Same index is present multiple times in FPS output during sample selection

[Luthaf]

Running this code

import numpy as np
import skcosmo.sample_selection

X = np.load("power-spectrum.npy")
print("shape =", X.shape)
print()

fps = skcosmo.sample_selection.FPS(n_to_select=35)
fps.fit(X)
print("selected_idx_ =", fps.selected_idx_)

outputs

shape = (640, 320)

selected_idx_ = [  0 247  45 105  16  56 176  25   9  38 152  72  54 192 131  88  64 168
 212 189 166 202  83 145  96 113 142 125 217 226 233   9   9   9   9]

There are enough samples to select more than 35 of them, but the last one is repeated multiple times in the output, which is unexpected. I'll try to double check the data to see if we are creating the same sample multiple time for some reason.

power-spectrum.npy.zip

[Luthaf]

So the data does contains the same entry multiple time, by blocks of 8 samples (the input structures contains 8 atoms per frame). This script:

start = 0
for _ in range(X.shape[0] // 8):
    stop = start + 8
    x = X[start:stop, :]

    print(f"{np.linalg.norm(x):.4}    {np.linalg.norm(x - x[0]):.4}")
    start = stop

outputs something like

243.6    1.634e-12
227.3    1.377e-12
210.5    1.213e-12
194.1    1.185e-12
178.9    1.275e-12
165.4    1.699e-12
154.2    1.409e-12
145.3    1.646e-12
138.7    1.114e-12
134.1    6.263e-13
130.7    1.202e-12
127.7    1.208e-12
125.3    1.173e-12
123.4    9.692e-13
121.8    7.84e-13
120.2    1.432e-12
118.6    1.155e-12
116.9    1.12e-12
114.8    1.026e-12
112.1    6.875e-13
108.7    1.746e-12
       ...

I.e. each block of 8 is different, but all samples in a block contains the same data, up to some numerical noise.