// file: crates/ksp-worker-api/src/lifecycle.rs
// version: 2

/// Current lifecycle state of one continuous Worker.
#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
#[non_exhaustive]
pub enum WorkerState {
    /// The Worker exists but has not started initialization.
    Created,
    /// The Worker is initializing resources before entering steady service.
    Starting,
    /// The Worker is actively providing its continuous service.
    Running,
    /// The Worker observed stop intent and is draining or releasing resources.
    Stopping,
    /// The Worker reached its normal terminal stopped state.
    Stopped,
    /// The Worker reached a terminal fault classified by one stable KSP error code.
    Faulted(crate::ErrorCode),
}

impl crate::WorkerState {
    /// Returns the stable safe lifecycle code without rendering Worker data.
    #[must_use]
    pub const fn code(&self) -> &'static str {
        return match self {
            Self::Created => "created",
            Self::Starting => "starting",
            Self::Running => "running",
            Self::Stopping => "stopping",
            Self::Stopped => "stopped",
            Self::Faulted(_) => "faulted",
        };
    }

    /// Returns the terminal fault code when the state is [`Self::Faulted`].
    #[must_use]
    pub const fn fault_code(&self) -> std::option::Option<crate::ErrorCode> {
        return match self {
            Self::Faulted(code) => std::option::Option::Some(*code),
            _ => std::option::Option::None,
        };
    }

    /// Reports whether no later lifecycle transition is permitted.
    #[must_use]
    pub const fn is_terminal(&self) -> bool {
        return matches!(self, Self::Stopped | Self::Faulted(_));
    }
}

/// Operational health classification independent from Worker lifecycle phase.
#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
#[non_exhaustive]
pub enum WorkerHealth {
    /// Health has not yet been established or cannot currently be classified.
    Unknown,
    /// The Worker is operating within its expected healthy envelope.
    Healthy,
    /// The Worker is operating with a known degradation while service remains available.
    Degraded,
    /// The Worker is currently unable to satisfy its expected service health contract.
    Unhealthy,
}

impl crate::WorkerHealth {
    /// Returns the stable safe code for this health classification.
    #[must_use]
    pub const fn code(&self) -> &'static str {
        return match self {
            Self::Unknown => "unknown",
            Self::Healthy => "healthy",
            Self::Degraded => "degraded",
            Self::Unhealthy => "unhealthy",
        };
    }
}

/// Minimal generic activity classification for a continuous Worker.
#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
#[non_exhaustive]
pub enum WorkerActivity {
    /// Activity has not yet been established or cannot currently be classified.
    Unknown,
    /// The Worker is alive but not currently processing concrete work.
    Idle,
    /// The Worker is currently processing concrete work.
    Active,
}

impl crate::WorkerActivity {
    /// Returns the stable safe code for this activity classification.
    #[must_use]
    pub const fn code(&self) -> &'static str {
        return match self {
            Self::Unknown => "unknown",
            Self::Idle => "idle",
            Self::Active => "active",
        };
    }
}

/// Passive owner of one Worker identity and its validated lifecycle state.
#[derive(Eq, PartialEq)]
pub struct WorkerLifecycle {
    id: crate::WorkerId,
    kind: crate::WorkerKindCode,
    state: crate::WorkerState,
}

impl crate::WorkerLifecycle {
    /// Creates one lifecycle in [`WorkerState::Created`] state.
    #[must_use]
    pub const fn new(id: crate::WorkerId, kind: crate::WorkerKindCode) -> Self {
        return Self { id, kind, state: crate::WorkerState::Created };
    }

    /// 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 current lifecycle state.
    #[must_use]
    pub const fn state(&self) -> crate::WorkerState {
        return self.state;
    }

    /// Begins initialization of a newly created Worker.
    pub fn start(&mut self) -> crate::Result<()> {
        return self.transition(crate::WorkerState::Starting);
    }

    /// Records that initialization completed and steady service is running.
    pub fn mark_running(&mut self) -> crate::Result<()> {
        return self.transition(crate::WorkerState::Running);
    }

    /// Records that a starting or running Worker observed cooperative stop intent.
    pub fn mark_stopping(&mut self) -> crate::Result<()> {
        return self.transition(crate::WorkerState::Stopping);
    }

    /// Records normal terminal stop after draining, or before initialization began.
    pub fn mark_stopped(&mut self) -> crate::Result<()> {
        return self.transition(crate::WorkerState::Stopped);
    }

    /// Records a terminal fault using only a stable KSP error code.
    pub fn fault(&mut self, code: crate::ErrorCode) -> crate::Result<()> {
        return self.transition(crate::WorkerState::Faulted(code));
    }

    fn transition(&mut self, target: crate::WorkerState) -> crate::Result<()> {
        if !allowed_transition(self.state, target) {
            return std::result::Result::Err(transition_error(self.state, target));
        }
        self.state = target;
        return std::result::Result::Ok(());
    }
}

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

fn allowed_transition(source: crate::WorkerState, target: crate::WorkerState) -> bool {
    return matches!(
        (source, target),
        (crate::WorkerState::Created, crate::WorkerState::Starting)
            | (crate::WorkerState::Created, crate::WorkerState::Stopped)
            | (crate::WorkerState::Starting, crate::WorkerState::Running)
            | (crate::WorkerState::Starting, crate::WorkerState::Stopping)
            | (crate::WorkerState::Starting, crate::WorkerState::Faulted(_))
            | (crate::WorkerState::Running, crate::WorkerState::Stopping)
            | (crate::WorkerState::Running, crate::WorkerState::Faulted(_))
            | (crate::WorkerState::Stopping, crate::WorkerState::Stopped)
            | (crate::WorkerState::Stopping, crate::WorkerState::Faulted(_))
    );
}

fn transition_error(source: crate::WorkerState, target: crate::WorkerState) -> crate::Error {
    return crate::Error::new(crate::ERROR_CODE_WORKER_TRANSITION_INVALID, "invalid Worker lifecycle transition")
        .with_context("source_state", source.code())
        .with_context("target_state", target.code());
}

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