quantrs2-core 0.1.3

//! Metal backend implementation ready for SciRS2 GPU migration
//!
//! This module implements Metal GPU acceleration in a way that's compatible
//! with the expected SciRS2 GPU abstractions in v0.1.0.
//!
//! NOTE: This is a forward-compatible implementation anticipating SciRS2 Metal support.

use crate::{
    error::{QuantRS2Error, QuantRS2Result},
    qubit::QubitId,
};
use scirs2_core::Complex64;
use std::sync::Arc;

// Placeholder for future SciRS2 Metal types
#[cfg(feature = "metal")]
pub mod scirs2_metal_placeholder {

    /// Placeholder for Metal device handle
    pub struct MetalDeviceHandle {
        pub name: String,
    }

    /// Placeholder for Metal command queue
    pub struct MetalCommandQueue;

    /// Placeholder for Metal buffer
    pub struct MetalBufferHandle;

    /// Placeholder for Metal compute pipeline
    pub struct MetalComputePipeline;

    /// Placeholder for SciRS2 MetalDevice
    pub struct MetalDevice {
        pub(crate) device: MetalDeviceHandle,
        pub(crate) command_queue: MetalCommandQueue,
    }

    /// Placeholder for SciRS2 MetalBuffer
    pub struct MetalBuffer<T> {
        pub buffer: MetalBufferHandle,
        pub length: usize,
        pub _phantom: std::marker::PhantomData<T>,
    }

    /// Placeholder for SciRS2 MetalKernel
    pub struct MetalKernel {
        pub pipeline: MetalComputePipeline,
        pub function_name: String,
    }
}

#[cfg(feature = "metal")]
use self::scirs2_metal_placeholder::*;

/// Metal shader library for quantum operations
pub const METAL_QUANTUM_SHADERS: &str = r"
#include <metal_stdlib>
using namespace metal;

// Complex number operations
struct Complex {
    float real;
    float imag;
};

Complex complex_mul(Complex a, Complex b) {
    return Complex{
        a.real * b.real - a.imag * b.imag,
        a.real * b.imag + a.imag * b.real
    };
}

Complex complex_add(Complex a, Complex b) {
    return Complex{a.real + b.real, a.imag + b.imag};
}

// Single qubit gate kernel
kernel void apply_single_qubit_gate(
    device Complex* state [[buffer(0)]],
    constant Complex* gate_matrix [[buffer(1)]],
    constant uint& target_qubit [[buffer(2)]],
    constant uint& num_qubits [[buffer(3)]],
    uint gid [[thread_position_in_grid]]
) {
    uint state_size = 1u << num_qubits;
    if (gid >= state_size / 2) return;

    uint mask = (1u << target_qubit) - 1u;
    uint idx0 = ((gid & ~mask) << 1u) | (gid & mask);
    uint idx1 = idx0 | (1u << target_qubit);

    Complex amp0 = state[idx0];
    Complex amp1 = state[idx1];

    state[idx0] = complex_add(
        complex_mul(gate_matrix[0], amp0),
        complex_mul(gate_matrix[1], amp1)
    );
    state[idx1] = complex_add(
        complex_mul(gate_matrix[2], amp0),
        complex_mul(gate_matrix[3], amp1)
    );
}

// Measurement probability kernel
kernel void compute_probabilities(
    device const Complex* state [[buffer(0)]],
    device float* probabilities [[buffer(1)]],
    constant uint& num_qubits [[buffer(2)]],
    uint gid [[thread_position_in_grid]]
) {
    uint state_size = 1u << num_qubits;
    if (gid >= state_size) return;

    Complex amp = state[gid];
    probabilities[gid] = amp.real * amp.real + amp.imag * amp.imag;
}
";

/// Metal-accelerated quantum state vector
pub struct MetalQuantumState {
    #[cfg(feature = "metal")]
    device: Arc<MetalDevice>,
    #[cfg(feature = "metal")]
    state_buffer: MetalBuffer<Complex64>,
    pub num_qubits: usize,
}

impl MetalQuantumState {
    /// Create a new Metal-accelerated quantum state
    #[cfg(feature = "metal")]
    pub fn new(num_qubits: usize) -> QuantRS2Result<Self> {
        // This is a placeholder implementation
        // In the future, this would use SciRS2's Metal device initialization

        // For now, we simulate Metal availability check
        if !is_metal_available() {
            return Err(QuantRS2Error::BackendExecutionFailed(
                "Metal support not available".to_string(),
            ));
        }

        // Create placeholder device
        let device = Arc::new(MetalDevice {
            device: MetalDeviceHandle {
                name: "Apple M1 GPU (Placeholder)".to_string(),
            },
            command_queue: MetalCommandQueue,
        });

        // Allocate state vector buffer (placeholder)
        let state_size = 1 << num_qubits;

        let state_buffer = MetalBuffer {
            buffer: MetalBufferHandle,
            length: state_size,
            _phantom: std::marker::PhantomData,
        };

        Ok(Self {
            device,
            state_buffer,
            num_qubits,
        })
    }

    /// Apply a single-qubit gate using Metal
    #[cfg(feature = "metal")]
    #[allow(clippy::missing_const_for_fn)] // Runtime GPU operations cannot be const
    pub fn apply_single_qubit_gate(
        &mut self,
        gate_matrix: &[Complex64; 4],
        target: QubitId,
    ) -> QuantRS2Result<()> {
        // This is a placeholder implementation
        // In the future, this would dispatch actual Metal compute kernels via SciRS2

        // Validate inputs
        if target.0 >= self.num_qubits as u32 {
            return Err(QuantRS2Error::InvalidQubitId(target.0));
        }

        // Log the operation (placeholder behavior)
        let _ = gate_matrix; // Suppress unused warning

        // In a real implementation, this would:
        // 1. Get or compile the Metal kernel via SciRS2
        // 2. Create command buffer and encoder
        // 3. Set the state buffer and gate matrix
        // 4. Dispatch the compute kernel
        // 5. Wait for completion

        // For now, we just return success
        Ok(())
    }

    /// Get or compile a Metal kernel
    #[cfg(feature = "metal")]
    pub fn get_or_compile_kernel(&self, function_name: &str) -> QuantRS2Result<MetalKernel> {
        // This is a placeholder implementation
        // In the future, this would use SciRS2's kernel registry

        // Validate that the requested kernel exists in our shader library
        let valid_kernels = ["apply_single_qubit_gate", "compute_probabilities"];
        if !valid_kernels.contains(&function_name) {
            return Err(QuantRS2Error::BackendExecutionFailed(format!(
                "Unknown kernel function: {function_name}"
            )));
        }

        // Return a placeholder kernel
        Ok(MetalKernel {
            pipeline: MetalComputePipeline,
            function_name: function_name.to_string(),
        })
    }

    #[cfg(not(feature = "metal"))]
    pub fn new(_num_qubits: usize) -> QuantRS2Result<Self> {
        Err(QuantRS2Error::UnsupportedOperation(
            "Metal support not compiled in. Please enable the 'metal' feature.".to_string(),
        ))
    }
}

/// Check if Metal is available on this system
pub const fn is_metal_available() -> bool {
    #[cfg(all(target_os = "macos", feature = "metal"))]
    {
        // This is a placeholder check
        // In the future, this would use SciRS2's GPU device detection
        // For now, we assume Metal is available on macOS
        true
    }

    #[cfg(not(all(target_os = "macos", feature = "metal")))]
    {
        false
    }
}

/// Get Metal device info
pub fn get_metal_device_info() -> Option<String> {
    #[cfg(all(target_os = "macos", feature = "metal"))]
    {
        // This is placeholder information
        // In the future, this would query actual device capabilities via SciRS2
        Some(
            "Metal Device: Apple GPU (Placeholder)\n\
             Max threads per threadgroup: 1024\n\
             Max buffer length: 256 GB\n\
             Note: This is placeholder information. Actual device info will be available via SciRS2.".to_string()
        )
    }

    #[cfg(not(all(target_os = "macos", feature = "metal")))]
    {
        None
    }
}

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

    #[test]
    fn test_metal_availability() {
        let available = is_metal_available();
        println!("Metal available: {}", available);

        if let Some(info) = get_metal_device_info() {
            println!("Metal device info:\n{}", info);
        }
    }
}