// file: crates/ksp-worker-api/src/snapshot.rs
// version: 3

/// Monotone sequence attached to one latest-value Worker snapshot stream.
#[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
pub struct WorkerSnapshotSequence(u64);

impl crate::WorkerSnapshotSequence {
    /// Creates the initial sequence position for one Worker snapshot stream.
    #[must_use]
    pub const fn initial() -> Self {
        return Self(0);
    }

    /// Returns the opaque numeric position carried by this sequence.
    #[must_use]
    pub const fn value(&self) -> u64 {
        return self.0;
    }

    /// Advances the sequence exactly once or reports exhaustion without wrapping.
    pub fn next(&self) -> crate::Result<Self> {
        let next = match self.0.checked_add(1) {
            std::option::Option::Some(value) => value,
            std::option::Option::None => {
                return std::result::Result::Err(
                    crate::Error::new(crate::ERROR_CODE_WORKER_SNAPSHOT_SEQUENCE_EXHAUSTED, "Worker snapshot sequence exhausted")
                        .with_context("sequence", self.0.to_string()),
                );
            },
        };
        return std::result::Result::Ok(Self(next));
    }

    /// Reports whether this sequence is strictly newer than an observed sequence.
    #[must_use]
    pub const fn is_after(&self, observed: Self) -> bool {
        return self.0 > observed.0;
    }

    #[cfg(test)]
    const fn exhausted_for_test() -> Self {
        return Self(u64::MAX);
    }
}

/// Fixed common latest-value snapshot exposed by every Worker implementation.
#[derive(Clone, Eq, PartialEq)]
pub struct WorkerSnapshot {
    id: crate::WorkerId,
    kind: crate::WorkerKindCode,
    sequence: crate::WorkerSnapshotSequence,
    state: crate::WorkerState,
    health: crate::WorkerHealth,
    activity: crate::WorkerActivity,
}

impl crate::WorkerSnapshot {
    /// Creates one immutable common Worker snapshot from already validated values.
    #[must_use]
    pub const fn new(
        id: crate::WorkerId,
        kind: crate::WorkerKindCode,
        sequence: crate::WorkerSnapshotSequence,
        state: crate::WorkerState,
        health: crate::WorkerHealth,
        activity: crate::WorkerActivity,
    ) -> Self {
        return Self { id, kind, sequence, state, health, activity };
    }

    /// Returns the logical Worker identity.
    #[must_use]
    pub const fn id(&self) -> &crate::WorkerId {
        return &self.id;
    }

    /// Returns the stable Worker family code.
    #[must_use]
    pub const fn kind(&self) -> &crate::WorkerKindCode {
        return &self.kind;
    }

    /// Returns the monotone sequence of this latest value.
    #[must_use]
    pub const fn sequence(&self) -> crate::WorkerSnapshotSequence {
        return self.sequence;
    }

    /// Returns the lifecycle state represented by this snapshot.
    #[must_use]
    pub const fn state(&self) -> crate::WorkerState {
        return self.state;
    }

    /// Returns the operational health represented by this snapshot.
    #[must_use]
    pub const fn health(&self) -> crate::WorkerHealth {
        return self.health;
    }

    /// Returns the generic activity represented by this snapshot.
    #[must_use]
    pub const fn activity(&self) -> crate::WorkerActivity {
        return self.activity;
    }
}

impl std::fmt::Debug for crate::WorkerSnapshot {
    fn fmt(&self, formatter: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        return formatter
            .debug_struct("WorkerSnapshot")
            .field("id", &self.id)
            .field("kind", &self.kind)
            .field("sequence", &self.sequence)
            .field("state", &self.state)
            .field("health", &self.health)
            .field("activity", &self.activity)
            .finish();
    }
}

/// Runtime-neutral future returned while observing a latest-value Worker snapshot source.
pub type WorkerSnapshotFuture<'a> = std::pin::Pin<std::boxed::Box<dyn std::future::Future<Output = crate::WorkerSnapshot> + std::marker::Send + 'a>>;

/// Runtime-neutral read and change-wait contract for one latest-value Worker snapshot stream.
pub trait WorkerSnapshotSource: std::marker::Send + std::marker::Sync {
    /// Returns the complete current common Worker snapshot without replaying prior updates.
    #[must_use]
    fn current(&self) -> crate::WorkerSnapshot;

    /// Waits for a snapshot newer than `observed`, returning the complete current value after coalescing intermediate updates.
    fn wait_for_change(&self, observed: crate::WorkerSnapshotSequence) -> crate::WorkerSnapshotFuture<'_>;
}

#[cfg(test)]
fn exhausted_snapshot_sequence() -> crate::WorkerSnapshotSequence {
    return crate::WorkerSnapshotSequence::exhausted_for_test();
}

#[cfg(test)]
#[path = "../unit_tests/snapshot.rs"]
mod tests;