1//===- llvm/Analysis/MaximumSpanningTree.h - Interface ----------*- C++ -*-===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This module provides means for calculating a maximum spanning tree for a
11// given set of weighted edges. The type parameter T is the type of a node.
12//
13//===----------------------------------------------------------------------===//
14
15#ifndef LLVM_ANALYSIS_MAXIMUMSPANNINGTREE_H
16#define LLVM_ANALYSIS_MAXIMUMSPANNINGTREE_H
17
18#include "llvm/ADT/EquivalenceClasses.h"
19#include "llvm/IR/BasicBlock.h"
20#include <algorithm>
21#include <vector>
22
23namespace llvm {
24
25  /// MaximumSpanningTree - A MST implementation.
26  /// The type parameter T determines the type of the nodes of the graph.
27  template <typename T>
28  class MaximumSpanningTree {
29  public:
30    typedef std::pair<const T*, const T*> Edge;
31    typedef std::pair<Edge, double> EdgeWeight;
32    typedef std::vector<EdgeWeight> EdgeWeights;
33  protected:
34    typedef std::vector<Edge> MaxSpanTree;
35
36    MaxSpanTree MST;
37
38  private:
39    // A comparing class for comparing weighted edges.
40    struct EdgeWeightCompare {
41      static bool getBlockSize(const T *X) {
42        const BasicBlock *BB = dyn_cast_or_null<BasicBlock>(X);
43        return BB ? BB->size() : 0;
44      }
45
46      bool operator()(EdgeWeight X, EdgeWeight Y) const {
47        if (X.second > Y.second) return true;
48        if (X.second < Y.second) return false;
49
50        // Equal edge weights: break ties by comparing block sizes.
51        size_t XSizeA = getBlockSize(X.first.first);
52        size_t YSizeA = getBlockSize(Y.first.first);
53        if (XSizeA > YSizeA) return true;
54        if (XSizeA < YSizeA) return false;
55
56        size_t XSizeB = getBlockSize(X.first.second);
57        size_t YSizeB = getBlockSize(Y.first.second);
58        if (XSizeB > YSizeB) return true;
59        if (XSizeB < YSizeB) return false;
60
61        return false;
62      }
63    };
64
65  public:
66    static char ID; // Class identification, replacement for typeinfo
67
68    /// MaximumSpanningTree() - Takes a vector of weighted edges and returns a
69    /// spanning tree.
70    MaximumSpanningTree(EdgeWeights &EdgeVector) {
71
72      std::stable_sort(EdgeVector.begin(), EdgeVector.end(), EdgeWeightCompare());
73
74      // Create spanning tree, Forest contains a special data structure
75      // that makes checking if two nodes are already in a common (sub-)tree
76      // fast and cheap.
77      EquivalenceClasses<const T*> Forest;
78      for (typename EdgeWeights::iterator EWi = EdgeVector.begin(),
79           EWe = EdgeVector.end(); EWi != EWe; ++EWi) {
80        Edge e = (*EWi).first;
81
82        Forest.insert(e.first);
83        Forest.insert(e.second);
84      }
85
86      // Iterate over the sorted edges, biggest first.
87      for (typename EdgeWeights::iterator EWi = EdgeVector.begin(),
88           EWe = EdgeVector.end(); EWi != EWe; ++EWi) {
89        Edge e = (*EWi).first;
90
91        if (Forest.findLeader(e.first) != Forest.findLeader(e.second)) {
92          Forest.unionSets(e.first, e.second);
93          // So we know now that the edge is not already in a subtree, so we push
94          // the edge to the MST.
95          MST.push_back(e);
96        }
97      }
98    }
99
100    typename MaxSpanTree::iterator begin() {
101      return MST.begin();
102    }
103
104    typename MaxSpanTree::iterator end() {
105      return MST.end();
106    }
107  };
108
109} // End llvm namespace
110
111#endif
112