llvm/ADT/SmallVector.h

//===- llvm/ADT/SmallVector.h - 'Normally small' vectors --------*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file was developed by Chris Lattner and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file defines the SmallVector class.
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_ADT_SMALLVECTOR_H
#define LLVM_ADT_SMALLVECTOR_H

#include <cassert>
#include <memory>

namespace llvm {

/// SmallVector - This is a 'vector' (really, a variable-sized array), optimized
/// for the case when the array is small.  It contains some number of elements
/// in-place, which allows it to avoid heap allocation when the actual number of
/// elements is below that threshold.  This allows normal "small" cases to be
/// fast without losing generality for large inputs.
///
/// Note that this does not attempt to be exception safe.
///
template <typename T, unsigned N>
class SmallVector {
  // Allocate raw space for N elements of type T.  If T has a ctor or dtor, we
  // don't want it to be automatically run, so we need to represent the space as
  // something else.  An array of char would work great, but might not be
  // aligned sufficiently.  Instead, we either use GCC extensions, or some
  // number of union instances for the space, which guarantee maximal alignment.
  union U {
    double D;
    long double LD;
    long long L;
    void *P;
  };

  /// InlineElts - These are the 'N' elements that are stored inline in the body
  /// of the vector
  U InlineElts[(sizeof(T)*N+sizeof(U)-1)/sizeof(U)];
  T *Begin, *End, *Capacity;
public:
  // Default ctor - Initialize to empty.
  SmallVector() : Begin((T*)InlineElts), End(Begin), Capacity(Begin+N) {
  }

  SmallVector(const SmallVector &RHS) {
    unsigned RHSSize = RHS.size();
    Begin = (T*)InlineElts;

    // Doesn't fit in the small case?  Allocate space.
    if (RHSSize > N) {
      End = Capacity = Begin;
      grow(RHSSize);
    }
    End = Begin+RHSSize;
    Capacity = Begin+N;
    std::uninitialized_copy(RHS.begin(), RHS.end(), Begin);
  }
  ~SmallVector() {
    // If this wasn't grown from the inline copy, deallocate the old space.
    if ((void*)Begin != (void*)InlineElts)
      delete[] (char*)Begin;
  }

  typedef size_t size_type;
  typedef T* iterator;
  typedef const T* const_iterator;
  typedef T& reference;
  typedef const T& const_reference;

  bool empty() const { return Begin == End; }
  size_type size() const { return End-Begin; }

  iterator begin() { return Begin; }
  const_iterator begin() const { return Begin; }

  iterator end() { return End; }
  const_iterator end() const { return End; }

  reference operator[](unsigned idx) {
    assert(idx < size() && "out of range reference!");
    return Begin[idx];
  }
  const_reference operator[](unsigned idx) const {
    assert(idx < size() && "out of range reference!");
    return Begin[idx];
  }

  reference back() {
    assert(!empty() && "SmallVector is empty!");
    return end()[-1];
  }
  const_reference back() const {
    assert(!empty() && "SmallVector is empty!");
    return end()[-1];
  }

  void push_back(const_reference Elt) {
    if (End < Capacity) {
  Retry:
      new (End) T(Elt);
      ++End;
      return;
    }
    grow();
    goto Retry;
  }

  const SmallVector &operator=(const SmallVector &RHS) {
    // Avoid self-assignment.
    if (this == &RHS) return *this;

    // If we already have sufficient space, assign the common elements, then
    // destroy any excess.
    unsigned RHSSize = RHS.size();
    unsigned CurSize = size();
    if (CurSize >= RHSSize) {
      // Assign common elements.
      for (unsigned i = 0; i != RHSSize; ++i)
        Begin[i] = RHS.Begin[i];

      // Destroy excess elements.
      for (unsigned i = RHSSize; i != CurSize; ++i)
        Begin[i].~T();

      // Trim.
      End = Begin + RHSSize;
      return *this;
    }

    // If we have to grow to have enough elements, destroy the current elements.
    // This allows us to avoid copying them during the grow.
    if (Capacity-Begin < RHSSize) {
      // Destroy current elements.
      for (T *I = Begin, E = End; I != E; ++I)
        I->~T();
      End = Begin;
      CurSize = 0;
      grow(RHSSize);
    } else {
      // Otherwise, use assignment for the already-constructed elements.
      for (unsigned i = 0; i != CurSize; ++i)
        Begin[i] = RHS.Begin[i];
    }

    // Copy construct the new elements in place.
    std::uninitialized_copy(RHS.Begin+CurSize, RHS.End, Begin+CurSize);

    // Set end.
    End = Begin+RHSSize;
  }

private:
  /// isSmall - Return true if this is a smallvector which has not had dynamic
  /// memory allocated for it.
  bool isSmall() const {
    return (void*)Begin == (void*)InlineElts;
  }

  /// grow - double the size of the allocated memory, guaranteeing space for at
  /// least one more element or MinSize if specified.
  void grow(unsigned MinSize = 0) {
    unsigned CurCapacity = Capacity-Begin;
    unsigned CurSize = size();
    unsigned NewCapacity = 2*CurCapacity;
    if (NewCapacity < MinSize)
      NewCapacity = MinSize;
    T *NewElts = reinterpret_cast<T*>(new char[NewCapacity*sizeof(T)]);

    // Copy the elements over.
    std::uninitialized_copy(Begin, End, NewElts);

    // Destroy the original elements.
    for (T *I = Begin, *E = End; I != E; ++I)
      I->~T();

    // If this wasn't grown from the inline copy, deallocate the old space.
    if (!isSmall())
      delete[] (char*)Begin;

    Begin = NewElts;
    End = NewElts+CurSize;
    Capacity = Begin+NewCapacity*2;
  }
};

} // End llvm namespace

#endif