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Posted to dev@lucene.apache.org by "Uwe Schindler (JIRA)" <ji...@apache.org> on 2018/10/18 10:40:00 UTC
[jira] [Updated] (LUCENE-8533) DataInput#readVInt() supports
negative numbers although not documented
[ https://issues.apache.org/jira/browse/LUCENE-8533?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]
Uwe Schindler updated LUCENE-8533:
----------------------------------
Summary: DataInput#readVInt() supports negative numbers although not documented (was: Incorrect readVInt: negative number could be returned)
> DataInput#readVInt() supports negative numbers although not documented
> ----------------------------------------------------------------------
>
> Key: LUCENE-8533
> URL: https://issues.apache.org/jira/browse/LUCENE-8533
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Vladimir Dolzhenko
> Assignee: Uwe Schindler
> Priority: Major
> Attachments: LUCENE-8533_fix_readVInt_javadoc.patch, readVInt.patch
>
>
> {{readVInt()}} has to return positive numbers (and zero), throw some exception in case of negative numbers.
> While for the sequence of bytes {{[-1, -1, -1, -1, 15]}} it returns {{-1}}.
> simplifying [readVInt|https://github.com/apache/lucene-solr/blob/1d85cd783863f75cea133fb9c452302214165a4d/lucene/core/src/java/org/apache/lucene/store/DataInput.java#L113] up to last readByte (exclusive):
> {code:java}
> int i = ((byte)-1) & 0x7F;
> i |= (((byte)-1) & 0x7F) << 7;
> i |= (((byte)-1) & 0x7F) << 14;
> i |= (((byte)-1) & 0x7F) << 21;
> {code}
> Here {{i = 268435455}} or in binary format is {{00001111_11111111_11111111_11111111}}
> Keeping in mind that {{int}} is a signed type we have only 3 more bits before overflow happens or in another words {{(Integer.MAX_VALUE - i) >> 28 == 7}} - that's max value could be stored in 5th byte to avoid overflow.
> Instead of
> {code:java}
> i |= (b & 0x0F) << 28;
> if ((b & 0xF0) == 0) return i;
> {code}
> has to be
> {code:java}
> i |= (b & 0x07) << 28;
> if ((b & 0xF8) == 0) return i;
> {code}
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