Add fmax/fmin overloads for scalars

Comparison operators are defined for Jet-scalar and scalar-Jet.
Add corresponding overloads for fmax and fmix so users
don't have to convert to Jet only to access these functions,
plus it can potentially save an unnecessary conversion.

Change-Id: I20cc2d5874935e019d4ecb2d6a8c2ae631c8a4d3

include/ceres/jet.h

diff --git a/include/ceres/jet.h b/include/ceres/jet.h
index 50aac1b..99cff79 100644
--- a/include/ceres/jet.h
+++ b/include/ceres/jet.h
@@ -687,11 +687,27 @@
 inline Jet<T, N> fmax(const Jet<T, N>& x, const Jet<T, N>& y) {
   return x < y ? y : x;
 }
+template <typename T, int N>
+inline Jet<T, N> fmax(const Jet<T, N>& x, const T& y) {
+  return x < y ? Jet<T, N>{y} : x;
+}
+template <typename T, int N>
+inline Jet<T, N> fmax(const T& x, const Jet<T, N>& y) {
+  return x < y ? y : Jet<T, N>{x};
+}
 
 template <typename T, int N>
 inline Jet<T, N> fmin(const Jet<T, N>& x, const Jet<T, N>& y) {
   return y < x ? y : x;
 }
+template <typename T, int N>
+inline Jet<T, N> fmin(const Jet<T, N>& x, const T& y) {
+  return y < x ? Jet<T, N>{y} : x;
+}
+template <typename T, int N>
+inline Jet<T, N> fmin(const T& x, const Jet<T, N>& y) {
+  return y < x ? y : Jet<T, N>{x};
+}
 
 // erf is defined as an integral that cannot be expressed analytically
 // however, the derivative is trivial to compute

internal/ceres/jet_test.cc

diff --git a/internal/ceres/jet_test.cc b/internal/ceres/jet_test.cc
index c65f68d..00a1227 100644
--- a/internal/ceres/jet_test.cc
+++ b/internal/ceres/jet_test.cc
@@ -736,12 +736,62 @@
     VL << "z = " << z;
     ExpectJetsClose(x, z);
   }
+  {
+    J z = fmax(y, x);
+    VL << "z = " << z;
+    ExpectJetsClose(x, z);
+  }
+  {
+    J z = fmax(x, y.a);
+    VL << "z = " << z;
+    ExpectJetsClose(x, z);
+  }
+  {
+    J z = fmax(y, x.a);
+    VL << "z = " << z;
+    ExpectJetsClose(J{x.a}, z);
+  }
+  {
+    J z = fmax(x.a, y);
+    VL << "z = " << z;
+    ExpectJetsClose(J{x.a}, z);
+  }
+  {
+    J z = fmax(y.a, x);
+    VL << "z = " << z;
+    ExpectJetsClose(x, z);
+  }
 
   {
     J z = fmin(x, y);
     VL << "z = " << z;
     ExpectJetsClose(y, z);
   }
+  {
+    J z = fmin(y, x);
+    VL << "z = " << z;
+    ExpectJetsClose(y, z);
+  }
+  {
+    J z = fmin(x, y.a);
+    VL << "z = " << z;
+    ExpectJetsClose(J{y.a}, z);
+  }
+  {
+    J z = fmin(y, x.a);
+    VL << "z = " << z;
+    ExpectJetsClose(y, z);
+  }
+  {
+    J z = fmin(x.a, y);
+    VL << "z = " << z;
+    ExpectJetsClose(y, z);
+  }
+  {
+    J z = fmin(y.a, x);
+    VL << "z = " << z;
+    ExpectJetsClose(J{y.a}, z);
+  }
 }
 
 TEST(Jet, JetsInEigenMatrices) {