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Merged
rcjackson merged 1 commit into from
Feb 17, 2026
Merged

normalized_central_derivative #77

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11 changes: 10 additions & 1 deletion pysp2/util/normalized_derivative_method.py
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Original file line number Diff line number Diff line change
@@ -1,7 +1,7 @@
import numpy as np
import xarray as xr

def central_difference(S, num_records=None):
def central_difference(S, num_records=None, normalize=True):

"""
Compute fourth order derivative S'(t) using the
Expand All @@ -20,6 +20,9 @@ def central_difference(S, num_records=None):
num_records: int or None
Only process first num_records datapoints.
Set to None to process all records.
normalize: bool
If True, normalize the derivative by the scattering signal
S(t) to get (1/S) * dS/dt.

Returns
-------
Expand Down Expand Up @@ -52,6 +55,12 @@ def central_difference(S, num_records=None):
d[:, n-2] = (25*y[:, n-2] - 48*y[:, n-3] + 36*y[:, n-4] - 16*y[:, n-5] + 3*y[:, n-6]) / (12*dt)
d[:, n-1] = (25*y[:, n-1] - 48*y[:, n-2] + 36*y[:, n-3] - 16*y[:, n-4] + 3*y[:, n-5]) / (12*dt)

if normalize:
with np.errstate(divide='ignore', invalid='ignore'):
d = np.where(y != 0, d / y, 0)
else:
d = d

dSdt[ch] = xr.DataArray(
d,
dims=('event_index', 'time'),
Expand Down
12 changes: 10 additions & 2 deletions tests/test_ndm.py
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Original file line number Diff line number Diff line change
Expand Up @@ -5,12 +5,20 @@ def test_central_difference():
my_sp2b = pysp2.io.read_sp2(pysp2.testing.EXAMPLE_SP2B)
my_ini = pysp2.io.read_config(pysp2.testing.EXAMPLE_INI)
my_binary = pysp2.util.gaussian_fit(my_sp2b, my_ini, parallel=False)
dSdt = pysp2.util.central_difference(my_binary)
dSdt = pysp2.util.central_difference(my_binary, normalize=False)

np.testing.assert_almost_equal(dSdt['Data_ch4'].isel(event_index=5876, time=0).item(),
8.3333333333e6, decimal=2)
np.testing.assert_almost_equal(dSdt['Data_ch4'].isel(event_index=5876, time=99).item(),
7.166666666e7, decimal=2)
np.testing.assert_almost_equal(dSdt['Data_ch4'].isel(event_index=5876, time=19).item(),
1.5e7, decimal=2)
assert np.isfinite(dSdt).all()
assert np.isfinite(dSdt).all()

dSdt_norm = pysp2.util.central_difference(my_binary, normalize=True)
np.testing.assert_almost_equal(dSdt_norm['Data_ch4'].isel(event_index=5876, time=0).item(),
8.3333333333e6/-30168, decimal=2)
np.testing.assert_almost_equal(dSdt_norm['Data_ch4'].isel(event_index=5876, time=99).item(),
7.166666666e7/-30152, decimal=2)
np.testing.assert_almost_equal(dSdt_norm['Data_ch4'].isel(event_index=5876, time=19).item(),
1.5e7/-30132, decimal=2)
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