Moved AutoDiffCodeGen macros to a separate (public) header

User defined cost functors have to include this new header
instead of internal/expression_ref.h. This hides some
complexity of ExpressionRef and reduces compile time outside
of code generation mode.

This patch also removes the dependency ExpressionRef->Jet.

Change-Id: Ie3f93648775e14881dc5cfab213bbc983c6cfeee

include/ceres/codegen/internal/code_generator.h

@@ -0,0 +1,123 @@
+// Ceres Solver - A fast non-linear least squares minimizer
+// Copyright 2019 Google Inc. All rights reserved.
+// http://code.google.com/p/ceres-solver/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are met:
+//
+// * Redistributions of source code must retain the above copyright notice,
+//   this list of conditions and the following disclaimer.
+// * Redistributions in binary form must reproduce the above copyright notice,
+//   this list of conditions and the following disclaimer in the documentation
+//   and/or other materials provided with the distribution.
+// * Neither the name of Google Inc. nor the names of its contributors may be
+//   used to endorse or promote products derived from this software without
+//   specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+// ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+// CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+// SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+// CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+// ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+// POSSIBILITY OF SUCH DAMAGE.
+//
+// Author: darius.rueckert@fau.de (Darius Rueckert)
+//
+#ifndef CERES_PUBLIC_CODEGEN_INTERNAL_CODE_GENERATOR_H_
+#define CERES_PUBLIC_CODEGEN_INTERNAL_CODE_GENERATOR_H_
+
+#include "ceres/codegen/internal/expression.h"
+#include "ceres/codegen/internal/expression_graph.h"
+
+#include <string>
+#include <vector>
+
+namespace ceres {
+namespace internal {
+
+// This class is used to convert an expression graph into a string. The typical
+// pipeline is:
+//
+// 1. Record ExpressionGraph
+// 2. Optimize ExpressionGraph
+// 3. Generate C++ code (this class here)
+//
+// The CodeGenerator operates in the following way:
+//
+// 1. Print Header
+//    - The header string is defined in the options.
+//    - This is usually the function name including the parameter list.
+//
+// 2. Print Declarations
+//    - Declare all used variables
+//    - Example:
+//        double v_0;
+//        double v_1;
+//        bool v_3;
+//        ...
+//
+// 3. Print Code
+//    - Convert each expression line by line to a string
+//    - Example:
+//        v_2 = v_0 + v_1
+//        if (v_5) {
+//          v_2 = v_0;
+//          ....
+//
+class CodeGenerator {
+ public:
+  struct Options {
+    // Name of the function.
+    // Example:
+    //   bool Evaluate(const double* x, double* res)
+    std::string function_name = "";
+
+    // Number of spaces added for each level of indentation.
+    int indentation_spaces_per_level = 2;
+
+    // The prefix added to each variable name.
+    std::string variable_prefix = "v_";
+  };
+
+  CodeGenerator(const ExpressionGraph& graph, const Options& options);
+
+  // Generate the C++ code in the steps (1)-(3) described above.
+  // The result is a vector of strings, where each element is exactly one line
+  // of code. The order is important and must not be changed.
+  std::vector<std::string> Generate();
+
+ private:
+  // Converts a single expression given by id to a string.
+  // The format depends on the ExpressionType.
+  // See ExpressionType in expression.h for more detailed how the different
+  // lines will look like.
+  std::string ExpressionToString(ExpressionId id);
+
+  // Helper function to get the name of an expression.
+  // If the expression does not have a valid name an error is generated.
+  std::string VariableForExpressionId(ExpressionId id);
+
+  // Returns the type as a string of the left hand side.
+  static std::string DataTypeForExpression(ExpressionType type);
+
+  // Adds one level of indentation. Called when an IF expression is encountered.
+  void PushIndentation();
+
+  // Removes one level of indentation. Currently only used by ENDIF.
+  void PopIndentation();
+
+  const ExpressionGraph& graph_;
+  const Options options_;
+  std::string indentation_ = "";
+  static constexpr int kFloatingPointPrecision = 25;
+};
+
+}  // namespace internal
+}  // namespace ceres
+
+#endif  // CERES_PUBLIC_CODEGEN_INTERNAL_CODE_GENERATOR_H_