added documentation to `PeekReader

Author: Tiago Marques

mavlink-core/src/peek_reader.rs

@@ -1,7 +1,29 @@
+//! This module implements a buffered/peekable reader.
+//!
+//! The purpose of the buffered/peekable reader is to allow for backtracking parsers.
+//!
+//! A reader implementing the standard librairy's [`std::io::BufRead`] trait seems like a good fit, but
+//! it does not allow for peeking a specific number of bytes, so it provides no way to request
+//! more data from the underlying reader without consuming the existing data.
+//!
+//! This API still tries to adhere to the [`std::io::BufRead`]'s trait philosophy.
+//!
+//! The main type `PeekReader`does not implement [`std::io::Read`] itself, as there is no added benefit
+//! in doing so.
+//!
+
 use std::io::{self, ErrorKind, Read};
 
+// The default chunk size to read from the underlying reader
 const DEFAULT_CHUNK_SIZE: usize = 1024;
 
+/// A buffered/peekable reader
+///
+/// This reader wraps a type implementing [`std::io::Read`] and adds buffering via an internal buffer.
+///
+/// It allows the user to `peek` a specified number of bytes (without consuming them),
+/// to `read` bytes (consuming them), or to `consume` them after `peek`ing.
+///
 pub struct PeekReader<R> {
     // Internal buffer
     buffer: Vec<u8>,
@@ -13,15 +35,17 @@ pub struct PeekReader<R> {
     reader: R,
     // Stashed error, if any.
     error: Option<io::Error>,
-    /// Whether we hit EOF on the underlying reader.
+    // Whether we hit EOF on the underlying reader.
     eof: bool,
 }
 
 impl<R: Read> PeekReader<R> {
+    /// Instanciates a new [`PeekReader`], wrapping the provided [`std::io::Read`]er and using the default chunk size
     pub fn new(reader: R) -> Self {
         Self::with_chunk_size(reader, DEFAULT_CHUNK_SIZE)
     }
 
+    /// Instanciates a new [`PeekReader`], wrapping the provided [`std::io::Read`]er and using the supplied chunk size
     pub fn with_chunk_size(reader: R, preferred_chunk_size: usize) -> Self {
         Self {
             buffer: Vec::with_capacity(preferred_chunk_size),
@@ -33,41 +57,95 @@ impl<R: Read> PeekReader<R> {
         }
     }
 
+    /// Peeks a specified amount of bytes from the internal buffer
+    ///
+    /// If the internal buffer does not contain enough data, this function will read
+    /// from the underlying [`std::io::Read`]er until it does, an error occurs or no more data can be read (EOF).
+    ///
+    /// This function does not consume  data from the buffer, so subsequent calls to `peek` or `read` functions
+    /// will still return the peeked data.
+    ///
     pub fn peek(&mut self, amount: usize) -> io::Result<&[u8]> {
         self.fetch(amount, false, false)
     }
 
+    /// Peeks an exact amount of bytes from the internal buffer
+    ///
+    /// If the internal buffer does not contain enough data, this function will read
+    /// from the underlying [`std::io::Read`]er until it does, an error occurs or no more data can be read (EOF).
+    ///
+    /// If an EOF occurs and the specified amount could not be read, this function will return an [`ErrorKind::UnexpectedEof`].
+    ///
+    /// This function does not consume data from the buffer, so subsequent calls to `peek` or `read` functions
+    /// will still return the peeked data.
+    ///
     pub fn peek_exact(&mut self, amount: usize) -> io::Result<&[u8]> {
         self.fetch(amount, true, false)
     }
 
+    /// Consumes a specified amount of bytes from the buffer
+    ///
+    /// If the internal buffer does not contain enough data, this function will consume as much data as is buffered.
+    ///
     pub fn consume(&mut self, amount: usize) -> usize {
         let amount = amount.min(self.buffer.len() - self.cursor);
         self.cursor += amount;
         amount
     }
 
+    /// Reads a specified amount of bytes from the internal buffer
+    ///
+    /// If the internal buffer does not contain enough data, this function will read
+    /// from the underlying [`std::io::Read`]er until it does, an error occurs or no more data can be read (EOF).
+    ///
+    /// This function consumes the data from the buffer, unless an error occurs, in which case no data is consumed.
+    ///
     pub fn read(&mut self, amount: usize) -> io::Result<&[u8]> {
         self.fetch(amount, false, true)
     }
 
+    /// Reads a specified amount of bytes from the internal buffer
+    ///
+    /// If the internal buffer does not contain enough data, this function will read
+    /// from the underlying [`std::io::Read`]er until it does, an error occurs or no more data can be read (EOF).
+    ///
+    /// If an EOF occurs and the specified amount could not be read, this function will return an [`ErrorKind::UnexpectedEof`].
+    ///
+    /// This function consumes the data from the buffer, unless an error occurs, in which case no data is consumed.
+    ///
     pub fn read_exact(&mut self, amount: usize) -> io::Result<&[u8]> {
         self.fetch(amount, true, true)
     }
 
+    /// Reads a byte from the internal buffer
+    ///
+    /// If the internal buffer does not contain enough data, this function will read
+    /// from the underlying [`std::io::Read`]er until it does, an error occurs or no more data can be read (EOF).
+    ///
+    /// If an EOF occurs and the specified amount could not be read, this function will return an [`ErrorKind::UnexpectedEof`].
+    ///
+    /// This function consumes the data from the buffer, unless an error occurs, in which case no data is consumed.
+    ///
     pub fn read_u8(&mut self) -> io::Result<u8> {
         let buf = self.read_exact(1)?;
         Ok(buf[0])
     }
 
+    /// Returns an immutable reference to the underlying [`std::io::Read`]er
+    ///
+    /// Reading directly from the underlying stream will cause data loss
     pub fn reader_ref(&mut self) -> &R {
         &self.reader
     }
 
+    /// Returns a mutable reference to the underlying [`std::io::Read`]er
+    ///
+    /// Reading directly from the underlying stream will cause data loss
     pub fn reader_mut(&mut self) -> &mut R {
         &mut self.reader
     }
 
+    /// Internal function to fetch data from the internal buffer and/or reader
     fn fetch(&mut self, amount: usize, exact: bool, consume: bool) -> io::Result<&[u8]> {
         let previous_len = self.buffer.len();
         let mut buffered = previous_len - self.cursor;
@@ -109,6 +187,7 @@ impl<R: Read> PeekReader<R> {
                     }
                 }
             }
+            // if some bytes were read, add them to the buffer
             if read > 0 {
                 if self.buffer.capacity() - previous_len < read {
                     // reallocate