dense_evolution/registry.py

#  Copyright (c) 2026 Salvatore Pennacchio <jtatopenn@libero.it>
#  Distributed under the Business Source License 1.1 (BSL 1.1)
#  See LICENSE.md in the project root for full license terms.


import subprocess
import sys
import os
import time
import psutil
import platform
import warnings
from typing import Optional, List, Dict, Any
import numpy as np
import matplotlib
import matplotlib.pyplot as plt

warnings.filterwarnings('ignore')

try:
    import cupy as cp
    HAS_CUPY = True
except ImportError:
    HAS_CUPY = False

try:
    import jax
    import jax.numpy as jnp
    HAS_JAX = True
    jax.config.update("jax_enable_x64", True)
except ImportError:
    HAS_JAX = False
    jnp = None


class QuantumHardwareRegistry:
    def __init__(self):
        self.processor = platform.processor()
        self.ram_total = psutil.virtual_memory().total / (1024**3)
        self.ram_avail = psutil.virtual_memory().available / (1024**3)
        self.has_cupy = HAS_CUPY
        self.has_jax = HAS_JAX
        self.has_gpu = self._detect_gpu()
        self.max_dense_qubits = self._get_qubit_limit()

    def _detect_gpu(self) -> bool:
        try:
            subprocess.check_output(['nvidia-smi'], stderr=subprocess.DEVNULL)
            return True
        except Exception:
            return False

    def _get_qubit_limit(self) -> int:
        if self.ram_total >= 50: return 28
        elif self.ram_total >= 12: return 24
        return 20

    def print_diagnostics(self):
        print(f"MAX_DENSE={self.max_dense_qubits}q | JAX={self.has_jax} | GPU={self.has_gpu}")


HARDWARE_REGISTRY = QuantumHardwareRegistry()
plt.style.use('dark_background')
matplotlib.rcParams.update({
    'figure.facecolor': '#010409',
    'axes.facecolor': '#0d1117',
    'axes.edgecolor': '#21262d',
    'grid.color': '#21262d',
    'font.family': 'monospace',
    'font.size': 9,
})


# ─────────────────────────────────────────────────────────────────────────────
# Internal helpers
# ─────────────────────────────────────────────────────────────────────────────

def _fresh_rng() -> np.random.Generator:
    """
    Create a hardware-entropy-seeded RNG.
    Combines os.urandom (CSPRNG) with a high-resolution nanosecond counter
    so two calls within the same microsecond still differ.
    """
    entropy_bytes = os.urandom(8)
    entropy_int   = int.from_bytes(entropy_bytes, byteorder='big')
    ns_counter    = time.perf_counter_ns() & 0xFFFF_FFFF_FFFF_FFFF
    seed          = (entropy_int ^ ns_counter) & 0xFFFF_FFFF_FFFF_FFFF
    return np.random.default_rng(seed)


def _qubit_index_pairs(dim: int, q: int):
    """
    Return (idx_0, idx_1) — two integer arrays of length dim/2 — where
    idx_0[i] has bit q == 0 and idx_1[i] = idx_0[i] | (1 << q).

    This is the correct and vectorised way to build qubit-pair indices.
    The original code used `xp.where()` which returns a *tuple*, then
    did `idx_1 = idx_0 | step` on that tuple — producing wrong indices
    for all models and making phaseflip look deterministic.
    """
    step   = 1 << q
    all_i  = np.arange(dim, dtype=np.intp)
    idx_0  = all_i[(all_i & step) == 0]          # shape: (dim//2,)
    idx_1  = idx_0 | step                          # shape: (dim//2,)
    return idx_0, idx_1


# ─────────────────────────────────────────────────────────────────────────────
# NoiseModel
# ─────────────────────────────────────────────────────────────────────────────

class NoiseModel:
    """
    Stochastic single-qubit Kraus channels applied directly to a statevector.

    All channels are mathematically correct Kraus maps:
    - trace is preserved (normalisation enforced at the end)
    - phaseflip applies Z with probability p per qubit (non-deterministic)
    - amplitude_damping applies the correct K0/K1 Kraus operators
    - combined is a true worst-case NISQ mixture of all three Pauli errors
      plus amplitude damping

    Supported models
    ----------------
    'ideal'            identity — no modification
    'depolarizing'     {√(1-p)I, √(p/3)X, √(p/3)Y, √(p/3)Z}
    'bitflip'          {√(1-p)I, √p·X}
    'phaseflip'        {√(1-p)I, √p·Z}    ← was broken, now fixed
    'amplitude_damping'{K0=diag(1,√(1-γ)), K1=[[0,√γ],[0,0]]}
    'combined'         depolarizing(p/2) + amplitude_damping(p/3), renormalised
    """

    MODELS = ['ideal', 'depolarizing', 'bitflip', 'phaseflip',
              'amplitude_damping', 'combined']

    @staticmethod
    def apply_to_sv(
        sv:       np.ndarray,
        n:        int,
        model:    str,
        p:        float,
        rng:      Optional[np.random.Generator] = None,
        qubits:   Optional[List[int]] = None,
        jax_key:  Optional[Any] = None,
    ) -> np.ndarray:
        """
        Apply a stochastic Kraus channel to statevector *sv* in-place
        (numpy path) or via functional updates (JAX path).

        Parameters
        ----------
        sv      : complex statevector of length 2**n
        n       : number of qubits
        model   : one of NoiseModel.MODELS
        p       : error probability (or damping rate γ for amplitude_damping)
        rng     : optional pre-seeded numpy Generator; created fresh if None
        qubits  : subset of qubits to apply the channel to; defaults to all
        jax_key : optional JAX PRNGKey; created fresh if None and sv is a JAX array

        Returns
        -------
        Normalised statevector (same array type as input).
        """
        if model == 'ideal' or p <= 0.0:
            return sv

        is_jax = HAS_JAX and isinstance(sv, jnp.ndarray)
        dim    = len(sv)

        # ── RNG initialisation ────────────────────────────────────────
        if is_jax:
            if jax_key is None:
                seed_bytes = os.urandom(4)
                jax_seed   = int.from_bytes(seed_bytes, byteorder='big')
                jax_seed  ^= time.perf_counter_ns() & 0xFFFF_FFFF
                key = jax.random.PRNGKey(jax_seed)
            else:
                key = jax_key
        else:
            if rng is None:
                rng = _fresh_rng()

        target_qubits = qubits if qubits is not None else list(range(n))
        sv_out = sv  # JAX: functional; NumPy: will be modified in-place copy

        if not is_jax:
            sv_out = sv.copy()  # never mutate the caller's array

        for q in target_qubits:
            # ── correct index pair construction ───────────────────────
            idx_0, idx_1 = _qubit_index_pairs(dim, q)
            half = len(idx_0)   # == dim // 2

            # ── draw random numbers ───────────────────────────────────
            if is_jax:
                key, subkey = jax.random.split(key)
                r = jax.random.uniform(subkey, shape=(half,), minval=0.0, maxval=1.0)
            else:
                r = rng.random(half)            # uniform [0, 1)

            # ── channel application ───────────────────────────────────
            if model == 'depolarizing':
                # Three equiprobable Pauli errors, each with rate p/3
                p3 = p / 3.0
                if is_jax:
                    key, subkey2 = jax.random.split(key)
                    ch = jax.random.uniform(subkey2, shape=(half,), minval=0.0, maxval=1.0)
                    v0, v1      = sv_out[idx_0], sv_out[idx_1]
                    fire        = r < p
                    x_gate      = fire & (ch < p3)
                    y_gate      = fire & (ch >= p3) & (ch < 2.0 * p3)
                    z_gate      = fire & (ch >= 2.0 * p3)
                    new_v0 = jnp.where(x_gate,  v1,
                             jnp.where(y_gate, -1j * v1, v0))
                    new_v1 = jnp.where(x_gate,  v0,
                             jnp.where(y_gate,  1j * v0,
                             jnp.where(z_gate, -v1, v1)))
                    sv_out = sv_out.at[idx_0].set(new_v0)
                    sv_out = sv_out.at[idx_1].set(new_v1)
                else:
                    ch     = rng.random(half)
                    v0, v1 = sv_out[idx_0].copy(), sv_out[idx_1].copy()
                    fire   = r < p
                    x_gate = fire & (ch < p3)
                    y_gate = fire & (ch >= p3) & (ch < 2.0 * p3)
                    z_gate = fire & (ch >= 2.0 * p3)
                    sv_out[idx_0] = np.where(x_gate,  v1,
                                    np.where(y_gate, -1j * v1, v0))
                    sv_out[idx_1] = np.where(x_gate,  v0,
                                    np.where(y_gate,  1j * v0,
                                    np.where(z_gate, -v1, v1)))

            elif model == 'bitflip':
                # X gate applied with probability p
                fire = r < p
                if is_jax:
                    v0, v1 = sv_out[idx_0], sv_out[idx_1]
                    sv_out = sv_out.at[idx_0].set(jnp.where(fire, v1, v0))
                    sv_out = sv_out.at[idx_1].set(jnp.where(fire, v0, v1))
                else:
                    v0, v1 = sv_out[idx_0].copy(), sv_out[idx_1].copy()
                    sv_out[idx_0] = np.where(fire, v1, v0)
                    sv_out[idx_1] = np.where(fire, v0, v1)

            elif model == 'phaseflip':
                # Z gate applied with probability p:
                # Z|0⟩ = |0⟩  →  no change to idx_0 amplitudes
                # Z|1⟩ = -|1⟩ →  negate idx_1 amplitudes when fired
                fire = r < p
                if is_jax:
                    v1     = sv_out[idx_1]
                    sv_out = sv_out.at[idx_1].set(jnp.where(fire, -v1, v1))
                else:
                    v1            = sv_out[idx_1].copy()
                    sv_out[idx_1] = np.where(fire, -v1, v1)

            elif model == 'amplitude_damping':
                # K0 = [[1, 0], [0, √(1-γ)]]  — no decay
                # K1 = [[0, √γ], [0, 0]]       — decay |1⟩ → |0⟩
                # Applied stochastically: with probability γ the qubit
                # decays (K1 path), otherwise K0 is applied.
                gamma = float(np.clip(p, 0.0, 1.0))
                decay = r < gamma
                if is_jax:
                    v0, v1 = sv_out[idx_0], sv_out[idx_1]
                    # decay path:    v0 += v1 * √γ,  v1 = 0
                    # no-decay path: v0 unchanged,   v1 *= √(1-γ)
                    sq_gamma    = jnp.sqrt(gamma)
                    sq_1m_gamma = jnp.sqrt(1.0 - gamma)
                    new_v0 = jnp.where(decay, v0 + v1 * sq_gamma, v0)
                    new_v1 = jnp.where(decay, 0.0 + 0j,           v1 * sq_1m_gamma)
                    sv_out = sv_out.at[idx_0].set(new_v0)
                    sv_out = sv_out.at[idx_1].set(new_v1)
                else:
                    v0, v1      = sv_out[idx_0].copy(), sv_out[idx_1].copy()
                    sq_gamma    = np.sqrt(gamma)
                    sq_1m_gamma = np.sqrt(1.0 - gamma)
                    sv_out[idx_0] = np.where(decay, v0 + v1 * sq_gamma, v0)
                    sv_out[idx_1] = np.where(decay, 0.0 + 0j,           v1 * sq_1m_gamma)

            elif model == 'combined':
                # Worst-case NISQ: depolarizing(p/2) + amplitude_damping(p/3)
                # applied sequentially on the same qubit.
                p_dep   = p * 0.5
                p_damp  = p * 0.333333
                p3      = p_dep / 3.0

                # — depolarizing sub-channel —
                if is_jax:
                    key, sk1, sk2 = jax.random.split(key, 3)
                    r_dep = jax.random.uniform(sk1, shape=(half,), minval=0.0, maxval=1.0)
                    ch    = jax.random.uniform(sk2, shape=(half,), minval=0.0, maxval=1.0)
                    v0, v1 = sv_out[idx_0], sv_out[idx_1]
                    fire   = r_dep < p_dep
                    x_gate = fire & (ch < p3)
                    y_gate = fire & (ch >= p3) & (ch < 2.0 * p3)
                    z_gate = fire & (ch >= 2.0 * p3)
                    new_v0 = jnp.where(x_gate,  v1,
                             jnp.where(y_gate, -1j * v1, v0))
                    new_v1 = jnp.where(x_gate,  v0,
                             jnp.where(y_gate,  1j * v0,
                             jnp.where(z_gate, -v1, v1)))
                    sv_out = sv_out.at[idx_0].set(new_v0)
                    sv_out = sv_out.at[idx_1].set(new_v1)
                    # — amplitude_damping sub-channel —
                    key, sk3   = jax.random.split(key)
                    r_damp     = jax.random.uniform(sk3, shape=(half,), minval=0.0, maxval=1.0)
                    decay      = r_damp < p_damp
                    v0, v1     = sv_out[idx_0], sv_out[idx_1]
                    sq_g       = jnp.sqrt(p_damp)
                    sq_1mg     = jnp.sqrt(1.0 - p_damp)
                    sv_out     = sv_out.at[idx_0].set(jnp.where(decay, v0 + v1 * sq_g, v0))
                    sv_out     = sv_out.at[idx_1].set(jnp.where(decay, 0.0 + 0j,       v1 * sq_1mg))
                else:
                    r_dep  = rng.random(half)
                    ch     = rng.random(half)
                    v0, v1 = sv_out[idx_0].copy(), sv_out[idx_1].copy()
                    fire   = r_dep < p_dep
                    x_gate = fire & (ch < p3)
                    y_gate = fire & (ch >= p3) & (ch < 2.0 * p3)
                    z_gate = fire & (ch >= 2.0 * p3)
                    sv_out[idx_0] = np.where(x_gate,  v1,
                                    np.where(y_gate, -1j * v1, v0))
                    sv_out[idx_1] = np.where(x_gate,  v0,
                                    np.where(y_gate,  1j * v0,
                                    np.where(z_gate, -v1, v1)))
                    r_damp        = rng.random(half)
                    decay         = r_damp < p_damp
                    v0, v1        = sv_out[idx_0].copy(), sv_out[idx_1].copy()
                    sq_g          = np.sqrt(p_damp)
                    sq_1mg        = np.sqrt(1.0 - p_damp)
                    sv_out[idx_0] = np.where(decay, v0 + v1 * sq_g, v0)
                    sv_out[idx_1] = np.where(decay, 0.0 + 0j,       v1 * sq_1mg)

        # ── normalise ─────────────────────────────────────────────────
        if is_jax:
            norm = jnp.linalg.norm(sv_out)
            return sv_out / (norm + 1e-15)
        else:
            norm = np.linalg.norm(sv_out)
            return sv_out / (norm + 1e-15)

    @staticmethod
    def kraus_description(model: str) -> Dict:
        desc = {
            'ideal': {
                'kraus': 1,
                'formula': 'K₀ = I',
                'physical': 'No noise',
            },
            'depolarizing': {
                'kraus': 4,
                'formula': 'K₀=√(1-p)I  K₁=√(p/3)X  K₂=√(p/3)Y  K₃=√(p/3)Z',
                'physical': 'Isotropic Pauli error — equiprobable X, Y, Z',
            },
            'bitflip': {
                'kraus': 2,
                'formula': 'K₀=√(1-p)I  K₁=√p·X',
                'physical': 'Bit flip σ_x with probability p',
            },
            'phaseflip': {
                'kraus': 2,
                'formula': 'K₀=√(1-p)I  K₁=√p·Z',
                'physical': 'Pure dephasing σ_z with probability p',
            },
            'amplitude_damping': {
                'kraus': 2,
                'formula': 'K₀=diag(1,√(1-γ))  K₁=[[0,√γ],[0,0]]',
                'physical': 'T₁ energy relaxation |1⟩→|0⟩ with rate γ',
            },
            'combined': {
                'kraus': 6,
                'formula': 'Depolarizing(p/2) ∘ AmplitudeDamping(p/3)',
                'physical': 'Worst-case NISQ: dephasing + relaxation',
            },
        }
        return desc.get(model, desc['ideal'])