quantrs2-core 0.2.1

//! 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.
    ///
    /// Real Metal device initialization (via `oxicuda-metal`) is DEFERRED. This
    /// returns an honest error rather than fabricating a placeholder device and
    /// buffers that cannot actually run kernels. When real Metal support is
    /// wired, construct the device/queue/state buffer here.
    #[cfg(feature = "metal")]
    pub fn new(num_qubits: usize) -> QuantRS2Result<Self> {
        let _ = num_qubits;
        Err(QuantRS2Error::UnsupportedOperation(
            "Metal quantum-state backend is not implemented (DEFERRED): real Metal device \
             initialization is not wired, refusing to fabricate a placeholder device"
                .to_string(),
        ))
    }

    /// Apply a single-qubit gate using Metal.
    ///
    /// The Metal compute kernel for gate application is DEFERRED (no real
    /// `oxicuda-metal` dispatch is wired yet). Inputs are validated, then this
    /// returns an honest error instead of silently returning success without
    /// transforming the state — a caller must never mistake a no-op for a real
    /// Metal gate application.
    #[cfg(feature = "metal")]
    pub fn apply_single_qubit_gate(
        &mut self,
        _gate_matrix: &[Complex64; 4],
        target: QubitId,
    ) -> QuantRS2Result<()> {
        if target.0 >= self.num_qubits as u32 {
            return Err(QuantRS2Error::InvalidQubitId(target.0));
        }

        Err(QuantRS2Error::UnsupportedOperation(
            "Metal single-qubit gate kernel is not implemented (DEFERRED): no real Metal compute \
             dispatch is wired, refusing to fabricate a no-op success"
                .to_string(),
        ))
    }

    /// Get or compile a Metal kernel.
    ///
    /// Real Metal kernel compilation is DEFERRED. Rather than return a
    /// placeholder pipeline that pretends a kernel was compiled, this validates
    /// the requested function name and then returns an honest error.
    #[cfg(feature = "metal")]
    pub fn get_or_compile_kernel(&self, function_name: &str) -> QuantRS2Result<MetalKernel> {
        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}"
            )));
        }

        Err(QuantRS2Error::UnsupportedOperation(format!(
            "Metal kernel `{function_name}` cannot be compiled: real Metal shader compilation is \
             DEFERRED (no placeholder pipeline is returned)"
        )))
    }

    #[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 whether a real Metal compute backend is available.
///
/// This build does not yet wire a real Metal backend (`oxicuda-metal` is not
/// linked and no device dispatch exists), so this honestly returns `false` on
/// every platform — it does NOT assume Metal is present just because the `metal`
/// feature is enabled on macOS. DEFERRED: when real Metal init is wired, probe
/// the actual device here.
pub const fn is_metal_available() -> bool {
    false
}

/// Get real Metal device info.
///
/// Returns `None` because no real Metal device is initialized in this build.
/// Previously this returned fabricated placeholder specs; that fabrication has
/// been removed. DEFERRED until a real Metal backend is wired.
pub const fn get_metal_device_info() -> Option<String> {
    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);
        }
    }
}