use std::{
    cmp::Ordering,
    collections::{HashMap, HashSet},
};

use uuid::Uuid;

#[derive(Debug, Clone, PartialEq, Eq, Default)]
pub struct VectorClock(HashMap<Uuid, u64>);

impl VectorClock {
    pub fn new() -> Self {
        Self(HashMap::new())
    }

    pub fn get(&self, actor_id: &Uuid) -> u64 {
        self.0.get(actor_id).unwrap_or(&0).clone()
    }

    pub fn inc(&self, actor_id: &Uuid) -> Self {
        let mut m = self.0.clone();
        m.insert(actor_id.clone(), self.get(actor_id) + 1);
        VectorClock(m)
    }

    /// Returns true if this clock is concurrent with another (neither happens before the other)
    pub fn is_concurrent_with(&self, other: &VectorClock) -> bool {
        self.partial_cmp(other).is_none()
    }
}

impl PartialOrd for VectorClock {
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        let orderings: HashSet<_> = self
            .0
            .keys()
            .chain(other.0.keys())
            .collect::<HashSet<_>>()
            .into_iter()
            .map(|actor_id| self.get(actor_id).cmp(&other.get(actor_id)))
            .collect();

        let less = orderings.contains(&Ordering::Less);
        let greater = orderings.contains(&Ordering::Greater);

        match (less, greater) {
            (true, true) => None,
            (true, false) => Some(Ordering::Less),
            (false, true) => Some(Ordering::Greater),
            (false, false) => Some(Ordering::Equal),
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn vector_clock_compare() {
        let alice_id = Uuid::now_v7();
        let bob_id = Uuid::now_v7();
        let carol_id = Uuid::now_v7();

        assert!(alice_id < bob_id);
        assert!(bob_id < carol_id);

        let mut alice = VectorClock::new();
        let mut bob = VectorClock::new();

        assert!(alice == bob);
        assert!(bob == alice);

        bob = bob.inc(&bob_id);

        assert!(alice < bob);
        assert!(bob > alice);

        alice = alice.inc(&alice_id);

        assert!(!(alice < bob));
        assert!(!(alice > bob));

        alice = alice.inc(&bob_id);
        bob = bob.inc(&alice_id);

        assert!(alice == bob);

        alice = alice.inc(&alice_id);

        assert!(alice > bob);
        assert!(bob < alice);

        bob = bob.inc(&alice_id);

        assert!(alice == bob);

        alice = alice.inc(&carol_id);

        assert!(alice > bob);
        assert!(bob < alice);

        bob = bob.inc(&bob_id);

        assert!(!(alice > bob));
        assert!(!(alice < bob));

        let clock_a = VectorClock::new().inc(&alice_id).inc(&carol_id);
        let clock_b = VectorClock::new().inc(&bob_id).inc(&carol_id);

        assert!(!(clock_a > clock_b));
        assert!(!(clock_a < clock_b));
    }

    #[test]
    fn concurrent_clocks() {
        let alice_id = Uuid::now_v7();
        let bob_id = Uuid::now_v7();
        let carol_id = Uuid::now_v7();

        // Equal clocks are not concurrent
        let clock1 = VectorClock::new();
        let clock2 = VectorClock::new();
        assert!(!clock1.is_concurrent_with(&clock2));

        // Causally ordered clocks are not concurrent
        let clock_before = VectorClock::new().inc(&alice_id);
        let clock_after = VectorClock::new().inc(&alice_id).inc(&bob_id);
        assert!(!clock_before.is_concurrent_with(&clock_after));
        assert!(!clock_after.is_concurrent_with(&clock_before));

        // Clocks from different actors are concurrent
        let alice_clock = VectorClock::new().inc(&alice_id);
        let bob_clock = VectorClock::new().inc(&bob_id);
        assert!(alice_clock.is_concurrent_with(&bob_clock));
        assert!(bob_clock.is_concurrent_with(&alice_clock));

        // Complex concurrent case: diverged branches
        let clock_a = VectorClock::new()
            .inc(&alice_id)
            .inc(&alice_id)
            .inc(&bob_id)
            .inc(&carol_id);

        let clock_b = VectorClock::new()
            .inc(&alice_id)
            .inc(&alice_id)
            .inc(&bob_id)
            .inc(&bob_id);

        // clock_a: {alice: 2, bob: 1, carol: 1}
        // clock_b: {alice: 2, bob: 2}
        // carol: 1 > 0, but bob: 1 < 2 → concurrent
        assert!(clock_a.is_concurrent_with(&clock_b));
    }
}