//! # AtomicCounter
//!
//! Atomic (thread-safe) counters for Rust.
//!
//! This crate contains an [`AtomicCounter`](trait.AtomicCounter.html) trait
//! that can safely be shared across threads.
//!
//! This crate provides two implementations:
//!
//! * [`RelaxedCounter`](struct.RelaxedCounter.html) which is suitable for
//!     e.g. collecting metrics or generate IDs, but which does not provide
//!     ["Sequential Consistency"](https://doc.rust-lang.org/nomicon/atomics.html#sequentially-consistent).
//!     `RelaxedCounter` uses [`Relaxed`](https://doc.rust-lang.org/std/sync/atomic/enum.Ordering.html#variant.Relaxed)
//!     memory ordering.
//!
//! * [`ConsistentCounter`](struct.ConsistentCounter.html) which provides the
//!     same interface but is sequentially consistent. Use this counter if the
//!     order of update from multiple threads is important.
//!     `ConsistentCounter` uses [`Sequentially Consistent`](https://doc.rust-lang.org/std/sync/atomic/enum.Ordering.html#variant.SeqCst)
//!     memory ordering.
//!
//! Both implementations are lock-free. Both are a very thin layer over
//! [`AtomicUsize`](https://doc.rust-lang.org/std/sync/atomic/struct.AtomicUsize.html)
//! which is more powerful but might be harder to use correctly.
//!
//! ## Which counter to use
//!
//! * If you are just collecting metrics, the `RelaxedCounter` is probably right choice.
//!
//! * If you are generating IDs, but don't make strong assumptions (like allocating
//!     memory based on the ID count), `RelaxedCounter` is probably the right choice.
//!
//! * If you are generating multiple IDs where you maintain an ordering
//!     invariant (e.g. ID `a` is always greater than ID `b`), you need "Sequential
//!     Consistency" and thus need to use `ConsistentCounter`. The same is true
//!     for all use cases where the _ordering_ of incrementing the counter is
//!     important.
//!
//! ## No updates are lost - It's just about the ordering!
//!
//! Note that in both implementations, _no count is lost_ and all operations are atomic.
//! The difference is _only_ in how the order of operations are observed by different
//! threads.
//!
//! ## Example:
//! Assume `a` is 5 and `b` is 4. You always want to maintain `a > b`.
//!
//! Thread 1 executes this code:
//!
//! ```rust,ignore
//!
//! a.inc();
//! b.inc();
//! ```
//!
//! Thread 2 gets counts:
//!
//! ```rust,ignore
//!
//! let a_local = a.get();
//! let b_local = b.get();
//! ```
//!
//! What are the values for `a_local` and `b_local`? That depends on the order
//! in which thread 1 and 2 have run:
//!
//! * `a_local` could still be 5 and `b_local` is still be 4 (e.g. if thread 2 ran before thread 1)
//! * `a_local` could be increment to 6 while `b_local` is still at 4 (e.g. if thread 1 and 2 ran in parallel)
//! * `a_local` could be increment to 6 and `b_local` be incremented to 5 (e.g. if thread 2 ran after thread 1).
//! * Additionally, if at least one counter is a `RelaxedCounter`, we cannot make
//!     assumption on the order of `a.inc()` and `b.inc()`. Thus, in this case
//!     thread 2 can also observe `a_local` to be 5 (not incremented yet) but
//!     `b_local` to be incremented to 5, _breaking the invariant_ `a > b`.
//!     Note that if thread 2 (or any other thread) `get()` the counts
//!     again, at some point they will observe both values to be incremented.
//!     No operations will be lost. It is only the _ordering_ of the operations
//!     that cannot be assumed if `Ordering` is `Relaxed`.
//!
//! So in order to maintain invariants such as `a > b` across multiple threads,
//! use `ConsistentCounter`.

use std::sync::atomic::AtomicUsize;
use std::sync::atomic::Ordering::{Relaxed, SeqCst};

/// Provides an atomic counter trait that can be shared across threads.
pub trait AtomicCounter: Send + Sync {
    /// Underlying primitive type that is being shared atomically.
    type PrimitiveType;

    /// Atomically increments the counter by one, returning the _previous_ value.
    fn inc(&self) -> Self::PrimitiveType;

    /// Atomically increments the counter by amount, returning the _previous_ value.
    fn add(&self, amount: Self::PrimitiveType) -> Self::PrimitiveType;

    /// Atomically gets the current value of the counter, without modifying the counter.
    fn get(&self) -> Self::PrimitiveType;

    /// Atomically returns the current value of the counter, while resetting to count to zero.
    fn reset(&self) -> Self::PrimitiveType;

    /// Consume the atomic counter and return the primitive type.
    ///
    /// This is safe because passing self by value guarantees that no other threads are concurrently accessing the atomic data.
    fn into_inner(self) -> Self::PrimitiveType;
}

/// Implementation of [`AtomicCounter`](trait.AtomicCounter.html) that uses
/// [`Relaxed`](https://doc.rust-lang.org/std/sync/atomic/enum.Ordering.html#variant.Relaxed)
/// memory ordering.
///
/// See [crate level documentation](index.html) for more details.
///
/// Note that all operations wrap if the counter is incremented beyond usize::max_value().
#[derive(Debug, Default)]
pub struct RelaxedCounter(AtomicUsize);

impl RelaxedCounter {

    /// Creates a new counter with initial_value
    pub fn new(initial_count: usize) -> RelaxedCounter {
        RelaxedCounter(AtomicUsize::new(initial_count))
    }
}

impl AtomicCounter for RelaxedCounter {
    type PrimitiveType = usize;

    fn inc(&self) -> usize {
        self.add(1)
    }

    fn add(&self, amount: usize) -> usize {
        self.0.fetch_add(amount, Relaxed)
    }

    fn get(&self) -> usize {
        self.0.load(Relaxed)
    }

    fn reset(&self) -> usize {
        self.0.swap(0, Relaxed)
    }

    fn into_inner(self) -> usize {
        self.0.into_inner()
    }
}

/// Implementation of [`AtomicCounter`](trait.AtomicCounter.html) that uses
/// [`Sequentially Consistent`](https://doc.rust-lang.org/std/sync/atomic/enum.Ordering.html#variant.SeqCst)
/// memory ordering.
///
/// See [crate level documentation](index.html) for more details.
///
/// Note that all operations wrap if the counter is incremented beyond usize::max_value().
#[derive(Debug, Default)]
pub struct ConsistentCounter(AtomicUsize);

impl ConsistentCounter {

    /// Creates a new counter with initial_value
    pub fn new(initial_count: usize) -> ConsistentCounter {
        ConsistentCounter(AtomicUsize::new(initial_count))
    }
}

impl AtomicCounter for ConsistentCounter {
    type PrimitiveType = usize;

    fn inc(&self) -> usize {
        self.add(1)
    }

    fn add(&self, amount: usize) -> usize {
        self.0.fetch_add(amount, SeqCst)
    }

    fn get(&self) -> usize {
        self.0.load(SeqCst)
    }

    fn reset(&self) -> usize {
        self.0.swap(0, SeqCst)
    }

    fn into_inner(self) -> usize {
        self.0.into_inner()
    }
}

#[cfg(test)]
mod tests {

    use std::fmt::Debug;
    use std::thread;
    use std::sync::Arc;
    use std::ops::Deref;

    use super::*;

    const NUM_THREADS: usize = 29;
    const NUM_ITERATIONS: usize = 7_000_000;

    fn test_simple_with<Counter>(counter: Counter)
        where Counter: AtomicCounter<PrimitiveType=usize>
    {
        counter.reset();
        assert_eq!(0, counter.add(5));
        assert_eq!(5, counter.add(3));
        assert_eq!(8, counter.inc());
        assert_eq!(9, counter.inc());
        assert_eq!(10, counter.get());
        assert_eq!(10, counter.get());
    }

    #[test]
    fn test_simple_relaxed() {
        test_simple_with(RelaxedCounter::new(0))
    }

    #[test]
    fn test_simple_consistent() {
        test_simple_with(ConsistentCounter::new(0))
    }

    fn test_inc_with<Counter>(counter: Arc<Counter>)
        where Counter: AtomicCounter<PrimitiveType=usize> + 'static + Debug
    {
        let mut join_handles = Vec::new();
        println!("test_inc: Spawning {} threads, each with {} iterations...",
                 NUM_THREADS,
                 NUM_ITERATIONS);
        for _ in 0..NUM_THREADS {
            let counter_ref = counter.clone();
            join_handles.push(thread::spawn(move || {
                //make sure we're not going though Arc on each iteration
                let counter: &Counter = counter_ref.deref();
                for _ in 0..NUM_ITERATIONS {
                    counter.inc();
                }
            }));
        }
        for handle in join_handles {
            handle.join().unwrap();
        }
        let count = Arc::try_unwrap(counter).unwrap().into_inner();
        println!("test_inc: Got count: {}", count);
        assert_eq!(NUM_THREADS * NUM_ITERATIONS, count);
    }

    #[test]
    fn test_inc_relaxed() {
        test_inc_with(Arc::new(RelaxedCounter::new(0)));
    }

    #[test]
    fn test_inc_consistent() {
        test_inc_with(Arc::new(ConsistentCounter::new(0)));
    }

    fn test_add_with<Counter>(counter: Arc<Counter>)
        where Counter: AtomicCounter<PrimitiveType=usize> + 'static + Debug
    {
        let mut join_handles = Vec::new();
        println!("test_add: Spawning {} threads, each with {} iterations...",
                 NUM_THREADS,
                 NUM_ITERATIONS);
        let mut expected_count = 0;
        for to_add in 0..NUM_THREADS {
            let counter_ref = counter.clone();
            expected_count += to_add * NUM_ITERATIONS;
            join_handles.push(thread::spawn(move || {
                //make sure we're not going though Arc on each iteration
                let counter: &Counter = counter_ref.deref();
                for _ in 0..NUM_ITERATIONS {
                    counter.add(to_add);
                }
            }));
        }
        for handle in join_handles {
            handle.join().unwrap();
        }
        let count = Arc::try_unwrap(counter).unwrap().into_inner();
        println!("test_add: Expected count: {}, got count: {}",
                 expected_count,
                 count);
        assert_eq!(expected_count, count);
    }

    #[test]
    fn test_add_relaxed() {
        test_add_with(Arc::new(RelaxedCounter::new(0)));
    }

    #[test]
    fn test_add_consistent() {
        test_add_with(Arc::new(ConsistentCounter::new(0)));
    }

    fn test_reset_with<Counter>(counter: Arc<Counter>)
        where Counter: AtomicCounter<PrimitiveType=usize> + 'static + Debug
    {
        let mut join_handles = Vec::new();
        println!("test_add_reset: Spawning {} threads, each with {} iterations...",
                 NUM_THREADS,
                 NUM_ITERATIONS);
        let mut expected_count = 0;
        for to_add in 0..NUM_THREADS {
            expected_count += to_add * NUM_ITERATIONS;
        }

        // setup thread that `reset()`s all the time
        let counter_ref = counter.clone();
        let reset_handle = thread::spawn(move || {
            // Usually, you would check for some better termination condition.
            // I don't want to pollute my test with thread synchronization
            // operations outside of AtomicCounter, hence this approach.
            let mut total_count = 0;
            let counter: &Counter = counter_ref.deref();
            while total_count < expected_count {
                total_count += counter.reset();
            }
            // Ok, now we got the total_count but this could just be lucky.
            // Better do some more resets to be sure... ;)
            for _ in 0..NUM_ITERATIONS {
                total_count += counter.reset();
            }
            total_count
        });

        for to_add in 0..NUM_THREADS {
            let counter_ref = counter.clone();

            join_handles.push(thread::spawn(move || {
                //make sure we're not going though Arc on each iteration
                let counter: &Counter = counter_ref.deref();
                for _ in 0..NUM_ITERATIONS {
                    counter.add(to_add);
                }
            }));
        }
        for handle in join_handles {
            handle.join().unwrap();
        }
        let actual_count = reset_handle.join().unwrap();
        println!("test_add_reset: Expected count: {}, got count: {}",
                 expected_count,
                 actual_count);
        assert_eq!(expected_count, actual_count);
    }

    #[test]
    fn test_reset_consistent() {
        test_reset_with(Arc::new(ConsistentCounter::new(0)));
    }

    #[test]
    fn test_reset_relaxed() {
        test_reset_with(Arc::new(RelaxedCounter::new(0)));
    }

}