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https://issues.apache.org/jira/browse/LUCENE-1990?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12803743#action_12803743
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Paul Elschot commented on LUCENE-1990:
--------------------------------------
The generated code in the patches has quite a few switch statements to decode a
single value.
These switch statements could be avoided by using something like this (adapted
from the 1410b patch):
{code}
/** Decode a value from the compressed array of b bit values by retrieving the
corresponding bits.
* Since numFrameBits is always smaller than the number of bits in an int,
* at most two ints in the buffer will be used.
*/
public int decodeCompressedValueBase(int compressedPos, int numBits) {
int compressedBitPos = numBits * compressedPos;
int intIndex = (compressedBitPos >> 5);
int firstBitPosition = compressedBitPos & 31;
int value = intBuffer.get(intIndex) >>> firstBitPosition;
if ((firstBitPosition + numBits) > 32) { // value does not fit in first int
intIndex++;
value |= (intBuffer.get(intIndex) << (32 - firstBitPosition));
}
final int maxValue = (int) ((1L << numBits) - 1);
return value & maxValue;
}
{code}
As maxValue is essentially a mask, it could also be looked up in an array.
Could that be faster than these generated switch statements?
> Add unsigned packed int impls in oal.util
> -----------------------------------------
>
> Key: LUCENE-1990
> URL: https://issues.apache.org/jira/browse/LUCENE-1990
> Project: Lucene - Java
> Issue Type: Improvement
> Components: Index
> Reporter: Michael McCandless
> Priority: Minor
> Attachments: LUCENE-1990-te20100122.patch, LUCENE-1990.patch,
> LUCENE-1990_PerformanceMeasurements20100104.zip
>
>
> There are various places in Lucene that could take advantage of an
> efficient packed unsigned int/long impl. EG the terms dict index in
> the standard codec in LUCENE-1458 could subsantially reduce it's RAM
> usage. FieldCache.StringIndex could as well. And I think "load into
> RAM" codecs like the one in TestExternalCodecs could use this too.
> I'm picturing something very basic like:
> {code}
> interface PackedUnsignedLongs {
> long get(long index);
> void set(long index, long value);
> }
> {code}
> Plus maybe an iterator for getting and maybe also for setting. If it
> helps, most of the usages of this inside Lucene will be "write once"
> so eg the set could make that an assumption/requirement.
> And a factory somewhere:
> {code}
> PackedUnsignedLongs create(int count, long maxValue);
> {code}
> I think we should simply autogen the code (we can start from the
> autogen code in LUCENE-1410), or, if there is an good existing impl
> that has a compatible license that'd be great.
> I don't have time near-term to do this... so if anyone has the itch,
> please jump!
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