//! Tools for implementing writer style serializers dedicated for `no_std` targets.
//!
//! Features a [trait][SerWrite] for objects which are byte-oriented sinks, akin to `std::io::Write`.
//!
//! Serializers can be implemented using this trait as a writer.
//!
//! Embedded or otherwise `no_std` projects can implement [`SerWrite`] for custom sinks.
//!
//! Some [implemenentations] for foreign types are provided depending on the enabled features.
//!
//! [implemenentations]: SerWrite#foreign-impls
#![no_std]
#![cfg_attr(docsrs, feature(doc_cfg))]

#[cfg(feature = "std")]
extern crate std;

#[cfg(all(feature = "alloc",not(feature = "std")))]
extern crate alloc;

use core::fmt;

mod foreign;

pub type SerResult<T> = Result<T, SerError>;

/// A simple error type that can be used for [`SerWrite::Error`] implementations.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
#[non_exhaustive]
pub enum SerError {
    /// Buffer is full
    BufferFull,
}

impl fmt::Display for SerError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            SerError::BufferFull => f.write_str("buffer is full"),
        }
    }
}

#[cfg(feature = "std")]
#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
impl std::error::Error for SerError {}

/// Serializers should write data to the implementations of this trait.
pub trait SerWrite {
    /// An error type returned from the trait methods.
    type Error;
    /// Write **all** bytes from `buf` to the internal buffer.
    ///
    /// Otherwise return an error.
    fn write(&mut self, buf: &[u8]) -> Result<(), Self::Error>;
    /// Write a single `byte` to the internal buffer.
    ///
    /// Return an error if the operation could not succeed.
    #[inline]
    fn write_byte(&mut self, byte: u8) -> Result<(), Self::Error> {
        self.write(core::slice::from_ref(&byte))
    }
    /// Write a **whole** string to the internal buffer.
    ///
    /// Otherwise return an error.
    #[inline]
    fn write_str(&mut self, s: &str) -> Result<(), Self::Error> {
        self.write(s.as_bytes())
    }
}

impl<T: SerWrite> SerWrite for &'_ mut T {
    type Error = T::Error;

    #[inline(always)]
    fn write(&mut self, buf: &[u8]) -> Result<(), Self::Error> {
        (*self).write(buf)
    }

    #[inline(always)]
    fn write_byte(&mut self, byte: u8) -> Result<(), Self::Error> {
        (*self).write_byte(byte)
    }

    #[inline(always)]
    fn write_str(&mut self, s: &str) -> Result<(), Self::Error> {
        (*self).write_str(s)
    }
}

/// A simple slice writer (example implementation)
#[derive(Debug, PartialEq)]
pub struct SliceWriter<'a> {
    pub buf: &'a mut [u8],
    pub len: usize
}

impl<'a> AsRef<[u8]> for SliceWriter<'a> {
    /// Returns a populated portion of the slice
    fn as_ref(&self) -> &[u8] {
        &self.buf[..self.len]
    }
}

impl<'a> AsMut<[u8]> for SliceWriter<'a> {
    /// Returns a populated portion of the slice
    fn as_mut(&mut self) -> &mut [u8] {
        &mut self.buf[..self.len]
    }
}

impl<'a> SliceWriter<'a> {
    /// Create a new instance
    pub fn new(buf: &'a mut [u8]) -> Self {
        SliceWriter { buf, len: 0 }
    }
    /// Return populated length
    pub fn len(&self) -> usize {
        self.len
    }
    /// Return whether the output is not populated.
    pub fn is_empty(&self) -> bool {
        self.len == 0
    }
    /// Return total capacity of the container
    pub fn capacity(&self) -> usize {
        self.buf.len()
    }
    /// Return remaining free capacity
    pub fn rem_capacity(&self) -> usize {
        self.buf.len() - self.len
    }
    /// Reset cursor to the beginning of a container slice
    pub fn clear(&mut self) {
        self.len = 0;
    }
    /// Split the underlying buffer and return the portion of the populated buffer
    /// with an underlying buffer's borrowed lifetime.
    ///
    /// Once a [`SliceWriter`] is dropped the slice stays borrowed as long as the
    /// original container lives.
    pub fn split(self) -> (&'a mut[u8], Self) {
        let (res, buf) = self.buf.split_at_mut(self.len);
        (res, Self { buf, len: 0 })
    }
}

impl SerWrite for SliceWriter<'_> {
    type Error = SerError;

    fn write(&mut self, buf: &[u8]) -> SerResult<()> {
        let end = self.len + buf.len();
        match self.buf.get_mut(self.len..end) {
            Some(chunk) => {
                chunk.copy_from_slice(buf);
                self.len = end;
                Ok(())
            }
            None => Err(SerError::BufferFull)
        }
    }
}

impl<'a> fmt::Write for SliceWriter<'a> {
    fn write_str(&mut self, s: &str) -> fmt::Result {
        SerWrite::write_str(self, s).map_err(|_| fmt::Error)
    }
}

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

    #[test]
    fn test_slice_writer() {
        let mut buf = [0u8;22];
        let mut writer = SliceWriter::new(&mut buf);
        writer.write(b"Hello World!").unwrap();
        writer.write_byte(b' ').unwrap();
        writer.write_str("Good Bye!").unwrap();
        let expected = b"Hello World! Good Bye!";
        assert_eq!(writer.as_ref(), expected);
        let (head, mut writer) = writer.split();
        assert_eq!(head, expected);
        assert_eq!(writer.write_byte(b' ').unwrap_err(), SerError::BufferFull);
    }
}