Dft reverse axis

xrft/tests/test_xrft.py

@@ -1252,3 +1252,17 @@ def test_constant_coordinates():
 
         with pytest.raises(ValueError):
             xrft.idft(s)
+
+
+def test_reversed_coordinates():
+    """Reversed coordinates should not impact dft with true_phase = True"""
+    N = 20
+    s = xr.DataArray(
+        np.random.rand(N) + 1j * np.random.rand(N),
+        dims="x",
+        coords={"x": np.arange(N // 2, -N // 2, -1) + 2},
+    )
+    s2 = s.sortby("x")
+    xrt.assert_allclose(
+        xrft.dft(s, dim="x", true_phase=True), xrft.dft(s2, dim="x", true_phase=True)
+    )

xrft/xrft.py

@@ -214,7 +214,11 @@ def _lag_coord(coord):
 
     v0 = coord.values[0]
     calendar = getattr(v0, "calendar", None)
-    lag = coord[(len(coord.data)) // 2]
+    if coord[-1] > coord[0]:
+        coord_data = coord.data
+    else:
+        coord_data = np.flip(coord.data, axis=-1)
+    lag = coord_data[len(coord.data) // 2]
     if calendar:
         import cftime
 
@@ -408,7 +412,12 @@ def dft(
         _, da = _apply_window(da, dim, window_type=window)
 
     if true_phase:
-        f = fft_fn(fft.ifftshift(da.data, axes=axis_num), axes=axis_num)
+        reversed_axis = [
+            da.get_axis_num(d) for d in dim if da[d][-1] < da[d][0]
+        ]  # handling decreasing coordinates
+        f = fft_fn(
+            fft.ifftshift(np.flip(da, axis=reversed_axis), axes=axis_num), axes=axis_num
+        )
     else:
         f = fft_fn(da.data, axes=axis_num)
 
@@ -566,7 +575,7 @@ def idft(
         delta = np.abs(diff[0])
         l = _lag_coord(daft[d]) if d is not real_dim else daft[d][0].data
         if not np.allclose(
-            diff, diff[0], rtol=spacing_tol
+            diff, delta, rtol=spacing_tol
         ):  # means that input is not on regular increasing grid
             reordered_coord = daft[d].copy()
             reordered_coord = reordered_coord.sortby(d)