Handled linear extrapolation exception when fewer than 2 interpolation points.

Author: pelson

stratify/_vinterp.pyx

@@ -277,13 +277,19 @@ cdef long linear_extrap(int direction, double[:] z_src,
                         double[:] fz_target) nogil except -1:
     """Linear extrapolation using either the first or last 2 values."""
     cdef unsigned int m = fz_src.shape[0]
+    cdef unsigned int n_src_pts = fz_src.shape[1]
     cdef unsigned int p0, p1, i
     cdef double frac
 
+    if n_src_pts < 2:
+        with gil:
+            raise ValueError('Linear extrapolation requires at least '
+                             '2 source points. Got {}.'.format(n_src_pts))
+
     if direction < 0:
         p0, p1 = 0, 1
     else:
-        p0, p1 = fz_src.shape[1] - 2, fz_src.shape[1] - 1
+        p0, p1 = n_src_pts - 2, n_src_pts - 1
 
     frac = ((level - z_src[p0]) /
             (z_src[p1] - z_src[p0]))

stratify/tests/test_vinterp.py

@@ -145,6 +145,19 @@ def test_high_precision(self):
         assert_array_almost_equal(self.interpolate([1.123456789]),
                                   [11.23456789], decimal=6)
 
+    def test_single_point(self):
+        # Test that a single input point that falls exactly on the target
+        # level triggers a shortcut that avoids the expectation of >=2 source
+        # points.
+        interpolation = stratify.INTERPOLATE_LINEAR
+        extrapolation = vinterp._TestableDirectionExtrapKernel()
+
+        r = stratify.interpolate([2], [2], [20],
+                                 interpolation=interpolation,
+                                 extrapolation=extrapolation,
+                                 rising=True)
+        self.assertEqual(r, 20) 
+
 
 class Test_INTERPOLATE_NEAREST(unittest.TestCase):
     def interpolate(self, x_target):
@@ -156,8 +169,8 @@ def interpolate(self, x_target):
 
         # Use -2 to test negative number support.
         return stratify.interpolate(np.array(x_target) - 2, x_src - 2, fx_src,
-                                   interpolation=interpolation,
-                                   extrapolation=extrapolation)
+                                    interpolation=interpolation,
+                                    extrapolation=extrapolation)
 
     def test_on_the_mark(self):
         assert_array_equal(self.interpolate([0, 1, 2, 3, 4]),
@@ -183,8 +196,8 @@ def interpolate(self, x_target):
 
         # Use -2 to test negative number support.
         return stratify.interpolate(np.array(x_target) - 2, x_src - 2, fx_src,
-                                   interpolation=interpolation,
-                                   extrapolation=extrapolation)
+                                    interpolation=interpolation,
+                                    extrapolation=extrapolation)
 
     def test_below(self):
         assert_array_equal(self.interpolate([-1]), [np.nan])
@@ -203,8 +216,8 @@ def interpolate(self, x_target):
 
         # Use -2 to test negative number support.
         return stratify.interpolate(np.array(x_target) - 2, x_src - 2, fx_src,
-                                   interpolation=interpolation,
-                                   extrapolation=extrapolation)
+                                    interpolation=interpolation,
+                                    extrapolation=extrapolation)
 
     def test_below(self):
         assert_array_equal(self.interpolate([-1]), [0.])
@@ -225,15 +238,27 @@ def interpolate(self, x_target):
 
         # Use -2 to test negative number support.
         return stratify.interpolate(np.array(x_target) - 2, x_src - 2, fx_src,
-                                   interpolation=interpolation,
-                                   extrapolation=extrapolation)
+                                    interpolation=interpolation,
+                                    extrapolation=extrapolation)
 
     def test_below(self):
         assert_array_equal(self.interpolate([-1]), [-10.])
 
     def test_above(self):
         assert_array_almost_equal(self.interpolate([15.123]), [151.23])
 
+    def test_npts(self):
+        interpolation = vinterp._TestableIndexInterpKernel()
+        extrapolation = stratify.EXTRAPOLATE_LINEAR
+
+        msg = (r'Linear extrapolation requires at least 2 '
+               r'source points. Got 1.')
+
+        with self.assertRaisesRegexp(ValueError, msg):
+            stratify.interpolate([1, 3.], [2], [20],
+                                 interpolation=interpolation,
+                                 extrapolation=extrapolation, rising=True)
+
 
 class Test__Interpolator(unittest.TestCase):
     def test_axis_m1(self):