#!/usr/bin/env python3
"""Split an image into two lossless, random-looking PNG fixture shares."""

from __future__ import annotations

import argparse
from io import BytesIO
from pathlib import Path
import secrets

from PIL import Image


def split_image(input_path: Path, output_dir: Path, index: int | None = None) -> tuple[Path, Path]:
    if index is None:
        index = 1
        while any((output_dir / f"image_{index}{part}.png").exists() for part in ("a", "b")):
            index += 1
    if index < 1:
        raise ValueError("Image index must be a positive integer")

    paths = (
        output_dir / f"image_{index}a.png",
        output_dir / f"image_{index}b.png",
    )
    for path in paths:
        if path.exists():
            raise FileExistsError(f"Refusing to overwrite {path}; choose another --index")

    # RGBA shares preserve every decoded pixel, including transparency. For animated
    # inputs, use the first frame, matching the dashboard's static image upload behavior.
    with Image.open(input_path) as source:
        image = source.convert("RGBA")
    pixels = image.tobytes()
    first = secrets.token_bytes(len(pixels))
    second = bytes(pixel ^ mask for pixel, mask in zip(pixels, first))

    output_dir.mkdir(parents=True, exist_ok=True)
    for path, share in zip(paths, (first, second)):
        # Exclusive creation also prevents overwriting a file created during splitting.
        encoded = BytesIO()
        Image.frombytes("RGBA", image.size, share).save(encoded, format="PNG")
        with path.open("xb") as output:
            output.write(encoded.getvalue())
    return paths


def main() -> None:
    parser = argparse.ArgumentParser(description=__doc__)
    parser.add_argument("input_image", type=Path)
    parser.add_argument("--output-dir", type=Path, default=Path("."))
    parser.add_argument("--index", type=int, help="Fixture number (default: first unused positive index)")
    args = parser.parse_args()
    try:
        paths = split_image(args.input_image, args.output_dir, args.index)
    except (OSError, ValueError) as error:
        parser.exit(1, f"Could not split image: {error}\n")
    for path in paths:
        print(path)


if __name__ == "__main__":
    main()