[numerics] Add previous value option to tabulated Func1 objects

include/cantera/numerics/Func1.h

@@ -279,7 +279,8 @@ class Pow1 : public Func1
 class Tabulated1 : public Func1
 {
 public:
-    Tabulated1(const std::vector<double>& tvec, const std::vector<double>& fvec) :
+    Tabulated1(const std::vector<double>& tvec, const std::vector<double>& fvec,
+               const std::string& method = "linear") :
         Func1() {
         if (tvec.size() != fvec.size()) {
             throw CanteraError("Tabulated1::Tabulated1",
@@ -291,6 +292,15 @@ class Tabulated1 : public Func1
         }
         m_tvec = tvec;
         m_fvec = fvec;
+        if (method == "linear") {
+            m_isLinear = true;
+        } else if (method == "previous") {
+            m_isLinear = false;
+        } else {
+            throw CanteraError("Tabulated1::Tabulated1",
+                               "interpolation method '{}' is not implemented",
+                               method);
+        }
     }
 
     Tabulated1(const Tabulated1& b) :
@@ -311,13 +321,18 @@ class Tabulated1 : public Func1
     }
     virtual double eval(double t) const;
     virtual Func1& duplicate() const {
-        return *(new Tabulated1(m_tvec, m_fvec));
+        if (m_isLinear) {
+            return *(new Tabulated1(m_tvec, m_fvec, "linear"));
+        } else {
+            return *(new Tabulated1(m_tvec, m_fvec, "previous"));
+        }
     }
 
     virtual Func1& derivative() const;
 private:
     std::vector<double> m_tvec;
     std::vector<double> m_fvec;
+    bool m_isLinear;
 };
 
 

interfaces/cython/cantera/_cantera.pxd

@@ -40,7 +40,7 @@ cdef extern from "cantera/cython/funcWrapper.h":
 
 cdef extern from "cantera/numerics/Func1.h":
     cdef cppclass CxxTabulated1 "Cantera::Tabulated1":
-        CxxTabulated1(vector[double]&, vector[double]&) except +translate_exception
+        CxxTabulated1(vector[double]&, vector[double]&, string) except +translate_exception
         double eval(double) except +translate_exception
 
     cdef cppclass CxxConst1 "Cantera::Const1":
@@ -1021,7 +1021,7 @@ cdef class Func1:
     cdef object exception
     cpdef void _set_callback(self, object) except *
     cpdef void _set_const(self, double) except *
-    cpdef void _set_tables(self, object, object) except *
+    cpdef void _set_tables(self, object, object, string) except *
 
 cdef class ReactorBase:
     cdef CxxReactorBase* rbase

interfaces/cython/cantera/func1.pyx

@@ -58,22 +58,29 @@ cdef class Func1:
     points and corresponding function values. Inputs are specified either by
     two iterable objects containing sample point location and function values,
     or a single array that concatenates those inputs in two columns. Between
-    sample points, values are evaluated using a linear interpolation, whereas
-    outside the sample interval, the value at the closest end point is returned.
+    sample points, values are evaluated based on the keyword argument
+    'interpolation'; options are 'linear' (linear interpolation, default) or
+    'previous' (nearest previous value). Outside the sample interval, the value
+    at the closest end point is returned.
+
     Examples for tabulated `Func1` object are::
 
         >>> t1 = Func1([[0, 2], [1, 1], [2, 0]])
         >>> [t1(v) for v in [-0.5, 0, 0.5, 1.5, 2, 2.5]]
         [2.0, 2.0, 1.5, 0.5, 0.0, 0.0]
 
-        >>> t2 = Func1([0, 1, 2], [2, 1, 0])
+        >>> t2 = Func1([[0, 2], [1, 1], [2, 0]], interpolation='previous')
         >>> [t2(v) for v in [-0.5, 0, 0.5, 1.5, 2, 2.5]]
-        [2.0, 2.0, 1.5, 0.5, 0.0, 0.0]
+        [2.0, 2.0, 2.0, 1.0, 0.0, 0.0]
 
-        >>> t3 = Func1(np.array([0, 1, 2]), (2, 1, 0))
+        >>> t3 = Func1([0, 1, 2], [2, 1, 0])
         >>> [t3(v) for v in [-0.5, 0, 0.5, 1.5, 2, 2.5]]
         [2.0, 2.0, 1.5, 0.5, 0.0, 0.0]
 
+        >>> t4 = Func1(np.array([0, 1, 2]), (2, 1, 0))
+        >>> [t4(v) for v in [-0.5, 0, 0.5, 1.5, 2, 2.5]]
+        [2.0, 2.0, 1.5, 0.5, 0.0, 0.0]
+
     Note that all methods which accept `Func1` objects will also accept the
     callable object and create the wrapper on their own, so it is generally
     unnecessary to explicitly create a `Func1` object.
@@ -82,7 +89,7 @@ cdef class Func1:
         self.exception = None
         self.callable = None
 
-    def __init__(self, *args):
+    def __init__(self, *args, **kwargs):
         if len(args) == 1:
             c = args[0]
             if hasattr(c, '__call__'):
@@ -104,7 +111,8 @@ cdef class Func1:
                         if arr.shape[1] == 2:
                             time = arr[:, 0]
                             fval = arr[:, 1]
-                            self._set_tables(time, fval)
+                            method = kwargs.get('interpolation', 'linear')
+                            self._set_tables(time, fval, stringify(method))
                         else:
                             raise ValueError(
                                 "Invalid dimensions: specification of "
@@ -121,7 +129,8 @@ cdef class Func1:
         elif len(args) == 2:
             # tabulated function (two arguments mimic C++ interface)
             time, fval = args
-            self._set_tables(time, fval)
+            method = kwargs.get('interpolation', 'linear')
+            self._set_tables(time, fval, stringify(method))
 
         else:
             raise ValueError("Invalid number of arguments")
@@ -134,13 +143,13 @@ cdef class Func1:
     cpdef void _set_const(self, double c) except *:
         self.func = <CxxFunc1*>(new CxxConst1(c))
 
-    cpdef void _set_tables(self, time, fval) except *:
+    cpdef void _set_tables(self, time, fval, string method) except *:
         cdef vector[double] tvec, fvec
         for t in time:
             tvec.push_back(t)
         for f in fval:
             fvec.push_back(f)
-        self.func = <CxxFunc1*>(new CxxTabulated1(tvec, fvec))
+        self.func = <CxxFunc1*>(new CxxTabulated1(tvec, fvec, method))
 
     def __dealloc__(self):
         del self.func

src/numerics/Func1.cpp

@@ -227,10 +227,14 @@ double Tabulated1::eval(double t) const {
             while (t > m_tvec[ix+1]) {
                 ix++;
             }
-            double df = m_fvec[ix+1] - m_fvec[ix];
-            df /= m_tvec[ix+1] - m_tvec[ix];
-            df *= t - m_tvec[ix];
-            return m_fvec[ix] + df;
+            if (m_isLinear) {
+                double df = m_fvec[ix+1] - m_fvec[ix];
+                df /= m_tvec[ix+1] - m_tvec[ix];
+                df *= t - m_tvec[ix];
+                return m_fvec[ix] + df;
+            } else {
+                return m_fvec[ix];
+            }
         }
     } else {
         return 0.;
@@ -241,20 +245,26 @@ Func1& Tabulated1::derivative() const {
     std::vector<double> tvec;
     std::vector<double> dvec;
     size_t siz = m_tvec.size();
-    if (siz>1) {
-        for (size_t i=1; i<siz; i++) {
-            double d = (m_fvec[i] - m_fvec[i-1]) /
-                (m_tvec[i] - m_tvec[i-1]);
-            tvec.push_back(m_tvec[i-1]);
-            dvec.push_back(d);
-            tvec.push_back(m_tvec[i]);
-            dvec.push_back(d);
+    if (m_isLinear) {
+        // piece-wise continuous derivative
+        if (siz>1) {
+            for (size_t i=1; i<siz; i++) {
+                double d = (m_fvec[i] - m_fvec[i-1]) /
+                  (m_tvec[i] - m_tvec[i-1]);
+                tvec.push_back(m_tvec[i-1]);
+                dvec.push_back(d);
+            }
         }
+        tvec.push_back(m_tvec[siz-1]);
+        dvec.push_back(0.);
     } else {
+        // derivative is zero (ignoring discontinuities)
+        tvec.push_back(m_tvec[0]);
         tvec.push_back(m_tvec[siz-1]);
         dvec.push_back(0.);
+        dvec.push_back(0.);
     }
-    return *(new Tabulated1(tvec, dvec));
+    return *(new Tabulated1(tvec, dvec, "previous"));
 }
 
 /******************************************************************************/