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

use std::{any::Any, cmp::max, sync::Arc};

use dynamo_llm::{
    block_manager::{
        BlockPool, NixlRegisterableStorage, Storage,
        block::{BlockMetadata, locality::LocalityProvider},
        config::should_bypass_cpu_cache,
        connector::protocol::{LeaderTransferRequest, RequestType, TransferType},
        distributed::{BlockTransferPool, BlockTransferRequest, KvbmLeader},
    },
    tokens::TokenBlock,
};
use dynamo_runtime::utils::task::CriticalTaskExecutionHandle;
use tokio_util::sync::CancellationToken;

use super::*;

#[derive(Debug, thiserror::Error)]
pub enum SlotError {
    #[error("slot not found")]
    NotFound,

    #[error("slot is in an invalid state: {0}")]
    InvalidState(String),

    #[error("slot operation failed: {0}")]
    InvalidOperation(String),

    #[error(transparent)]
    BlockPoolError(#[from] BlockPoolError),
}

pub trait SlotManager<R: RequestKey>: Send + Sync {
    type SlotType: Slot + ?Sized;

    fn has_slot(&self, request_id: &R) -> bool;

    /// Create a new slot for the given request ID, initial tokens and salt hash.
    fn create_slot(
        &self,
        request_id: &R,
        tokens: Vec<u32>,
        salt_hash: SaltHash,
    ) -> Result<(), SlotError>;

    fn get_slot(&self, request_id: &R) -> Result<Arc<Mutex<Self::SlotType>>, SlotError>;
    fn remove_slot(&self, request_id: &R) -> Result<(), SlotError>;
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[allow(dead_code)]
pub enum SlotState {
    /// The slot was not scheduled in the previous iteration.
    Initialized,

    /// The slot is prepared to load kv blocks from external storage; however, the onboarding operation
    /// has not been triggered yet. The usize is the number of tokens that are ready for onboarding.
    OnboardStaged(usize),

    /// The slot is actively copying blocks to device storage from some external storage(s).
    /// The usize is the number of tokens that are being onboarded.
    Onboarding(usize),

    /// The slot is actively prefilling the sequence.
    Prefilling,

    /// The slot is skipped prefill.
    SkippedPrefill,

    /// The slot is actively participating in a forward pass which will result in one more more tokens
    /// to be applied to the sequence.
    Decoding,

    /// The slot is skipped decoding.
    SkippedDecode,

    /// The slot is marked as finished, but not all resources have been released.
    Finishing,

    /// The slot is finished and all resources have been released.
    Finished,

    /// The slot is preempted and is waiting for the next iteration to resume.
    Preempted,
}

#[allow(dead_code)]
pub trait Slot: std::fmt::Debug {
    fn request_id(&self) -> &str;

    fn state(&self) -> SlotState;

    fn sequence(&self) -> &TokenBlockSequence;

    /// The number of tokens that have been computed on the device, i.e. the number of tokens for which we have ownership
    /// of computed kv blocks in the device storage.
    fn computed_tokens(&self) -> usize;

    fn apply_scheduler_output(
        &mut self,
        tokens: &[u32],
        block_ids: &[usize],
        num_computed_tokens: usize,
        num_scheduled_tokens: usize,
    ) -> Result<(), SlotError>;

    // TRT-LLM does not include scheduled tokens in the scheduler output.
    // Ideally, we should have a dedicated implementation for the TRT-LLM slot.
    // However, since only this single function needs to be rewritten for now,
    // we keep it as a separate function in Slot.
    fn apply_scheduler_output_with_computed_position(
        &mut self,
        tokens: &[u32],
        block_ids: &[usize],
        computed_position: usize,
        is_new_request: bool,
    ) -> Result<(), SlotError>;

    fn record_start_iteration(&mut self, iteration: u64) -> Result<(), SlotError>;

    fn mark_as_prefilling(&mut self, iteration: u64) -> Result<(), SlotError>;
    fn mark_as_decoding(&mut self, iteration: u64) -> Result<(), SlotError>;

    fn mark_as_finished(&mut self, iteration: u64) -> Result<(), SlotError>;

    /// The number of device blocks that have been allocated to the slot.
    fn num_device_blocks_allocated(&self) -> usize;

    /// Find all possible block matches for remaining known tokens in some local storage, i.e. look up and take ownership
    /// of any kv blocks for tokens in the isl that are not already in memory on the device, but on some local storage.
    ///
    /// If external tokens are matched, then the slot will transition to the [`SlotState::Onboarding`] state.
    /// `num_computed_tokens` is the number of tokens that have been computed on the device, this indicated the number of
    /// blocks in the ISL sequence that we should skip before we start looking for matches.
    fn acquire_local_matches(&mut self, num_computed_tokens: usize) -> Result<(), SlotError>;

    /// Trigger the onboarding operation for the slot.
    fn trigger_onboarding(&mut self, num_external_tokens: usize) -> Result<(), SlotError>;

    /// Take all pending operations for the slot.
    fn take_pending_operations(&mut self) -> Option<Vec<WorkerTransferRequest>>;

    /// Record the number of tokens that were cached on the device.
    fn record_cached_device_tokens(&mut self, num_tokens: usize);

    /// Record the number of tokens that were cached on the host.
    fn record_cached_host_tokens(&mut self, num_tokens: usize);

    /// Record the number of tokens that were cached on the disk.
    fn record_cached_disk_tokens(&mut self, num_tokens: usize);

    /// Reset the slot after preemption.
    fn reset_after_preemption(&mut self);

    /// Reset the slot.
    fn reset(&mut self);

    /// Get a mutable reference to the slot as a dynamic Any.
    fn as_any_mut(&mut self) -> &mut dyn Any;
}

pub trait ExternallyManagedDeviceSlot: Slot {
    /// Since we do not control the device pool, nor do we have insight in how the device pool is managed,
    /// we must accept external updates to the computed position.
    fn advance_computed_position(&mut self, num_tokens: usize) -> Result<(), SlotError>;

    /// Append the given block ids to the slot.
    ///
    /// The external device block manager has provided a set of mutable blocks to the slot.
    fn append_mutable_device_blocks(&mut self, block_ids: &[BlockId]) -> Result<(), SlotError>;
}

pub struct ConnectorSlotManager<R: RequestKey> {
    slots: Mutex<HashMap<R, Arc<Mutex<VllmConnectorSlot>>>>,
    block_manager: VllmBlockManager,
    /// use this to issue [`LocalTransferRequest`]s to the transfer engine
    xfer_tx: mpsc::UnboundedSender<LocalTransferRequest>,
    _transfer_engine_handle: Option<CriticalTaskExecutionHandle>,
}

impl std::fmt::Debug for ConnectorSlotManager<String> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("ConnectorSlotManager").finish()
    }
}

impl<R: RequestKey> ConnectorSlotManager<R> {
    pub fn new(
        block_manager: VllmBlockManager,
        leader: Arc<KvbmLeader>,
        drt: DistributedRuntime,
        kvbm_metrics: KvbmMetrics,
    ) -> Self {
        tracing::debug!(
            "creating slot manager with block size: {}",
            block_manager.block_size()
        );

        let (xfer_tx, xfer_rx) = mpsc::unbounded_channel();

        let mut xfer_engine = LocalTransferEngine::new(block_manager.clone(), leader, xfer_rx);
        let primary_token = drt.primary_token();
        let runtime_primary = drt.runtime().primary();

        let drt_for_task = drt;

        let xfer_engine_task = CriticalTaskExecutionHandle::new_with_runtime(
            |cancellation_token| async move {
                xfer_engine
                    .execute(cancellation_token, drt_for_task, kvbm_metrics)
                    .await
            },
            primary_token,
            "LocalTransferEngine",
            &runtime_primary,
        )
        .unwrap();

        Self {
            slots: Mutex::new(HashMap::new()),
            block_manager,
            xfer_tx,
            _transfer_engine_handle: Some(xfer_engine_task),
        }
    }
}

impl<R: RequestKey> SlotManager<R> for ConnectorSlotManager<R> {
    type SlotType = dyn ExternallyManagedDeviceSlot;

    fn has_slot(&self, request_id: &R) -> bool {
        self.slots.lock().unwrap().contains_key(request_id)
    }

    fn create_slot(
        &self,
        request_id: &R,
        tokens: Vec<u32>,
        salt_hash: SaltHash,
    ) -> Result<(), SlotError> {
        tracing::debug!(
            "creating slot with request_id: {}, num_tokens: {}",
            request_id,
            tokens.len()
        );
        let slot = VllmConnectorSlot::new(
            request_id.to_string(),
            tokens.into(),
            salt_hash,
            self.block_manager.clone(),
            self.xfer_tx.clone(),
        );
        self.slots
            .lock()
            .unwrap()
            .insert(request_id.clone(), Arc::new(Mutex::new(slot)));
        Ok(())
    }

    fn get_slot(&self, request_id: &R) -> Result<Arc<Mutex<Self::SlotType>>, SlotError> {
        let slots = self.slots.lock().unwrap();
        let slot = slots.get(request_id).ok_or(SlotError::NotFound)?;
        Ok(slot.clone())
    }

    fn remove_slot(&self, request_id: &R) -> Result<(), SlotError> {
        self.slots.lock().unwrap().remove(request_id);
        Ok(())
    }
}

impl<R: RequestKey> Drop for ConnectorSlotManager<R> {
    fn drop(&mut self) {
        if let Some(task) = self._transfer_engine_handle.take() {
            task.cancel();
            task.detach();
        }
    }
}

pub struct VllmConnectorSlot {
    request_id: String,

    /// The state of the slot.
    state: SlotState,

    // /// Current position in the sequence of tokens that have been computed.
    // /// When the slot is initialized, we populate the sequence with the prefill tokens.
    // /// However, those tokens are not yet prefilled, so they are not yet represented
    // /// in the sequence_position.
    // computed_position: usize,
    /// The sequence of token blocks
    sequence: TokenBlockSequence,

    /// The mutable blocks id (device)
    device_blocks: Vec<BlockId>,

    /// Blocks to be onboarded from the host
    /// We must hold these blocks in the slot state until the scheduler trigger the onboarding.
    staging_from_host: Option<Vec<ImmutableBlock<PinnedStorage, VllmLocality, BasicMetadata>>>,

    /// Blocks to be onboarded from the disk
    /// We must hold these blocks in the slot state until the scheduler trigger the onboarding.
    staging_from_disk: Option<Vec<ImmutableBlock<DiskStorage, VllmLocality, BasicMetadata>>>,

    /// The number of blocks cached from the device
    tokens_cached_from_device: usize,

    /// The number of blocks cached from the host
    tokens_cached_from_host: usize,

    /// The number of blocks cached from the disk
    tokens_cached_from_disk: usize,

    /// Phantom data to ensure the storage type is correct.
    block_manager: VllmBlockManager,

    block_size: usize,

    iteration_first_scheduled: Option<u64>,

    pending_operations: Option<Vec<WorkerTransferRequest>>,

    /// use this to issue [`LocalTransferRequest`]s to the transfer engine
    xfer_tx: mpsc::UnboundedSender<LocalTransferRequest>,

    /// This is the current position for which we are applying some number of active/scheduled tokens.
    /// On application, then we decide what actions we take.
    /// This the point that we will call our generic policy object.
    current_position: usize,

    /// The number of blocks that have been evaluated by the policy.
    /// Each policy evaluation will skip the already evaluated blocks.
    evaluated_blocks: usize,
}

impl VllmConnectorSlot {
    fn new(
        request_id: String,
        tokens: Tokens,
        salt_hash: SaltHash,
        block_manager: VllmBlockManager,
        xfer_tx: mpsc::UnboundedSender<LocalTransferRequest>,
    ) -> Self {
        assert!(!tokens.is_empty(), "tokens must be non-empty");
        let block_size = block_manager.block_size();
        debug_assert!(block_size.is_power_of_two() && block_size <= 1024);
        let sequence = TokenBlockSequence::new(tokens, block_size as u32, Some(salt_hash));

        Self {
            request_id,
            sequence,
            block_manager,
            block_size,
            xfer_tx,
            // default values
            state: SlotState::Initialized,
            iteration_first_scheduled: None,
            current_position: 0,
            evaluated_blocks: 0,
            device_blocks: Vec::new(),
            staging_from_host: None,
            staging_from_disk: None,
            pending_operations: None,
            tokens_cached_from_device: 0,
            tokens_cached_from_host: 0,
            tokens_cached_from_disk: 0,
        }
    }

    fn mark_as_skipped_prefill(&mut self) -> Result<(), SlotError> {
        if self.state != SlotState::Prefilling {
            return Err(SlotError::InvalidState(format!(
                "cannot mark slot as skipped prefill in state {:?}",
                self.state
            )));
        }
        self.state = SlotState::SkippedPrefill;
        Ok(())
    }

    fn mark_as_skipped_decode(&mut self) -> Result<(), SlotError> {
        if self.state != SlotState::Decoding {
            return Err(SlotError::InvalidState(format!(
                "cannot mark slot as skipped decode in state {:?}",
                self.state
            )));
        }
        self.state = SlotState::SkippedDecode;
        Ok(())
    }

    pub fn mark_as_skipped(&mut self) -> Result<(), SlotError> {
        match self.state {
            SlotState::Prefilling => self.mark_as_skipped_prefill(),
            SlotState::Decoding => self.mark_as_skipped_decode(),
            SlotState::SkippedPrefill => Ok(()), // already skipped
            SlotState::SkippedDecode => Ok(()),  // already skipped
            _ => {
                tracing::debug!(
                    "slot is in the {:?} state; will not explicitly mark as skipped, request_id: {}",
                    self.state,
                    self.request_id
                );
                Ok(())
            }
        }
    }
}

impl std::fmt::Debug for VllmConnectorSlot {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("VllmConnectorSlot")
            .field("state", &self.state)
            .field("current_position", &self.current_position)
            .field("num_tokens", &self.sequence.total_tokens())
            .finish()
    }
}

impl Slot for VllmConnectorSlot {
    fn request_id(&self) -> &str {
        &self.request_id
    }

    fn state(&self) -> SlotState {
        self.state
    }

    fn reset_after_preemption(&mut self) {
        assert!(self.staging_from_disk.is_none());
        assert!(self.staging_from_host.is_none());
        assert!(self.pending_operations.is_none());

        self.state = SlotState::Preempted;
        self.iteration_first_scheduled = None;
        self.current_position = 0;
        self.evaluated_blocks = 0;
        self.device_blocks.clear();
        self.tokens_cached_from_device = 0;
        self.tokens_cached_from_host = 0;
        self.tokens_cached_from_disk = 0;
    }

    fn reset(&mut self) {
        self.reset_after_preemption();
        self.state = SlotState::Initialized;
    }

    fn mark_as_prefilling(&mut self, _iteration: u64) -> Result<(), SlotError> {
        self.state = SlotState::Prefilling;
        Ok(())
    }

    fn mark_as_decoding(&mut self, _iteration: u64) -> Result<(), SlotError> {
        self.state = SlotState::Decoding;
        Ok(())
    }

    fn record_cached_device_tokens(&mut self, num_tokens: usize) {
        self.tokens_cached_from_device = num_tokens;
        tracing::debug!("recording {} cached device tokens", num_tokens,);
    }

    fn record_cached_host_tokens(&mut self, num_tokens: usize) {
        self.tokens_cached_from_host = num_tokens;
        tracing::debug!("recording {} cached host tokens", num_tokens);
    }

    fn record_cached_disk_tokens(&mut self, num_tokens: usize) {
        self.tokens_cached_from_disk = num_tokens;
        tracing::debug!("recording {} cached disk tokens", num_tokens);
    }

    #[tracing::instrument(level = "debug", skip_all, fields(request_id = self.request_id.as_str()))]
    fn apply_scheduler_output(
        &mut self,
        tokens: &[u32],
        block_ids: &[BlockId],
        num_computed_tokens: usize,
        num_scheduled_tokens: usize,
    ) -> Result<(), SlotError> {
        if !tokens.is_empty() {
            tracing::debug!(
                "appending {} newly decoded tokens to sequence",
                tokens.len()
            );
            self.state = SlotState::Decoding;
            self.sequence.extend(tokens.into()).unwrap();
        } else {
            self.state = SlotState::Prefilling;
        }

        // Use max to advance both current_position and evaluated_blocks at least by num_computed_tokens.
        // This logic is to prevent redundant block offloading.
        self.current_position = max(self.current_position, num_computed_tokens);
        self.evaluated_blocks = max(self.evaluated_blocks, num_computed_tokens / self.block_size);

        // apply new block_ids
        if !block_ids.is_empty() {
            tracing::debug!("assigning {} new device blocks slot", block_ids.len());
            self.device_blocks.extend(block_ids);
        }

        // we should have enough device blocks to cover the newly scheduled tokens
        let next_position = self.current_position + num_scheduled_tokens;
        assert!(
            next_position <= self.device_blocks.len() * self.block_size,
            "next_position: {} > device_blocks.len() {} * block_size {}",
            next_position,
            self.device_blocks.len(),
            self.block_size
        );

        if next_position > self.sequence.total_tokens() {
            // vllm stopped providing tokens, so we are done
            self.state = SlotState::Decoding;
            tracing::debug!(
                "connector source stopped providing tokens; no further evaluation possible"
            );
            return Ok(());
        }

        // now we decide what we should do from the current position to the num_scheduled_tokens
        tracing::debug!(
            "applying kv cache policy at current_position: {}; num_scheduled_tokens: {}; num_evaluated_blocks: {}",
            self.current_position,
            num_scheduled_tokens,
            self.evaluated_blocks
        );

        // TODO(ryan) - apply policy
        let next_position = self.current_position + num_scheduled_tokens;

        debug_assert!(next_position / self.block_size >= self.evaluated_blocks);

        let num_candidate_blocks = (next_position / self.block_size) - self.evaluated_blocks;

        tracing::debug!(
            "evaluating policy with the following parameters: state: {:?}; current_position: {}; num_candidate_blocks: {}; num_scheduled_tokens: {}",
            self.state,
            self.current_position,
            num_candidate_blocks,
            num_scheduled_tokens
        );

        if num_candidate_blocks != 0 {
            // do we have a mechanism for skipping gpu cache hit blocks?  not sure yet.
            // for now, offload all the blocks to the host
            let offload_block_ids: Vec<usize> = self
                .device_blocks
                .iter()
                .skip(self.evaluated_blocks)
                .take(num_candidate_blocks)
                .copied()
                .collect::<Vec<_>>();

            assert_eq!(
                offload_block_ids.len(),
                num_candidate_blocks,
                "device block overflow - candidate blocks exceed block count at offset {}",
                self.evaluated_blocks
            );

            let offload_token_blocks: Vec<TokenBlock> = self
                .sequence
                .blocks()
                .iter()
                .skip(self.evaluated_blocks)
                .take(num_candidate_blocks)
                .cloned()
                .collect::<Vec<_>>();

            self.offload_blocks(&offload_block_ids, &offload_token_blocks)
                .expect("failed to offload blocks");

            self.evaluated_blocks += num_candidate_blocks;
        }

        // done applying policy
        tracing::debug!(
            "done applying kv cache policy at current_position: {}; num_scheduled_tokens: {}",
            self.current_position,
            num_scheduled_tokens
        );

        // advance current and computed position
        self.current_position += num_scheduled_tokens;

        Ok(())
    }

    #[tracing::instrument(level = "debug", skip_all, fields(request_id = self.request_id.as_str()))]
    fn apply_scheduler_output_with_computed_position(
        &mut self,
        tokens: &[u32],
        block_ids: &[usize],
        computed_position: usize,
        is_new_request: bool,
    ) -> Result<(), SlotError> {
        // TRTLLM's KV Connector Manager will have (computed_position - external matches)
        // in onborading case
        if computed_position < self.current_position {
            tracing::debug!(
                "computed_position={} < current_position={}, so we are onboarding during prefilling phase",
                computed_position,
                self.current_position
            );
            return Ok(());
        }

        // now we decide what we should do for the new computed tokens
        tracing::debug!(
            "applying scheduler output, computed_position={}, sequence_total_tokens={}",
            computed_position,
            self.sequence.total_tokens()
        );

        if computed_position < self.sequence.total_tokens() {
            // no need to apply new tokens, since it's applied when created the slot during prefilling
            self.state = SlotState::Prefilling;
        } else {
            tracing::debug!(
                "appending {} newly decoded tokens to sequence",
                tokens.len()
            );
            self.sequence.extend(tokens.into()).unwrap();
            self.state = SlotState::Decoding;
        }

        // apply new block_ids, this should be applied for both prefilling and decoding
        // because this is unknown when creating the slot
        if !block_ids.is_empty() {
            tracing::debug!("assigning {} new device blocks slot", block_ids.len());
            self.device_blocks.extend(block_ids);
        }

        // This approach is fragile, but it’s the only way currently to skip evaluating
        // the device matched blocks and to avoid offloading them again.
        // TODO: Consider adding an indicator in the scheduler output to distinguish between
        // matched and unmatched device blocks/tokens from the scheduler.
        let maybe_have_device_matched_blocks =
            is_new_request && computed_position > 0 && self.evaluated_blocks == 0;

        if maybe_have_device_matched_blocks {
            self.evaluated_blocks = (computed_position + 1) / self.block_size;
        }

        let num_candidate_blocks =
            ((computed_position + 1) / self.block_size).saturating_sub(self.evaluated_blocks);

        if num_candidate_blocks > 0 {
            // do we have a mechanism for skipping gpu cache hit blocks?  not sure yet.
            // for now, offload all the blocks to the host
            let offload_block_ids: Vec<usize> = self
                .device_blocks
                .iter()
                .skip(self.evaluated_blocks)
                .take(num_candidate_blocks)
                .copied()
                .collect::<Vec<_>>();

            assert_eq!(
                offload_block_ids.len(),
                num_candidate_blocks,
                "device block overflow - candidate blocks exceed block count at offset {}",
                self.evaluated_blocks
            );

            let offload_token_blocks: Vec<TokenBlock> = self
                .sequence
                .blocks()
                .iter()
                .skip(self.evaluated_blocks)
                .take(num_candidate_blocks)
                .cloned()
                .collect::<Vec<_>>();

            self.offload_blocks(&offload_block_ids, &offload_token_blocks)
                .expect("failed to offload blocks");

            self.evaluated_blocks += num_candidate_blocks;
        }

        // done applying policy
        tracing::debug!(
            "done applying kv cache policy at current_position: {}; computed_position: {}",
            self.current_position,
            computed_position,
        );

        // advance current position to computed position
        self.current_position = computed_position;

        Ok(())
    }

    fn record_start_iteration(&mut self, iteration: u64) -> Result<(), SlotError> {
        if self.iteration_first_scheduled.is_none() {
            self.iteration_first_scheduled = Some(iteration);
        }
        Ok(())
    }

    fn mark_as_finished(&mut self, _iteration: u64) -> Result<(), SlotError> {
        self.state = SlotState::Finishing;
        tracing::info!(
            request_id = %self.request_id,
            "request set to finish: cached_gpu_tokens: {}; cached_host_tokens: {}; cached_disk_tokens: {}",
            self.tokens_cached_from_device,
            self.tokens_cached_from_host,
            self.tokens_cached_from_disk
        );
        Ok(())
    }

    fn sequence(&self) -> &TokenBlockSequence {
        &self.sequence
    }

    fn computed_tokens(&self) -> usize {
        self.current_position
    }

    fn num_device_blocks_allocated(&self) -> usize {
        self.device_blocks.len()
    }

    fn take_pending_operations(&mut self) -> Option<Vec<WorkerTransferRequest>> {
        self.pending_operations.take()
    }

    #[tracing::instrument(level = "debug", skip_all)]
    fn acquire_local_matches(&mut self, num_computed_tokens: usize) -> Result<(), SlotError> {
        if matches!(self.state(), SlotState::OnboardStaged(_)) {
            tracing::debug!("slot is already in the OnboardStaged state; skipping lookup");
            return Ok(());
        }

        if !matches!(self.state(), SlotState::Initialized | SlotState::Preempted) {
            return Err(SlotError::InvalidOperation(format!(
                "slot must be in the NotScheduled or Preempted state to acquire local matches; got {:?}",
                self.state()
            )));
        }

        if matches!(self.state(), SlotState::Preempted) {
            tracing::info!("slot is in the Preempted state; we get another chance to match");
        }

        let block_size = self.block_manager.block_size();
        let num_computed_blocks = num_computed_tokens / block_size;
        debug_assert!(num_computed_tokens % block_size == 0);

        let sequence_hashes = self
            .sequence()
            .blocks()
            .iter()
            .map(|b| b.sequence_hash())
            .collect::<Vec<_>>();

        // we start matching non-device blocks after the device blocks
        let search_offset = num_computed_blocks;

        tracing::debug!(
            "matching against {} block hashes",
            sequence_hashes[search_offset..].len()
        );

        // we should do this opportunistically after this operation is done
        // ideally it was triggered by the match_sequence_hashes_blocking calls directly

        // if let Some(host) = self.block_manager.host() {
        //     host.touch_blocks_blocking(&sequence_hashes)?;
        // }

        // if let Some(disk) = self.block_manager.disk() {
        //     disk.touch_blocks_blocking(&sequence_hashes)?;
        // }

        let mut host_blocks = self
            .block_manager
            .host()
            .map(|host| host.match_sequence_hashes_blocking(&sequence_hashes[search_offset..]))
            .transpose()?
            .unwrap_or_default();

        let num_matched_host_blocks = host_blocks.len();
        self.record_cached_host_tokens(num_matched_host_blocks * block_size);

        // advance the search offset by the number of matched host blocks
        let search_offset = search_offset + num_matched_host_blocks;

        // start at host offset
        let mut disk_blocks = self
            .block_manager
            .disk()
            .map(|disk| disk.match_sequence_hashes_blocking(&sequence_hashes[search_offset..]))
            .transpose()?
            .unwrap_or_default();

        let num_matched_disk_blocks = disk_blocks.len();
        self.record_cached_disk_tokens(num_matched_disk_blocks * block_size);

        let num_matched_blocks = num_matched_host_blocks + num_matched_disk_blocks;

        tracing::debug!(
            "successfully matched {} host blocks and {} disk blocks; {} total blocks",
            num_matched_host_blocks,
            num_matched_disk_blocks,
            num_matched_blocks
        );

        // early exit if we did not match any blocks
        if num_matched_blocks == 0 {
            return Ok(());
        }

        let mut num_new_matched_tokens = num_matched_blocks * block_size;

        // we are on a block boundary, so we need to throw away the last block
        if (num_computed_tokens + num_new_matched_tokens) == self.sequence().total_tokens() {
            tracing::debug!("on a block boundary, throwing away the last block");

            // we should have matched at least one block
            assert!(!host_blocks.is_empty() || !disk_blocks.is_empty());

            // pop from disk, or if there are none, then from host
            if disk_blocks.is_empty() {
                host_blocks.pop();
            } else {
                disk_blocks.pop();
            }

            // decrement the number of new matched tokens by the block size
            num_new_matched_tokens -= block_size;
        }

        // early exit if we need to onboard 0 blocks (after potentially dropping the last block)
        if num_new_matched_tokens == 0 {
            return Ok(());
        }

        self.staging_from_host = if !host_blocks.is_empty() {
            Some(host_blocks)
        } else {
            None
        };
        self.staging_from_disk = if !disk_blocks.is_empty() {
            Some(disk_blocks)
        } else {
            None
        };

        self.state = SlotState::OnboardStaged(num_new_matched_tokens);

        Ok(())
    }

    fn trigger_onboarding(&mut self, num_external_tokens: usize) -> Result<(), SlotError> {
        if !matches!(self.state(), SlotState::OnboardStaged(_)) {
            return Err(SlotError::InvalidOperation(format!(
                "slot must be in the OnboardStaged state to trigger onboarding; got {:?}",
                self.state()
            )));
        }

        debug_assert_eq!(self.evaluated_blocks, 0);
        debug_assert_eq!(self.current_position % self.block_size, 0);
        debug_assert_eq!(num_external_tokens % self.block_size, 0);

        let num_computed_blocks = self.current_position / self.block_size;

        // shift the evaluated blocks position to the end of the computed/cached blocks
        self.evaluated_blocks = num_computed_blocks;

        // match the host / disk blocks to the newly assigned mutable device blocks
        if let Some(host_blocks) = self.staging_from_host.take() {
            let num_host_blocks = host_blocks.len();

            // get device block ids
            let dst_block_ids = self
                .device_blocks
                .iter()
                .skip(self.evaluated_blocks)
                .take(num_host_blocks)
                .copied()
                .collect::<Vec<_>>();

            debug_assert_eq!(dst_block_ids.len(), num_host_blocks);

            // construct offload requests - transfer engine + worker
            let src_blocks = Box::new(AnyImmutableBlocks::<PinnedStorage, _, _>::new(host_blocks));

            self.onboard_blocks(src_blocks, dst_block_ids)?;

            // shift the evaluated blocks position to the end of the computed/cached blocks
            self.evaluated_blocks += num_host_blocks;
        }

        if let Some(disk_blocks) = self.staging_from_disk.take() {
            let num_disk_blocks = disk_blocks.len();

            // get device block ids
            let dst_block_ids = self
                .device_blocks
                .iter()
                .skip(self.evaluated_blocks)
                .take(num_disk_blocks)
                .copied()
                .collect::<Vec<_>>();

            debug_assert_eq!(dst_block_ids.len(), num_disk_blocks);

            // construct offload requests - transfer engine + worker
            let src_blocks = Box::new(AnyImmutableBlocks::<DiskStorage, _, _>::new(disk_blocks));

            self.onboard_blocks(src_blocks, dst_block_ids)?;

            // shift the evaluated blocks position to the end of the computed/cached blocks
            self.evaluated_blocks += num_disk_blocks;
        }

        self.state = SlotState::Onboarding(num_external_tokens);
        self.advance_computed_position(num_external_tokens)?;

        Ok(())
    }

    fn as_any_mut(&mut self) -> &mut dyn Any {
        self
    }
}

impl ExternallyManagedDeviceSlot for VllmConnectorSlot {
    fn advance_computed_position(&mut self, num_tokens: usize) -> Result<(), SlotError> {
        if self.current_position + num_tokens > self.sequence().total_tokens() {
            return Err(SlotError::InvalidOperation(format!(
                "cannot advance computed position from {} by {num_tokens} tokens, total tokens is {}",
                self.current_position,
                self.sequence().total_tokens()
            )));
        }

        tracing::debug!(
            "advancing computed position by {} tokens from {} to {}",
            num_tokens,
            self.current_position,
            self.current_position + num_tokens
        );

        self.current_position += num_tokens;
        Ok(())
    }

    #[tracing::instrument(level = "debug", skip_all, fields(request_id = self.request_id))]
    fn append_mutable_device_blocks(&mut self, block_ids: &[BlockId]) -> Result<(), SlotError> {
        let count = block_ids.len();
        self.device_blocks.extend(block_ids);
        tracing::debug!(
            "appended {} mutable device blocks to slot; total device blocks: {}",
            count,
            self.num_device_blocks_allocated()
        );

        Ok(())
    }
}

impl VllmConnectorSlot {
    /// this method does two things which are related:
    /// 1. creates transfer engine offload request
    /// 2. creates matching connector worker transfer request
    ///
    /// these requests share the same uuid.
    ///
    /// the worker request triggers the transfer when sufficient forward pass progress has been made.
    fn offload_blocks(
        &mut self,
        block_ids: &[BlockId],
        token_blocks: &[TokenBlock],
    ) -> Result<(), SlotError> {
        assert!(block_ids.len() == token_blocks.len());
        let operation_id = uuid::Uuid::new_v4();

        let xfer_req = LocalTransferRequest::Offload(LocalOffloadRequest::new(
            self.request_id.clone(),
            block_ids.to_vec(),
            token_blocks.to_vec(),
            operation_id,
        ));

        let worker_req = WorkerTransferRequest {
            request_id: self.request_id.clone(),
            uuid: operation_id,
            transfer_type: TransferType::Store,
            request_type: RequestType::Scheduled,
        };

        if let Err(e) = self.xfer_tx.send(xfer_req) {
            tracing::error!("Failed to send transfer request: {:?}", e);
            return Err(SlotError::InvalidOperation(format!(
                "Transfer engine unavailable: {}; aborting offload",
                e
            )));
        }

        self.append_pending_operation(worker_req);

        tracing::debug!(
            request_id = self.request_id,
            operation_id = %operation_id,
            "offloading {} blocks to host",
            block_ids.len()
        );

        Ok(())
    }

    fn onboard_blocks(
        &mut self,
        src_blocks: Box<dyn AnyBlocks>,
        dst_block_ids: Vec<BlockId>,
    ) -> Result<(), SlotError> {
        debug_assert_eq!(src_blocks.len(), dst_block_ids.len());

        let num_blocks = src_blocks.len();
        let src_storage_pool = src_blocks.storage_pool();
        let operation_id = uuid::Uuid::new_v4();

        let xfer_req = LocalTransferRequest::Onboard(LocalOnboardRequest::new(
            self.request_id.clone(),
            src_blocks,
            dst_block_ids,
            operation_id,
        ));

        let worker_req = WorkerTransferRequest {
            request_id: self.request_id.clone(),
            uuid: operation_id,
            transfer_type: TransferType::Load,
            request_type: RequestType::Immediate,
        };

        if let Err(e) = self.xfer_tx.send(xfer_req) {
            tracing::error!("Failed to send transfer request: {:?}", e);
            return Err(SlotError::InvalidOperation(format!(
                "Transfer engine unavailable: {}; aborting offload",
                e
            )));
        }

        self.append_pending_operation(worker_req);

        tracing::debug!(
            request_id = self.request_id,
            operation_id = %operation_id,
            "start onboarding {} blocks from {:?} to device",
            num_blocks,
            src_storage_pool,
        );

        Ok(())
    }

    fn append_pending_operation(&mut self, operation: WorkerTransferRequest) {
        if let Some(pending_operations) = self.pending_operations.as_mut() {
            pending_operations.push(operation);
        } else {
            self.pending_operations = Some(vec![operation]);
        }
    }
}

enum LocalTransferRequest {
    Offload(LocalOffloadRequest),
    Onboard(LocalOnboardRequest),
}

struct LocalOffloadRequest {
    request_id: String,
    block_ids: Vec<BlockId>,
    token_blocks: Vec<TokenBlock>,
    operation_id: uuid::Uuid,
}

impl LocalOffloadRequest {
    pub fn new(
        request_id: String,
        block_ids: Vec<BlockId>,
        token_blocks: Vec<TokenBlock>,
        operation_id: uuid::Uuid,
    ) -> Self {
        debug_assert!(block_ids.len() == token_blocks.len());
        Self {
            request_id,
            block_ids,
            token_blocks,
            operation_id,
        }
    }
}

struct LocalOnboardRequest {
    request_id: String,
    src_blocks: Box<dyn AnyBlocks>,
    dst_block_ids: Vec<BlockId>,
    operation_id: uuid::Uuid,
}

impl LocalOnboardRequest {
    pub fn new(
        request_id: String,
        src_blocks: Box<dyn AnyBlocks>,
        dst_block_ids: Vec<BlockId>,
        operation_id: uuid::Uuid,
    ) -> Self {
        debug_assert!(src_blocks.len() == dst_block_ids.len());
        Self {
            request_id,
            src_blocks,
            dst_block_ids,
            operation_id,
        }
    }
}

struct LocalTransferEngine {
    block_manager: VllmBlockManager,
    leader: Arc<KvbmLeader>,
    xfer_rx: mpsc::UnboundedReceiver<LocalTransferRequest>,
}

impl LocalTransferEngine {
    pub fn new(
        block_manager: VllmBlockManager,
        leader: Arc<KvbmLeader>,
        xfer_rx: mpsc::UnboundedReceiver<LocalTransferRequest>,
    ) -> Self {
        Self {
            block_manager,
            leader,
            xfer_rx,
        }
    }

    // build an adapted TaskTracker:
    // https://docs.rs/tokio-util/latest/tokio_util/task/task_tracker/struct.TaskTracker.html
    //
    // this should track completions via atomic counters using the dynamo prometheus metrics
    // - critical_tasks: labels - success, failure, cancelled
    //
    // should spawn any task/future that returns either any task that can be converted to a
    // Result<CompletionStatus, String> where CompletionStatus is an enum with Ok and Cancelled.
    // anyhow::Result<()> can be considered non-cancellable and coerced to Ok(CompletionStatus::Ok)
    // tasks allowed to cancel should return a CompletionStatus.
    //
    // This should be a composable unit that we can layer on specialized types of critical tasks
    // with their own sets of custom metrics.
    async fn execute(
        &mut self,
        cancellation_token: CancellationToken,
        drt: DistributedRuntime,
        kvbm_metrics: KvbmMetrics,
    ) -> anyhow::Result<()> {
        let (onboard_tx, mut onboard_rx) = mpsc::unbounded_channel();
        let (offload_tx, mut offload_rx) = mpsc::unbounded_channel();
        let drt_clone = drt.clone();

        // Clone resources needed for tasks
        let block_manager_offload = self.block_manager.clone();
        let leader_offload = Arc::clone(&self.leader);
        let leader_onboard = Arc::clone(&self.leader);

        let kvbm_metrics_onboard = kvbm_metrics.clone();
        let kvbm_metrics_offload = kvbm_metrics.clone();

        let onboard_task = CriticalTaskExecutionHandle::new_with_runtime(
            |cancellation_token_onboard| async move {
                while let Some(req) = onboard_rx.recv().await {
                    if cancellation_token_onboard.is_cancelled() {
                        tracing::debug!("LocalOnboardTask: received cancellation signal");
                        break;
                    }
                    if let Err(e) =
                        process_onboard_request(req, &leader_onboard, kvbm_metrics_onboard.clone())
                            .await
                    {
                        tracing::error!("LocalOnboardTask: error processing request: {:?}", e);
                    }
                }
                Ok(())
            },
            drt.primary_token(),
            "LocalOnboardTask",
            &drt.runtime().primary(),
        )
        .unwrap();
        let offload_task = CriticalTaskExecutionHandle::new_with_runtime(
            |cancellation_token_offload| async move {
                while let Some(req) = offload_rx.recv().await {
                    if cancellation_token_offload.is_cancelled() {
                        tracing::debug!("LocalOffloadTask: received cancellation signal");
                        break;
                    }
                    if let Err(e) = process_offload_request(
                        req,
                        &block_manager_offload,
                        &leader_offload,
                        kvbm_metrics_offload.clone(),
                    )
                    .await
                    {
                        tracing::error!("LocalOffloadTask: error processing request: {:?}", e);
                    }
                }
                Ok(())
            },
            drt_clone.primary_token(),
            "LocalOffloadTask",
            &drt_clone.runtime().primary(),
        )
        .unwrap();

        loop {
            tokio::select! {
                _ = cancellation_token.cancelled() => {
                    tracing::debug!("LocalTransferEngine: received cancellation signal");
                    break;
                }
                req = self.xfer_rx.recv() => {
                    match req {
                        Some(req) => {
                            match req {
                                LocalTransferRequest::Offload(offload_req) => {
                                    if let Err(e) = offload_tx.send(offload_req) {
                                        tracing::error!("LocalTransferEngine: error sending offload request: {:?}", e);
                                    }
                                }
                                LocalTransferRequest::Onboard(onboard_req) => {
                                    if let Err(e) = onboard_tx.send(onboard_req) {
                                        tracing::error!("LocalTransferEngine: error sending onboard request: {:?}", e);
                                    }
                                }
                            }
                        }
                        None => {
                            tracing::debug!("LocalTransferEngine: channel closed");
                            break;
                        }
                    }
                }
            }
        }

        tracing::debug!("LocalTransferEngine: shutting down");

        // drop all tx channels
        drop(onboard_tx);
        drop(offload_tx);

        onboard_task.cancel();
        offload_task.cancel();

        if let Err(e) = onboard_task.join().await {
            tracing::error!("LocalOnboardTask failed: {:?}", e);
        }
        if let Err(e) = offload_task.join().await {
            tracing::error!("LocalOffloadTask failed: {:?}", e);
        }

        tracing::debug!("LocalTransferEngine: shutdown complete");
        Ok(())
    }
}

async fn process_offload_request(
    offload_req: LocalOffloadRequest,
    block_manager: &VllmBlockManager,
    leader: &Arc<KvbmLeader>,
    kvbm_metrics: KvbmMetrics,
) -> anyhow::Result<()> {
    let request_id = offload_req.request_id.clone();
    let operation_id = offload_req.operation_id;

    tracing::debug!(
        "Processing offload request for {} blocks",
        offload_req.block_ids.len()
    );

    // Determine if we should bypass CPU memory (G2) and offload directly from GPU (G1) to Disk (G3)
    let bypass_cpu_mem = should_bypass_cpu_cache();

    if bypass_cpu_mem {
        // Direct G1 -> G3 path (Device to Disk, bypassing Host)
        kvbm_metrics
            .offload_blocks_d2d
            .inc_by(offload_req.block_ids.len() as u64);

        tracing::debug!(
            request_id = %request_id,
            operation_id = %operation_id,
            "offloading directly to disk (bypassing host)"
        );

        process_offload_to_storage(
            offload_req,
            block_manager.disk().unwrap(),
            BlockTransferPool::Disk,
            leader,
            &request_id,
            &operation_id,
            "disk",
        )
        .await?;
    } else {
        // Standard path: G1 -> G2 (Device to Host)
        kvbm_metrics
            .offload_blocks_d2h
            .inc_by(offload_req.block_ids.len() as u64);

        process_offload_to_storage(
            offload_req,
            block_manager.host().unwrap(),
            BlockTransferPool::Host,
            leader,
            &request_id,
            &operation_id,
            "host",
        )
        .await?;
    }

    Ok(())
}

async fn process_offload_to_storage<S, L, M>(
    offload_req: LocalOffloadRequest,
    storage_pool: &dyn BlockPool<S, L, M>,
    transfer_pool: BlockTransferPool,
    leader: &Arc<KvbmLeader>,
    request_id: &str,
    operation_id: &uuid::Uuid,
    storage_name: &str,
) -> anyhow::Result<()>
where
    S: Storage + NixlRegisterableStorage,
    L: LocalityProvider,
    M: BlockMetadata,
{
    // 1. Acquire mutable blocks
    let blocks = storage_pool
        .allocate_blocks(offload_req.block_ids.len())
        .await?;
    let token_blocks = offload_req.token_blocks;

    let allocated_block_ids: Vec<usize> = blocks.iter().map(|b| b.block_id()).collect();
    let block_pairs: Vec<(usize, usize)> = offload_req
        .block_ids
        .into_iter()
        .zip(allocated_block_ids.into_iter())
        .collect();

    tracing::debug!(
        request_id = request_id,
        operation_id = %operation_id,
        "offload to {} - stage 1 complete",
        storage_name
    );

    // 2. Apply token blocks
    let mut blocks_to_register = Vec::new();
    let zipped_blocks = blocks.into_iter().zip(token_blocks.into_iter());

    for (mut mutable_block, token_block) in zipped_blocks {
        mutable_block
            .apply_token_block(token_block.clone())
            .map_err(|e| anyhow::anyhow!("failed to apply token block: {:?}", e))?;

        blocks_to_register.push(mutable_block);
    }
    tracing::debug!(
        request_id = request_id,
        operation_id = %operation_id,
        "offload to {} - stage 2 complete",
        storage_name
    );

    // 3. Issue the offload request using `leader`
    let block_xfer_req = BlockTransferRequest {
        from_pool: BlockTransferPool::Device,
        to_pool: transfer_pool,
        blocks: block_pairs,
        connector_req: Some(LeaderTransferRequest {
            request_id: offload_req.request_id.clone(),
            uuid: offload_req.operation_id,
            requirement: None,
            request_type: RequestType::Scheduled,
        }),
    };
    let notify_receiver = leader.transfer_blocks_request(block_xfer_req).await?;
    tracing::debug!(
        request_id = request_id,
        operation_id = %operation_id,
        "offload to {} - stage 3 complete",
        storage_name
    );

    // 4. Wait for the offload request to complete
    match notify_receiver.await {
        Ok(_) => {
            tracing::debug!(
                "Offloading transfer to {} completed successfully",
                storage_name
            );
        }
        Err(_) => {
            return Err(anyhow::anyhow!(
                "Offloading transfer completion notification failed"
            ));
        }
    }
    tracing::debug!(
        request_id = request_id,
        operation_id = %operation_id,
        "offload to {} - stage 4 complete",
        storage_name
    );

    // 5. Register the mutable blocks
    let immutable_blocks = storage_pool.register_blocks(blocks_to_register).await?;

    tracing::debug!(
        request_id = request_id,
        operation_id = %operation_id,
        "registered {} blocks to {}",
        immutable_blocks.len(),
        storage_name
    );

    Ok(())
}

async fn process_onboard_request(
    onboard_req: LocalOnboardRequest,
    leader: &Arc<KvbmLeader>,
    kvbm_metrics: KvbmMetrics,
) -> anyhow::Result<()> {
    if onboard_req.src_blocks.storage_pool() == BlockTransferPool::Host {
        kvbm_metrics
            .onboard_blocks_h2d
            .inc_by(onboard_req.src_blocks.len() as u64);
    } else if onboard_req.src_blocks.storage_pool() == BlockTransferPool::Disk {
        kvbm_metrics
            .onboard_blocks_d2d
            .inc_by(onboard_req.src_blocks.len() as u64);
    }

    let request_id = &onboard_req.request_id;
    let operation_id = &onboard_req.operation_id;

    // extract source block ids
    let src_block_ids = onboard_req.src_blocks.block_ids();

    // create block pairs
    let block_pairs = src_block_ids
        .iter()
        .zip(onboard_req.dst_block_ids.iter())
        .map(|(src, dst)| (*src, *dst))
        .collect::<Vec<_>>();

    // create transfer request
    let block_xfer_req = BlockTransferRequest {
        from_pool: onboard_req.src_blocks.storage_pool(),
        to_pool: BlockTransferPool::Device,
        blocks: block_pairs,
        connector_req: Some(LeaderTransferRequest {
            request_id: request_id.clone(),
            uuid: *operation_id,
            requirement: None,
            request_type: RequestType::Immediate,
        }),
    };

    let notify_receiver = leader.transfer_blocks_request(block_xfer_req).await?;

    match notify_receiver.await {
        Ok(_) => {
            tracing::debug!("Onboarding transfer completed successfully");
        }
        Err(_) => {
            return Err(anyhow::anyhow!(
                "Onboarding transfer completion notification failed"
            ));
        }
    }

    Ok(())
}

// todo move to core lib
pub trait AnyBlocks: Send {
    fn len(&self) -> usize;
    fn storage_pool(&self) -> BlockTransferPool;
    fn block_ids(&self) -> Vec<BlockId>;
}

struct AnyImmutableBlocks<S: Storage, L: LocalityProvider, M: BlockMetadata> {
    blocks: Vec<ImmutableBlock<S, L, M>>,
    storage_pool: BlockTransferPool,
}

impl<L: LocalityProvider, M: BlockMetadata> AnyImmutableBlocks<PinnedStorage, L, M> {
    pub fn new(blocks: Vec<ImmutableBlock<PinnedStorage, L, M>>) -> Self {
        Self {
            blocks,
            storage_pool: BlockTransferPool::Host,
        }
    }
}

impl<L: LocalityProvider, M: BlockMetadata> AnyImmutableBlocks<DiskStorage, L, M> {
    pub fn new(blocks: Vec<ImmutableBlock<DiskStorage, L, M>>) -> Self {
        Self {
            blocks,
            storage_pool: BlockTransferPool::Disk,
        }
    }
}

impl<S: Storage, L: LocalityProvider, M: BlockMetadata> AnyImmutableBlocks<S, L, M> {
    pub fn storage_pool(&self) -> BlockTransferPool {
        self.storage_pool
    }

    pub fn block_ids(&self) -> Vec<BlockId> {
        self.blocks.iter().map(|b| b.block_id()).collect()
    }

    fn len(&self) -> usize {
        self.blocks.len()
    }
}

impl<S: Storage, L: LocalityProvider, M: BlockMetadata> AnyBlocks for AnyImmutableBlocks<S, L, M> {
    fn len(&self) -> usize {
        self.len()
    }

    fn storage_pool(&self) -> BlockTransferPool {
        self.storage_pool()
    }

    fn block_ids(&self) -> Vec<BlockId> {
        self.block_ids()
    }
}
