"""Qiskit backend provider for HIQ.

This module implements Qiskit's provider and backend interfaces, allowing
users to execute Arvak circuits through Qiskit's familiar backend.run() API.
"""

from typing import List, Optional, Union, TYPE_CHECKING
import warnings

if TYPE_CHECKING:
    from qiskit import QuantumCircuit
    from qiskit.providers import BackendV2, JobV1, Options


class ArvakProvider:
    """Qiskit provider for HIQ backends.

    This provider allows users to access HIQ execution capabilities through
    Qiskit's standard provider interface.

    Example:
        >>> from arvak.integrations.qiskit import ArvakProvider
        >>> provider = ArvakProvider()
        >>> backend = provider.get_backend('sim')
        >>> job = backend.run(qiskit_circuit, shots=1000)
        >>> result = job.result()
    """

    def __init__(self):
        """Initialize the HIQ provider."""
        self._backends = {}

    def backends(self, name: Optional[str] = None, **filters) -> List['BackendV2']:
        """Get list of available backends.

        Args:
            name: Optional backend name filter
            **filters: Additional filters (currently unused)

        Returns:
            List of ArvakBackend instances

        Example:
            >>> provider = ArvakProvider()
            >>> all_backends = provider.backends()
            >>> sim_backend = provider.backends(name='sim')
        """
        # Lazy initialization of backends
        if not self._backends:
            self._backends = {
                'sim': HIQSimulatorBackend(provider=self),
                # Future: Add more backends (iqm, ibm, etc.)
            }

        if name:
            backend = self._backends.get(name)
            return [backend] if backend else []

        return list(self._backends.values())

    def get_backend(self, name: str = 'sim') -> 'BackendV2':
        """Get a specific backend by name.

        Args:
            name: Backend name (default: 'sim')

        Returns:
            ArvakBackend instance

        Raises:
            ValueError: If backend name is unknown

        Example:
            >>> provider = ArvakProvider()
            >>> backend = provider.get_backend('sim')
        """
        backends = self.backends(name=name)
        if not backends:
            available = list(self._backends.keys())
            raise ValueError(
                f"Unknown backend: {name}. "
                f"Available backends: {', '.join(available)}"
            )
        return backends[0]

    def __repr__(self) -> str:
        """String representation of the provider."""
        return f"<ArvakProvider(backends={list(self._backends.keys())})>"


class HIQSimulatorBackend:
    """HIQ simulator backend with Qiskit-compatible interface.

    This backend wraps HIQ's simulation capabilities with Qiskit's BackendV2
    interface, allowing seamless integration with Qiskit workflows.

    Note:
        This is a simplified backend implementation. Full BackendV2 compliance
        would require implementing all BackendV2 abstract methods and properties.
    """

    def __init__(self, provider: ArvakProvider):
        """Initialize the simulator backend.

        Args:
            provider: Parent ArvakProvider instance
        """
        self._provider = provider
        self.name = 'hiq_simulator'
        self.description = 'HIQ quantum circuit simulator'
        self.online_date = '2024-01-01'
        self.backend_version = '0.1.0'

    @property
    def max_circuits(self) -> Optional[int]:
        """Maximum number of circuits that can be run in a single job."""
        return None  # No limit

    @property
    def num_qubits(self) -> int:
        """Number of qubits supported by the backend."""
        return 32  # Configurable, but reasonable default

    @property
    def basis_gates(self) -> List[str]:
        """List of basis gate names supported by the backend."""
        return ['id', 'rz', 'sx', 'x', 'cx', 'measure']

    @property
    def coupling_map(self) -> Optional[List[List[int]]]:
        """Coupling map for the backend (None = all-to-all connectivity)."""
        return None  # Simulator supports all-to-all

    def run(self, circuits: Union['QuantumCircuit', List['QuantumCircuit']],
            shots: int = 1024, **options) -> 'JobV1':
        """Run circuits on the simulator.

        Args:
            circuits: Single circuit or list of circuits to execute
            shots: Number of measurement shots (default: 1024)
            **options: Additional execution options

        Returns:
            Job instance representing the execution

        Example:
            >>> backend = provider.get_backend('sim')
            >>> job = backend.run(qiskit_circuit, shots=1000)
            >>> result = job.result()
            >>> counts = result.get_counts()
        """
        warnings.warn(
            "HIQ backend execution through Qiskit is not yet fully implemented. "
            "For now, please use HIQ CLI for execution: "
            "'hiq run circuit.qasm --backend sim --shots 1000'. "
            "This backend interface will return mock results.",
            RuntimeWarning
        )

        # Ensure circuits is a list
        if not isinstance(circuits, list):
            circuits = [circuits]

        # Import here to avoid circular dependency
        from .converter import qiskit_to_arvak
        import arvak

        # Convert circuits to Arvak format
        hiq_circuits = [qiskit_to_arvak(qc) for qc in circuits]

        # Create a mock job (real execution would happen here)
        job = HIQJob(
            backend=self,
            circuits=hiq_circuits,
            shots=shots,
            options=options
        )

        return job

    def __repr__(self) -> str:
        """String representation of the backend."""
        return f"<HIQSimulatorBackend('{self.name}')>"


class HIQJob:
    """Mock job for HIQ backend execution.

    This is a placeholder that will be replaced with actual asynchronous
    execution once HIQ backend execution is exposed to Python.
    """

    def __init__(self, backend, circuits, shots, options):
        """Initialize the job.

        Args:
            backend: Backend instance
            circuits: List of Arvak circuits
            shots: Number of shots
            options: Execution options
        """
        self._backend = backend
        self._circuits = circuits
        self._shots = shots
        self._options = options
        self._result = None

    def result(self) -> 'HIQResult':
        """Get job result.

        Returns:
            HIQResult instance with mock data
        """
        if self._result is None:
            self._result = HIQResult(
                backend_name=self._backend.name,
                circuits=self._circuits,
                shots=self._shots
            )
        return self._result

    def status(self) -> str:
        """Get job status."""
        return "DONE"

    def __repr__(self) -> str:
        """String representation of the job."""
        return f"<HIQJob(circuits={len(self._circuits)}, shots={self._shots})>"


class HIQResult:
    """Mock result for HIQ backend execution.

    This is a placeholder that returns mock data. Real implementation would
    parse actual execution results from HIQ backend.
    """

    def __init__(self, backend_name, circuits, shots):
        """Initialize the result.

        Args:
            backend_name: Name of the backend
            circuits: List of circuits
            shots: Number of shots
        """
        self.backend_name = backend_name
        self._circuits = circuits
        self._shots = shots

    def get_counts(self, circuit=None):
        """Get measurement counts.

        Args:
            circuit: Optional circuit index (default: 0)

        Returns:
            Dictionary of measurement counts (mock data)
        """
        warnings.warn(
            "Returning mock results. Use HIQ CLI for actual execution: "
            "'hiq run circuit.qasm --backend sim --shots 1000'",
            RuntimeWarning
        )

        # Return mock data for demonstration
        # In a real implementation, this would parse HIQ execution results
        circuit_idx = 0 if circuit is None else circuit
        if circuit_idx >= len(self._circuits):
            circuit_idx = 0

        # Mock Bell state results
        return {
            '00': self._shots // 2,
            '11': self._shots // 2,
        }

    def __repr__(self) -> str:
        """String representation of the result."""
        return f"<HIQResult(backend='{self.backend_name}', circuits={len(self._circuits)})>"
