use std::sync::{
    atomic::{AtomicUsize, Ordering},
    Arc,
};

/// A small, cloneable counter that can be safely shared across threads.
#[derive(Clone, Debug, Default)]
pub struct Counter {
    value: Arc<AtomicUsize>,
}

impl Counter {
    /// Creates a counter starting at zero.
    pub fn new() -> Self {
        Self::with_value(0)
    }

    /// Creates a counter with an initial value.
    pub fn with_value(value: usize) -> Self {
        Self {
            value: Arc::new(AtomicUsize::new(value)),
        }
    }

    /// Increments the counter by one and returns the new value.
    pub fn increment(&self) -> usize {
        self.add(1)
    }

    /// Adds `amount` and returns the new value.
    pub fn add(&self, amount: usize) -> usize {
        self.value.fetch_add(amount, Ordering::Relaxed) + amount
    }

    /// Returns the current value.
    pub fn get(&self) -> usize {
        self.value.load(Ordering::Relaxed)
    }

    /// Sets the counter back to zero.
    pub fn reset(&self) {
        self.value.store(0, Ordering::Relaxed);
    }
}

#[cfg(test)]
mod tests {
    use super::Counter;
    use std::thread;

    #[test]
    fn increments_and_resets() {
        let counter = Counter::new();

        assert_eq!(counter.increment(), 1);
        assert_eq!(counter.add(4), 5);
        assert_eq!(counter.get(), 5);

        counter.reset();
        assert_eq!(counter.get(), 0);
    }

    #[test]
    fn counts_across_threads() {
        let counter = Counter::new();
        let mut handles = Vec::new();

        for _ in 0..8 {
            let counter = counter.clone();
            handles.push(thread::spawn(move || {
                for _ in 0..1_000 {
                    counter.increment();
                }
            }));
        }

        for handle in handles {
            handle.join().unwrap();
        }

        assert_eq!(counter.get(), 8_000);
    }
}