Add truncate as a BibTex key modifier

[k3KAW8Pnf7mkmdSMPHz27]

Fixes #3915. Adds a truncate modifier to the available BibTex key modifiers. For "A Title" the bracketed pattern [title:truncate3] produces the result "A T".

• The syntax is truncateN ([title:truncate5]) to mimic the pattern of authN etc.

Remaining

• See if layered expressions are reasonable to implement, e.g., [[auth][title]:truncate10]
• Update changelog
• Documentation

Things I am not sure about

• Should truncate be a formatter(org.jabref/logic/formatter)? Currently, formatters does not accept arguments
• The interaction between BibTex key generation and truncate might lead to unexpected results as whitespaces are removed after truncation. Unless this is considered obvious, it should perhaps be mentioned in the documentation of truncate. E.g., for "A Title", [title:truncate3] will lead to the bibtexkey "AT"
• Should this be included in the changelog?
• There were a BracketedPatternTest in org.jabref/logic/util/. Is there a correct location for the BracketedPatternTest?

Others

• Change in CHANGELOG.md described (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.

[Siedlerchr]

1. Changelog, yes please
2. Ideally it would be a formatter, maybe we need to adapt the interface to allow arguments.
3. Whitespace: At least it should be documented
4. BrackedPatternTest, yes would be nice if you could integrate your tests there, as they test with a bibtex entry and multiple formatters

Otherwise looks already good as starting point

[k3KAW8Pnf7mkmdSMPHz27]

2. Ideally it would be a formatter, maybe we need to adapt the interface to allow arguments.

I missed how RegexFormatter is implemented, I'll just mimic that and it shouldn't be a problem.

[k3KAW8Pnf7mkmdSMPHz27]

Updates to documentation.

[k3KAW8Pnf7mkmdSMPHz27]

In a nutshell.

1. truncateN has been added as a Formatter.
2. Directory names are truncated to a length of 255 characters using a utility function for filenames.

• I don't think I should change the parser, there is a large risk I will create more issues than I solve
• It might be possible to shorten the code in java/org/jabref/logic/bibtexkeypattern/BracketedPattern.java quite a bit if larger parts of the code is turned into Formatters and Formatters can accept arguments (either in the constructor or in the format method, both are currently being used).

[k3KAW8Pnf7mkmdSMPHz27]

When I try to import a file from an URL using a pattern that creates a java.nio.file.FileSystemException. The directory cannot be created but the file is still downloaded. Should I try to fix that as part of this pull request as well?

If a file cannot be downloaded to the location specified by a file pattern it makes more sense (to me) that the file isn't downloaded at all.

[Siedlerchr]

your changes look so far, is this ready then?

[k3KAW8Pnf7mkmdSMPHz27]

Yes. The last conflicts should be resolved now.

[k3KAW8Pnf7mkmdSMPHz27]

The last change (change of test names) can be removed but it seems more consistent with the guidelines?

[Siedlerchr]

For me it's fine. For external contributors we have the rule that at least two developers should review a PR. So when a second dev gives his okay, it's ready for merge!

[tobiasdiez]

The code looks good to me, so I'll merge now. Thanks for your contribution - we look forward to your next PR!