test: adding a test to unsqueeze squeezed data

Author: Luiskitsu

news/test_squeeze.rst

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+**Added:**
+
+* Squeeze test: given a squeezed signal, unsqueezing recovers the original
+
+**Changed:**
+
+* <news item>
+
+**Deprecated:**
+
+* <news item>
+
+**Removed:**
+
+* <news item>
+
+**Fixed:**
+
+* <news item>
+
+**Security:**
+
+* <news item>

tests/test_morphsqueeze.py

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+import matplotlib.pyplot as plt
+import numpy as np
+
+
+def test_morphsqueeze():
+    """
+    Test that we can unsqueeze squeezed data.
+    The test inputs are an expected uniform target (e.g. synchrotron data)
+    and a squeezed version of the target (e.g. XFEL data). The squeezed data
+    is created by applying a nonlinear distortion to the uniform target.
+    Both input data are in the same uniform x-axis grid.
+    Then we unsqueeze the squeezed data by doing the inverse transformation
+    using interpolatiion.
+    Finally we check that the unsqueezed data matches the expected uniform
+    target.
+    """
+
+    # 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)
+
+    # Apply squeeze parameters to uniform data to get the squeezed data
+    squeeze_1 = 0.003
+    squeeze_2 = 0.004
+    x_squeezed = x + squeeze_1 * x**2 + squeeze_2 * x**3
+    y_squeezed = np.sin(x_squeezed)
+
+    # Unsqueeze the data by interpolating back to uniform grid
+    y_unsqueezed = np.interp(x, x_squeezed, y_squeezed)
+    y_actual = y_unsqueezed
+
+    # Check that the unsqueezed (actual) data matches the expected data
+    # Including tolerance error because I was having issues
+    # with y_actual == y_expected. I think is because interpolation?
+    assert np.allclose(y_actual, y_expected, atol=1e-3)
+
+    # Plot to verify what we are doing
+    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()