From 1666155d25de9bf0222f9a1e3fde350909b62aff Mon Sep 17 00:00:00 2001
From: Christopher Lackner <clackner@cerbsim.com>
Date: Sat, 19 Sep 2020 17:39:03 +0200
Subject: [PATCH] add range adaptors (filter, transform)

---
 libsrc/core/CMakeLists.txt |   2 +-
 libsrc/core/ranges.hpp     | 109 +++++++++++++++++++++++++++++++++++++
 tests/catch/CMakeLists.txt |   1 +
 tests/catch/ranges.cpp     |  18 ++++++
 4 files changed, 129 insertions(+), 1 deletion(-)
 create mode 100644 libsrc/core/ranges.hpp
 create mode 100644 tests/catch/ranges.cpp

diff --git a/libsrc/core/CMakeLists.txt b/libsrc/core/CMakeLists.txt
index 16c43a82..a2873ed9 100644
--- a/libsrc/core/CMakeLists.txt
+++ b/libsrc/core/CMakeLists.txt
@@ -65,7 +65,7 @@ target_link_libraries(ngcore PUBLIC netgen_mpi PRIVATE netgen_python ${CMAKE_THR
 install(FILES ngcore.hpp archive.hpp type_traits.hpp version.hpp ngcore_api.hpp logging.hpp
   exception.hpp symboltable.hpp paje_trace.hpp utils.hpp profiler.hpp mpi_wrapper.hpp
   array.hpp taskmanager.hpp concurrentqueue.h localheap.hpp python_ngcore.hpp flags.hpp
-  xbool.hpp signal.hpp bitarray.hpp table.hpp hashtable.hpp
+  xbool.hpp signal.hpp bitarray.hpp table.hpp hashtable.hpp ranges.hpp
   DESTINATION ${NG_INSTALL_DIR_INCLUDE}/core COMPONENT netgen_devel)
 
 if(ENABLE_CPP_CORE_GUIDELINES_CHECK)
diff --git a/libsrc/core/ranges.hpp b/libsrc/core/ranges.hpp
new file mode 100644
index 00000000..658ee3d0
--- /dev/null
+++ b/libsrc/core/ranges.hpp
@@ -0,0 +1,109 @@
+#ifndef NETGEN_CORE_RANGES_HPP
+#define NETGEN_CORE_RANGES_HPP
+
+#include <iterator>
+
+namespace ngcore
+{
+  template<typename Iterator>
+  class AdapterRange
+  {
+    Iterator _begin,_end;
+  public:
+    AdapterRange(Iterator abegin, Iterator aend) : _begin(abegin), _end(aend) { ; }
+    Iterator begin() const { return _begin; }
+    Iterator end() const { return _end; }
+  };
+
+  template<typename FUNC>
+  class FilterAdapter
+  {
+    FUNC f;
+  public:
+    FilterAdapter(FUNC af) : f(af) { ; }
+    FUNC GetFunction() const { return f; }
+  };
+
+  template<typename FUNC, typename Iterator>
+  class FilterIterator
+  {
+    Iterator iter;
+    Iterator end;
+    FUNC f;
+  public:
+    FilterIterator(FUNC af, Iterator aiter, Iterator aend)
+      :  f(af), iter(aiter), end(aend)
+    {
+      while(iter!=end && !f(*iter))
+        ++iter;
+    }
+    inline FilterIterator& operator ++()
+    {
+      ++iter;
+      while(iter!=end && !f(*iter))
+        ++iter;
+      return *this;
+    }
+
+    inline bool operator !=(FilterIterator other)
+    {
+      return iter != other.iter;
+    }
+
+    inline bool operator ==(FilterIterator other)
+    {
+      return iter == other.iter;
+    }
+
+    inline decltype(auto) operator *() const
+    {
+      return *iter;
+    }
+  };
+
+  template<typename FUNC>
+  FilterAdapter<FUNC> filter(FUNC f) { return {f}; }
+
+  template<typename Range, typename FUNC>
+  auto operator |(Range&& range, FilterAdapter<FUNC> adapter)
+    -> AdapterRange<FilterIterator<FUNC,decltype(std::begin(range))>>
+  {
+    return {{adapter.GetFunction(),std::begin(range),std::end(range)},
+        {adapter.GetFunction(), std::end(range), std::end(range)}};
+  }
+
+  template<typename FUNC, typename Iterator>
+  class TransformIterator
+  {
+    FUNC f;
+    Iterator iter;
+  public:
+    TransformIterator(FUNC af, Iterator aiter) : f(af), iter(aiter) { ; }
+
+    TransformIterator& operator++() { ++iter; }
+    bool operator !=(TransformIterator other) { return iter != other.iter; }
+    decltype(auto) operator *() const { return f(*iter); }
+  };
+
+  template<typename FUNC>
+  class TransformAdapter
+  {
+    FUNC f;
+  public:
+    TransformAdapter(FUNC af) : f(af) { ; }
+    FUNC GetFunction() const { return f; }
+  };
+
+  template<typename FUNC>
+  TransformAdapter<FUNC> transform(FUNC f) { return {f}; }
+
+  template<typename Range, typename FUNC>
+  auto operator |(Range&& range, TransformAdapter<FUNC> adapter)
+    -> AdapterRange<TransformIterator<FUNC,decltype(std::begin(range))>>
+  {
+    return {{adapter.GetFunction(), std::begin(range)},
+        {adapter.GetFunction(),std::end(range)}};
+  }
+} // namespace ngcore
+
+#endif // NETGEN_CORE_RANGES_HPP
diff --git a/tests/catch/CMakeLists.txt b/tests/catch/CMakeLists.txt
index c3a6525d..12a665df 100644
--- a/tests/catch/CMakeLists.txt
+++ b/tests/catch/CMakeLists.txt
@@ -27,6 +27,7 @@ endmacro()
 
 add_unit_test(archive archive.cpp)
 add_unit_test(array array.cpp)
+add_unit_test(ranges ranges.cpp)
 add_unit_test(symboltable symboltable.cpp)
 add_unit_test(utils utils.cpp)
 add_unit_test(version version.cpp)
diff --git a/tests/catch/ranges.cpp b/tests/catch/ranges.cpp
new file mode 100644
index 00000000..b4559de0
--- /dev/null
+++ b/tests/catch/ranges.cpp
@@ -0,0 +1,18 @@
+
+#include "catch.hpp"
+
+#include <core/array.hpp>
+#include <core/ranges.hpp>
+
+using namespace ngcore;
+
+TEST_CASE("ranges")
+{
+  Array<int> a { 3, -1, 10, -5 };
+  Array<int> positive { 3, 10 };
+  int i = 0;
+  for(auto pos_val : a | filter([](auto val) { return val >= 0; }))
+    {
+      CHECK(pos_val == positive[i++]);
+    }
+}