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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 
     43   using VIterator = typename SmallVector<T, N>::const_iterator;
     44   using mutable_iterator = typename SmallVector<T, N>::iterator;
     45 
     46   // In small mode SmallPtrSet uses linear search for the elements, so it is
     47   // not a good idea to choose this value too high. You may consider using a
     48   // DenseSet<> instead if you expect many elements in the set.
     49   static_assert(N <= 32, "N should be small");
     50 
     51 public:
     52   using size_type = size_t;
     53 
     54   SmallSet() = default;
     55 
     56   LLVM_NODISCARD bool empty() const {
     57     return Vector.empty() && Set.empty();
     58   }
     59 
     60   size_type size() const {
     61     return isSmall() ? Vector.size() : Set.size();
     62   }
     63 
     64   /// count - Return 1 if the element is in the set, 0 otherwise.
     65   size_type count(const T &V) const {
     66     if (isSmall()) {
     67       // Since the collection is small, just do a linear search.
     68       return vfind(V) == Vector.end() ? 0 : 1;
     69     } else {
     70       return Set.count(V);
     71     }
     72   }
     73 
     74   /// insert - Insert an element into the set if it isn't already there.
     75   /// Returns true if the element is inserted (it was not in the set before).
     76   /// The first value of the returned pair is unused and provided for
     77   /// partial compatibility with the standard library self-associative container
     78   /// concept.
     79   // FIXME: Add iterators that abstract over the small and large form, and then
     80   // return those here.
     81   std::pair<NoneType, bool> insert(const T &V) {
     82     if (!isSmall())
     83       return std::make_pair(None, Set.insert(V).second);
     84 
     85     VIterator I = vfind(V);
     86     if (I != Vector.end())    // Don't reinsert if it already exists.
     87       return std::make_pair(None, false);
     88     if (Vector.size() < N) {
     89       Vector.push_back(V);
     90       return std::make_pair(None, true);
     91     }
     92 
     93     // Otherwise, grow from vector to set.
     94     while (!Vector.empty()) {
     95       Set.insert(Vector.back());
     96       Vector.pop_back();
     97     }
     98     Set.insert(V);
     99     return std::make_pair(None, true);
    100   }
    101 
    102   template <typename IterT>
    103   void insert(IterT I, IterT E) {
    104     for (; I != E; ++I)
    105       insert(*I);
    106   }
    107 
    108   bool erase(const T &V) {
    109     if (!isSmall())
    110       return Set.erase(V);
    111     for (mutable_iterator I = Vector.begin(), E = Vector.end(); I != E; ++I)
    112       if (*I == V) {
    113         Vector.erase(I);
    114         return true;
    115       }
    116     return false;
    117   }
    118 
    119   void clear() {
    120     Vector.clear();
    121     Set.clear();
    122   }
    123 
    124 private:
    125   bool isSmall() const { return Set.empty(); }
    126 
    127   VIterator vfind(const T &V) const {
    128     for (VIterator I = Vector.begin(), E = Vector.end(); I != E; ++I)
    129       if (*I == V)
    130         return I;
    131     return Vector.end();
    132   }
    133 };
    134 
    135 /// If this set is of pointer values, transparently switch over to using
    136 /// SmallPtrSet for performance.
    137 template <typename PointeeType, unsigned N>
    138 class SmallSet<PointeeType*, N> : public SmallPtrSet<PointeeType*, N> {};
    139 
    140 } // end namespace llvm
    141 
    142 #endif // LLVM_ADT_SMALLSET_H
    143