source: vbox/trunk/src/libs/xpcom18a4/xpcom/ds/nsDeque.cpp@ 102005

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#include "nsDeque.h"
#include "nsCRT.h"
#ifdef DEBUG_rickg
#include <stdio.h>
#endif

/**
 * 07/02/2001  09:17p 509,104 clangref.pdf from openwatcom's site
 * Watcom C Language Reference Edition 11.0c
 * page 118 of 297
 *
 * The % symbol yields the remainder from the division of the first operand
 * by the second operand. The operands of % must have integral type.
 *
 * When both operands of % are positive, the result is a positive value
 * smaller than the second operand. When one or both operands is negative,
 * whether the result is positive or negative is implementation-defined.
 *
 */
/* Ok, so first of all, C is underspecified. joy.
 * The following functions do not provide a correct implementation of modulus
 * They provide functionality for x>-y.
 * There are risks of 2*y being greater than max int, which is part of the
 * reason no multiplication is used and other operations are avoided.
 *
 * modasgn
 * @param x variable
 * @param y expression
 * approximately equivalent to x %= y
 *
 * modulus
 * @param x expression
 * @param y expression
 * approximately equivalent to x % y
 */
#define modasgn(x,y) if (x<0) x+=y; x%=y
#define modulus(x,y) ((x<0)?(x+y)%(y):(x)%(y))

MOZ_DECL_CTOR_COUNTER(nsDeque)

/**
 * Standard constructor
 */
nsDeque::nsDeque() {
  MOZ_COUNT_CTOR(nsDeque);
  mOrigin=mSize=0;
  mData=mBuffer; // don't allocate space until you must
  mCapacity=sizeof(mBuffer)/sizeof(mBuffer[0]);
  memset(mData, 0, mCapacity*sizeof(mBuffer[0]));
}

/**
 * Destructor
 */
nsDeque::~nsDeque() {
  MOZ_COUNT_DTOR(nsDeque);

  Erase();
  if (mData && (mData!=mBuffer)) {
    delete [] mData;
  }
  mData=0;
}

/**
 * Remove all items from container without destroying them.
 *
 * @return  *this
 */
nsDeque& nsDeque::Empty() {
  if (mSize && mData) {
    memset(mData, 0, mCapacity*sizeof(*mData));
  }
  mSize=0;
  mOrigin=0;
  return *this;
}

/**
 * Remove and delete all items from container
 *
 * @return  *this
 */
nsDeque& nsDeque::Erase() {
  return Empty();
}

/**
 * This method quadruples the size of the deque
 * Elements in the deque are resequenced so that elements
 * in the deque are stored sequentially
 *
 * If the deque actually overflows, there's very little we can do.
 * Perhaps this function should return PRBool/nsresult indicating success/failure.
 *
 * @return  capacity of the deque
 *          If the deque did not grow,
 *          and you knew its capacity beforehand,
 *          then this would be a way to indicate the failure.
 */
PRInt32 nsDeque::GrowCapacity() {
  PRInt32 theNewSize=mCapacity<<2;
  NS_ASSERTION(theNewSize>mCapacity, "Overflow");
  if (theNewSize<=mCapacity)
    return mCapacity;
  void** temp=new void*[theNewSize];

  //Here's the interesting part: You can't just move the elements
  //directly (in situ) from the old buffer to the new one.
  //Since capacity has changed, the old origin doesn't make
  //sense anymore. It's better to resequence the elements now.

  if (temp) {
    PRInt32 tempi=0;
    PRInt32 i=0;
    PRInt32 j=0;
    for (i=mOrigin; i<mCapacity; i++) {
      temp[tempi++]=mData[i]; //copy the leading elements...
    }
    for (j=0;j<mOrigin;j++) {
      temp[tempi++]=mData[j]; //copy the trailing elements...
    }
    if (mData != mBuffer) {
      delete [] mData;
    }
    mCapacity=theNewSize;
    mOrigin=0; //now realign the origin...
    mData=temp;
  }
  return mCapacity;
}

/**
 * This method adds an item to the end of the deque.
 * This operation has the potential to cause the
 * underlying buffer to resize.
 *
 * @param   aItem: new item to be added to deque
 * @return  *this
 */
nsDeque& nsDeque::Push(void* aItem) {
  if (mSize==mCapacity) {
    GrowCapacity();
  }
  mData[modulus(mOrigin + mSize, mCapacity)]=aItem;
  mSize++;
  return *this;
}

/**
 * This method adds an item to the front of the deque.
 * This operation has the potential to cause the
 * underlying buffer to resize.
 *
 * --Commments for GrowCapacity() case
 * We've grown and shifted which means that the old
 * final element in the deque is now the first element
 * in the deque.  This is temporary.
 * We haven't inserted the new element at the front.
 *
 * To continue with the idea of having the front at zero
 * after a grow, we move the old final item (which through
 * the voodoo of mOrigin-- is now the first) to its final
 * position which is conveniently the old length.
 *
 * Note that this case only happens when the deque is full.
 * [And that pieces of this magic only work if the deque is full.]
 * picture:
 *   [ABCDEFGH] @[mOrigin:3]:D.
 * Task: PushFront("Z")
 * shift mOrigin so, @[mOrigin:2]:C
 * stretch and rearrange: (mOrigin:0)
 *   [CDEFGHAB ________ ________ ________]
 * copy: (The second C is currently out of bounds)
 *   [CDEFGHAB C_______ ________ ________]
 * later we will insert Z:
 *   [ZDEFGHAB C_______ ________ ________]
 * and increment size: 9. (C is no longer out of bounds)
 * --
 * @param   aItem: new item to be added to deque
 * @return  *this
 */
nsDeque& nsDeque::PushFront(void* aItem) {
  mOrigin--;
  modasgn(mOrigin,mCapacity);
  if (mSize==mCapacity) {
    GrowCapacity();
    /* Comments explaining this are above*/
    mData[mSize]=mData[mOrigin];
  }
  mData[mOrigin]=aItem;
  mSize++;
  return *this;
}

/**
 * Remove and return the last item in the container.
 *
 * @return  ptr to last item in container
 */
void* nsDeque::Pop() {
  void* result=0;
  if (mSize>0) {
    --mSize;
    PRInt32 offset=modulus(mSize + mOrigin, mCapacity);
    result=mData[offset];
    mData[offset]=0;
    if (!mSize) {
      mOrigin=0;
    }
  }
  return result;
}

/**
 * This method gets called you want to remove and return
 * the first member in the container.
 *
 * @return  last item in container
 */
void* nsDeque::PopFront() {
  void* result=0;
  if (mSize>0) {
    NS_ASSERTION(mOrigin < mCapacity, "Error: Bad origin");
    result=mData[mOrigin];
    mData[mOrigin++]=0;     //zero it out for debugging purposes.
    mSize--;
    // Cycle around if we pop off the end
    // and reset origin if when we pop the last element
    if (mCapacity==mOrigin || !mSize) {
      mOrigin=0;
    }
  }
  return result;
}

/**
 * This method gets called you want to peek at the bottom
 * member without removing it.
 *
 * @return  last item in container
 */
void* nsDeque::Peek() {
  void* result=0;
  if (mSize>0) {
    result = mData[modulus(mSize - 1 + mOrigin, mCapacity)];
  }
  return result;
} 

/**
 * This method gets called you want to peek at the topmost
 * member without removing it.
 *
 * @return  last item in container
 */
void* nsDeque::PeekFront() {
  void* result=0;
  if (mSize>0) {
    result=mData[mOrigin];
  }
  return result;
}

/**
 * Call this to retrieve the ith element from this container.
 * Keep in mind that accessing the underlying elements is
 * done in a relative fashion. Object 0 is not necessarily
 * the first element (the first element is at mOrigin).
 *
 * @param   aIndex : 0 relative offset of item you want
 * @return  void* or null
 */
void* nsDeque::ObjectAt(PRInt32 aIndex) const {
  void* result=0;
  if ((aIndex>=0) && (aIndex<mSize)) {
    result=mData[modulus(mOrigin + aIndex, mCapacity)];
  }
  return result;
}