Issue #15460 has been updated by gettalong (Thomas Leitner).


Eregon (Benoit Daloze) wrote:
> Why are values higher in HexaPDF? Does it mean multiple bytes need to be written or is it enough to throw away the high bits?
> I suspect the latter is rarely correct.

This actually happens in the predictor filter of HexaPDF (see https://github.com/gettalong/hexapdf/blob/master/lib/hexapdf/filter/predictor.rb#L166) which is an implementation of the PNG filter types (see https://www.w3.org/TR/2003/REC-PNG-20031110/#9Filter-types).

The PNG spec says that all operations are to be taken as unsigned arithmetic modulo 256 and since this is what `String#setbyte` originally did, I just left out the additional module operation.

It is not a huge problem for me and I will release a new HexaPDF version with the fix soon.

As for whether the modulo operation should be done by `String#setbyte`: Many methods automatically convert arguments to an expected type if possible, e.g. with `#to_str`. Converting an integer to a byte using modulo 256 seems like the most straight-forward way to do this for `String#setbyte`.

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Bug #15460: Behaviour of String#setbyte changed
https://bugs.ruby-lang.org/issues/15460#change-75945

* Author: gettalong (Thomas Leitner)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.6.0p0 (2018-12-25 revision 66547) [x86_64-linux]
* Backport: 2.4: DONTNEED, 2.5: DONTNEED, 2.6: REQUIRED
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I just installed Ruby 2.6.0 for benchmarking reasons and found that the change [string.c: setbyte silently ignores upper bits](https://bugs.ruby-lang.org/projects/ruby-trunk/repository/revisions/65804) broke my library/application HexaPDF.

Before using String#setbyte I tested how it would respond to values lower than 0 or greater than 255 and found that it automatically performed the needed modulo 256 operation (at least up to Ruby 2.5.3). Therefore I left out the explicit modulo operation for performance reasons.

Would it make sense to change the String#setbyte implementation to perform the modulo operation? This would restore compatibility with prior Ruby versions and may be what people would expect.



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