Add threads and channels exercise

exercise/threads_channels/Cargo.toml

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+[package]
+name = "threads_channels"
+version = "0.1.0"
+authors = ["Nathan Stocks <nathan@agileperception.com>"]
+edition = "2018"
+
+[dependencies]
+crossbeam = "0.8"

exercise/threads_channels/src/main.rs

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+// Silence some warnings so they don't distract from the exercise.
+#![allow(dead_code, unused_imports, unused_variables)]
+use crossbeam::channel;
+use std::thread;
+use std::time::Duration;
+
+fn sleep_ms(ms: u64) {
+    thread::sleep(Duration::from_millis(ms));
+}
+
+fn expensive_sum(v: Vec<i32>) -> i32 {
+    // Pretend our fancy little filter-map-sum is expensive and takes 500ms
+    sleep_ms(500);
+    println!("Child thread: just about finished");
+    v.iter().filter(|&x| x % 2 == 0).map(|x| x * x).sum()
+}
+
+fn main() {
+    let my_vector = vec![2, 5, 1, 0, 4, 3];
+
+    // 1. Spawn a child thread and have it call `expensive_sum(my_vector)`.  Store the returned
+    // join handle in a variable called `handle`. Once you've done this you should be able to run
+    // the code and see the Child thread output in the middle of the main thread's letters
+    //
+    //let handle = ...
+    let handle = thread::spawn(|| expensive_sum(my_vector));
+
+    // While the child thread is running, the main thread will also do some work
+    for letter in vec!["a", "b", "c", "d", "e", "f"] {
+        println!("Main thread: Letter {}", letter);
+        sleep_ms(200);
+    }
+
+    // 2. Let's retrieve the value returned by the child thread once it has exited.
+    // - Uncomment the code below.
+    // - Using the `handle` variable you stored the join handle in earlier, call .join() to wait for
+    //   the thread to exit with a `Result<i32, Err>`.
+    // - Get the i32 out of the Result and store it in a `sum` variable.
+
+    // let result =
+    // let sum =
+    // println!("The child thread's expensive sum is {}", sum);
+
+    // 3. Time for some fun with channels!
+    // - Uncomment the block comment below (Find and remove the `/*` and `*/`).
+    // - Create variables `tx` and `rx` and assign them to the sending and receiving ends of an
+    //   unbounded channel.
+
+    /*
+    let (tx, rx) = channel::unbounded();
+    // Cloning a channel makes another variable connected to that end of the channel so that you can
+    // send it to another thread.
+    let tx2 = tx.clone();
+
+    // 4. Examine the flow of execution of "Thread A" and "Thread B" below. Do you see how their
+    // output will mix with each other?
+    // - Alter the values passed to the `pause_ms()` calls in "Thread A" so that both the "Thread B"
+    //   outputs occur before the "Thread A" outputs.
+
+    let handle_a = thread::spawn(move || {
+        pause_ms(0);
+        tx2.send("Thread A: 1").unwrap();
+        pause_ms(200);
+        tx2.send("Thread A: 2").unwrap();
+    });
+
+    pause_ms(100); // Make sure Thread A has time to get going before we spawn Thread B
+
+    let handle_b = thread::spawn(move || {
+        pause_ms(0);
+        tx.send("Thread B: 1").unwrap();
+        pause_ms(200);
+        tx.send("Thread B: 2").unwrap();
+    });
+
+    // Using a Receiver channel as an iterator is a convenient way to get values until the channel
+    // gets closed.  A Receiver channel is automatically closed once all Sender channels have been
+    // closed.  Both our threads automatically close their Sender channels when they exit and the
+    // destructors for the channels get automatically called.
+    for msg in rx {
+        println!("Main thread: Received {}", msg);
+    }
+
+    // 5. Oops, we forgot to join "Thread A" and "Thread B". That's bad hygiene!
+    // - Use the thread handles to join both threads without getting any compiler warnings.
+    */
+
+    // Challenge: Make two child threads and give them each a receiving end to a channel.  From the
+    // main thread loop through several values and print each out and then send it to the channel.
+    // On the child threads print out the values you receive. Close the sending side in the main
+    // thread by calling `drop(tx)` (assuming you named your sender channel variable `tx`).  Join
+    // the child threads.
+    println!("Main thread: Exiting.")
+}