//! Pipeline Parallelism for Large Model Training
//!
//! This module implements pipeline parallelism, which splits a model into stages

#![allow(unused_variables)] // Distributed parallelism implementation with reserved parameters
//! across multiple devices and processes microbatches in a pipelined manner.

use super::model_parallel::{
    ModelParallelContext, PipelineOp, PipelineSchedule, PipelineScheduleType,
};
use crate::errors::{runtime_error, Result};
use crate::Tensor;
use parking_lot::{Mutex, RwLock};
use std::collections::{HashMap, VecDeque};
use std::sync::Arc;

/// Layer wrapper for pipeline stages
pub trait PipelineLayer: Send + Sync {
    fn forward(&self, input: &Tensor) -> Result<Tensor>;
    fn backward(&mut self, grad_output: &Tensor) -> Result<Tensor>;
}

/// A single stage in the pipeline
pub struct PipelineStage {
    /// Stage ID (0-indexed)
    pub stage_id: usize,
    /// Layers in this stage
    pub layers: Vec<Box<dyn PipelineLayer>>,
    /// Device ID for this stage
    pub device_id: usize,
    /// Whether this stage requires gradient computation
    pub requires_grad: bool,
}

impl PipelineStage {
    pub fn new(stage_id: usize, device_id: usize) -> Self {
        Self {
            stage_id,
            layers: Vec::new(),
            device_id,
            requires_grad: true,
        }
    }

    pub fn add_layer(&mut self, layer: Box<dyn PipelineLayer>) {
        self.layers.push(layer);
    }

    /// Forward pass through all layers in the stage
    pub fn forward(&self, input: &Tensor) -> Result<Tensor> {
        let mut output = input.clone();
        for layer in &self.layers {
            output = layer.forward(&output)?;
        }
        Ok(output)
    }

    /// Backward pass through all layers in the stage
    pub fn backward(&mut self, grad_output: &Tensor) -> Result<Tensor> {
        let mut grad = grad_output.clone();
        // Process layers in reverse order
        for layer in self.layers.iter_mut().rev() {
            grad = layer.backward(&grad)?;
        }
        Ok(grad)
    }
}

/// Model split into pipeline stages
pub struct PipelineModel {
    /// All pipeline stages
    pub stages: Vec<PipelineStage>,
    /// Model parallel context
    pub mp_context: Arc<ModelParallelContext>,
    /// Stage assignment for this rank
    pub local_stage_id: Option<usize>,
}

impl PipelineModel {
    pub fn new(mp_context: Arc<ModelParallelContext>) -> Self {
        Self {
            stages: Vec::new(),
            mp_context,
            local_stage_id: None,
        }
    }

    /// Add a stage to the pipeline
    pub fn add_stage(&mut self, stage: PipelineStage) {
        if stage.device_id == self.mp_context.rank() {
            self.local_stage_id = Some(stage.stage_id);
        }
        self.stages.push(stage);
    }

    /// Get the local stage for this rank
    pub fn local_stage(&self) -> Result<&PipelineStage> {
        let stage_id =
            self.local_stage_id.ok_or_else(|| runtime_error("No local stage assigned"))?;
        self.stages.get(stage_id).ok_or_else(|| runtime_error("Invalid stage ID"))
    }

    /// Get mutable local stage
    pub fn local_stage_mut(&mut self) -> Result<&mut PipelineStage> {
        let stage_id =
            self.local_stage_id.ok_or_else(|| runtime_error("No local stage assigned"))?;
        self.stages.get_mut(stage_id).ok_or_else(|| runtime_error("Invalid stage ID"))
    }

    /// Get total number of stages
    pub fn num_stages(&self) -> usize {
        self.stages.len()
    }
}

/// Microbatch data structure
#[derive(Clone)]
pub struct Microbatch {
    /// Microbatch ID
    pub id: usize,
    /// Input tensor
    pub input: Option<Tensor>,
    /// Output tensor (activations)
    pub output: Option<Tensor>,
    /// Gradient w.r.t output
    pub grad_output: Option<Tensor>,
    /// Gradient w.r.t input
    pub grad_input: Option<Tensor>,
    /// Labels for loss computation (only for last stage)
    pub labels: Option<Tensor>,
}

impl Microbatch {
    pub fn new(id: usize) -> Self {
        Self {
            id,
            input: None,
            output: None,
            grad_output: None,
            grad_input: None,
            labels: None,
        }
    }
}

/// Manages microbatches across pipeline stages
pub struct MicrobatchManager {
    /// All microbatches
    microbatches: Vec<Microbatch>,
    /// Activation checkpointing enabled
    checkpoint_activations: bool,
    /// Queue of pending forward passes
    forward_queue: VecDeque<usize>,
    /// Queue of pending backward passes
    backward_queue: VecDeque<usize>,
}

impl MicrobatchManager {
    pub fn new(num_microbatches: usize, checkpoint_activations: bool) -> Self {
        let microbatches = (0..num_microbatches).map(Microbatch::new).collect();

        Self {
            microbatches,
            checkpoint_activations,
            forward_queue: VecDeque::new(),
            backward_queue: VecDeque::new(),
        }
    }

    /// Get microbatch by ID
    pub fn get(&self, id: usize) -> Result<&Microbatch> {
        self.microbatches
            .get(id)
            .ok_or_else(|| runtime_error(format!("Invalid microbatch ID: {}", id)))
    }

    /// Get mutable microbatch
    pub fn get_mut(&mut self, id: usize) -> Result<&mut Microbatch> {
        self.microbatches
            .get_mut(id)
            .ok_or_else(|| runtime_error(format!("Invalid microbatch ID: {}", id)))
    }

    /// Add microbatch to forward queue
    pub fn enqueue_forward(&mut self, mb_id: usize) {
        self.forward_queue.push_back(mb_id);
    }

    /// Add microbatch to backward queue
    pub fn enqueue_backward(&mut self, mb_id: usize) {
        self.backward_queue.push_back(mb_id);
    }

    /// Get next forward microbatch
    pub fn dequeue_forward(&mut self) -> Option<usize> {
        self.forward_queue.pop_front()
    }

    /// Get next backward microbatch
    pub fn dequeue_backward(&mut self) -> Option<usize> {
        self.backward_queue.pop_front()
    }

    /// Clear activation if checkpointing is enabled
    pub fn maybe_clear_activation(&mut self, mb_id: usize) -> Result<()> {
        if self.checkpoint_activations {
            let mb = self.get_mut(mb_id)?;
            mb.output = None; // Clear to save memory
        }
        Ok(())
    }

    /// Recompute activation if needed
    pub fn maybe_recompute_activation(
        &mut self,
        mb_id: usize,
        stage: &PipelineStage,
    ) -> Result<()> {
        let should_recompute = self.checkpoint_activations;
        let mb = self.get_mut(mb_id)?;
        if should_recompute && mb.output.is_none() {
            // Recompute forward pass
            if let Some(input) = &mb.input {
                mb.output = Some(stage.forward(input)?);
            }
        }
        Ok(())
    }
}

/// Pipeline executor that manages the execution schedule
pub struct PipelineExecutor {
    /// Pipeline model
    model: Arc<RwLock<PipelineModel>>,
    /// Pipeline schedule
    schedule: PipelineSchedule,
    /// Microbatch manager
    mb_manager: Arc<Mutex<MicrobatchManager>>,
    /// Communication buffers
    #[allow(dead_code)]
    send_buffers: HashMap<usize, Tensor>,
    _recv_buffers: HashMap<usize, Tensor>,
}

impl PipelineExecutor {
    pub fn new(
        model: Arc<RwLock<PipelineModel>>,
        num_microbatches: usize,
        checkpoint_activations: bool,
    ) -> Result<Self> {
        let num_stages = {
            let model_read = model.read();
            model_read.num_stages()
        };

        let schedule = PipelineSchedule::new(
            num_stages,
            num_microbatches,
            PipelineScheduleType::OneForwardOneBackward,
        );

        let mb_manager = Arc::new(Mutex::new(MicrobatchManager::new(
            num_microbatches,
            checkpoint_activations,
        )));

        Ok(Self {
            model,
            schedule,
            mb_manager,
            send_buffers: HashMap::new(),
            _recv_buffers: HashMap::new(),
        })
    }

    /// Execute one training step
    pub fn execute_step(&mut self, inputs: Vec<Tensor>, labels: Vec<Tensor>) -> Result<f32> {
        let num_inputs = inputs.len();

        // Split inputs into microbatches
        self.prepare_microbatches(inputs, labels)?;

        // Get schedule for local stage
        let stage_id = {
            let model = self.model.read();
            model.local_stage_id.ok_or_else(|| runtime_error("No local stage"))?
        };

        let ops = self.schedule.get_stage_schedule(stage_id);

        // Execute operations according to schedule
        let mut total_loss = 0.0;
        for op in ops {
            match op {
                PipelineOp::Forward { microbatch_id } => {
                    self.execute_forward(microbatch_id)?;
                },
                PipelineOp::Backward { microbatch_id } => {
                    let loss = self.execute_backward(microbatch_id)?;
                    total_loss += loss;
                },
                PipelineOp::SendActivation { to_stage } => {
                    self.send_activation(to_stage)?;
                },
                PipelineOp::RecvActivation { from_stage } => {
                    self.recv_activation(from_stage)?;
                },
                PipelineOp::SendGradient { to_stage } => {
                    self.send_gradient(to_stage)?;
                },
                PipelineOp::RecvGradient { from_stage } => {
                    self.recv_gradient(from_stage)?;
                },
            }
        }

        Ok(total_loss / num_inputs as f32)
    }

    /// Prepare microbatches from full batch
    fn prepare_microbatches(&mut self, inputs: Vec<Tensor>, labels: Vec<Tensor>) -> Result<()> {
        let mut mb_manager = self.mb_manager.lock();

        for (i, (input, label)) in inputs.into_iter().zip(labels).enumerate() {
            let mb = mb_manager.get_mut(i)?;
            mb.input = Some(input);
            mb.labels = Some(label);
            mb_manager.enqueue_forward(i);
        }

        Ok(())
    }

    /// Execute forward pass for a microbatch
    fn execute_forward(&mut self, mb_id: usize) -> Result<()> {
        let mut model = self.model.write();
        let stage = model.local_stage_mut()?;

        let mut mb_manager = self.mb_manager.lock();
        let mb = mb_manager.get_mut(mb_id)?;

        // Get input (from previous stage or initial input)
        let input = if stage.stage_id == 0 {
            mb.input.as_ref().ok_or_else(|| runtime_error("Missing input"))?
        } else {
            // Would receive from previous stage
            mb.output.as_ref().ok_or_else(|| runtime_error("Missing activation"))?
        };

        // Forward pass
        let output = stage.forward(input)?;
        mb.output = Some(output);

        // Maybe clear activation for checkpointing
        mb_manager.maybe_clear_activation(mb_id)?;

        Ok(())
    }

    /// Execute backward pass for a microbatch
    fn execute_backward(&mut self, mb_id: usize) -> Result<f32> {
        let (is_last_stage, stage_id) = {
            let model = self.model.read();
            let stage = model.local_stage()?;
            (stage.stage_id == model.num_stages() - 1, stage.stage_id)
        };

        let mut model = self.model.write();
        let stage = model.local_stage_mut()?;

        let mut mb_manager = self.mb_manager.lock();

        // Recompute activation if needed
        mb_manager.maybe_recompute_activation(mb_id, stage)?;

        let mb = mb_manager.get_mut(mb_id)?;

        // Compute loss and gradient for last stage
        let loss = if is_last_stage {
            // Compute loss (simplified - would use actual loss function)
            1.0
        } else {
            0.0
        };

        // Get gradient w.r.t output
        let grad_output = if is_last_stage {
            // Compute gradient from loss
            mb.output.as_ref().ok_or_else(|| runtime_error("Missing output"))?.clone()
        } else {
            // Would receive from next stage
            mb.grad_output
                .as_ref()
                .ok_or_else(|| runtime_error("Missing grad_output"))?
                .clone()
        };

        // Backward pass
        let grad_input = stage.backward(&grad_output)?;
        mb.grad_input = Some(grad_input);

        Ok(loss)
    }

    /// Send activation to next stage
    fn send_activation(&mut self, to_stage: usize) -> Result<()> {
        // In practice, would use MPI/NCCL for communication
        Ok(())
    }

    /// Receive activation from previous stage
    fn recv_activation(&mut self, from_stage: usize) -> Result<()> {
        // In practice, would use MPI/NCCL for communication
        Ok(())
    }

    /// Send gradient to previous stage
    fn send_gradient(&mut self, to_stage: usize) -> Result<()> {
        // In practice, would use MPI/NCCL for communication
        Ok(())
    }

    /// Receive gradient from next stage
    fn recv_gradient(&mut self, from_stage: usize) -> Result<()> {
        // In practice, would use MPI/NCCL for communication
        Ok(())
    }
}

/// Optimizer for pipeline parallel training
pub struct PipelineOptimizer {
    /// Learning rate
    #[allow(dead_code)]
    lr: f32,
    /// Weight decay
    _weight_decay: f32,
    /// Gradient accumulation steps
    accumulation_steps: usize,
    /// Current accumulation step
    current_step: usize,
    /// Accumulated gradients
    accumulated_grads: HashMap<String, Tensor>,
}

impl PipelineOptimizer {
    pub fn new(lr: f32, weight_decay: f32, accumulation_steps: usize) -> Self {
        Self {
            lr,
            _weight_decay: weight_decay,
            accumulation_steps,
            current_step: 0,
            accumulated_grads: HashMap::new(),
        }
    }

    /// Accumulate gradients from microbatch
    pub fn accumulate_gradients(&mut self, grads: HashMap<String, Tensor>) -> Result<()> {
        for (name, grad) in grads {
            if let Some(acc_grad) = self.accumulated_grads.get_mut(&name) {
                *acc_grad = acc_grad.add(&grad)?;
            } else {
                self.accumulated_grads.insert(name, grad);
            }
        }

        self.current_step += 1;
        Ok(())
    }

    /// Apply gradients if accumulation is complete
    pub fn step(&mut self, model: &mut PipelineModel) -> Result<bool> {
        if self.current_step < self.accumulation_steps {
            return Ok(false);
        }

        // Apply accumulated gradients
        let scale = 1.0 / self.accumulation_steps as f32;

        // In practice, would update model parameters
        // For now, just clear accumulated gradients
        self.accumulated_grads.clear();
        self.current_step = 0;

        Ok(true)
    }
}

/// Builder for creating pipeline models
pub struct PipelineModelBuilder {
    mp_context: Arc<ModelParallelContext>,
    stages: Vec<PipelineStage>,
    layers_per_stage: Option<usize>,
}

impl PipelineModelBuilder {
    pub fn new(mp_context: Arc<ModelParallelContext>) -> Self {
        Self {
            mp_context,
            stages: Vec::new(),
            layers_per_stage: None,
        }
    }

    /// Set number of layers per stage (for automatic partitioning)
    pub fn layers_per_stage(mut self, layers_per_stage: usize) -> Self {
        self.layers_per_stage = Some(layers_per_stage);
        self
    }

    /// Add a pre-configured stage
    pub fn add_stage(mut self, stage: PipelineStage) -> Self {
        self.stages.push(stage);
        self
    }

    /// Build the pipeline model
    pub fn build(self) -> Result<PipelineModel> {
        let mut model = PipelineModel::new(self.mp_context);

        for stage in self.stages {
            model.add_stage(stage);
        }

        Ok(model)
    }
}

#[cfg(test)]
mod tests {
    use super::super::model_parallel::{
        CommunicationBackend, ModelParallelConfig, ModelParallelStrategy,
    };
    use super::*;

    #[test]
    fn test_pipeline_stage() {
        let stage = PipelineStage::new(0, 0);
        assert_eq!(stage.stage_id, 0);
        assert_eq!(stage.device_id, 0);
        assert!(stage.requires_grad);
    }

    #[test]
    fn test_microbatch_manager() {
        let mut manager = MicrobatchManager::new(4, true);

        manager.enqueue_forward(0);
        manager.enqueue_forward(1);

        assert_eq!(manager.dequeue_forward(), Some(0));
        assert_eq!(manager.dequeue_forward(), Some(1));
        assert_eq!(manager.dequeue_forward(), None);
    }

    #[test]
    fn test_pipeline_model_builder() {
        let config = ModelParallelConfig {
            num_devices: 4,
            device_ids: vec![0, 1, 2, 3],
            strategy: ModelParallelStrategy::Pipeline,
            comm_backend: CommunicationBackend::Custom,
            ..Default::default()
        };

        let mp_context = Arc::new(ModelParallelContext::new(config).unwrap());

        let model = PipelineModelBuilder::new(mp_context)
            .add_stage(PipelineStage::new(0, 0))
            .add_stage(PipelineStage::new(1, 1))
            .build()
            .unwrap();

        assert_eq!(model.num_stages(), 2);
    }
}
