MAINT: fix two more issues with padding length 0 and size-0 axis input

pywt/_dwt.py

@@ -461,16 +461,27 @@ def pad(x, pad_widths, mode):
         xp = np.pad(x, pad_widths, mode='edge')
     elif mode == 'smooth':
         def pad_smooth(vector, pad_width, iaxis, kwargs):
+            # Note: indices are measured from the start of the vector so that
+            # a pad width of 0 gives an empty slice rather than the full one.
+            iright = vector.size - pad_width[1] - 1  # last unpadded sample
+            vsize_nonpad = iright + 1 - pad_width[0]
+            if vsize_nonpad < 2:
+                # The slope is undefined for a signal shorter than two
+                # samples, so use constant edge extension instead. This is
+                # what the transforms do as well (see the MODE_SMOOTH case in
+                # convolution.template.c).
+                if vsize_nonpad == 1:
+                    vector[:pad_width[0]] = vector[iright]
+                    vector[iright + 1:] = vector[iright]
+                return vector
+
             # smooth extension to left
             left = vector[pad_width[0]]
             slope_left = (left - vector[pad_width[0] + 1])
             vector[:pad_width[0]] = \
                 left + np.arange(pad_width[0], 0, -1) * slope_left
 
             # smooth extension to right
-            # Note: indices are measured from the start of the vector so that
-            # a pad width of 0 gives an empty slice rather than the full one.
-            iright = vector.size - pad_width[1] - 1
             right = vector[iright]
             slope_right = (right - vector[iright - 1])
             vector[iright + 1:] = \
@@ -479,10 +490,13 @@ def pad_smooth(vector, pad_width, iaxis, kwargs):
         xp = np.pad(x, pad_widths, pad_smooth)
     elif mode == 'antisymmetric':
         def pad_antisymmetric(vector, pad_width, iaxis, kwargs):
-            # smooth extension to left
-            # implement by flipping portions symmetric padding
+            # implement by flipping portions of symmetric padding
             npad_l, npad_r = pad_width
             vsize_nonpad = vector.size - npad_l - npad_r
+            if vsize_nonpad == 0:
+                # Nothing to reflect. The reflected segments below would have
+                # zero width, so the loops over them would never terminate.
+                return vector
             # Note: must modify vector in-place
             vector[:] = np.pad(vector[npad_l:vector.size - npad_r],
                                pad_width, mode='symmetric')

pywt/tests/test_dwt_idwt.py

@@ -290,6 +290,19 @@ def test_pad_zero_width():
             assert_array_equal(pywt.pad(x, 0, mode), x,
                                err_msg=f"mode={mode}, ndim={ndim}")
 
+    assert_array_equal(pywt.pad([1, 2, 3], 0, 'periodization'), [1, 2, 3, 3])
+
+
+def test_pad_zero_size_axis():
+    # a zero-size axis has nothing to extend from, so a zero pad width leaves
+    # it alone rather than raising or (for 'antisymmetric') hanging (gh-589)
+    x = np.ones((0, 4))
+    for mode in pywt.Modes.modes:
+        assert_array_equal(pywt.pad(x, 0, mode), x, err_msg=f"mode={mode}")
+
+    # padding along the other axis only
+    assert_(pywt.pad(x, ((0, 0), (2, 2)), 'antisymmetric').shape == (0, 8))
+
 
 def test_pad_one_sided():
     # a zero pad width on only one side of the axis (gh-589)
@@ -310,6 +323,25 @@ def test_pad_one_sided():
                            err_msg=f"mode={mode}")
 
 
+def test_pad_smooth_short_signal():
+    # the slope used by 'smooth' is undefined for a signal shorter than two
+    # samples, so constant edge extension is used instead (gh-589)
+    assert_array_equal(pywt.pad([1.], 0, 'smooth'), [1.])
+    assert_array_equal(pywt.pad([1.], (2, 3), 'smooth'),
+                       pywt.pad([1.], (2, 3), 'constant'))
+
+    # only the length 1 axis of an nd array falls back
+    x = np.arange(3.0).reshape(1, 3)
+    assert_array_equal(pywt.pad(x, ((0, 0), (2, 2)), 'smooth'),
+                       [[-2, -1, 0, 1, 2, 3, 4]])
+    assert_array_equal(pywt.pad(x, ((2, 2), (0, 0)), 'smooth'),
+                       np.tile(x, (5, 1)))
+
+    # matches the fallback used by the transforms
+    assert_allclose(pywt.dwt([1.], 'db2', 'smooth'),
+                    pywt.dwt([1.], 'db2', 'constant'))
+
+
 def test_pad_errors():
     # negative pad width
     x = [1, 2, 3]