codec/
codec.rs

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#[derive(Debug)]
pub enum DecodeError {
    Invalid,
    Incomplete
}

impl From<()> for DecodeError {
    fn from(_: ()) -> Self {
        DecodeError::Invalid
    }
}

#[derive(Debug)]
pub enum EncodeError {
    Overrun,
    Encoding
}

pub trait EncodeBuffer {
    fn push(&mut self, byte: u8) -> Result<(), EncodeError>;
    fn push_slice(&mut self, slice: &[u8]) -> Result<(), EncodeError>;
    fn get_encoded_len(&self) -> usize;
    fn get_raw<'a>(&'a self) -> &'a [u8];
}

pub trait DecodeBuffer {
    fn read(&mut self) -> Result<u8, DecodeError>;
    fn read_slice<'a>(&'a mut self, size: usize) -> Result<&'a [u8], DecodeError>;
    fn get_read_len(&self) -> usize;
    fn get_raw<'a>(&'a self) -> &'a [u8];
}

pub struct StaticBuffer<'a> {
    ptr: usize,
    buf: &'a mut [u8]
}

impl<'a> StaticBuffer<'a> {
    pub fn new(buf: &'a mut [u8]) -> Self {
        Self { ptr: 0, buf }
    }
    pub fn reset(&mut self) {
        self.ptr = 0;
    }
}

impl<'b> EncodeBuffer for StaticBuffer<'b> {
    fn get_encoded_len(&self) -> usize {self.ptr}
    fn get_raw<'a>(&'a self) -> &'a [u8] {&self.buf[0..self.ptr]}
    fn push(&mut self, byte: u8) -> Result<(), EncodeError> {
        if self.ptr >= self.buf.len() {return Err(EncodeError::Overrun);}

        self.buf[self.ptr] = byte;
        self.ptr+=1;

        Ok(())
    }
    fn push_slice(&mut self, slice: &[u8]) -> Result<(), EncodeError> {
        if self.ptr + slice.len() > self.buf.len() {return Err(EncodeError::Overrun)}

        self.buf[self.ptr..self.ptr+slice.len()].copy_from_slice(slice);
        self.ptr += slice.len();
        Ok(())
    }
}

impl<'b> DecodeBuffer for StaticBuffer<'b> {
    fn get_read_len(&self) -> usize {self.ptr}
    fn get_raw<'a>(&'a self) -> &'a [u8] {&self.buf[0..self.ptr]}
    fn read(&mut self) -> Result<u8, DecodeError> {
        if self.buf.len() <= self.ptr {return Err(DecodeError::Incomplete);}

        self.ptr+=1;
        Ok(self.buf[self.ptr-1])
    }
    fn read_slice<'a>(&'a mut self, size: usize) -> Result<&'a [u8], DecodeError> {
        if self.buf.len() < self.ptr+size {return Err(DecodeError::Incomplete);}

        self.ptr+=size;
        Ok(&self.buf[self.ptr-size..self.ptr])
    }
}

pub trait Codec<Context>
where Self: Sized {
    fn encode(self, buf: &mut impl EncodeBuffer, ctx: Context) -> Result<(), EncodeError>;
    fn decode(buf: &mut impl DecodeBuffer, ctx: Context) -> Result<Self, DecodeError>;
}

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

    #[test]
    fn static_buffer_read_singles() {
        let mut slice: [u8;10] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
        let mut buf = StaticBuffer::new(&mut slice);
        assert_eq!(buf.get_read_len(), 0);
        for i in 0..10 {
            assert_eq!(buf.read().unwrap(), i);
        }
        buf.read().expect_err("Buf should be empty");
        assert_eq!(buf.get_read_len(), 10);
    }

    #[test]
    fn static_buffer_read_batches() {
        let mut slice: [u8;10] = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
        let mut buf = StaticBuffer::new(&mut slice);
        assert_eq!(buf.get_read_len(), 0);
        for i in 0..5 {
            assert_eq!(buf.read_slice(2).unwrap(), [i*2, i*2 + 1])
        }
        buf.read().expect_err("Buf should be empty");
        assert_eq!(buf.get_read_len(), 10);
    }

    #[test]
    fn static_buffer_write_singles() {
        let mut slice: [u8;10] = [0; 10];
        {
            let mut buf = StaticBuffer::new(&mut slice);
            assert_eq!(buf.get_encoded_len(), 0);
            for i in 0..10 {
                buf.push(i).unwrap();
            }
            buf.push(69).expect_err("Buf should be full");
            assert_eq!(buf.get_encoded_len(), 10);
        }
        assert_eq!(slice, [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
    }

    #[test]
    fn static_buffer_write_batches() {
        let mut slice: [u8;10] = [0; 10];
        {
            let mut buf = StaticBuffer::new(&mut slice);
            assert_eq!(buf.get_encoded_len(), 0);
            for i in 0..5 {
                buf.push_slice(&[i*2, i*2 + 1]).unwrap();
            }
            buf.push(69).expect_err("Buf should be full");
            assert_eq!(buf.get_encoded_len(), 10);
        }
        assert_eq!(slice, [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
    }

    #[test]
    fn static_buffer_reset() {
        let mut slice: [u8;10] = [0; 10];
        {
            let mut buf = StaticBuffer::new(&mut slice);
            assert_eq!(buf.get_encoded_len(), 0);
            for i in 0..10 {
                buf.push(i).unwrap();
            }
            assert_eq!(buf.get_encoded_len(), 10);
            buf.reset();
            assert_eq!(buf.get_encoded_len(), 0);
            for i in 10..20 {
                buf.push(i).unwrap();
            }
            assert_eq!(buf.get_encoded_len(), 10);
        }
        assert_eq!(slice, [10, 11, 12, 13, 14, 15, 16, 17, 18, 19]);
    }
}