import numpy as np

PALETTE = np.array([
    [0, 0, 0],
    [100, 0, 100],
    [255, 255, 255],
    [0, 255, 0],
    [255, 255, 0],
    [0, 0, 255],
    [255, 0, 0],
    [255, 0, 255],
    [0, 255, 255],
], dtype=np.int32)

def decode_mask(mask_rgb):
    h, w = mask_rgb.shape[:2]
    flat = mask_rgb.reshape(-1, 3).astype(np.int32)
    d2 = (
        (flat[:, None, :] - PALETTE[None, :, :]) ** 2
    ).sum(-1)
    classes = d2.argmin(axis=1).astype(np.uint8)
    return classes.reshape(h, w)