#!/usr/bin/env python3
"""Merge new questions into an existing question file, deduplicating by ID.

Usage:
    python3 merge_questions.py <existing_file> <new_file> <output_file>

If existing_file does not exist or is empty, treats it as having no questions.
Outputs a JSON summary to stdout with counts.

Exit codes: 0 = success, 1 = error, 2 = usage error.
"""
import json
import sys
from pathlib import Path


def load_json(path: str) -> dict | None:
    """Load JSON file, return None if missing or empty."""
    p = Path(path)
    if not p.exists() or p.stat().st_size == 0:
        return None
    with open(p, encoding="utf-8") as f:
        return json.load(f)


def merge(existing: dict | None, new_data: dict) -> dict:
    """Merge new questions into existing, deduplicating by ID."""
    existing_questions = (existing or {}).get("questions", [])
    new_questions = new_data.get("questions", [])

    existing_ids = {q["id"] for q in existing_questions}
    added = [q for q in new_questions if q["id"] not in existing_ids]
    skipped = len(new_questions) - len(added)
    merged = existing_questions + added

    # Use new_data as base, replace questions with merged
    result = dict(new_data)
    result["questions"] = merged

    return result, {
        "existing_count": len(existing_questions),
        "new_count": len(new_questions),
        "added_count": len(added),
        "skipped_count": skipped,
        "total_count": len(merged),
    }


def main():
    if len(sys.argv) != 4:
        print(f"Usage: {sys.argv[0]} <existing_file> <new_file> <output_file>",
              file=sys.stderr)
        sys.exit(2)

    existing_path = sys.argv[1]
    new_path = sys.argv[2]
    output_path = sys.argv[3]

    existing = load_json(existing_path)

    try:
        with open(new_path, encoding="utf-8") as f:
            new_data = json.load(f)
    except (FileNotFoundError, json.JSONDecodeError) as e:
        print(f"❌ Cannot read new questions file: {e}", file=sys.stderr)
        sys.exit(1)

    merged, stats = merge(existing, new_data)

    with open(output_path, "w", encoding="utf-8") as f:
        json.dump(merged, f, ensure_ascii=False, indent=2)

    print(json.dumps(stats, ensure_ascii=False))
    sys.exit(0)


if __name__ == "__main__":
    main()