utils/mnist_reader.py

from __future__ import annotations

import gzip
import struct
from pathlib import Path

import numpy as np


def load_mnist(path: str | Path, kind: str = "train") -> tuple[np.ndarray, np.ndarray]:
    """Load UFO-MNIST IDX gzip files.

    Parameters
    ----------
    path:
        Directory containing `*-images-idx3-ubyte.gz` and `*-labels-idx1-ubyte.gz`.
    kind:
        Either `train` or `t10k`.
    """
    path = Path(path)
    labels_path = path / f"{kind}-labels-idx1-ubyte.gz"
    images_path = path / f"{kind}-images-idx3-ubyte.gz"

    with gzip.open(labels_path, "rb") as handle:
        magic, count = struct.unpack(">II", handle.read(8))
        if magic != 2049:
            raise ValueError(f"Invalid label file magic number: {magic}")
        labels = np.frombuffer(handle.read(), dtype=np.uint8)
        if labels.shape[0] != count:
            raise ValueError(f"Expected {count} labels, found {labels.shape[0]}")

    with gzip.open(images_path, "rb") as handle:
        magic, count, rows, cols = struct.unpack(">IIII", handle.read(16))
        if magic != 2051:
            raise ValueError(f"Invalid image file magic number: {magic}")
        images = np.frombuffer(handle.read(), dtype=np.uint8).reshape(count, rows * cols)

    if images.shape[0] != labels.shape[0]:
        raise ValueError("Image and label counts differ")
    return images, labels