Improve surface_elevation internal method selection (#340)

* Test: Determine method using input frequency index

* Feat: Use sum of sines if ifft is not computable

This change allows `surface_elevation` to return a result if the user
inputs a spectrum with a frequency index that does not have a zero
frequency.

If the non zero frequency index condition is found when the method is
`ifft` we warn the user and change the method to `sum_of_sines`

* Fix: Use previously found frequency index

S.index may not exist for some input datasets, but f[0] does and we
should use the value of f[0] here.

* Test: Warn when using ifft with a non zero frequency

* Lint

mhkit/tests/wave/test_resource_spectrum.py

@@ -8,6 +8,7 @@
 import pandas as pd
 import numpy as np
 import unittest
+import pytest
 import os
 
 
@@ -158,6 +159,36 @@ def test_ifft_sum_of_sines(self):
 
         assert_allclose(eta_ifft, eta_sos)
 
+    def test_surface_elevation_uses_sum_of_sines_when_input_frequency_index_does_not_have_zero(
+        self,
+    ):
+        f = np.linspace(1 / 30, 1 / 2, 32)
+        S = wave.resource.jonswap_spectrum(f, self.Tp, self.Hs)
+
+        eta_default = wave.resource.surface_elevation(S, self.t, seed=1)
+        eta_sos = wave.resource.surface_elevation(
+            S, self.t, seed=1, method="sum_of_sines"
+        )
+
+        assert_allclose(eta_default, eta_sos)
+
+    def test_surface_elevation_warn_user_if_zero_frequency_not_defined_and_using_ifft(
+        self,
+    ):
+        f = np.linspace(1 / 30, 1 / 2, 32)
+        S = wave.resource.jonswap_spectrum(f, self.Tp, self.Hs)
+
+        with pytest.warns(UserWarning):
+            wave.resource.surface_elevation(S, self.t, seed=1, method="ifft")
+
+    def test_surface_elevation_uses_ifft_when_input_frequency_index_has_zero(self):
+        S = wave.resource.jonswap_spectrum(self.f, self.Tp, self.Hs)
+
+        eta_default = wave.resource.surface_elevation(S, self.t, seed=1)
+        eta_ifft = wave.resource.surface_elevation(S, self.t, seed=1, method="ifft")
+
+        assert_allclose(eta_default, eta_ifft)
+
     def test_plot_spectrum(self):
         filename = abspath(join(plotdir, "wave_plot_spectrum.png"))
         if isfile(filename):

mhkit/wave/resource.py

@@ -1,3 +1,5 @@
+import warnings
+
 from scipy.optimize import fsolve as _fsolve
 from scipy import signal as _signal
 import pandas as pd
@@ -335,9 +337,10 @@ def surface_elevation(
 
     if method == "ifft":
         if not f[0] == 0:
-            raise ValueError(
-                f"ifft method must have zero frequency defined. Lowest frequency is: {S.index.values[0]}"
+            warnings.warn(
+                f"ifft method must have zero frequency defined. Lowest frequency is: {f[0].values}. Setting method to less efficient `sum_of_sines` method."
             )
+            method = "sum_of_sines"
 
     if frequency_bins is None:
         delta_f = f.values[1] - f.values[0]