add MultiReader and TeeReader

chapter3/Cargo.toml

@@ -86,3 +86,9 @@ path = "src/3_6_3/main.rs"
 [[bin]]
 name = "3_6_2"
 path = "src/3_6_2/main.rs"
+[[bin]]
+name = "3_7_multi"
+path = "src/3_7/multi_reader.rs"
+[[bin]]
+name = "3_7_tee"
+path = "src/3_7/tee_reader.rs"

chapter3/src/3_7/multi_reader.rs

@@ -0,0 +1,14 @@
+use std::io::{copy, stdout};
+
+use lib::io::MultiReader;
+
+fn main() -> std::io::Result<()> {
+    let header = "---- HEADER ----\n".as_bytes();
+    let content = "Example of MultiReader\n".as_bytes();
+    let footer = "---- FOOTER ----\n".as_bytes();
+    let mut multi_reader =
+        MultiReader::new(vec![Box::new(header), Box::new(content), Box::new(footer)]);
+    copy(&mut multi_reader, &mut stdout())?;
+
+    Ok(())
+}

chapter3/src/3_7/tee_reader.rs

@@ -0,0 +1,20 @@
+use lib::io::TeeReader;
+use std::io::Read;
+
+fn main() -> std::io::Result<()> {
+    // TeeReader を使用した場合の例。
+    let mut buf = Vec::new();
+    let reader = "Example of TeeReader".as_bytes();
+    let mut tee_reader = TeeReader::new(reader, &mut buf);
+    // データを読み捨てる。
+    let _ = tee_reader.read_to_end(&mut Vec::new())?;
+
+    println!(
+        "{}",
+        // けどバッファには残っている。
+        String::from_utf8(buf)
+            .expect("UTF-8 形式でない文字列の可能性があります。UTF-8 にしてください。")
+    );
+
+    Ok(())
+}

lib/src/io.rs

@@ -1,7 +1,11 @@
 mod limited_reader;
+mod multi_reader;
 mod multi_writer;
 mod section_reader;
+mod tee_reader;
 
 pub use limited_reader::LimitedReader;
+pub use multi_reader::MultiReader;
 pub use multi_writer::MultiWriter;
 pub use section_reader::SectionReader;
+pub use tee_reader::TeeReader;

lib/src/io/multi_reader.rs

@@ -0,0 +1,58 @@
+use std::{collections::VecDeque, io::Read};
+
+/// Takes multiple `std::io::Read` at once.
+/// This is inspired by `io.MultiReader` in Go.
+///
+/// # Example
+///
+/// ```
+/// use std::io::{copy, stdout};
+/// use lib::io::MultiReader;
+///
+/// fn main() -> std::io::Result<()> {
+///     let header = "---- HEADER ----\n".as_bytes();
+///     let content = "Example of MultiReader\n".as_bytes();
+///     let footer = "---- FOOTER ----\n".as_bytes();
+///     let mut multi_reader =
+///         MultiReader::new(vec![Box::new(header), Box::new(content), Box::new(footer)]);
+///     copy(&mut multi_reader, &mut stdout())?;
+///     Ok(())
+/// }
+/// ```
+pub struct MultiReader {
+    readers: VecDeque<Box<dyn Read>>,
+    /// Points to current element while reading.
+    current: Option<Box<dyn Read>>,
+}
+
+impl MultiReader {
+    /// Constructs `MultiReader`. `current` is set to the first element that is popped out from `VecDeque`.
+    pub fn new(readers: Vec<Box<dyn Read>>) -> Self {
+        let mut deque: VecDeque<Box<dyn Read>> = readers.into();
+        let current = deque.pop_front();
+        Self {
+            readers: deque,
+            current,
+        }
+    }
+}
+
+impl Read for MultiReader {
+    fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
+        // Doesn't move to the next one until the first one is completely read.
+        // If the processing ends in the middle of reading the first one,
+        // it returns the control at that point.
+        loop {
+            match self.current {
+                Some(ref mut r) => {
+                    let n = r.read(buf)?;
+                    if n > 0 {
+                        return Ok(n);
+                    }
+                }
+                None => return Ok(0),
+            }
+            self.current = self.readers.pop_front();
+        }
+    }
+}

lib/src/io/tee_reader.rs

@@ -0,0 +1,32 @@
+use std::io::{Read, Write};
+
+pub struct TeeReader<R, W>
+where
+    R: Read,
+    W: Write,
+{
+    reader: R,
+    writer: W,
+}
+
+impl<R, W> TeeReader<R, W>
+where
+    R: Read,
+    W: Write,
+{
+    pub fn new(reader: R, writer: W) -> Self {
+        Self { reader, writer }
+    }
+}
+
+impl<R, W> Read for TeeReader<R, W>
+where
+    R: Read,
+    W: Write,
+{
+    fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
+        let n = self.reader.read(buf)?;
+        self.writer.write_all(&buf[..n])?;
+        Ok(n)
+    }
+}