#!/usr/bin/env python3
"""
download_streamflow.py — fetch the NWIS instantaneous discharge record for a
gauge over an event window (same convention as the released packages:
24 h before begin to 72 h after end) and write streamflow.csv
(columns: datetime_utc, value [cfs]).

Example
-------
  python download_streamflow.py --gauge 08165500 \
      --begin 2025-07-04T00:00 --end 2025-07-05T00:00 --out streamflow.csv
"""
import argparse
from datetime import timedelta

from torrent_tools import fetch_iv, parse_utc


def main():
    ap = argparse.ArgumentParser()
    ap.add_argument("--gauge", required=True)
    ap.add_argument("--begin", required=True, help="episode begin (UTC)")
    ap.add_argument("--end", required=True, help="episode end (UTC)")
    ap.add_argument("--pre-hours", type=float, default=24.0)
    ap.add_argument("--post-hours", type=float, default=72.0)
    ap.add_argument("--parameter", default="00060", help="00060 discharge / 00065 stage")
    ap.add_argument("--out", default="streamflow.csv")
    args = ap.parse_args()

    b = parse_utc(args.begin) - timedelta(hours=args.pre_hours)
    e = parse_utc(args.end) + timedelta(hours=args.post_hours)
    df = fetch_iv(args.gauge.zfill(8), b, e, args.parameter)
    if df.empty:
        raise SystemExit(f"no {args.parameter} IV data for {args.gauge} in window")
    df.to_csv(args.out, index=False)
    print(f"{args.gauge}: {len(df)} samples "
          f"{df.datetime_utc.iloc[0]} -> {df.datetime_utc.iloc[-1]}")
    print(f"wrote {args.out}")


if __name__ == "__main__":
    main()