Introduces CRCs (#98)

* Introduces CRCs

A `crc` feature has been added that brings in the `crc` crate and a new `CrcModifier` flavour. This modifier flavour is similar to the COBS implementation in how it wraps another flavour.

* Convenience functions

Convenience functions for serde usage

* Optionally include paste

The paste crate is included now only if `use-crc` feature is enabled. This may help improve build performance where CRCs are not required.

* Fix use of paste macro

This uses the correct digest sizes, and adds a test for
a random CRC8 variant.

---------

Co-authored-by: James Munns <[email protected]>

.gitignore

@@ -1,3 +1,4 @@
 /target
 **/*.rs.bk
-Cargo.lock
+Cargo.lock
+.vscode

Cargo.toml

@@ -52,13 +52,22 @@ path = "./postcard-derive"
 version = "0.1.1"
 optional = true
 
+[dependencies.crc]
+version = "3.0.1"
+optional = true
+
+[dependencies.paste]
+version = "1.0.12"
+optional = true
+
 [features]
 default = ["heapless-cas"]
 
 use-std = ["serde/std", "alloc"]
 heapless-cas = ["heapless", "heapless/cas"]
 alloc = ["serde/alloc"]
 use-defmt = ["defmt"]
+use-crc = ["crc", "paste"]
 
 # Experimental features!
 #

src/de/flavors.rs

@@ -163,120 +163,139 @@ impl<'de> Flavor<'de> for Slice<'de> {
     }
 }
 
-// This is a terrible checksum implementation to make sure that we can effectively
-// use the deserialization flavor. This is kept as a test (and not published)
-// because an 8-bit checksum is not ACTUALLY useful for almost anything.
-//
-// You could certainly do something similar with a CRC32, cryptographic sig,
-// or something else
-#[cfg(test)]
-mod test {
-    use super::*;
-    use serde::{Deserialize, Serialize};
+////////////////////////////////////////
+// CRC
+////////////////////////////////////////
 
-    struct Checksum<'de, F>
-    where
-        F: Flavor<'de> + 'de,
-    {
-        flav: F,
-        checksum: u8,
-        _plt: PhantomData<&'de ()>,
-    }
+/// This Cyclic Redundancy Check flavor applies [the CRC crate's `Algorithm`](https://docs.rs/crc/latest/crc/struct.Algorithm.html) struct on
+/// the serialized data. The flavor will check the CRC assuming that it has been appended to the bytes.
+///
+/// CRCs are used for error detection when reading data back.
+///
+/// The `crc` feature requires enabling to use this module.
+///
+/// More on CRCs: <https://en.wikipedia.org/wiki/Cyclic_redundancy_check>.
+#[cfg(feature = "use-crc")]
+pub mod crc {
+    use core::convert::TryInto;
+
+    use crc::Digest;
+    use crc::Width;
+    use serde::Deserialize;
 
-    impl<'de, F> Checksum<'de, F>
+    use super::Flavor;
+    use super::Slice;
+
+    use crate::Deserializer;
+    use crate::Error;
+    use crate::Result;
+    use paste::paste;
+
+    /// Manages CRC modifications as a flavor.
+    pub struct CrcModifier<'de, B, W>
     where
-        F: Flavor<'de> + 'de,
+        B: Flavor<'de>,
+        W: Width,
     {
-        pub fn from_flav(flav: F) -> Self {
-            Self {
-                flav,
-                checksum: 0,
-                _plt: PhantomData,
-            }
-        }
+        flav: B,
+        digest: Digest<'de, W>,
     }
 
-    impl<'de, F> Flavor<'de> for Checksum<'de, F>
+    impl<'de, B, W> CrcModifier<'de, B, W>
     where
-        F: Flavor<'de> + 'de,
+        B: Flavor<'de>,
+        W: Width,
     {
-        type Remainder = (<F as Flavor<'de>>::Remainder, u8);
-        type Source = F;
-
-        fn pop(&mut self) -> Result<u8> {
-            match self.flav.pop() {
-                Ok(u) => {
-                    self.checksum = self.checksum.wrapping_add(u);
-                    Ok(u)
-                }
-                Err(e) => Err(e),
-            }
-        }
-        fn try_take_n(&mut self, ct: usize) -> Result<&'de [u8]> {
-            match self.flav.try_take_n(ct) {
-                Ok(u) => {
-                    u.iter().for_each(|u| {
-                        self.checksum = self.checksum.wrapping_add(*u);
-                    });
-                    Ok(u)
-                }
-                Err(e) => Err(e),
-            }
-        }
-        fn finalize(self) -> Result<Self::Remainder> {
-            Ok((self.flav.finalize()?, self.checksum))
+        /// Create a new Crc modifier Flavor.
+        pub fn new(bee: B, digest: Digest<'de, W>) -> Self {
+            Self { flav: bee, digest }
         }
     }
 
-    #[derive(Serialize, Deserialize, PartialEq, Eq, Debug)]
-    pub struct SomeData<'a> {
-        #[serde(borrow)]
-        sli: &'a [u8],
-        sts: &'a str,
-        foo: u64,
-        bar: u128,
-    }
+    macro_rules! impl_flavor {
+        ($( $int:ty ),*) => {
+            $(
+                paste! {
+                    impl<'de, B> Flavor<'de> for CrcModifier<'de, B, $int>
+                    where
+                        B: Flavor<'de>,
+                    {
+                        type Remainder = B::Remainder;
 
-    #[test]
-    fn smoke() {
-        const EXPECTED: &[u8] = &[
-            4, 255, 1, 34, 51, 19, 116, 104, 105, 115, 32, 105, 115, 32, 97, 32, 103, 111, 111,
-            100, 32, 116, 101, 115, 116, 170, 213, 170, 213, 170, 213, 170, 213, 170, 1, 255, 255,
-            255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 127,
-        ];
+                        type Source = B::Source;
 
-        // Calculate simple 8-bit checksum
-        let mut check: u8 = 0;
-        EXPECTED.iter().for_each(|u| check = check.wrapping_add(*u));
+                        #[inline]
+                        fn pop(&mut self) -> Result<u8> {
+                            match self.flav.pop() {
+                                Ok(byte) => {
+                                    self.digest.update(&[byte]);
+                                    Ok(byte)
+                                }
+                                e @ Err(_) => e,
+                            }
+                        }
 
-        let mut buf = [0u8; 256];
-        let data = SomeData {
-            sli: &[0xFF, 0x01, 0x22, 0x33],
-            sts: "this is a good test",
-            foo: (u64::MAX / 3) * 2,
-            bar: u128::MAX / 4,
-        };
-        let used = crate::to_slice(&data, &mut buf).unwrap();
-        assert_eq!(used, EXPECTED);
-        let used = used.len();
+                        #[inline]
+                        fn try_take_n(&mut self, ct: usize) -> Result<&'de [u8]> {
+                            match self.flav.try_take_n(ct) {
+                                Ok(bytes) => {
+                                    self.digest.update(bytes);
+                                    Ok(bytes)
+                                }
+                                e @ Err(_) => e,
+                            }
+                        }
 
-        // Put the checksum at the end
-        buf[used] = check;
+                        fn finalize(mut self) -> Result<Self::Remainder> {
+                            match self.flav.try_take_n(core::mem::size_of::<$int>()) {
+                                Ok(prev_crc_bytes) => match self.flav.finalize() {
+                                    Ok(remainder) => {
+                                        let crc = self.digest.finalize();
+                                        let le_bytes = prev_crc_bytes
+                                            .try_into()
+                                            .map_err(|_| Error::DeserializeBadEncoding)?;
+                                        let prev_crc = <$int>::from_le_bytes(le_bytes);
+                                        if crc == prev_crc {
+                                            Ok(remainder)
+                                        } else {
+                                            Err(Error::DeserializeBadEncoding)
+                                        }
+                                    }
+                                    e @ Err(_) => e,
+                                },
+                                Err(e) => Err(e),
+                            }
+                        }
+                    }
 
-        let mut deser = crate::de::Deserializer::from_flavor(Checksum::from_flav(Slice::new(&buf)));
+                    /// Deserialize a message of type `T` from a byte slice with a Crc. The unused portion (if any)
+                    /// of the byte slice is not returned.
+                    pub fn [<from_bytes_ $int>]<'a, T>(s: &'a [u8], digest: Digest<'a, $int>) -> Result<T>
+                    where
+                        T: Deserialize<'a>,
+                    {
+                        let flav = CrcModifier::new(Slice::new(s), digest);
+                        let mut deserializer = Deserializer::from_flavor(flav);
+                        let r = T::deserialize(&mut deserializer)?;
+                        let _ = deserializer.finalize()?;
+                        Ok(r)
+                    }
 
-        let t = SomeData::<'_>::deserialize(&mut deser).unwrap();
-        assert_eq!(t, data);
-
-        // Normally, you'd probably expect the check
-        let (rem, cksm) = deser.finalize().unwrap();
-
-        // The pre-calculated checksum we stuffed at the end is the
-        // first "unused" byte
-        assert_eq!(rem[0], check);
-
-        // the one we calculated during serialization matches the
-        // pre-calculated one
-        assert_eq!(cksm, check);
+                    /// Deserialize a message of type `T` from a byte slice with a Crc. The unused portion (if any)
+                    /// of the byte slice is returned for further usage
+                    pub fn [<take_from_bytes_ $int>]<'a, T>(s: &'a [u8], digest: Digest<'a, $int>) -> Result<(T, &'a [u8])>
+                    where
+                        T: Deserialize<'a>,
+                    {
+                        let flav = CrcModifier::new(Slice::new(s), digest);
+                        let mut deserializer = Deserializer::from_flavor(flav);
+                        let t = T::deserialize(&mut deserializer)?;
+                        Ok((t, deserializer.finalize()?))
+                    }
+                }
+            )*
+        };
     }
+
+    impl_flavor![u8, u16, u32, u64, u128];
 }

src/de/mod.rs

@@ -20,7 +20,7 @@ where
 
 /// Deserialize a message of type `T` from a cobs-encoded byte slice. The
 /// unused portion (if any) of the byte slice is not returned.
-/// The used portion of the input slice is modified during deserialization (even if an error is returned). 
+/// The used portion of the input slice is modified during deserialization (even if an error is returned).
 /// Therefore, if this is not desired, pass a clone of the original slice.
 pub fn from_bytes_cobs<'a, T>(s: &'a mut [u8]) -> Result<T>
 where
@@ -32,7 +32,7 @@ where
 
 /// Deserialize a message of type `T` from a cobs-encoded byte slice. The
 /// unused portion (if any) of the byte slice is returned for further usage.
-/// The used portion of the input slice is modified during deserialization (even if an error is returned). 
+/// The used portion of the input slice is modified during deserialization (even if an error is returned).
 /// Therefore, if this is not desired, pass a clone of the original slice.
 pub fn take_from_bytes_cobs<'a, T>(s: &'a mut [u8]) -> Result<(T, &'a mut [u8])>
 where
@@ -67,6 +67,22 @@ where
     Ok((t, deserializer.finalize()?))
 }
 
+/// Conveniently deserialize a message of type `T` from a byte slice with a Crc. The unused portion (if any)
+/// of the byte slice is not returned.
+///
+/// See the `de_flavors::crc` module for the complete set of functions.
+/// 
+#[cfg(feature = "use-crc")]
+pub use flavors::crc::from_bytes_u32 as from_bytes_crc32;
+
+/// Conveniently deserialize a message of type `T` from a byte slice with a Crc. The unused portion (if any)
+/// of the byte slice is returned for further usage
+///
+/// See the `de_flavors::crc` module for the complete set of functions.
+/// 
+#[cfg(feature = "use-crc")]
+pub use flavors::crc::take_from_bytes_u32 as take_from_bytes_crc32;
+
 ////////////////////////////////////////////////////////////////////////////////
 
 #[cfg(feature = "heapless")]

src/lib.rs

@@ -91,6 +91,21 @@ pub use ser::{to_stdvec, to_stdvec_cobs};
 #[cfg(feature = "alloc")]
 pub use ser::{to_allocvec, to_allocvec_cobs};
 
+#[cfg(feature = "use-crc")]
+pub use {
+    de::{from_bytes_crc32, take_from_bytes_crc32},
+    ser::to_slice_crc32,
+};
+
+#[cfg(all(feature = "use-crc", feature = "heapless"))]
+pub use ser::to_vec_crc32;
+
+#[cfg(all(feature = "use-crc", feature = "use-std"))]
+pub use ser::to_stdvec_crc32;
+
+#[cfg(all(feature = "use-crc", feature = "alloc"))]
+pub use ser::to_allocvec_crc32;
+
 #[cfg(test)]
 mod test {
     #[test]