Re[6]: New portion of improvements for Dual-Pivot Quicksort

Fri May 14 19:54:06 UTC 2010

Hello,

I've updated the class, please, review the changes.

Vladimir

Fri, 14 May 2010 01:48:11 +0700 письмо от Dmytro Sheyko <dmytro_sheyko at hotmail.com>:

> Yes. I prefer F (Find First zero using binary search) over C (Count negatives) and S (Smart Scan for zero).
> 
> > From: iaroslavski at mail.ru
> > To: dmytro_sheyko at hotmail.com
> > CC: jjb at google.com; core-libs-dev at openjdk.java.net; iaroslavski at mail.ru
> > Subject: Re[4]: New portion of improvements for Dual-Pivot Quicksort
> > Date: Thu, 13 May 2010 21:34:54 +0400
> > 
> > Dmytro,
> > 
> > I've tested your suggested variants, and found that case "C"
> > (very interesting approach to find first position of zero
> > by counting negative elements) works slower than original
> > or two other cases.
> > 
> > Implementations "F" and "S" are very close to each other
> > and little bit faster than original. I prefer case "F":
> > it is shorter and more clear. Do you agree?
> > 
> > I'll prepare updated DualPivotQuicksort file and send it
> > tomorrow.
> > 
> > Thank you,
> > Vladimir
> > 
> > Wed, 12 May 2010 17:04:52 +0700 письмо от Dmytro Sheyko <dmytro_sheyko at hotmail.com>:
> > 
> > > Vladimir,
> > > 
> > > Your changes are good for me.
> > > 
> > > Additionally I have some comments/proposals regarding dealing with negative zeros.
> > > 
> > > 1. Scanning for the first zero we can avoid range check (i >= left) if we have at least one negative value.
> > > --- DualPivotQuicksort.java Tue May 11 09:04:19 2010
> > > +++ DualPivotQuicksortS.java Wed May 12 12:10:46 2010
> > > @@ -1705,10 +1705,15 @@
> > > }
> > > 
> > > // Find first zero element
> > > - int zeroIndex = findAnyZero(a, left, n);
> > > + int zeroIndex = 0;
> > > 
> > > - for (int i = zeroIndex - 1; i >= left && a[i] == 0.0f; i--) {
> > > - zeroIndex = i;
> > > + if (a[left] < 0.0f) {
> > > + zeroIndex = findAnyZero(a, left, n);
> > > +
> > > + // there is at least one negative value, so range check is not needed
> > > + for (int i = zeroIndex - 1; /*i >= left &&*/ a[i] == 0.0f; i--) {
> > > + zeroIndex = i;
> > > + }
> > > }
> > > 
> > > // Turn the right number of positive zeros back into negative zeros
> > > 
> > > 2. We can find the position of the first zero by counting negative values during preprocessing phase.
> > > --- DualPivotQuicksort.java Tue May 11 09:04:19 2010
> > > +++ DualPivotQuicksortC.java Wed May 12 12:01:24 2010
> > > @@ -1678,7 +1678,7 @@
> > > * Phase 1: Count negative zeros and move NaNs to end of array.
> > > */ 
> > > final int NEGATIVE_ZERO = Float.floatToIntBits(-0.0f);
> > > - int numNegativeZeros = 0;
> > > + int numNegativeZeros = 0, numNegativeValues = 0;
> > > int n = right;
> > > 
> > > for (int k = left; k <= n; k++) {
> > > @@ -1689,6 +1689,8 @@
> > > } else if (ak != ak) { // i.e., ak is NaN
> > > a[k--] = a[n];
> > > a[n--] = Float.NaN;
> > > + } else if (ak < 0.0f) {
> > > + numNegativeValues++;
> > > }
> > > }
> > > 
> > > @@ -1705,7 +1707,7 @@
> > > }
> > > 
> > > // Find first zero element
> > > - int zeroIndex = findAnyZero(a, left, n);
> > > + int zeroIndex = numNegativeValues;
> > > 
> > > for (int i = zeroIndex - 1; i >= left && a[i] == 0.0f; i--) {
> > > zeroIndex = i;
> > > 
> > > 3. We can use binary search to find the first zero and thus avoid linear scan.
> > > --- DualPivotQuicksort.java Tue May 11 09:04:19 2010
> > > +++ DualPivotQuicksortF.java Wed May 12 12:03:58 2010
> > > @@ -1705,11 +1705,7 @@
> > > }
> > > 
> > > // Find first zero element
> > > - int zeroIndex = findAnyZero(a, left, n);
> > > -
> > > - for (int i = zeroIndex - 1; i >= left && a[i] == 0.0f; i--) {
> > > - zeroIndex = i;
> > > - }
> > > + int zeroIndex = findFirstZero(a, left, n);
> > > 
> > > // Turn the right number of positive zeros back into negative zeros
> > > for (int i = zeroIndex, m = zeroIndex + numNegativeZeros; i < m; i++) {
> > > @@ -1718,7 +1714,7 @@
> > > }
> > > 
> > > /**
> > > - * Returns the index of some zero element in the specified range via
> > > + * Returns the index of the first zero element in the specified range via
> > > * binary search. The range is assumed to be sorted, and must contain
> > > * at least one zero.
> > > *
> > > @@ -1726,18 +1722,17 @@
> > > * @param low the index of the first element, inclusive, to be searched
> > > * @param high the index of the last element, inclusive, to be searched
> > > */ 
> > > - private static int findAnyZero(float[] a, int low, int high) {
> > > - while (true) {
> > > + private static int findFirstZero(float[] a, int low, int high) {
> > > + while (low < high) {
> > > int middle = (low + high) >>> 1;
> > > float middleValue = a[middle];
> > > 
> > > if (middleValue < 0.0f) {
> > > low = middle + 1;
> > > - } else if (middleValue > 0.0f) {
> > > - high = middle - 1;
> > > - } else { // middleValue == 0.0f
> > > - return middle;
> > > + } else { // middleValue >= 0.0f
> > > + high = middle;
> > > }
> > > + return low;
> > > }
> > > }
> > > 
> > > Counting negative values appeared more expensive than any other variants. 
> > > The last proposal seems to me as efficient as the current solution is in its worst case - when we have only one negative zero (in the half of array).
> > > And it shows the best result if we have many zeros.
> > > 
> > > Regards,
> > > Dmytro Sheyko
> > > 
> > > > From: iaroslavski at mail.ru
> > > > To: jjb at google.com; dmytro_sheyko at hotmail.com
> > > > CC: core-libs-dev at openjdk.java.net; iaroslavski at mail.ru
> > > > Subject: Re[2]: New portion of improvements for Dual-Pivot Quicksort
> > > > Date: Sun, 9 May 2010 23:51:27 +0400
> > > > 
> > > > Josh,
> > > > Dmytro,
> > > > 
> > > > I have done more thoroughly testing "great - less > 5 * seventh" vs. "less < e1 && great > e5",
> > > > and found that more symmetric code "less < e1 && great > e5" is little bit faster, ~0.5..0.7%
> > > > on both VMs. Other code has not been changed.
> > > > 
> > > > Please, take the latest version in attachment.
> > > > 
> > > > Vladimir
> > > > 
> > > > Tue, 4 May 2010 21:57:42 -0700 письмо от Joshua Bloch <jjb at google.com>:
> > > > 
> > > > > Vladimir,
> > > > > 
> > > > > Old:
> > > > > 
> > > > >298 if (less < e1 && great > e5) {
> > > > > 
> > > > > New:
> > > > > 
> > > > >256 if (great - less > 5 * seventh) {
> > > > 
> > > > >Regards,
> > > > >Josh
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