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https://issues.apache.org/jira/browse/LANG-1606?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17188393#comment-17188393
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Sebb commented on LANG-1606:
----------------------------
There are two ways to count matches: overlapping or non-overlapping.
The code currently correctly counts non-overlapping matches.
I agree that the Javadoc is not clear on this, however I don't think it is
actually wrong
Given that users may rely on the current behaviour, generally it is the Javadoc
that must be changed rather than the code.
> StringUtils.countMatches returns incorrect value while handling intersecting
> substrings
> ---------------------------------------------------------------------------------------
>
> Key: LANG-1606
> URL: https://issues.apache.org/jira/browse/LANG-1606
> Project: Commons Lang
> Issue Type: Bug
> Components: lang.*
> Affects Versions: 3.11
> Reporter: Rustem Galiev
> Priority: Major
> Time Spent: 20m
> Remaining Estimate: 0h
>
> Steps to reproduce:
> 1. Call the method like that:
> {code:java}
> int count = StringUtils.countMatches("abaabaababaab", "aba");
> {code}
> Actual result: the value of count variable equals 3
> Expected result: the value of count variable equals 4
> The substrings are highlighted in red:
> {color:#ff0000}aba{color}abaababaab
> aba{color:#ff0000}aba{color}ababaab
> abaaba{color:#ff0000}aba{color}baab
> abaabaab{color:#ff0000}aba{color}ab
> Method returns incorrect value because of this code:
> {code:java}
> while ((idx = CharSequenceUtils.indexOf(str, sub, idx)) != INDEX_NOT_FOUND) {
> count++;
> idx += sub.length();
> }
> {code}
> This looks like a greedy algorithm - but increasing the idx variable by the
> length of substring could lead to the problems like in example:
> Let's say that idx = 6, so we try to find a substring in the highlighted
> suffix:
> abaaba{color:#ff0000}ababaab{color}
> We found the substring, so idx now becomes idx + 3 = 9. So now this suffix
> will be used for searching substring in it:
> abaabaaba{color:#ff0000}baab{color}
> But because of increasing the value of idx by 3 we won't find the substring
> (abaabaab{color:#ff0000}aba{color}ab) which intersects with the already found
> substring on the last step.
> Basically, this method will work incorrectly with any substrings that
> intersect with each other.
> There is also a unit test with incorrect expected value:
> {code:java}
> assertEquals(4,
> StringUtils.countMatches("oooooooooooo", "ooo"));
> {code}
> If this behavior (counting substrings that do not intersect) is intended,
> please update the JavaDoc to reflect it. Right now it looks like that:
> {code:java}
> Counts how many times the substring appears in the larger string.
> {code}
> Link for the PR: https://github.com/apache/commons-lang/pull/615
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