-// Rust-101, Part 15: Interior Mutability (cont.), RefCell, Cell, Drop
+// Rust-101, Part 15: Mutex, Interior Mutability (cont.), RwLock, Sync
// ===================================================================
+use std::sync::{Arc, Mutex};
+use std::thread;
+use std::time::Duration;
+
+
+// The derived `Clone` implementation will clone the `Arc`, so all clones will actually talk about the same counter.
+#[derive(Clone)]
+struct ConcurrentCounter(Arc<Mutex<usize>>);
+
+impl ConcurrentCounter {
+ // The constructor just wraps the constructors of `Arc` and `Mutex`.
+ pub fn new(val: usize) -> Self {
+ unimplemented!()
+ }
+
+ // The core operation is, of course, `increment`.
+ pub fn increment(&self, by: usize) {
+ // `lock` on a mutex returns a guard, very much like `RefCell`. The guard gives access to the data contained in the mutex.
+ let mut counter = self.0.lock().unwrap();
+ *counter = *counter + by;
+ }
+
+ // The function `get` returns the current value of the counter.
+ pub fn get(&self) -> usize {
+ unimplemented!()
+ }
+}
+
+// Now our counter is ready for action.
+pub fn main() {
+ let counter = ConcurrentCounter::new(0);
+
+ // We clone the counter for the first thread, which increments it by 2 every 15ms.
+ let counter1 = counter.clone();
+ let handle1 = thread::spawn(move || {
+ for _ in 0..10 {
+ thread::sleep(Duration::from_millis(15));
+ counter1.increment(2);
+ }
+ });
+
+ // The second thread increments the counter by 3 every 20ms.
+ let counter2 = counter.clone();
+ let handle2 = thread::spawn(move || {
+ for _ in 0..10 {
+ thread::sleep(Duration::from_millis(20));
+ counter2.increment(3);
+ }
+ });
+
+ // Now we watch the threads working on the counter.
+ for _ in 0..50 {
+ thread::sleep(Duration::from_millis(5));
+ println!("Current value: {}", counter.get());
+ }
+
+ // Finally, we wait for all the threads to finish to be sure we can catch the counter's final value.
+ handle1.join().unwrap();
+ handle2.join().unwrap();
+ println!("Final value: {}", counter.get());
+}
+
+// **Exercise 15.1**: Add an operation `compare_and_inc(&self, test: usize, by: usize)` that increments the counter by
+// `by` *only if* the current value is `test`.
+//
+// **Exercise 15.2**: Rather than panicking in case the lock is poisoned, we can use `into_inner` on the error to recover
+// the data inside the lock. Change the code above to do that. Try using `unwrap_or_else` for this job.
+
+
+// **Exercise 15.3**: Change the code above to use `RwLock`, such that multiple calls to `get` can be executed at the same time.
+