include/llvm/ADT/GraphTraits.h

//===- llvm/ADT/GraphTraits.h - Graph traits template -----------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file defines the little GraphTraits<X> template class that should be
// specialized by classes that want to be iteratable by generic graph iterators.
//
// This file also defines the marker class Inverse that is used to iterate over
// graphs in a graph defined, inverse ordering...
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_ADT_GRAPHTRAITS_H
#define LLVM_ADT_GRAPHTRAITS_H

#include "llvm/ADT/iterator_range.h"

namespace llvm {

// GraphTraits - This class should be specialized by different graph types...
// which is why the default version is empty.
//
// This template evolved from supporting `BasicBlock` to also later supporting
// more complex types (e.g. CFG and DomTree).
//
// GraphTraits can be used to create a view over a graph interpreting it
// differently without requiring a copy of the original graph. This could
// be achieved by carrying more data in NodeRef. See LoopBodyTraits for one
// example.
template<class GraphType>
struct GraphTraits {
  // Elements to provide:

  // typedef NodeRef           - Type of Node token in the graph, which should
  //                             be cheap to copy.
  // typedef ChildIteratorType - Type used to iterate over children in graph,
  //                             dereference to a NodeRef.

  // static NodeRef getEntryNode(const GraphType &)
  //    Return the entry node of the graph

  // static ChildIteratorType child_begin(NodeRef)
  // static ChildIteratorType child_end  (NodeRef)
  //    Return iterators that point to the beginning and ending of the child
  //    node list for the specified node.

  // typedef  ...iterator nodes_iterator; - dereference to a NodeRef
  // static nodes_iterator nodes_begin(GraphType *G)
  // static nodes_iterator nodes_end  (GraphType *G)
  //    nodes_iterator/begin/end - Allow iteration over all nodes in the graph

  // typedef EdgeRef           - Type of Edge token in the graph, which should
  //                             be cheap to copy.
  // typedef ChildEdgeIteratorType - Type used to iterate over children edges in
  //                             graph, dereference to a EdgeRef.

  // static ChildEdgeIteratorType child_edge_begin(NodeRef)
  // static ChildEdgeIteratorType child_edge_end(NodeRef)
  //     Return iterators that point to the beginning and ending of the
  //     edge list for the given callgraph node.
  //
  // static NodeRef edge_dest(EdgeRef)
  //     Return the destination node of an edge.

  // static unsigned       size       (GraphType *G)
  //    Return total number of nodes in the graph

  // If anyone tries to use this class without having an appropriate
  // specialization, make an error.  If you get this error, it's because you
  // need to include the appropriate specialization of GraphTraits<> for your
  // graph, or you need to define it for a new graph type. Either that or
  // your argument to XXX_begin(...) is unknown or needs to have the proper .h
  // file #include'd.
  using NodeRef = typename GraphType::UnknownGraphTypeError;
};

// Inverse - This class is used as a little marker class to tell the graph
// iterator to iterate over the graph in a graph defined "Inverse" ordering.
// Not all graphs define an inverse ordering, and if they do, it depends on
// the graph exactly what that is.  Here's an example of usage with the
// df_iterator:
//
// idf_iterator<Method*> I = idf_begin(M), E = idf_end(M);
// for (; I != E; ++I) { ... }
//
// Which is equivalent to:
// df_iterator<Inverse<Method*>> I = idf_begin(M), E = idf_end(M);
// for (; I != E; ++I) { ... }
//
template <class GraphType>
struct Inverse {
  const GraphType &Graph;

  inline Inverse(const GraphType &G) : Graph(G) {}
};

// Provide a partial specialization of GraphTraits so that the inverse of an
// inverse falls back to the original graph.
template <class T> struct GraphTraits<Inverse<Inverse<T>>> : GraphTraits<T> {};

// Provide iterator ranges for the graph traits nodes and children
template <class GraphType>
iterator_range<typename GraphTraits<GraphType>::nodes_iterator>
nodes(const GraphType &G) {
  return make_range(GraphTraits<GraphType>::nodes_begin(G),
                    GraphTraits<GraphType>::nodes_end(G));
}
template <class GraphType>
iterator_range<typename GraphTraits<Inverse<GraphType>>::nodes_iterator>
inverse_nodes(const GraphType &G) {
  return make_range(GraphTraits<Inverse<GraphType>>::nodes_begin(G),
                    GraphTraits<Inverse<GraphType>>::nodes_end(G));
}

template <class GraphType>
iterator_range<typename GraphTraits<GraphType>::ChildIteratorType>
children(const typename GraphTraits<GraphType>::NodeRef &G) {
  return make_range(GraphTraits<GraphType>::child_begin(G),
                    GraphTraits<GraphType>::child_end(G));
}

template <class GraphType>
iterator_range<typename GraphTraits<Inverse<GraphType>>::ChildIteratorType>
inverse_children(const typename GraphTraits<GraphType>::NodeRef &G) {
  return make_range(GraphTraits<Inverse<GraphType>>::child_begin(G),
                    GraphTraits<Inverse<GraphType>>::child_end(G));
}

template <class GraphType>
iterator_range<typename GraphTraits<GraphType>::ChildEdgeIteratorType>
children_edges(const typename GraphTraits<GraphType>::NodeRef &G) {
  return make_range(GraphTraits<GraphType>::child_edge_begin(G),
                    GraphTraits<GraphType>::child_edge_end(G));
}

} // end namespace llvm

#endif // LLVM_ADT_GRAPHTRAITS_H