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      1 //===- llvm/ADT/SmallSet.h - 'Normally small' sets --------------*- 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 file defines the SmallSet class.
     11 //
     12 //===----------------------------------------------------------------------===//
     13 
     14 #ifndef LLVM_ADT_SMALLSET_H
     15 #define LLVM_ADT_SMALLSET_H
     16 
     17 #include "llvm/ADT/None.h"
     18 #include "llvm/ADT/SmallPtrSet.h"
     19 #include "llvm/ADT/SmallVector.h"
     20 #include "llvm/Support/Compiler.h"
     21 #include <cstddef>
     22 #include <functional>
     23 #include <set>
     24 #include <utility>
     25 
     26 namespace llvm {
     27 
     28 /// SmallSet - This maintains a set of unique values, optimizing for the case
     29 /// when the set is small (less than N).  In this case, the set can be
     30 /// maintained with no mallocs.  If the set gets large, we expand to using an
     31 /// std::set to maintain reasonable lookup times.
     32 ///
     33 /// Note that this set does not provide a way to iterate over members in the
     34 /// set.
     35 template <typename T, unsigned N, typename C = std::less<T>>
     36 class SmallSet {
     37   /// Use a SmallVector to hold the elements here (even though it will never
     38   /// reach its 'large' stage) to avoid calling the default ctors of elements
     39   /// we will never use.
     40   SmallVector<T, N> Vector;
     41   std::set<T, C> Set;
     42   typedef typename SmallVector<T, N>::const_iterator VIterator;
     43   typedef typename SmallVector<T, N>::iterator mutable_iterator;
     44 
     45   // In small mode SmallPtrSet uses linear search for the elements, so it is
     46   // not a good idea to choose this value too high. You may consider using a
     47   // DenseSet<> instead if you expect many elements in the set.
     48   static_assert(N <= 32, "N should be small");
     49 
     50 public:
     51   typedef size_t size_type;
     52 
     53   SmallSet() = default;
     54 
     55   LLVM_NODISCARD bool empty() const {
     56     return Vector.empty() && Set.empty();
     57   }
     58 
     59   size_type size() const {
     60     return isSmall() ? Vector.size() : Set.size();
     61   }
     62 
     63   /// count - Return 1 if the element is in the set, 0 otherwise.
     64   size_type count(const T &V) const {
     65     if (isSmall()) {
     66       // Since the collection is small, just do a linear search.
     67       return vfind(V) == Vector.end() ? 0 : 1;
     68     } else {
     69       return Set.count(V);
     70     }
     71   }
     72 
     73   /// insert - Insert an element into the set if it isn't already there.
     74   /// Returns true if the element is inserted (it was not in the set before).
     75   /// The first value of the returned pair is unused and provided for
     76   /// partial compatibility with the standard library self-associative container
     77   /// concept.
     78   // FIXME: Add iterators that abstract over the small and large form, and then
     79   // return those here.
     80   std::pair<NoneType, bool> insert(const T &V) {
     81     if (!isSmall())
     82       return std::make_pair(None, Set.insert(V).second);
     83 
     84     VIterator I = vfind(V);
     85     if (I != Vector.end())    // Don't reinsert if it already exists.
     86       return std::make_pair(None, false);
     87     if (Vector.size() < N) {
     88       Vector.push_back(V);
     89       return std::make_pair(None, true);
     90     }
     91 
     92     // Otherwise, grow from vector to set.
     93     while (!Vector.empty()) {
     94       Set.insert(Vector.back());
     95       Vector.pop_back();
     96     }
     97     Set.insert(V);
     98     return std::make_pair(None, true);
     99   }
    100 
    101   template <typename IterT>
    102   void insert(IterT I, IterT E) {
    103     for (; I != E; ++I)
    104       insert(*I);
    105   }
    106 
    107   bool erase(const T &V) {
    108     if (!isSmall())
    109       return Set.erase(V);
    110     for (mutable_iterator I = Vector.begin(), E = Vector.end(); I != E; ++I)
    111       if (*I == V) {
    112         Vector.erase(I);
    113         return true;
    114       }
    115     return false;
    116   }
    117 
    118   void clear() {
    119     Vector.clear();
    120     Set.clear();
    121   }
    122 
    123 private:
    124   bool isSmall() const { return Set.empty(); }
    125 
    126   VIterator vfind(const T &V) const {
    127     for (VIterator I = Vector.begin(), E = Vector.end(); I != E; ++I)
    128       if (*I == V)
    129         return I;
    130     return Vector.end();
    131   }
    132 };
    133 
    134 /// If this set is of pointer values, transparently switch over to using
    135 /// SmallPtrSet for performance.
    136 template <typename PointeeType, unsigned N>
    137 class SmallSet<PointeeType*, N> : public SmallPtrSet<PointeeType*, N> {};
    138 
    139 } // end namespace llvm
    140 
    141 #endif // LLVM_ADT_SMALLSET_H
    142