text: //===- llvm/ADT/SmallSet.h - 'Normally small' sets --------------*- 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 SmallSet class.
  //
  //===----------------------------------------------------------------------===//
  
  #ifndef LLVM_ADT_SMALLSET_H
  #define LLVM_ADT_SMALLSET_H
  
  #include "llvm/ADT/None.h"
  #include "llvm/ADT/SmallPtrSet.h"
  #include "llvm/ADT/SmallVector.h"
  #include "llvm/ADT/iterator.h"
  #include "llvm/Support/Compiler.h"
  #include "llvm/Support/type_traits.h"
  #include <cstddef>
  #include <functional>
  #include <set>
  #include <type_traits>
  #include <utility>
  
  namespace llvm {
  
  /// SmallSetIterator - This class implements a const_iterator for SmallSet by
  /// delegating to the underlying SmallVector or Set iterators.
  template <typename T, unsigned N, typename C>
  class SmallSetIterator
      : public iterator_facade_base<SmallSetIterator<T, N, C>,
                                    std::forward_iterator_tag, T> {
  private:
    using SetIterTy = typename std::set<T, C>::const_iterator;
    using VecIterTy = typename SmallVector<T, N>::const_iterator;
    using SelfTy = SmallSetIterator<T, N, C>;
  
    /// Iterators to the parts of the SmallSet containing the data. They are set
    /// depending on isSmall.
    union {
      SetIterTy SetIter;
      VecIterTy VecIter;
    };
  
    bool isSmall;
  
  public:
    SmallSetIterator(SetIterTy SetIter) : SetIter(SetIter), isSmall(false) {}
  
    SmallSetIterator(VecIterTy VecIter) : VecIter(VecIter), isSmall(true) {}
  
    // Spell out destructor, copy/move constructor and assignment operators for
    // MSVC STL, where set<T>::const_iterator is not trivially copy constructible.
    ~SmallSetIterator() {
      if (isSmall)
        VecIter.~VecIterTy();
      else
        SetIter.~SetIterTy();
    }
  
    SmallSetIterator(const SmallSetIterator &Other) : isSmall(Other.isSmall) {
      if (isSmall)
        VecIter = Other.VecIter;
      else
        // Use placement new, to make sure SetIter is properly constructed, even
        // if it is not trivially copy-able (e.g. in MSVC).
        new (&SetIter) SetIterTy(Other.SetIter);
    }
  
    SmallSetIterator(SmallSetIterator &&Other) : isSmall(Other.isSmall) {
      if (isSmall)
        VecIter = std::move(Other.VecIter);
      else
        // Use placement new, to make sure SetIter is properly constructed, even
        // if it is not trivially copy-able (e.g. in MSVC).
        new (&SetIter) SetIterTy(std::move(Other.SetIter));
    }
  
    SmallSetIterator& operator=(const SmallSetIterator& Other) {
      // Call destructor for SetIter, so it gets properly destroyed if it is
      // not trivially destructible in case we are setting VecIter.
      if (!isSmall)
        SetIter.~SetIterTy();
  
      isSmall = Other.isSmall;
      if (isSmall)
        VecIter = Other.VecIter;
      else
        new (&SetIter) SetIterTy(Other.SetIter);
      return *this;
    }
  
    SmallSetIterator& operator=(SmallSetIterator&& Other) {
      // Call destructor for SetIter, so it gets properly destroyed if it is
      // not trivially destructible in case we are setting VecIter.
      if (!isSmall)
        SetIter.~SetIterTy();
  
      isSmall = Other.isSmall;
      if (isSmall)
        VecIter = std::move(Other.VecIter);
      else
        new (&SetIter) SetIterTy(std::move(Other.SetIter));
      return *this;
    }
  
    bool operator==(const SmallSetIterator &RHS) const {
      if (isSmall != RHS.isSmall)
        return false;
      if (isSmall)
        return VecIter == RHS.VecIter;
      return SetIter == RHS.SetIter;
    }
  
    SmallSetIterator &operator++() { // Preincrement
      if (isSmall)
        VecIter++;
      else
        SetIter++;
      return *this;
    }
  
    const T &operator*() const { return isSmall ? *VecIter : *SetIter; }
  };
  
  /// 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.
  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;
    using const_iterator = SmallSetIterator<T, N, C>;
  
    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();
    }
  
    const_iterator begin() const {
      if (isSmall())
        return {Vector.begin()};
      return {Set.begin()};
    }
  
    const_iterator end() const {
      if (isSmall())
        return {Vector.end()};
      return {Set.end()};
    }
  
    /// Check if the SmallSet contains the given element.
    bool contains(const T &V) const {
      if (isSmall())
        return vfind(V) != Vector.end();
      return Set.find(V) != Set.end();
    }
  
  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> {};
  
  /// Equality comparison for SmallSet.
  ///
  /// Iterates over elements of LHS confirming that each element is also a member
  /// of RHS, and that RHS contains no additional values.
  /// Equivalent to N calls to RHS.count.
  /// For small-set mode amortized complexity is O(N^2)
  /// For large-set mode amortized complexity is linear, worst case is O(N^2) (if
  /// every hash collides).
  template <typename T, unsigned LN, unsigned RN, typename C>
  bool operator==(const SmallSet<T, LN, C> &LHS, const SmallSet<T, RN, C> &RHS) {
    if (LHS.size() != RHS.size())
      return false;
  
    // All elements in LHS must also be in RHS
    return all_of(LHS, [&RHS](const T &E) { return RHS.count(E); });
  }
  
  /// Inequality comparison for SmallSet.
  ///
  /// Equivalent to !(LHS == RHS). See operator== for performance notes.
  template <typename T, unsigned LN, unsigned RN, typename C>
  bool operator!=(const SmallSet<T, LN, C> &LHS, const SmallSet<T, RN, C> &RHS) {
    return !(LHS == RHS);
  }
  
  } // end namespace llvm
  
  #endif // LLVM_ADT_SMALLSET_H
  