moar fancy comments

members/mpsc/src/main.rs

@@ -6,17 +6,20 @@ use std::str::FromStr;
 use std::sync::{Arc, Barrier};
 use std::{sync::mpsc, thread};
 
+// simulate a complex datatype with special meaning being sent somewhere
 #[derive(Clone, Debug)]
 struct Message {
     payload: String,
 }
 
+// be able to print the Message
 impl std::fmt::Display for Message {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         write!(f, "Message {{{}}}", self.payload)
     }
 }
 
+// make a new Message from a string
 impl std::str::FromStr for Message {
     type Err = std::convert::Infallible;
     fn from_str(s: &str) -> std::prelude::v1::Result<Self, Self::Err> {
@@ -26,37 +29,76 @@ impl std::str::FromStr for Message {
     }
 }
 
+// printer thread function, will print any message received via the mpsc channel
 fn printer(receiver: mpsc::Receiver<Message>, barrier: Arc<Barrier>) -> Result<()> {
     let mut stdout = std::io::stdout();
-    loop {
-        let msg = receiver.recv()?;
+
+    // run as long as we can receive something (err always means all senders have been dropped)
+    while let Ok(msg) = receiver.recv() {
         println!("{msg}");
         stdout.flush()?;
-        barrier.wait(); // wait until the main thread wants us to print
+        barrier.wait(); // done with printing, the main thread can continue
     }
+    Ok(())
 }
 
 fn main() -> Result<()> {
+    // channel that can be split across threads to send values between them
+    // mpsc means Multi-producer, single-consumer
     let (sender, receiver) = mpsc::channel();
-    let barrier = Arc::new(Barrier::new(2));
-    let barrier_printer = barrier.clone();
-    thread::spawn(|| printer(receiver, barrier_printer).expect("printer error"));
-    let mut msg;
-    let mut buf = String::new();
-    let stdin = std::io::stdin();
-    let mut stdout = std::io::stdout();
+
+    // we need to wait for the printer thread to be done before we print the prompt "> " to stdout,
+    // otherwise we mix up the prints.
+    //
+    // We put it into an Arc, so that we can share it across threads.
+    let barrier = Arc::new(Barrier::new(
+        2, /* number of threads, continue when this many are waiting */
+    ));
+    let barrier_printer = barrier.clone(); // second one for the printer, this contains a reference
+                                           // to our original barrier. (An Arc is a special kind of reference)
+
+    // we spawn a thread and give it something to run
+    let _handle = thread::spawn(|| printer(receiver, barrier_printer).expect("printer error"));
+
+    let mut msg; // we store our messages here
+    let mut buf = String::new(); // we put the contents of the stdin here
+    let stdin = std::io::stdin(); // we read user input from here
+    let mut stdout = std::io::stdout(); // we need this to flush explicitly
+
+    // do this forever
     loop {
-        buf.clear();
+        buf.clear(); // we want an empty buf at the start
+
+        // print a prompt while staying in the same line
         print!("> ");
-        stdout.flush()?;
-        let _ = stdin.read_line(&mut buf)?;
-        buf = buf.replace('\n', "");
-        if buf.is_empty() {
+        stdout.flush()?; // make sure that the stdout gets printed now instead of waiting for a
+                         // newline (stdout flushes automatically at newlines)
+
+        let _ = stdin.read_line(&mut buf)?; // read the user input
+
+        // check for special inputs
+        if buf == "\n" {
+            // enter
             continue;
+        } else if buf.to_lowercase() == "exit\n" || buf.is_empty() {
+            // exit or ctrl-d
+            break;
         }
+
+        buf = buf.replace('\n', ""); // we don't need the newline, just accept the user input when
+                                     // the user presses return
+                                     // if the user input was empty start anew
+
+        // convert the user input into a message (we could also just send a String, but this
+        // simulates a more complex behavior)
         msg = Message::from_str(&buf).unwrap();
+
+        // send the `Message` to the printer thread
         sender.send(msg)?;
-        barrier.wait(); // wait until the printer is done printing the message, so we dont mix
+
+        // wait until the printer is done printing the message, so we dont mix
         // stdout prints (we use print instead of println)
+        barrier.wait();
     }
+    Ok(())
 }