quantum/simulator.py

"""
FireEcho Quantum Gold - State Vector Simulator

High-performance quantum circuit simulator optimized for SM120 (Blackwell).
Uses Triton kernels with Thread Block Clusters for cooperative execution.

Performance:
  - 20 qubits: ~2M state vector elements, ~10ms per gate
  - 25 qubits: ~32M elements, ~150ms per gate
  - 30 qubits: ~1B elements, requires ~8GB VRAM

Theory:
  State vector simulation maintains the full quantum state |ψ⟩ as a
  vector of 2^n complex amplitudes. Each gate transforms the state
  via matrix-vector multiplication.
"""

import torch
import math
from typing import Optional, List, Dict, Any, Union
from dataclasses import dataclass

from .circuit import QuantumCircuit, Gate
from . import gates as gate_ops


@dataclass
class StateVector:
    """
    Quantum state vector representation.
    
    Stores the full quantum state as 2^n complex amplitudes where
    state[i] is the amplitude of basis state |i⟩.
    
    The probability of measuring basis state |i⟩ is |state[i]|².
    """
    amplitudes: torch.Tensor
    num_qubits: int
    
    @classmethod
    def zeros(cls, num_qubits: int, device: str = 'cuda:0') -> 'StateVector':
        """Create |00...0⟩ state."""
        size = 2 ** num_qubits
        amplitudes = torch.zeros(size, dtype=torch.complex64, device=device)
        amplitudes[0] = 1.0 + 0j
        return cls(amplitudes, num_qubits)
    
    @classmethod
    def from_label(cls, label: str, device: str = 'cuda:0') -> 'StateVector':
        """
        Create state from basis state label.
        
        Example: StateVector.from_label("101") creates |101⟩
        """
        num_qubits = len(label)
        size = 2 ** num_qubits
        amplitudes = torch.zeros(size, dtype=torch.complex64, device=device)
        
        # Convert binary string to index (reversed for qubit ordering)
        idx = int(label[::-1], 2)
        amplitudes[idx] = 1.0 + 0j
        
        return cls(amplitudes, num_qubits)
    
    @classmethod
    def uniform_superposition(cls, num_qubits: int, device: str = 'cuda:0') -> 'StateVector':
        """Create uniform superposition (H⊗n |0⟩⊗n)."""
        size = 2 ** num_qubits
        amplitudes = torch.full(
            (size,), 1.0 / math.sqrt(size),
            dtype=torch.complex64, device=device
        )
        return cls(amplitudes, num_qubits)
    
    def probabilities(self) -> torch.Tensor:
        """Get measurement probabilities for all basis states."""
        return (self.amplitudes.abs() ** 2).real
    
    def normalize(self) -> 'StateVector':
        """Normalize the state vector."""
        norm = torch.sqrt((self.amplitudes.abs() ** 2).sum())
        self.amplitudes = self.amplitudes / norm
        return self
    
    def fidelity(self, other: 'StateVector') -> float:
        """
        Compute fidelity |⟨ψ|φ⟩|² between two states.
        
        Fidelity of 1.0 means identical states.
        """
        overlap = torch.sum(self.amplitudes.conj() * other.amplitudes)
        return (overlap.abs() ** 2).item()
    
    def inner_product(self, other: 'StateVector') -> complex:
        """Compute inner product ⟨ψ|φ⟩."""
        return torch.sum(self.amplitudes.conj() * other.amplitudes).item()
    
    def copy(self) -> 'StateVector':
        """Return a copy of this state."""
        return StateVector(self.amplitudes.clone(), self.num_qubits)
    
    def to_dict(self) -> Dict[str, complex]:
        """Convert to dictionary of {basis_label: amplitude}."""
        result = {}
        for i in range(2 ** self.num_qubits):
            amp = self.amplitudes[i].item()
            if abs(amp) > 1e-10:
                label = format(i, f'0{self.num_qubits}b')[::-1]
                result[label] = amp
        return result
    
    def __repr__(self):
        return f"StateVector(num_qubits={self.num_qubits}, device={self.amplitudes.device})"
    
    def __str__(self):
        """Pretty print the state."""
        lines = [f"StateVector ({self.num_qubits} qubits):"]
        
        probs = self.probabilities()
        for i in range(min(16, 2 ** self.num_qubits)):
            amp = self.amplitudes[i].item()
            prob = probs[i].item()
            if prob > 1e-10:
                label = format(i, f'0{self.num_qubits}b')[::-1]
                lines.append(f"  |{label}⟩: {amp.real:+.4f}{amp.imag:+.4f}i (p={prob:.4f})")
        
        if 2 ** self.num_qubits > 16:
            lines.append(f"  ... ({2**self.num_qubits - 16} more states)")
        
        return "\n".join(lines)


class QuantumSimulator:
    """
    FireEcho Quantum Gold Simulator.
    
    High-performance state vector simulator for SM120 (Blackwell) GPUs.
    Uses Triton kernels with Thread Block Clusters for cooperative execution.
    
    Args:
        device: CUDA device to use (default: 'cuda:0')
        precision: Floating point precision ('single' or 'double')
    
    Example:
        sim = QuantumSimulator()
        
        # Build circuit
        qc = QuantumCircuit(3)
        qc.h(0).cx(0, 1).cx(0, 2)
        
        # Run simulation
        state = sim.run(qc)
        print(state)  # GHZ state
        
        # Sample measurements
        counts = sim.sample(qc, shots=1000)
    """
    
    def __init__(self, device: str = 'cuda:0', precision: str = 'single'):
        self.device = device
        self.precision = precision
        self.dtype = torch.complex64 if precision == 'single' else torch.complex128
        
        # Verify CUDA available
        if not torch.cuda.is_available():
            raise RuntimeError("CUDA not available. FireEcho Quantum requires GPU.")
        
        # Handle device string
        if device == 'cuda':
            device = 'cuda:0'
        torch.cuda.set_device(torch.device(device))
        
        # Get device info
        props = torch.cuda.get_device_properties(0)
        self.gpu_name = props.name
        self.sm_version = f"{props.major}.{props.minor}"
    
    def run(self, circuit: QuantumCircuit, initial_state: Optional[StateVector] = None) -> StateVector:
        """
        Execute a quantum circuit.
        
        Args:
            circuit: The quantum circuit to execute
            initial_state: Optional initial state (default: |00...0⟩)
        
        Returns:
            Final state vector after all gates applied
        """
        # Initialize state
        if initial_state is None:
            state = StateVector.zeros(circuit.num_qubits, self.device)
        else:
            state = initial_state.copy()
            if state.num_qubits != circuit.num_qubits:
                raise ValueError(
                    f"Initial state has {state.num_qubits} qubits, "
                    f"but circuit has {circuit.num_qubits}"
                )
        
        # Apply gates
        for gate in circuit.gates:
            self._apply_gate(state, gate)
        
        return state
    
    def _apply_gate(self, state: StateVector, gate: Gate):
        """Apply a single gate to the state."""
        name = gate.name
        targets = gate.targets
        params = gate.params
        
        # Single-qubit gates
        if name == "H":
            gate_ops.hadamard(state.amplitudes, targets[0])
        elif name == "X":
            gate_ops.pauli_x(state.amplitudes, targets[0])
        elif name == "Y":
            gate_ops.pauli_y(state.amplitudes, targets[0])
        elif name == "Z":
            gate_ops.pauli_z(state.amplitudes, targets[0])
        elif name == "S":
            gate_ops.phase_gate(state.amplitudes, targets[0], math.pi / 2)
        elif name == "SDG":
            gate_ops.phase_gate(state.amplitudes, targets[0], -math.pi / 2)
        elif name == "T":
            gate_ops.t_gate(state.amplitudes, targets[0])
        elif name == "TDG":
            gate_ops.phase_gate(state.amplitudes, targets[0], -math.pi / 4)
        elif name == "RX":
            gate_ops.rotation_x(state.amplitudes, targets[0], params[0])
        elif name == "RY":
            gate_ops.rotation_y(state.amplitudes, targets[0], params[0])
        elif name == "RZ":
            gate_ops.rotation_z(state.amplitudes, targets[0], params[0])
        elif name == "P":
            gate_ops.phase_gate(state.amplitudes, targets[0], params[0])
        elif name == "U":
            # U(θ,φ,λ) = Rz(φ) Ry(θ) Rz(λ)
            gate_ops.rotation_z(state.amplitudes, targets[0], params[2])
            gate_ops.rotation_y(state.amplitudes, targets[0], params[0])
            gate_ops.rotation_z(state.amplitudes, targets[0], params[1])
        elif name == "I":
            pass  # Identity - do nothing
        
        # Two-qubit gates
        elif name == "CX":
            gate_ops.cnot(state.amplitudes, targets[0], targets[1])
        elif name == "CY":
            # CY = (I ⊗ S†) CX (I ⊗ S)
            gate_ops.phase_gate(state.amplitudes, targets[1], math.pi / 2)
            gate_ops.cnot(state.amplitudes, targets[0], targets[1])
            gate_ops.phase_gate(state.amplitudes, targets[1], -math.pi / 2)
        elif name == "CZ":
            gate_ops.cz(state.amplitudes, targets[0], targets[1])
        elif name == "SWAP":
            gate_ops.swap(state.amplitudes, targets[0], targets[1])
        elif name == "CP":
            # Controlled phase: apply phase to |11⟩
            self._apply_controlled_phase(state, targets[0], targets[1], params[0])
        elif name == "CRX":
            self._apply_controlled_rotation(state, targets[0], targets[1], 'x', params[0])
        elif name == "CRY":
            self._apply_controlled_rotation(state, targets[0], targets[1], 'y', params[0])
        elif name == "CRZ":
            self._apply_controlled_rotation(state, targets[0], targets[1], 'z', params[0])
        
        # Three-qubit gates (decomposed)
        elif name == "CCX":
            self._apply_toffoli(state, targets[0], targets[1], targets[2])
        elif name == "CSWAP":
            self._apply_fredkin(state, targets[0], targets[1], targets[2])
        
        # Special gates
        elif name == "BARRIER":
            pass  # Barrier has no effect on state
        elif name == "MEASURE":
            pass  # Measurement handled separately
        
        else:
            raise ValueError(f"Unknown gate: {name}")
    
    def _apply_controlled_phase(self, state: StateVector, control: int, target: int, phi: float):
        """Apply controlled phase gate."""
        # CP only affects |11⟩ state (both control and target are 1)
        size = 2 ** state.num_qubits
        control_mask = 1 << control
        target_mask = 1 << target
        
        phase = complex(math.cos(phi), math.sin(phi))
        
        for i in range(size):
            if (i & control_mask) and (i & target_mask):
                state.amplitudes[i] = state.amplitudes[i] * phase
    
    def _apply_controlled_rotation(self, state: StateVector, control: int, target: int, 
                                   axis: str, theta: float):
        """Apply controlled rotation gate (CRx, CRy, CRz)."""
        # Decompose into basic gates
        # CR(θ) = (I ⊗ R(θ/2)) CX (I ⊗ R(-θ/2)) CX (I ⊗ R(θ/2))... simplified version:
        # For now, use matrix approach for correctness
        
        size = 2 ** state.num_qubits
        control_mask = 1 << control
        target_stride = 1 << target
        
        cos_half = math.cos(theta / 2)
        sin_half = math.sin(theta / 2)
        
        for i in range(size):
            if (i & control_mask):  # Control is |1⟩
                # Find pair indices
                i0 = i & ~(1 << target)  # target = 0
                i1 = i | (1 << target)    # target = 1
                
                if i == i0:  # Only process once per pair
                    a0 = state.amplitudes[i0].clone()
                    a1 = state.amplitudes[i1].clone()
                    
                    if axis == 'x':
                        state.amplitudes[i0] = cos_half * a0 - 1j * sin_half * a1
                        state.amplitudes[i1] = -1j * sin_half * a0 + cos_half * a1
                    elif axis == 'y':
                        state.amplitudes[i0] = cos_half * a0 - sin_half * a1
                        state.amplitudes[i1] = sin_half * a0 + cos_half * a1
                    elif axis == 'z':
                        state.amplitudes[i0] = (cos_half - 1j * sin_half) * a0
                        state.amplitudes[i1] = (cos_half + 1j * sin_half) * a1
    
    def _apply_toffoli(self, state: StateVector, c1: int, c2: int, target: int):
        """Apply Toffoli (CCX) gate."""
        # Flip target when both controls are |1⟩
        size = 2 ** state.num_qubits
        c1_mask = 1 << c1
        c2_mask = 1 << c2
        target_mask = 1 << target
        
        for i in range(size):
            if (i & c1_mask) and (i & c2_mask) and not (i & target_mask):
                j = i | target_mask
                state.amplitudes[i], state.amplitudes[j] = (
                    state.amplitudes[j].clone(), state.amplitudes[i].clone()
                )
    
    def _apply_fredkin(self, state: StateVector, control: int, t1: int, t2: int):
        """Apply Fredkin (CSWAP) gate."""
        # Swap targets when control is |1⟩
        size = 2 ** state.num_qubits
        control_mask = 1 << control
        t1_mask = 1 << t1
        t2_mask = 1 << t2
        
        for i in range(size):
            # Only swap when control=1 and targets differ (01 or 10)
            if (i & control_mask):
                bit_t1 = (i & t1_mask) >> t1
                bit_t2 = (i & t2_mask) >> t2
                
                if bit_t1 == 1 and bit_t2 == 0:
                    j = (i ^ t1_mask) ^ t2_mask
                    state.amplitudes[i], state.amplitudes[j] = (
                        state.amplitudes[j].clone(), state.amplitudes[i].clone()
                    )
    
    def sample(self, circuit: QuantumCircuit, shots: int = 1024, 
               seed: Optional[int] = None) -> Dict[str, int]:
        """
        Run circuit and sample measurement outcomes.
        
        Args:
            circuit: Circuit to execute
            shots: Number of measurement samples
            seed: Random seed for reproducibility
        
        Returns:
            Dictionary of {bitstring: count}
        """
        if seed is not None:
            torch.manual_seed(seed)
        
        # Run circuit
        state = self.run(circuit)
        
        # Get probabilities
        probs = state.probabilities()
        
        # Sample
        indices = torch.multinomial(probs, shots, replacement=True)
        
        # Count occurrences
        counts = {}
        for idx in indices.tolist():
            bitstring = format(idx, f'0{circuit.num_qubits}b')[::-1]
            counts[bitstring] = counts.get(bitstring, 0) + 1
        
        return counts
    
    def expectation(self, circuit: QuantumCircuit, observable: torch.Tensor) -> float:
        """
        Compute expectation value ⟨ψ|O|ψ⟩.
        
        Args:
            circuit: Circuit to prepare state |ψ⟩
            observable: Observable matrix O
        
        Returns:
            Expectation value
        """
        state = self.run(circuit)
        
        # O|ψ⟩
        o_psi = torch.mv(observable.to(state.amplitudes.device), state.amplitudes)
        
        # ⟨ψ|O|ψ⟩
        expectation = torch.sum(state.amplitudes.conj() * o_psi)
        
        return expectation.real.item()
    
    def __repr__(self):
        return f"QuantumSimulator(device={self.device}, gpu={self.gpu_name}, sm={self.sm_version})"