Move eigen to third party (#282)

* remove useless statement

* Add eigen to third_party dir

* remove reducdant lines

third_party/eigen/doc/examples/nullary_indexing.cpp

@@ -0,0 +1,73 @@
+#include <Eigen/Core>
+#include <iostream>
+
+using namespace Eigen;
+
+// [functor]
+template <class ArgType, class RowIndexType, class ColIndexType>
+class indexing_functor {
+  const ArgType& m_arg;
+  const RowIndexType& m_rowIndices;
+  const ColIndexType& m_colIndices;
+
+ public:
+  typedef Matrix<typename ArgType::Scalar, RowIndexType::SizeAtCompileTime,
+                 ColIndexType::SizeAtCompileTime,
+                 ArgType::Flags & RowMajorBit ? RowMajor : ColMajor,
+                 RowIndexType::MaxSizeAtCompileTime,
+                 ColIndexType::MaxSizeAtCompileTime>
+      MatrixType;
+
+  indexing_functor(const ArgType& arg, const RowIndexType& row_indices,
+                   const ColIndexType& col_indices)
+      : m_arg(arg), m_rowIndices(row_indices), m_colIndices(col_indices) {}
+
+  const typename ArgType::Scalar& operator()(Index row, Index col) const {
+    return m_arg(m_rowIndices[row], m_colIndices[col]);
+  }
+};
+// [functor]
+
+// [function]
+template <class ArgType, class RowIndexType, class ColIndexType>
+CwiseNullaryOp<
+    indexing_functor<ArgType, RowIndexType, ColIndexType>,
+    typename indexing_functor<ArgType, RowIndexType, ColIndexType>::MatrixType>
+mat_indexing(const Eigen::MatrixBase<ArgType>& arg,
+             const RowIndexType& row_indices, const ColIndexType& col_indices) {
+  typedef indexing_functor<ArgType, RowIndexType, ColIndexType> Func;
+  typedef typename Func::MatrixType MatrixType;
+  return MatrixType::NullaryExpr(row_indices.size(), col_indices.size(),
+                                 Func(arg.derived(), row_indices, col_indices));
+}
+// [function]
+
+int main() {
+  std::cout << "[main1]\n";
+  Eigen::MatrixXi A = Eigen::MatrixXi::Random(4, 4);
+  Array3i ri(1, 2, 1);
+  ArrayXi ci(6);
+  ci << 3, 2, 1, 0, 0, 2;
+  Eigen::MatrixXi B = mat_indexing(A, ri, ci);
+  std::cout << "A =" << std::endl;
+  std::cout << A << std::endl << std::endl;
+  std::cout << "A([" << ri.transpose() << "], [" << ci.transpose()
+            << "]) =" << std::endl;
+  std::cout << B << std::endl;
+  std::cout << "[main1]\n";
+
+  std::cout << "[main2]\n";
+  B = mat_indexing(A, ri + 1, ci);
+  std::cout << "A(ri+1,ci) =" << std::endl;
+  std::cout << B << std::endl << std::endl;
+#if EIGEN_COMP_CXXVER >= 11
+  B = mat_indexing(
+      A, ArrayXi::LinSpaced(13, 0, 12).unaryExpr([](int x) { return x % 4; }),
+      ArrayXi::LinSpaced(4, 0, 3));
+  std::cout << "A(ArrayXi::LinSpaced(13,0,12).unaryExpr([](int x){return "
+               "x%4;}), ArrayXi::LinSpaced(4,0,3)) ="
+            << std::endl;
+  std::cout << B << std::endl << std::endl;
+#endif
+  std::cout << "[main2]\n";
+}