test: adding a test to unsqueeze squeezed data

news/test_squeeze.rst

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+**Added:**
+
+* Polynomial squeeze of x-axis of morphed data
+
+**Changed:**
+
+* <news item>
+
+**Deprecated:**
+
+* <news item>
+
+**Removed:**
+
+* <news item>
+
+**Fixed:**
+
+* <news item>
+
+**Security:**
+
+* <news item>

tests/test_morphsqueeze.py

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+import numpy as np
+import pytest
+from numpy.polynomial import Polynomial
+from scipy.interpolate import interp1d
+
+
+@pytest.mark.parametrize(
+    "squeeze_coeffs",
+    [
+        # The order of coefficients is [a0, a1, a2, ..., an]
+        # Negative cubic squeeze coefficients
+        [-0.2, -0.01, -0.001, -0.001],
+        # Positive cubic squeeze coefficients
+        [0.2, 0.01, 0.001, 0.001],
+        # Positive and negative cubic squeeze coefficients
+        [0.2, -0.01, 0.001, -0.001],
+        # Quadratic squeeze coefficients
+        [-0.2, 0.005, -0.003],
+        # Linear squeeze coefficients
+        [0.1, 0.3],
+        # 4th order squeeze coefficients
+        [0.2, -0.01, 0.001, -0.001, 0.0001],
+    ],
+)
+def test_morphsqueeze(squeeze_coeffs):
+    # Uniform x-axis grid. This is the same x-axis for all data.
+    x = np.linspace(0, 10, 1000)
+    # Expected uniform target
+    y_expected = np.sin(x)
+
+    # Create polynomial based on a list of values for polynomial coefficients
+    squeeze_polynomial = Polynomial(squeeze_coeffs)
+    # Apply squeeze parameters to uniform data to get the squeezed data
+    x_squeezed = x + squeeze_polynomial(x)
+    y_squeezed = np.sin(x_squeezed)
+
+    # Unsqueeze the data by interpolating back to uniform grid
+    y_unsqueezed = interp1d(
+        x_squeezed,
+        y_squeezed,
+        kind="cubic",
+        bounds_error=False,
+        fill_value="extrapolate",
+    )(x)
+    y_actual = y_unsqueezed
+
+    # Check that the unsqueezed (actual) data matches the expected data
+    # Including tolerance error because of extrapolation error
+    assert np.allclose(y_actual, y_expected, atol=1)
+
+    # This plotting code was used for the comments in the github
+    # PR https://github.com/diffpy/diffpy.morph/pull/180
+    # plt.figure(figsize=(7, 4))
+    # plt.plot(x, y_expected, color="black", label="Expected uniform data")
+    # plt.plot(x, y_squeezed, "--", color="purple", label="Squeezed data")
+    # plt.plot(x, y_unsqueezed, "--", color="gold", label="Unsqueezed data")
+    # plt.xlabel("x")
+    # plt.ylabel("y")
+    # plt.legend()
+    # plt.show()