgencyEpoch as u8) << 4;
+        let res = TimeProviderCcsdsEpoch::from_bytes(&buf);
+        assert!(res.is_err());
+        let err = res.unwrap_err();
+        if let TimestampError::InvalidTimeCode(code, raw) = err {
+            assert_eq!(code, CcsdsTimeCodes::CucCcsdsEpoch);
+            assert_eq!(raw, CcsdsTimeCodes::CucAgencyEpoch as u8);
+        } else {
+            panic!("unexpected error: {}", err);
+        }
+    }
+
+    #[test]
+    fn test_write_with_coarse_fractions() {
+        let mut buf: [u8; 16] = [0; 16];
+        let cuc = TimeProviderCcsdsEpoch::new_with_coarse_fractions(0x30201060, 120);
+        assert!(cuc.fractions.is_some());
+        assert_eq!(cuc.fractions.unwrap().1, 120);
+        assert_eq!(cuc.fractions.unwrap().0, FractionalResolution::FourMs);
+        let res = cuc.write_to_bytes(&mut buf);
+        assert!(res.is_ok());
+        let written = res.unwrap();
+        assert_eq!(written, 6);
+        assert_eq!(buf[5], 120);
+        assert_eq!(buf[6], 0);
+        assert_eq!(
+            u32::from_be_bytes(buf[1..5].try_into().unwrap()),
+            0x30201060
+        );
+    }
+
+    #[test]
+    fn test_read_with_coarse_fractions() {
+        let mut buf: [u8; 16] = [0; 16];
+        let cuc = TimeProviderCcsdsEpoch::new_with_coarse_fractions(0x30201060, 120);
+        let res = cuc.write_to_bytes(&mut buf);
+        assert!(res.is_ok());
+        let res = TimeProviderCcsdsEpoch::from_bytes(&buf);
+        assert!(res.is_ok());
+        let read_back = res.unwrap();
+        assert_eq!(read_back, cuc);
+    }
+
+    #[test]
+    fn test_write_with_medium_fractions() {
+        let mut buf: [u8; 16] = [0; 16];
+        let cuc = TimeProviderCcsdsEpoch::new_with_medium_fractions(0x30303030, 30000);
+        let res = cuc.write_to_bytes(&mut buf);
+        assert!(res.is_ok());
+        let written = res.unwrap();
+        assert_eq!(written, 7);
+        assert_eq!(u16::from_be_bytes(buf[5..7].try_into().unwrap()), 30000);
+        assert_eq!(buf[7], 0);
+    }
+
+    #[test]
+    fn test_read_with_medium_fractions() {
+        let mut buf: [u8; 16] = [0; 16];
+        let cuc = TimeProviderCcsdsEpoch::new_with_medium_fractions(0x30303030, 30000);
+        let res = cuc.write_to_bytes(&mut buf);
+        assert!(res.is_ok());
+        let res = TimeProviderCcsdsEpoch::from_bytes(&buf);
+        assert!(res.is_ok());
+        let cuc_read_back = res.unwrap();
+        assert_eq!(cuc_read_back, cuc);
+    }
+
+    #[test]
+    fn test_write_with_fine_fractions() {
+        let mut buf: [u8; 16] = [0; 16];
+        let cuc =
+            TimeProviderCcsdsEpoch::new_with_fine_fractions(0x30303030, u16::MAX as u32 + 60000);
+        assert!(cuc.is_ok());
+        let cuc = cuc.unwrap();
+        let res = cuc.write_to_bytes(&mut buf);
+        let written = res.unwrap();
+        assert_eq!(written, 8);
+        let mut dummy_buf: [u8; 4] = [0; 4];
+        dummy_buf[1..4].copy_from_slice(&buf[5..8]);
+        assert_eq!(u32::from_be_bytes(dummy_buf), u16::MAX as u32 + 60000);
+        assert_eq!(buf[8], 0);
+    }
+
+    #[test]
+    fn test_read_with_fine_fractions() {
+        let mut buf: [u8; 16] = [0; 16];
+        let cuc =
+            TimeProviderCcsdsEpoch::new_with_fine_fractions(0x30303030, u16::MAX as u32 + 60000);
+        assert!(cuc.is_ok());
+        let cuc = cuc.unwrap();
+        let res = cuc.write_to_bytes(&mut buf);
+        assert!(res.is_ok());
+        let res = TimeProviderCcsdsEpoch::from_bytes(&buf);
+        assert!(res.is_ok());
+        let cuc_read_back = res.unwrap();
+        assert_eq!(cuc_read_back, cuc);
+    }
+
+    #[test]
+    fn test_fractional_converter() {
+        let ns = convert_fractional_part_to_ns(FractionalPart(FractionalResolution::FourMs, 2));
+        // The formula for this is 2/255 * 10e9 = 7.843.137.
+        assert_eq!(ns, 7843137);
+        // This is the largest value we should be able to pass without this function panicking.
+        let ns = convert_fractional_part_to_ns(FractionalPart(
+            FractionalResolution::SixtyNs,
+            2_u32.pow(24) - 2,
+        ));
+        assert_eq!(ns, 999999940);
+    }
+
+    #[test]
+    #[should_panic]
+    fn test_fractional_converter_invalid_input() {
+        convert_fractional_part_to_ns(FractionalPart(FractionalResolution::FourMs, 256));
+    }
+
+    #[test]
+    #[should_panic]
+    fn test_fractional_converter_invalid_input_2() {
+        convert_fractional_part_to_ns(FractionalPart(
+            FractionalResolution::SixtyNs,
+            2_u32.pow(32) - 1,
+        ));
+    }
+
+    #[test]
+    fn fractional_part_formula() {
+        let fractional_part =
+            7843137 / (10_u64.pow(9) / fractional_res_to_div(FractionalResolution::FourMs) as u64);
+        assert_eq!(fractional_part, 2);
+    }
+
+    #[test]
+    fn fractional_part_formula_2() {
+        let fractional_part =
+            12000000 / (10_u64.pow(9) / fractional_res_to_div(FractionalResolution::FourMs) as u64);
+        assert_eq!(fractional_part, 3);
+    }
+
+    #[test]
+    fn fractional_part_formula_3() {
+        let one_fraction_with_width_two_in_ns = 10_u64.pow(9) / (2_u32.pow(8 * 2) - 1) as u64;
+        assert_eq!(one_fraction_with_width_two_in_ns, 15259);
+        let hundred_fractions_and_some = 100 * one_fraction_with_width_two_in_ns + 7000;
+        let fractional_part = hundred_fractions_and_some
+            / (10_u64.pow(9) / fractional_res_to_div(FractionalResolution::FifteenUs) as u64);
+        assert_eq!(fractional_part, 100);
+        let hundred_and_one_fractions = 101 * one_fraction_with_width_two_in_ns;
+        let fractional_part = hundred_and_one_fractions
+            / (10_u64.pow(9) / fractional_res_to_div(FractionalResolution::FifteenUs) as u64);
+        assert_eq!(fractional_part, 101);
+    }
+
+    #[test]
+    fn update_fractions() {
+        let mut stamp = TimeProviderCcsdsEpoch::new(2000);
+        let res = stamp.set_fractions(FractionalPart(FractionalResolution::SixtyNs, 5000));
+        assert!(res.is_ok());
+        assert!(stamp.fractions.is_some());
+        let fractions = stamp.fractions.unwrap();
+        assert_eq!(fractions.0, FractionalResolution::SixtyNs);
+        assert_eq!(fractions.1, 5000);
+    }
+
+    #[test]
+    fn set_fract_resolution() {
+        let mut stamp = TimeProviderCcsdsEpoch::new(2000);
+        stamp.set_fractional_resolution(FractionalResolution::SixtyNs);
+        assert!(stamp.fractions.is_some());
+        let fractions = stamp.fractions.unwrap();
+        assert_eq!(fractions.0, FractionalResolution::SixtyNs);
+        assert_eq!(fractions.1, 0);
+        let res = stamp.update_from_now();
+        assert!(res.is_ok());
+    }
+}

src/time/mod.rs

@@ -7,7 +7,6 @@ use core::fmt::{Display, Formatter};
 #[cfg(not(feature = "std"))]
 use num_traits::float::FloatCore;
 
-use crate::time::cuc::CucError;
 #[cfg(feature = "serde")]
 use serde::{Deserialize, Serialize};
 #[cfg(feature = "std")]
@@ -73,7 +72,7 @@ impl From<cds::CdsError> for TimestampError {
 }
 
 impl From<cuc::CucError> for TimestampError {
-    fn from(e: CucError) -> Self {
+    fn from(e: cuc::CucError) -> Self {
         TimestampError::CucError(e)
     }
 }
@@ -163,6 +162,16 @@ pub const fn ccsds_to_unix_days(ccsds_days: i64) -> i64 {
     ccsds_days + DAYS_CCSDS_TO_UNIX as i64
 }
 
+/// Similar to [unix_to_ccsds_days] but converts the epoch instead, which is the number of elpased
+/// seconds since the CCSDS and UNIX epoch times.
+pub const fn unix_epoch_to_ccsds_epoch(unix_epoch: u64) -> u64 {
+    (unix_epoch as i64 - (DAYS_CCSDS_TO_UNIX as i64 * SECONDS_PER_DAY as i64)) as u64
+}
+
+pub const fn ccsds_epoch_to_unix_epoch(ccsds_epoch: u64) -> u64 {
+    (ccsds_epoch as i64 + (DAYS_CCSDS_TO_UNIX as i64 * SECONDS_PER_DAY as i64)) as u64
+}
+
 #[cfg(feature = "std")]
 #[cfg_attr(doc_cfg, doc(cfg(feature = "std")))]
 pub fn ms_of_day_using_sysclock() -> u32 {
@@ -218,4 +227,17 @@ mod tests {
         let sec_floats = seconds_since_epoch();
         assert!(sec_floats > 0.0);
     }
+
+    #[test]
+    fn test_ccsds_epoch() {
+        let now = SystemTime::now()
+            .duration_since(SystemTime::UNIX_EPOCH)
+            .unwrap();
+        let unix_epoch = now.as_secs();
+        let ccsds_epoch = unix_epoch_to_ccsds_epoch(now.as_secs());
+        assert!(ccsds_epoch > unix_epoch);
+        assert_eq!((ccsds_epoch - unix_epoch) % SECONDS_PER_DAY as u64, 0);
+        let days_diff = (ccsds_epoch - unix_epoch) / SECONDS_PER_DAY as u64;
+        assert_eq!(days_diff, -DAYS_CCSDS_TO_UNIX as u64);
+    }
 }