Autodiff Codegen Part 2: Conditionals

- Expression types for if/else blocks
- CERES_IF/ELSE macros
- Multiple assignment to the same variable

Change-Id: If529516f243f31823d1ef7b8827bb6f2390e418d

include/ceres/internal/expression_ref.h

@@ -52,8 +52,26 @@ struct ExpressionRef {
   // it's automatically converted to the correct expression.
   explicit ExpressionRef(double compile_time_constant);
 
-  // Returns v_id
-  std::string ToString() const;
+  // Create an ASSIGNMENT expression from other to this.
+  //
+  // For example:
+  //   a = b;        // With a.id = 5 and b.id = 3
+  // will generate the following assignment:
+  //   v_5 = v_3;
+  //
+  // If this (lhs) ExpressionRef is currently not pointing to a variable
+  // (id==invalid), then we can eliminate the assignment by just letting "this"
+  // point to the same variable as "other".
+  //
+  // Example:
+  //   a = b;       // With a.id = invalid and b.id = 3
+  // will generate NO expression, but after this line the following will be
+  // true:
+  //    a.id == b.id == 3
+  //
+  // If 'other' is not pointing to a variable (id==invalid), we found an
+  // uninitialized assignment, which is handled as an error.
+  ExpressionRef& operator=(const ExpressionRef& other);
 
   // Compound operators
   ExpressionRef& operator+=(ExpressionRef x);
@@ -61,6 +79,8 @@ struct ExpressionRef {
   ExpressionRef& operator*=(ExpressionRef x);
   ExpressionRef& operator/=(ExpressionRef x);
 
+  bool IsInitialized() const { return id != kInvalidExpressionId; }
+
   // The index into the ExpressionGraph data array.
   ExpressionId id = kInvalidExpressionId;
 
@@ -152,6 +172,22 @@ inline typename RuntimeConstant<T>::ReturnType MakeRuntimeConstant(
 
 #define CERES_EXPRESSION_RUNTIME_CONSTANT(_v) \
   ceres::internal::MakeRuntimeConstant<T>(_v, #_v)
+
+// The CERES_CODEGEN macro is defined by the build system only during code
+// generation. In all other cases the CERES_IF/ELSE macros just expand to the
+// if/else keywords.
+#ifdef CERES_CODEGEN
+#define CERES_IF(condition_) Expression::CreateIf((condition_).id);
+#define CERES_ELSE Expression::CreateElse();
+#define CERES_ENDIF Expression::CreateEndIf();
+#else
+// clang-format off
+#define CERES_IF(condition_) if (condition_) {
+#define CERES_ELSE } else {
+#define CERES_ENDIF }
+// clang-format on
+#endif
+
 }  // namespace internal
 
 // See jet.h for more info on this type.