kmath-functions/src/commonMain/kotlin/space/kscience/kmath/functions/Piecewise.kt

@@ -79,4 +79,13 @@ public class OrderedPiecewisePolynomial<T : Comparable<T>>(delimiter: T) :
 public fun <T : Comparable<T>, C : Ring<T>> PiecewisePolynomial<T>.value(ring: C, arg: T): T? =
     findPiece(arg)?.value(ring, arg)
 
+/**
+ * Convert this polynomial to a function returning nullable value (null if argument is outside piecewise range).
+ */
 public fun <T : Comparable<T>, C : Ring<T>> PiecewisePolynomial<T>.asFunction(ring: C): (T) -> T? = { value(ring, it) }
+
+/**
+ * Convert this polynomial to a function using [defaultValue] for arguments outside the piecewise range.
+ */
+public fun <T : Comparable<T>, C : Ring<T>> PiecewisePolynomial<T>.asFunction(ring: C, defaultValue: T): (T) -> T =
+    { value(ring, it) ?: defaultValue }

kmath-functions/src/commonMain/kotlin/space/kscience/kmath/interpolation/SplineInterpolator.kt

@@ -42,9 +42,11 @@ public class SplineInterpolator<T : Comparable<T>>(
             val g = 2.0 * (points.x[i + 1] - points.x[i - 1]) - h[i - 1] * mu[i - 1]
             mu[i] = h[i] / g
 
-            z[i] =
+            z[i] = (3.0 * (points.y[i + 1] * h[i - 1]
-                (3.0 * (points.y[i + 1] * h[i - 1] - points.x[i] * (points.x[i + 1] - points.x[i - 1]) + points.y[i - 1] * h[i]) / (h[i - 1] * h[i])
+                    - points.x[i] * (points.x[i + 1] - points.x[i - 1])
-                        - h[i - 1] * z[i - 1]) / g
+                    + points.y[i - 1] * h[i]) / (h[i - 1] * h[i])
+                    - h[i - 1] * z[i - 1]
+                    ) / g
         }
 
         // cubic spline coefficients --  b is linear, c quadratic, d is cubic (original y's are constants)