text: //===- llvm/ADT/SmallSet.h - 'Normally small' sets --------------*- C++ -*-===//
  //
  //                     The LLVM Compiler Infrastructure
  //
  // This file is distributed under the University of Illinois Open Source
  // License. See LICENSE.TXT for details.
  //
  //===----------------------------------------------------------------------===//
  //
  // This file defines the SmallSet class.
  //
  //===----------------------------------------------------------------------===//
  
  #ifndef WPIUTIL_WPI_SMALLSET_H
  #define WPIUTIL_WPI_SMALLSET_H
  
  #include "wpi/None.h"
  #include "wpi/SmallPtrSet.h"
  #include "wpi/SmallVector.h"
  #include "wpi/Compiler.h"
  #include <cstddef>
  #include <functional>
  #include <set>
  #include <utility>
  
  namespace wpi {
  
  /// SmallSet - This maintains a set of unique values, optimizing for the case
  /// when the set is small (less than N).  In this case, the set can be
  /// maintained with no mallocs.  If the set gets large, we expand to using an
  /// std::set to maintain reasonable lookup times.
  ///
  /// Note that this set does not provide a way to iterate over members in the
  /// set.
  template <typename T, unsigned N, typename C = std::less<T>>
  class SmallSet {
    /// Use a SmallVector to hold the elements here (even though it will never
    /// reach its 'large' stage) to avoid calling the default ctors of elements
    /// we will never use.
    SmallVector<T, N> Vector;
    std::set<T, C> Set;
  
    using VIterator = typename SmallVector<T, N>::const_iterator;
    using mutable_iterator = typename SmallVector<T, N>::iterator;
  
    // In small mode SmallPtrSet uses linear search for the elements, so it is
    // not a good idea to choose this value too high. You may consider using a
    // DenseSet<> instead if you expect many elements in the set.
    static_assert(N <= 32, "N should be small");
  
  public:
    using size_type = size_t;
  
    SmallSet() = default;
  
    LLVM_NODISCARD bool empty() const {
      return Vector.empty() && Set.empty();
    }
  
    size_type size() const {
      return isSmall() ? Vector.size() : Set.size();
    }
  
    /// count - Return 1 if the element is in the set, 0 otherwise.
    size_type count(const T &V) const {
      if (isSmall()) {
        // Since the collection is small, just do a linear search.
        return vfind(V) == Vector.end() ? 0 : 1;
      } else {
        return Set.count(V);
      }
    }
  
    /// insert - Insert an element into the set if it isn't already there.
    /// Returns true if the element is inserted (it was not in the set before).
    /// The first value of the returned pair is unused and provided for
    /// partial compatibility with the standard library self-associative container
    /// concept.
    // FIXME: Add iterators that abstract over the small and large form, and then
    // return those here.
    std::pair<NoneType, bool> insert(const T &V) {
      if (!isSmall())
        return std::make_pair(None, Set.insert(V).second);
  
      VIterator I = vfind(V);
      if (I != Vector.end())    // Don't reinsert if it already exists.
        return std::make_pair(None, false);
      if (Vector.size() < N) {
        Vector.push_back(V);
        return std::make_pair(None, true);
      }
  
      // Otherwise, grow from vector to set.
      while (!Vector.empty()) {
        Set.insert(Vector.back());
        Vector.pop_back();
      }
      Set.insert(V);
      return std::make_pair(None, true);
    }
  
    template <typename IterT>
    void insert(IterT I, IterT E) {
      for (; I != E; ++I)
        insert(*I);
    }
  
    bool erase(const T &V) {
      if (!isSmall())
        return Set.erase(V);
      for (mutable_iterator I = Vector.begin(), E = Vector.end(); I != E; ++I)
        if (*I == V) {
          Vector.erase(I);
          return true;
        }
      return false;
    }
  
    void clear() {
      Vector.clear();
      Set.clear();
    }
  
  private:
    bool isSmall() const { return Set.empty(); }
  
    VIterator vfind(const T &V) const {
      for (VIterator I = Vector.begin(), E = Vector.end(); I != E; ++I)
        if (*I == V)
          return I;
      return Vector.end();
    }
  };
  
  /// If this set is of pointer values, transparently switch over to using
  /// SmallPtrSet for performance.
  template <typename PointeeType, unsigned N>
  class SmallSet<PointeeType*, N> : public SmallPtrSet<PointeeType*, N> {};
  
  } // end namespace wpi
  
  #endif // LLVM_ADT_SMALLSET_H
  