app/decoder.py

from __future__ import annotations

from typing import Dict, List


def baseline_decoder(syndrome: List[int], n_qubits: int) -> Dict[int, str]:
    """
    Simple deterministic decoder used for /baseline.

    It maps active syndrome bits to nearby qubit indices and applies alternating
    X/Z corrections, which creates a stable non-adaptive reference policy.
    """
    corrections: Dict[int, str] = {}
    if not syndrome:
        return corrections

    for idx, bit in enumerate(syndrome):
        if bit != 1:
            continue
        q = idx % max(1, n_qubits)
        corrections[q] = "X" if (idx % 2 == 0) else "Z"

    # Cap correction count to keep action bounded and low latency.
    if len(corrections) > 12:
        items = list(corrections.items())[:12]
        corrections = dict(items)

    return corrections