//! Device abstraction replacing `candle_core::Device`.
//!
//! Provides `MlDevice` enum with CPU and CUDA variants.
//! The CUDA variant holds `Arc<CudaDevice>` and `Arc<CudaStream>` for
//! direct cudarc interop.

use std::sync::Arc;

use crate::MLError;

#[cfg(feature = "cuda")]
use cudarc::driver::{CudaContext, CudaStream};

/// Device abstraction for the ML pipeline.
///
/// Replaces `candle_core::Device`. Only two variants: CPU (for checkpoint
/// serialization and preprocessing) and CUDA (for training and inference).
#[derive(Clone)]
pub enum MlDevice {
    /// Host CPU — used for checkpoint I/O and lightweight preprocessing.
    Cpu,
    /// CUDA GPU with context + stream handles.
    #[cfg(feature = "cuda")]
    Cuda {
        context: Arc<CudaContext>,
        stream: Arc<CudaStream>,
    },
}

impl std::fmt::Debug for MlDevice {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            MlDevice::Cpu => write!(f, "MlDevice::Cpu"),
            #[cfg(feature = "cuda")]
            MlDevice::Cuda { .. } => write!(f, "MlDevice::Cuda"),
        }
    }
}

impl MlDevice {
    /// Create a CUDA device with the given ordinal (0-indexed).
    ///
    /// Initializes a `CudaDevice` and forks a non-default stream for all
    /// subsequent operations.
    #[cfg(feature = "cuda")]
    pub fn cuda(ordinal: usize) -> Result<Self, MLError> {
        let context = CudaContext::new(ordinal).map_err(|e| {
            MLError::DeviceError(format!("Failed to open CUDA device {ordinal}: {e}"))
        })?;
        // Check for stale CUDA errors from previous context users in the same process.
        // CudaContext::new reuses the primary context (cuDevicePrimaryCtxRetain),
        // so deferred errors from a previous test's Drop persist.
        if let Err(e) = context.check_err() {
            tracing::warn!("Stale CUDA error on device {ordinal} (may indicate prior kernel failure): {e}");
        }
        let stream = context.new_stream().map_err(|e| {
            MLError::DeviceError(format!("Failed to create CUDA stream on device {ordinal}: {e}"))
        })?;
        Ok(MlDevice::Cuda {
            context,
            stream,
        })
    }

    /// Try to create a CUDA device, falling back to CPU if unavailable.
    ///
    /// This is the recommended way to get a device when you want GPU
    /// acceleration but can tolerate CPU fallback.
    #[cfg(feature = "cuda")]
    pub fn cuda_if_available(ordinal: usize) -> Self {
        match Self::cuda(ordinal) {
            Ok(dev) => dev,
            Err(e) => {
                tracing::warn!("CUDA device {ordinal} unavailable ({e}), falling back to CPU");
                MlDevice::Cpu
            }
        }
    }

    /// CPU-only fallback for `cuda_if_available` when CUDA feature is disabled.
    #[cfg(not(feature = "cuda"))]
    pub fn cuda_if_available(_ordinal: usize) -> Self {
        MlDevice::Cpu
    }

    /// Alias for `cuda()` — some callers use `new_cuda()`.
    #[cfg(feature = "cuda")]
    pub fn new_cuda(ordinal: usize) -> Result<Self, MLError> {
        Self::cuda(ordinal)
    }

    /// Identity accessor for compatibility — returns a reference to self.
    ///
    /// Some callers migrated from Candle use `obj.device()` to get the device
    /// handle. For `MlDevice` this is a no-op identity.
    pub fn device(&self) -> &Self {
        self
    }

    /// Returns `true` if this is a CUDA device.
    pub fn is_cuda(&self) -> bool {
        match self {
            MlDevice::Cpu => false,
            #[cfg(feature = "cuda")]
            MlDevice::Cuda { .. } => true,
        }
    }

    /// Returns `true` if this is the CPU device.
    pub fn is_cpu(&self) -> bool {
        matches!(self, MlDevice::Cpu)
    }

    /// Get the CUDA stream, or error if this is a CPU device.
    #[cfg(feature = "cuda")]
    pub fn cuda_stream(&self) -> Result<&Arc<CudaStream>, MLError> {
        match self {
            MlDevice::Cuda { stream, .. } => Ok(stream),
            MlDevice::Cpu => Err(MLError::DeviceError(
                "cuda_stream() called on CPU device".to_owned(),
            )),
        }
    }

    /// Get the CudaContext handle, or error if this is a CPU device.
    #[cfg(feature = "cuda")]
    pub fn cuda_context(&self) -> Result<&Arc<CudaContext>, MLError> {
        match self {
            MlDevice::Cuda { context, .. } => Ok(context),
            MlDevice::Cpu => Err(MLError::DeviceError(
                "cuda_context() called on CPU device".to_owned(),
            )),
        }
    }
}

impl std::fmt::Display for MlDevice {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            MlDevice::Cpu => write!(f, "cpu"),
            #[cfg(feature = "cuda")]
            MlDevice::Cuda { .. } => write!(f, "cuda"),
        }
    }
}