Add fft frequency transformation (#62)

* add fft frequency transformation for data where the sampling rate must be at least twice the signal bandwidth

* add fft frequency transformation for data where the sampling rate isnt at least twice the signal bandwidth

* fft frequency transofrmation

* fft freq unit test

* fft freq unit test

* fix lint

* fix lint

Author: SaraPido

sigpro/transformations/frequency/fftfreq.py

@@ -16,6 +16,7 @@ def fft_freq(amplitude_values, sampling_frequency):
             * `amplitude_values (numpy.ndarray)`
             * `frequency_values (numpy.ndarray)`
     """
-    frequency_values = np.fft.fftfreq(len(amplitude_values), 1 / sampling_frequency)
+    # frequency_values = np.fft.fftfreq(len(amplitude_values), 1 / sampling_frequency)
+    frequency_values = np.arange(0, len(amplitude_values)) * sampling_frequency
 
     return np.array(amplitude_values), np.array(frequency_values)

tests/unit/transformations/frequency/test_fftfreq.py

@@ -0,0 +1,19 @@
+"""Tests for sigpro.transformations.frequency module."""
+import numpy as np
+
+from sigpro.transformations.frequency.fftfreq import fft_freq
+
+
+def test_fft_freq():
+    # setup
+    amplitude_values = [1.5, -0.5, 2.0, 0.5, -1.0]
+    sampling_frequency = 2
+
+    # run
+    amplitude_result, frequency_result = fft_freq(amplitude_values, sampling_frequency)
+
+    # assert
+    expected_amplitude_values = np.array([1.5, -0.5, 2.0, 0.5, -1.0])
+    expected_frequency_values = np.array([0., 2., 4., 6., 8.])
+    np.testing.assert_array_almost_equal(amplitude_result, expected_amplitude_values)
+    np.testing.assert_array_almost_equal(frequency_result, expected_frequency_values)