"""Snapshot diff logic."""
from typing import Any, Optional


def diff_snapshots(
    previous: Optional[dict[str, Any]],
    current: dict[str, Any],
) -> list[dict[str, Any]]:
    """Return list of rate changes between previous and current snapshots.
    Returns empty list if previous is None (first check)."""
    if not previous:
        return []
    old_groups: dict[str, Any] = previous.get("groups") or {}
    new_groups: dict[str, Any] = current.get("groups") or {}
    changes: list[dict[str, Any]] = []
    for gid, new_g in sorted(new_groups.items()):
        if not isinstance(new_g, dict):
            continue
        old_g = old_groups.get(gid)
        old_rate = old_g.get("rate") if isinstance(old_g, dict) else None
        new_rate = new_g.get("rate")
        if old_rate != new_rate:
            changes.append({
                "group_id": gid,
                "group_name": new_g.get("group_name", ""),
                "platform": new_g.get("platform", ""),
                "old_rate": old_rate,
                "new_rate": new_rate,
            })
    for gid, old_g in sorted(old_groups.items()):
        if gid not in new_groups and isinstance(old_g, dict):
            changes.append({
                "group_id": gid,
                "group_name": old_g.get("group_name", ""),
                "platform": old_g.get("platform", ""),
                "old_rate": old_g.get("rate"),
                "new_rate": None,
            })
    return changes