//! Burn backend integration for the Aphelion framework.
//!
//! This module provides a placeholder implementation for the [Burn](https://burn.dev/) deep
//! learning framework. Burn is a Rust-native deep learning library that supports multiple
//! backends including CPU (NdArray), CUDA, Metal, Vulkan, and WebGPU.
//!
//! # Feature Flag
//!
//! This module is only available when the `burn` feature is enabled:
//!
//! ```toml
//! [dependencies]
//! aphelion-core = { version = "1.1", features = ["burn"] }
//! ```
//!
//! # Current Status
//!
//! **This is a placeholder implementation.** The actual Burn dependency is not yet integrated.
//! This module provides the API structure and type definitions that will be used when Burn
//! integration is completed. The implementation:
//!
//! - Defines device types (`BurnDevice`) matching Burn's supported backends
//! - Provides configuration structures for backend setup
//! - Implements the `Backend` trait with stub methods
//! - Reports CPU as available, GPU backends as unavailable until real integration
//!
//! # Future Integration
//!
//! When the Burn dependency is enabled, this backend will provide:
//!
//! - Automatic device detection and selection
//! - Hardware-accelerated tensor operations
//! - Autodiff support for training
//! - Model serialization compatible with Burn's format
//! - Integration with Burn's module system
//!
//! # Example (Future API)
//!
//! ```ignore
//! use aphelion_core::burn_backend::{BurnBackend, BurnBackendConfig, BurnDevice};
//! use aphelion_core::backend::Backend;
//!
//! // Create a GPU-accelerated backend
//! let config = BurnBackendConfig {
//!     device: BurnDevice::Cuda(0),
//!     allow_tf32: true,
//! };
//! let mut backend = BurnBackend::new(config);
//!
//! // Initialize the backend
//! backend.initialize().expect("Failed to initialize Burn backend");
//!
//! assert_eq!(backend.name(), "burn");
//! assert!(backend.is_available());
//!
//! // Clean up
//! backend.shutdown().expect("Failed to shutdown");
//! ```

use crate::backend::{Backend, DeviceCapabilities};
use crate::error::AphelionResult;
use std::sync::atomic::{AtomicBool, Ordering};
use std::sync::Arc;

/// Supported Burn device selectors.
///
/// `BurnDevice` specifies the computational device for Burn operations.
/// This enum maps to Burn's backend types and will be used for device
/// selection once the Burn dependency is integrated.
///
/// # Variants
///
/// - `Cpu` - CPU execution using NdArray backend (always available)
/// - `Cuda(u32)` - NVIDIA CUDA GPU with device index
/// - `Metal(u32)` - Apple Metal GPU with device index (macOS/iOS only)
/// - `Vulkan(u32)` - Vulkan GPU with device index (cross-platform)
///
/// # Examples
///
/// ```ignore
/// use aphelion_core::burn_backend::BurnDevice;
///
/// let cpu = BurnDevice::Cpu;
/// let gpu = BurnDevice::Cuda(0);
///
/// assert_eq!(cpu.as_label(), "cpu");
/// assert_eq!(gpu.as_label(), "cuda:0");
/// ```
#[derive(Debug, Clone, PartialEq, Eq, Default)]
pub enum BurnDevice {
    /// CPU execution using NdArray backend (always available)
    #[default]
    Cpu,
    /// NVIDIA CUDA GPU execution with device index
    Cuda(u32),
    /// Apple Metal GPU execution with device index (macOS/iOS only)
    Metal(u32),
    /// Vulkan GPU execution with device index (cross-platform)
    Vulkan(u32),
}

impl BurnDevice {
    /// Returns a human-readable label for this device.
    ///
    /// # Examples
    ///
    /// ```ignore
    /// use aphelion_core::burn_backend::BurnDevice;
    ///
    /// assert_eq!(BurnDevice::Cpu.as_label(), "cpu");
    /// assert_eq!(BurnDevice::Cuda(0).as_label(), "cuda:0");
    /// assert_eq!(BurnDevice::Metal(1).as_label(), "metal:1");
    /// assert_eq!(BurnDevice::Vulkan(0).as_label(), "vulkan:0");
    /// ```
    pub fn as_label(&self) -> String {
        match self {
            BurnDevice::Cpu => "cpu".to_string(),
            BurnDevice::Cuda(id) => format!("cuda:{}", id),
            BurnDevice::Metal(id) => format!("metal:{}", id),
            BurnDevice::Vulkan(id) => format!("vulkan:{}", id),
        }
    }

    /// Returns whether this is a CPU device.
    pub fn is_cpu(&self) -> bool {
        matches!(self, BurnDevice::Cpu)
    }

    /// Returns whether this is a GPU device.
    pub fn is_gpu(&self) -> bool {
        !self.is_cpu()
    }
}

/// Configuration for the Burn backend.
///
/// `BurnBackendConfig` contains all settings needed to initialize a Burn backend,
/// including device selection and compute precision options.
///
/// # Examples
///
/// ```ignore
/// use aphelion_core::burn_backend::{BurnBackendConfig, BurnDevice};
///
/// // Default configuration (CPU)
/// let default_config = BurnBackendConfig::default();
/// assert!(default_config.device.is_cpu());
///
/// // GPU configuration with TF32 enabled
/// let gpu_config = BurnBackendConfig {
///     device: BurnDevice::Cuda(0),
///     allow_tf32: true,
/// };
/// ```
#[derive(Debug, Clone)]
pub struct BurnBackendConfig {
    /// The computational device to use
    pub device: BurnDevice,
    /// Allow TensorFloat-32 (TF32) for faster but slightly less precise computation
    /// on NVIDIA Ampere+ GPUs
    pub allow_tf32: bool,
}

impl Default for BurnBackendConfig {
    fn default() -> Self {
        Self {
            device: BurnDevice::Cpu,
            allow_tf32: false,
        }
    }
}

impl BurnBackendConfig {
    /// Creates a new configuration with the specified device.
    ///
    /// # Arguments
    ///
    /// * `device` - The computational device to use
    ///
    /// # Examples
    ///
    /// ```ignore
    /// use aphelion_core::burn_backend::{BurnBackendConfig, BurnDevice};
    ///
    /// let config = BurnBackendConfig::new(BurnDevice::Cuda(0));
    /// assert!(!config.allow_tf32);
    /// ```
    pub fn new(device: BurnDevice) -> Self {
        Self {
            device,
            allow_tf32: false,
        }
    }

    /// Creates a CPU configuration.
    ///
    /// # Examples
    ///
    /// ```ignore
    /// use aphelion_core::burn_backend::BurnBackendConfig;
    ///
    /// let config = BurnBackendConfig::cpu();
    /// assert!(config.device.is_cpu());
    /// ```
    pub fn cpu() -> Self {
        Self::new(BurnDevice::Cpu)
    }

    /// Creates a CUDA GPU configuration.
    ///
    /// # Arguments
    ///
    /// * `device_id` - CUDA device index (0 for first GPU)
    ///
    /// # Examples
    ///
    /// ```ignore
    /// use aphelion_core::burn_backend::BurnBackendConfig;
    ///
    /// let config = BurnBackendConfig::cuda(0);
    /// assert!(config.device.is_gpu());
    /// ```
    pub fn cuda(device_id: u32) -> Self {
        Self::new(BurnDevice::Cuda(device_id))
    }

    /// Enables or disables TF32 mode.
    ///
    /// # Arguments
    ///
    /// * `allow` - Whether to allow TF32 computation
    ///
    /// # Examples
    ///
    /// ```ignore
    /// use aphelion_core::burn_backend::BurnBackendConfig;
    ///
    /// let config = BurnBackendConfig::cuda(0).with_tf32(true);
    /// assert!(config.allow_tf32);
    /// ```
    pub fn with_tf32(mut self, allow: bool) -> Self {
        self.allow_tf32 = allow;
        self
    }
}

/// Burn backend implementation for the Aphelion framework.
///
/// `BurnBackend` provides integration with the Burn deep learning framework.
/// This is currently a placeholder implementation that will be connected to
/// the actual Burn library when the dependency is enabled.
///
/// # Thread Safety
///
/// `BurnBackend` is thread-safe and can be shared across threads. The internal
/// state uses atomic operations for the initialization flag.
///
/// # Examples
///
/// ```ignore
/// use aphelion_core::burn_backend::{BurnBackend, BurnBackendConfig, BurnDevice};
/// use aphelion_core::backend::Backend;
///
/// let backend = BurnBackend::new(BurnBackendConfig::default());
/// assert_eq!(backend.name(), "burn");
/// assert_eq!(backend.device(), "cpu");
/// ```
#[derive(Debug)]
pub struct BurnBackend {
    config: BurnBackendConfig,
    initialized: Arc<AtomicBool>,
}

impl Clone for BurnBackend {
    fn clone(&self) -> Self {
        Self {
            config: self.config.clone(),
            initialized: Arc::new(AtomicBool::new(self.initialized.load(Ordering::SeqCst))),
        }
    }
}

impl BurnBackend {
    /// Creates a new Burn backend with the specified configuration.
    ///
    /// # Arguments
    ///
    /// * `config` - Backend configuration
    ///
    /// # Examples
    ///
    /// ```ignore
    /// use aphelion_core::burn_backend::{BurnBackend, BurnBackendConfig};
    ///
    /// let backend = BurnBackend::new(BurnBackendConfig::default());
    /// ```
    pub fn new(config: BurnBackendConfig) -> Self {
        Self {
            config,
            initialized: Arc::new(AtomicBool::new(false)),
        }
    }

    /// Creates a CPU backend with default configuration.
    ///
    /// # Examples
    ///
    /// ```ignore
    /// use aphelion_core::burn_backend::BurnBackend;
    /// use aphelion_core::backend::Backend;
    ///
    /// let backend = BurnBackend::cpu();
    /// assert_eq!(backend.device(), "cpu");
    /// ```
    pub fn cpu() -> Self {
        Self::new(BurnBackendConfig::cpu())
    }

    /// Creates a CUDA GPU backend.
    ///
    /// # Arguments
    ///
    /// * `device_id` - CUDA device index (0 for first GPU)
    ///
    /// # Examples
    ///
    /// ```ignore
    /// use aphelion_core::burn_backend::BurnBackend;
    /// use aphelion_core::backend::Backend;
    ///
    /// let backend = BurnBackend::cuda(0);
    /// assert_eq!(backend.device(), "cuda");
    /// ```
    pub fn cuda(device_id: u32) -> Self {
        Self::new(BurnBackendConfig::cuda(device_id))
    }

    /// Returns the backend configuration.
    ///
    /// # Examples
    ///
    /// ```ignore
    /// use aphelion_core::burn_backend::{BurnBackend, BurnBackendConfig, BurnDevice};
    ///
    /// let backend = BurnBackend::cpu();
    /// assert!(backend.config().device.is_cpu());
    /// ```
    pub fn config(&self) -> &BurnBackendConfig {
        &self.config
    }

    /// Returns whether the backend has been initialized.
    ///
    /// # Examples
    ///
    /// ```ignore
    /// use aphelion_core::burn_backend::BurnBackend;
    ///
    /// let backend = BurnBackend::cpu();
    /// assert!(!backend.is_initialized());
    /// ```
    pub fn is_initialized(&self) -> bool {
        self.initialized.load(Ordering::SeqCst)
    }

    /// Checks if the configured device is available on this system.
    ///
    /// # Note
    ///
    /// In the placeholder implementation:
    /// - CPU is always available
    /// - CUDA reports unavailable (no actual CUDA detection yet)
    /// - Metal reports available only on macOS
    /// - Vulkan reports unavailable (no actual Vulkan detection yet)
    fn check_device_availability(&self) -> bool {
        match &self.config.device {
            BurnDevice::Cpu => true,
            BurnDevice::Cuda(_) => {
                // In actual implementation, this would check CUDA availability
                // via Burn's device detection. For now, report as unavailable.
                false
            }
            BurnDevice::Metal(_) => {
                // Metal is only available on macOS/iOS
                cfg!(target_os = "macos")
            }
            BurnDevice::Vulkan(_) => {
                // In actual implementation, this would check Vulkan availability.
                // For now, report as unavailable.
                false
            }
        }
    }
}

impl Default for BurnBackend {
    fn default() -> Self {
        Self::cpu()
    }
}

impl Backend for BurnBackend {
    fn name(&self) -> &str {
        "burn"
    }

    fn device(&self) -> &str {
        match &self.config.device {
            BurnDevice::Cpu => "cpu",
            BurnDevice::Cuda(_) => "cuda",
            BurnDevice::Metal(_) => "metal",
            BurnDevice::Vulkan(_) => "vulkan",
        }
    }

    fn capabilities(&self) -> DeviceCapabilities {
        match &self.config.device {
            BurnDevice::Cpu => DeviceCapabilities {
                supports_f16: false,
                supports_bf16: false,
                supports_tf32: false,
                max_memory_bytes: None,
                compute_units: None,
            },
            BurnDevice::Cuda(_) => DeviceCapabilities {
                supports_f16: true,
                supports_bf16: true,
                supports_tf32: self.config.allow_tf32,
                // Placeholder values - actual implementation would query the device
                max_memory_bytes: Some(8 * 1024 * 1024 * 1024), // 8 GB placeholder
                compute_units: Some(128),                       // Placeholder
            },
            BurnDevice::Metal(_) => DeviceCapabilities {
                supports_f16: true,
                supports_bf16: false,
                supports_tf32: false,
                max_memory_bytes: None,
                compute_units: None,
            },
            BurnDevice::Vulkan(_) => DeviceCapabilities {
                supports_f16: true,
                supports_bf16: false,
                supports_tf32: false,
                max_memory_bytes: None,
                compute_units: None,
            },
        }
    }

    fn is_available(&self) -> bool {
        self.check_device_availability()
    }

    fn initialize(&mut self) -> AphelionResult<()> {
        if self.initialized.load(Ordering::SeqCst) {
            return Ok(());
        }

        if !self.check_device_availability() {
            return Err(crate::error::AphelionError::backend(format!(
                "Burn device {} is not available",
                self.config.device.as_label()
            )));
        }

        // In actual implementation, this would initialize the Burn backend:
        // - Create the device handle
        // - Set up autodiff context if needed
        // - Configure TF32 settings
        // For now, just mark as initialized
        self.initialized.store(true, Ordering::SeqCst);

        tracing::info!(
            backend = "burn",
            device = %self.config.device.as_label(),
            tf32 = self.config.allow_tf32,
            "Burn backend initialized (placeholder)"
        );

        Ok(())
    }

    fn shutdown(&mut self) -> AphelionResult<()> {
        if !self.initialized.load(Ordering::SeqCst) {
            return Ok(());
        }

        // In actual implementation, this would:
        // - Release device handles
        // - Clear cached tensors
        // - Synchronize pending operations
        self.initialized.store(false, Ordering::SeqCst);

        tracing::info!(
            backend = "burn",
            device = %self.config.device.as_label(),
            "Burn backend shutdown (placeholder)"
        );

        Ok(())
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_burn_device_as_label() {
        assert_eq!(BurnDevice::Cpu.as_label(), "cpu");
        assert_eq!(BurnDevice::Cuda(0).as_label(), "cuda:0");
        assert_eq!(BurnDevice::Cuda(1).as_label(), "cuda:1");
        assert_eq!(BurnDevice::Metal(0).as_label(), "metal:0");
        assert_eq!(BurnDevice::Vulkan(2).as_label(), "vulkan:2");
    }

    #[test]
    fn test_burn_device_default() {
        let device = BurnDevice::default();
        assert_eq!(device, BurnDevice::Cpu);
    }

    #[test]
    fn test_burn_device_is_cpu_gpu() {
        assert!(BurnDevice::Cpu.is_cpu());
        assert!(!BurnDevice::Cpu.is_gpu());

        assert!(!BurnDevice::Cuda(0).is_cpu());
        assert!(BurnDevice::Cuda(0).is_gpu());

        assert!(!BurnDevice::Metal(0).is_cpu());
        assert!(BurnDevice::Metal(0).is_gpu());

        assert!(!BurnDevice::Vulkan(0).is_cpu());
        assert!(BurnDevice::Vulkan(0).is_gpu());
    }

    #[test]
    fn test_burn_backend_config_default() {
        let config = BurnBackendConfig::default();
        assert_eq!(config.device, BurnDevice::Cpu);
        assert!(!config.allow_tf32);
    }

    #[test]
    fn test_burn_backend_config_builder() {
        let config = BurnBackendConfig::cuda(0).with_tf32(true);
        assert_eq!(config.device, BurnDevice::Cuda(0));
        assert!(config.allow_tf32);
    }

    #[test]
    fn test_burn_backend_new() {
        let backend = BurnBackend::new(BurnBackendConfig::default());
        assert_eq!(backend.name(), "burn");
        assert_eq!(backend.device(), "cpu");
        assert!(!backend.is_initialized());
    }

    #[test]
    fn test_burn_backend_cpu() {
        let backend = BurnBackend::cpu();
        assert_eq!(backend.device(), "cpu");
        assert!(backend.config().device.is_cpu());
    }

    #[test]
    fn test_burn_backend_cuda() {
        let backend = BurnBackend::cuda(0);
        assert_eq!(backend.device(), "cuda");
        assert!(backend.config().device.is_gpu());
    }

    #[test]
    fn test_burn_backend_default() {
        let backend = BurnBackend::default();
        assert_eq!(backend.device(), "cpu");
    }

    #[test]
    fn test_burn_backend_capabilities_cpu() {
        let backend = BurnBackend::cpu();
        let caps = backend.capabilities();
        assert!(!caps.supports_f16);
        assert!(!caps.supports_bf16);
        assert!(!caps.supports_tf32);
    }

    #[test]
    fn test_burn_backend_capabilities_cuda() {
        let backend = BurnBackend::new(BurnBackendConfig::cuda(0).with_tf32(true));
        let caps = backend.capabilities();
        assert!(caps.supports_f16);
        assert!(caps.supports_bf16);
        assert!(caps.supports_tf32);
        assert!(caps.max_memory_bytes.is_some());
    }

    #[test]
    fn test_burn_backend_is_available_cpu() {
        let backend = BurnBackend::cpu();
        assert!(backend.is_available());
    }

    #[test]
    fn test_burn_backend_is_available_cuda() {
        let backend = BurnBackend::cuda(0);
        // CUDA is not available in placeholder implementation
        assert!(!backend.is_available());
    }

    #[test]
    fn test_burn_backend_initialize_cpu() {
        let mut backend = BurnBackend::cpu();
        assert!(!backend.is_initialized());

        let result = backend.initialize();
        assert!(result.is_ok());
        assert!(backend.is_initialized());

        // Double initialization should be ok
        let result = backend.initialize();
        assert!(result.is_ok());
    }

    #[test]
    fn test_burn_backend_initialize_cuda_fails() {
        let mut backend = BurnBackend::cuda(0);
        let result = backend.initialize();
        // Should fail because CUDA is not available in placeholder
        assert!(result.is_err());
    }

    #[test]
    fn test_burn_backend_shutdown() {
        let mut backend = BurnBackend::cpu();
        backend.initialize().unwrap();
        assert!(backend.is_initialized());

        let result = backend.shutdown();
        assert!(result.is_ok());
        assert!(!backend.is_initialized());

        // Double shutdown should be ok
        let result = backend.shutdown();
        assert!(result.is_ok());
    }

    #[test]
    fn test_burn_backend_clone() {
        let mut backend = BurnBackend::cpu();
        backend.initialize().unwrap();

        let cloned = backend.clone();
        // Cloned backend has its own initialization state
        assert!(cloned.is_initialized());
        assert_eq!(cloned.device(), backend.device());
    }
}
