// SPDX-FileCopyrightText: Copyright (c) 2025-2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
// SPDX-License-Identifier: Apache-2.0

use std::ops::Range;
use std::sync::Arc;

use serde::{Deserialize, Serialize};

use crate::{BlockId, G2, InstanceId, SequenceHash};
use kvbm_logical::blocks::ImmutableBlock;
use kvbm_physical::manager::LayoutHandle;

use super::SessionId;

/// Messages exchanged between leaders during onboarding sessions.
///
/// Phase 2 protocol (G2-only):
/// 1. Initiator sends CreateSession to multiple responders
/// 2. Each responder searches local G2 and sends G2Results back
/// 3. Initiator applies first-responder-wins and sends HoldBlocks to each
/// 4. Responders send Acknowledged after releasing unwanted blocks
///
/// Phase 3 protocol (G3 staging):
/// 5. Responders search G3 and send G3Results
/// 6. Initiator sends StageBlocks with blocks to stage G3->G2
/// 7. Responders stage blocks and send BlocksReady when complete
#[derive(Debug, Clone, Serialize, Deserialize)]
pub enum OnboardMessage {
    /// Initiator creates a new onboarding session.
    CreateSession {
        requester: InstanceId,
        session_id: SessionId,
        sequence_hashes: Vec<SequenceHash>,
    },

    /// Responder signals local search (G2 and G3) is complete.
    SearchComplete {
        responder: InstanceId,
        session_id: SessionId,
    },

    /// Responder reports G2 search results.
    /// - sequence_hashes: ordered list of matched sequence hashes
    /// - block_ids: parallel list of block IDs (can be zipped with sequence_hashes)
    G2Results {
        responder: InstanceId,
        session_id: SessionId,
        sequence_hashes: Vec<SequenceHash>,
        block_ids: Vec<BlockId>,
    },

    /// Responder reports G3 search results.
    /// - sequence_hashes: ordered list of matched sequence hashes (no block IDs)
    G3Results {
        responder: InstanceId,
        session_id: SessionId,
        sequence_hashes: Vec<SequenceHash>,
    },

    /// Initiator tells responder which sequence hashes to hold/drop.
    /// Works across G2 and G3 tiers.
    HoldBlocks {
        requester: InstanceId,
        session_id: SessionId,
        hold_hashes: Vec<SequenceHash>,
        drop_hashes: Vec<SequenceHash>,
    },

    /// Initiator tells responder which G3 sequence hashes to stage to G2.
    /// Any G3 blocks with these hashes should be staged to G2.
    StageBlocks {
        requester: InstanceId,
        session_id: SessionId,
        stage_hashes: Vec<SequenceHash>,
    },

    /// Responder reports newly staged blocks are ready in G2 (after G3->G2 staging).
    /// Only reports blocks that were just staged, not all G2 blocks.
    /// - sequence_hashes: newly staged blocks
    /// - block_ids: parallel to sequence_hashes
    BlocksReady {
        responder: InstanceId,
        session_id: SessionId,
        sequence_hashes: Vec<SequenceHash>,
        block_ids: Vec<BlockId>,
    },

    /// Responder acknowledges hold/drop request.
    Acknowledged {
        responder: InstanceId,
        session_id: SessionId,
    },

    /// Initiator tells responder to release specific sequence hashes that weren't selected.
    /// Works across G2 and G3 tiers.
    ReleaseBlocks {
        requester: InstanceId,
        session_id: SessionId,
        release_hashes: Vec<SequenceHash>,
    },

    /// Initiator tells responder session is complete, responder can cleanup.
    CloseSession {
        requester: InstanceId,
        session_id: SessionId,
    },

    // =========================================================================
    // G4/Object Storage Messages (Internal - not sent over network)
    // =========================================================================
    /// G4 search results from object storage `has_blocks`.
    ///
    /// Internal message sent via mpsc channel from the G4 search task
    /// to the initiator session. Contains hashes found in object storage
    /// with their sizes.
    G4Results {
        session_id: SessionId,
        /// Hashes found in G4 with their sizes in bytes
        found_hashes: Vec<(SequenceHash, usize)>,
    },

    /// G4 load completion results from object storage `get_blocks`.
    ///
    /// Internal message sent via mpsc channel from the G4 load task
    /// to the initiator session. Contains per-block success/failure.
    G4LoadComplete {
        session_id: SessionId,
        /// Successfully loaded hashes
        success: Vec<SequenceHash>,
        /// Failed hashes with error messages
        failures: Vec<(SequenceHash, String)>,
        /// Successfully loaded and registered G2 blocks.
        /// These are ready to be added to local_g2_blocks.
        /// Wrapped in Arc for Clone derivation (internal message only).
        #[serde(skip)]
        blocks: Arc<Vec<ImmutableBlock<G2>>>,
    },
    // TODO: Add heartbeat/TTL mechanism for handling unresponsive initiators
    // Heartbeat {
    //     requester: InstanceId,
    //     session_id: SessionId,
    //     timestamp: u64,
    // },
    // TTL resets with each heartbeat. If TTL expires:
    // - Responder releases all held blocks
    // - Responder cleans up session state
    // - Session task exits
}

/// Represents a block match found during search.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct BlockMatch {
    pub sequence_hash: SequenceHash,
    pub block_id: BlockId,
}

impl OnboardMessage {
    /// Extract the session ID from any message variant.
    pub fn session_id(&self) -> SessionId {
        match self {
            OnboardMessage::CreateSession { session_id, .. }
            | OnboardMessage::SearchComplete { session_id, .. }
            | OnboardMessage::G2Results { session_id, .. }
            | OnboardMessage::G3Results { session_id, .. }
            | OnboardMessage::HoldBlocks { session_id, .. }
            | OnboardMessage::StageBlocks { session_id, .. }
            | OnboardMessage::BlocksReady { session_id, .. }
            | OnboardMessage::Acknowledged { session_id, .. }
            | OnboardMessage::ReleaseBlocks { session_id, .. }
            | OnboardMessage::CloseSession { session_id, .. }
            | OnboardMessage::G4Results { session_id, .. }
            | OnboardMessage::G4LoadComplete { session_id, .. } => *session_id,
        }
    }

    /// Extract the requester/responder instance ID from the message.
    ///
    /// # Panics
    /// Panics if called on G4 messages (internal only, no instance ID).
    pub fn instance_id(&self) -> InstanceId {
        match self {
            OnboardMessage::CreateSession { requester, .. }
            | OnboardMessage::HoldBlocks { requester, .. }
            | OnboardMessage::StageBlocks { requester, .. }
            | OnboardMessage::ReleaseBlocks { requester, .. }
            | OnboardMessage::CloseSession { requester, .. } => *requester,
            OnboardMessage::SearchComplete { responder, .. }
            | OnboardMessage::G2Results { responder, .. }
            | OnboardMessage::G3Results { responder, .. }
            | OnboardMessage::BlocksReady { responder, .. }
            | OnboardMessage::Acknowledged { responder, .. } => *responder,
            OnboardMessage::G4Results { .. } | OnboardMessage::G4LoadComplete { .. } => {
                panic!("G4 messages are internal and do not have an instance ID")
            }
        }
    }

    /// Get the variant name as a string for logging.
    pub fn variant_name(&self) -> &'static str {
        match self {
            OnboardMessage::CreateSession { .. } => "CreateSession",
            OnboardMessage::SearchComplete { .. } => "SearchComplete",
            OnboardMessage::G2Results { .. } => "G2Results",
            OnboardMessage::G3Results { .. } => "G3Results",
            OnboardMessage::HoldBlocks { .. } => "HoldBlocks",
            OnboardMessage::StageBlocks { .. } => "StageBlocks",
            OnboardMessage::BlocksReady { .. } => "BlocksReady",
            OnboardMessage::Acknowledged { .. } => "Acknowledged",
            OnboardMessage::ReleaseBlocks { .. } => "ReleaseBlocks",
            OnboardMessage::CloseSession { .. } => "CloseSession",
            OnboardMessage::G4Results { .. } => "G4Results",
            OnboardMessage::G4LoadComplete { .. } => "G4LoadComplete",
        }
    }
}

// =============================================================================
// Unified Session Protocol
// =============================================================================
//
// These types support the unified session model where sessions can dynamically
// transition between control roles.

use super::state::{ControlRole, SessionPhase};

/// Unified session message protocol.
///
/// Unified, bidirectional protocol that supports dynamic control transfer.
///
/// # Protocol Overview
///
/// 1. **Connection**: `Attach`/`Detach` for peer management
/// 2. **Control Transfer**: `YieldControl`/`AcquireControl` for bidirectional role changes
/// 3. **Block Operations**: Commands from controller to controllee
/// 4. **State Sync**: Responses from controllee to controller
/// 5. **Lifecycle**: `Close`/`Error` for termination
#[derive(Debug, Clone, Serialize, Deserialize)]
pub enum SessionMessage {
    // =========================================================================
    // Connection Management
    // =========================================================================
    /// Attach to a session as a specific role.
    ///
    /// Sent by a peer to establish the session relationship.
    Attach {
        /// The instance ID of the attaching peer.
        peer: InstanceId,
        /// The session to attach to.
        session_id: SessionId,
        /// The role this peer will assume (typically `Controllee` or `Controller`).
        as_role: ControlRole,
    },

    /// Detach from a session.
    ///
    /// Graceful disconnection. The session may continue if other peers are attached.
    Detach {
        /// The instance ID of the detaching peer.
        peer: InstanceId,
        /// The session to detach from.
        session_id: SessionId,
    },

    // =========================================================================
    // Control Transfer (Bidirectional)
    // =========================================================================
    /// Yield control to peer.
    ///
    /// The sender transitions from `Controller` to `Neutral`.
    /// The receiver (if in `Controllee`) can then `AcquireControl` or remain passive.
    YieldControl {
        /// The instance ID of the yielding peer.
        peer: InstanceId,
        /// The session.
        session_id: SessionId,
    },

    /// Acquire control from peer.
    ///
    /// The sender attempts to become `Controller`.
    /// Valid when sender is `Neutral` or `Controllee` and peer is `Neutral`.
    AcquireControl {
        /// The instance ID of the peer acquiring control.
        peer: InstanceId,
        /// The session.
        session_id: SessionId,
    },

    // =========================================================================
    // Block Operations (Controller → Controllee)
    // =========================================================================
    /// Trigger staging of blocks (e.g., G3→G2).
    TriggerStaging {
        /// The session.
        session_id: SessionId,
    },

    /// Request that specific blocks be held (kept alive).
    HoldBlocks {
        /// The session.
        session_id: SessionId,
        /// Sequence hashes of blocks to hold.
        hold_hashes: Vec<SequenceHash>,
    },

    /// Release specific blocks (they can now be evicted).
    ReleaseBlocks {
        /// The session.
        session_id: SessionId,
        /// Sequence hashes of blocks to release.
        release_hashes: Vec<SequenceHash>,
    },

    /// Notify that blocks have been pulled via RDMA.
    ///
    /// The controllee can release these blocks from its hold.
    BlocksPulled {
        /// The session.
        session_id: SessionId,
        /// Sequence hashes of blocks that were pulled.
        pulled_hashes: Vec<SequenceHash>,
    },

    // =========================================================================
    // State Synchronization (Controllee → Controller)
    // =========================================================================
    /// Full state snapshot.
    ///
    /// Sent after attachment and periodically on state changes.
    StateResponse {
        /// The session.
        session_id: SessionId,
        /// Complete state snapshot.
        state: SessionStateSnapshot,
    },

    /// Notification that blocks have been staged.
    ///
    /// This message supports layerwise transfer by optionally specifying
    /// which layer range is ready. When `layer_range` is `None`, all layers
    /// of the staged blocks are ready for transfer.
    BlocksStaged {
        /// The session.
        session_id: SessionId,
        /// Newly staged blocks (now in target tier).
        staged_blocks: Vec<BlockInfo>,
        /// Count of blocks remaining to stage.
        remaining: usize,
        /// Layer range that is ready for transfer.
        ///
        /// - `None`: All layers are ready (default behavior)
        /// - `Some(0..1)`: Only layer 0 is ready
        /// - `Some(0..60)`: Layers 0-59 are ready
        ///
        /// This enables layerwise streaming where the sender computes
        /// layer-by-layer and notifies the receiver as each layer completes.
        layer_range: Option<Range<usize>>,
    },

    // =========================================================================
    // Lifecycle
    // =========================================================================
    /// Close the session gracefully.
    Close {
        /// The session.
        session_id: SessionId,
    },

    /// Report an error.
    Error {
        /// The session.
        session_id: SessionId,
        /// Error description.
        message: String,
    },
}

impl SessionMessage {
    /// Extract the session ID from any message variant.
    pub fn session_id(&self) -> SessionId {
        match self {
            SessionMessage::Attach { session_id, .. }
            | SessionMessage::Detach { session_id, .. }
            | SessionMessage::YieldControl { session_id, .. }
            | SessionMessage::AcquireControl { session_id, .. }
            | SessionMessage::TriggerStaging { session_id, .. }
            | SessionMessage::HoldBlocks { session_id, .. }
            | SessionMessage::ReleaseBlocks { session_id, .. }
            | SessionMessage::BlocksPulled { session_id, .. }
            | SessionMessage::StateResponse { session_id, .. }
            | SessionMessage::BlocksStaged { session_id, .. }
            | SessionMessage::Close { session_id, .. }
            | SessionMessage::Error { session_id, .. } => *session_id,
        }
    }

    /// Extract the peer instance ID if present.
    pub fn peer(&self) -> Option<InstanceId> {
        match self {
            SessionMessage::Attach { peer, .. }
            | SessionMessage::Detach { peer, .. }
            | SessionMessage::YieldControl { peer, .. }
            | SessionMessage::AcquireControl { peer, .. } => Some(*peer),
            _ => None,
        }
    }

    /// Check if this is a control command (sent by controller).
    pub fn is_control_command(&self) -> bool {
        matches!(
            self,
            SessionMessage::TriggerStaging { .. }
                | SessionMessage::HoldBlocks { .. }
                | SessionMessage::ReleaseBlocks { .. }
                | SessionMessage::BlocksPulled { .. }
        )
    }

    /// Check if this is a state response (sent by controllee).
    pub fn is_state_response(&self) -> bool {
        matches!(
            self,
            SessionMessage::StateResponse { .. } | SessionMessage::BlocksStaged { .. }
        )
    }

    /// Get the variant name as a string for logging.
    pub fn variant_name(&self) -> &'static str {
        match self {
            SessionMessage::Attach { .. } => "Attach",
            SessionMessage::Detach { .. } => "Detach",
            SessionMessage::YieldControl { .. } => "YieldControl",
            SessionMessage::AcquireControl { .. } => "AcquireControl",
            SessionMessage::TriggerStaging { .. } => "TriggerStaging",
            SessionMessage::HoldBlocks { .. } => "HoldBlocks",
            SessionMessage::ReleaseBlocks { .. } => "ReleaseBlocks",
            SessionMessage::BlocksPulled { .. } => "BlocksPulled",
            SessionMessage::StateResponse { .. } => "StateResponse",
            SessionMessage::BlocksStaged { .. } => "BlocksStaged",
            SessionMessage::Close { .. } => "Close",
            SessionMessage::Error { .. } => "Error",
        }
    }
}

/// Complete session state snapshot.
///
/// Sent in `SessionMessage::StateResponse` to provide the controller
/// with full visibility into the controllee's state.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SessionStateSnapshot {
    /// Current session phase.
    pub phase: SessionPhase,
    /// Current control role of the sender.
    pub control_role: ControlRole,
    /// Blocks currently in G2 (ready for RDMA pull).
    pub g2_blocks: Vec<BlockInfo>,
    /// Count of blocks pending staging to G2.
    pub g3_pending: usize,
    /// Layer range that is ready for transfer.
    ///
    /// - `None`: All layers are ready (or not applicable)
    /// - `Some(0..1)`: Only layer 0 is ready
    /// - `Some(0..60)`: Layers 0-59 are ready
    ///
    /// This is updated when receiving `BlocksStaged` messages with `layer_range`.
    /// The controller can use this to know which layers can be pulled.
    #[serde(default)]
    pub ready_layer_range: Option<Range<usize>>,
}

/// Block information for session messages.
///
/// Contains the metadata needed to identify and transfer a block.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct BlockInfo {
    /// Physical block ID in the layout.
    pub block_id: BlockId,
    /// Logical sequence hash.
    pub sequence_hash: SequenceHash,
    /// Layout handle for RDMA operations.
    pub layout_handle: LayoutHandle,
}