#!/usr/bin/env python3
"""
get_watershed.py — retrieve the NLDI upstream contributing watershed for a USGS
gauge and write watershed.geojson. Retries transient NLDI failures (the service
throttles at roughly 800 requests/hour; most failures are transient).

Example
-------
  python get_watershed.py --gauge 08165500 --out watershed.geojson
"""
import argparse
import json

from torrent_tools import fetch_basin


def main():
    ap = argparse.ArgumentParser()
    ap.add_argument("--gauge", required=True)
    ap.add_argument("--out", default="watershed.geojson")
    args = ap.parse_args()

    gj = fetch_basin(args.gauge.zfill(8))
    with open(args.out, "w") as f:
        json.dump(gj, f)
    n = len(gj.get("features", []))
    print(f"wrote {args.out} ({n} feature(s))")


if __name__ == "__main__":
    main()