use std::sync::atomic::{AtomicU64, Ordering};

/// A thread-safe counter backed by an atomic integer.
#[derive(Debug, Default)]
pub struct Counter {
    value: AtomicU64,
}

impl Counter {
    /// Creates a counter with the given initial value.
    pub const fn new(initial: u64) -> Self {
        Self {
            value: AtomicU64::new(initial),
        }
    }

    /// Returns the current value.
    pub fn get(&self) -> u64 {
        self.value.load(Ordering::SeqCst)
    }

    /// Increments the counter by one and returns the updated value.
    pub fn increment(&self) -> u64 {
        self.add(1)
    }

    /// Adds `amount` and returns the updated value.
    pub fn add(&self, amount: u64) -> u64 {
        self.value.fetch_add(amount, Ordering::SeqCst) + amount
    }

    /// Resets the counter to zero and returns the previous value.
    pub fn reset(&self) -> u64 {
        self.value.swap(0, Ordering::SeqCst)
    }
}

impl From<u64> for Counter {
    fn from(value: u64) -> Self {
        Self::new(value)
    }
}

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

    #[test]
    fn basic_operations_work() {
        let counter = Counter::new(5);

        assert_eq!(counter.get(), 5);
        assert_eq!(counter.increment(), 6);
        assert_eq!(counter.add(4), 10);
        assert_eq!(counter.reset(), 10);
        assert_eq!(counter.get(), 0);
    }

    #[test]
    fn counts_safely_across_threads() {
        let counter = Arc::new(Counter::default());

        let handles: Vec<_> = (0..8)
            .map(|_| {
                let counter = Arc::clone(&counter);
                thread::spawn(move || {
                    for _ in 0..10_000 {
                        counter.increment();
                    }
                })
            })
            .collect();

        for handle in handles {
            handle.join().expect("worker thread panicked");
        }

        assert_eq!(counter.get(), 80_000);
    }
}