추가만 하는 원본 — 상태 말고 행위를 기록해

# One JSON object per line, appended, never rewritten.
# {"ts": "...", "type": "like", "article_id": 4812}
# {"ts": "...", "type": "save", "article_id": 4812}
# {"ts": "...", "type": "unsave", "article_id": 4812}

EVENT_KINDS = (
    "read", "like", "unlike", "dislike", "undislike",
    "save", "unsave", "share", "queue", "unqueue",
)


def append_event(kind: str, article_id: int, **extra) -> None:
    """The only write path into ground truth. A reversal is a NEW event,
    never an edit of the one it reverses -- that is what keeps a
    projection a pure function of the log."""
    if kind not in EVENT_KINDS:
        raise ValueError(f"unknown event kind: {kind}")
    row = {"ts": utcnow(), "type": kind, "article_id": article_id, **extra}
    with open(EVENT_LOG_PATH, "a", encoding="utf-8") as fh:
        fh.write(json.dumps(row, ensure_ascii=False) + "\n")


def project_state(log):
    """Every shelf, badge and score is one of these -- a fold over the
    log. Drop the database and this still returns the right answer."""
    state = {}
    for e in log:                      # chronological; later wins
        st = state.setdefault(e["article_id"], {})
        if e["type"] == "save":       st["saved_at"] = e["ts"]
        elif e["type"] == "unsave":   st["saved_at"] = None
        elif e["type"] == "like":     st["liked_at"], st["disliked_at"] = e["ts"], None
        elif e["type"] == "dislike":  st["disliked_at"], st["liked_at"] = e["ts"], None
    return state