#!/usr/bin/env python3
"""JSON to CSV converter with field selection and CLI."""
from __future__ import annotations
import argparse, csv, json, sys
from pathlib import Path

def load_rows(path: Path):
    data = json.loads(path.read_text(encoding='utf-8'))
    if isinstance(data, dict):
        # common wrappers
        for k in ('data','items','results','records'):
            if isinstance(data.get(k), list):
                data = data[k]
                break
        else:
            data = [data]
    if not isinstance(data, list):
        raise SystemExit('JSON root must be a list or object with a list field')
    return data

def main():
    p = argparse.ArgumentParser(description='Convert JSON array to CSV')
    p.add_argument('input', type=Path)
    p.add_argument('-o','--output', type=Path, required=True)
    p.add_argument('-f','--fields', help='Comma-separated fields (default: union of keys)')
    args = p.parse_args()
    rows = load_rows(args.input)
    if not rows:
        print('empty input', file=sys.stderr); return 1
    if args.fields:
        fields = [x.strip() for x in args.fields.split(',') if x.strip()]
    else:
        fields = []
        seen=set()
        for row in rows:
            if isinstance(row, dict):
                for k in row:
                    if k not in seen:
                        seen.add(k); fields.append(k)
    args.output.parent.mkdir(parents=True, exist_ok=True)
    with args.output.open('w', newline='', encoding='utf-8') as f:
        w = csv.DictWriter(f, fieldnames=fields, extrasaction='ignore')
        w.writeheader()
        for row in rows:
            if not isinstance(row, dict):
                raise SystemExit('rows must be objects')
            w.writerow({k: row.get(k, '') for k in fields})
    print(f'wrote {len(rows)} rows -> {args.output}')
    return 0

if __name__ == '__main__':
    raise SystemExit(main())