Support GS1 for datamatrix

[gijsbotje]

As #42 has been stale for years, this adds support for the GS1 hints used by the minimal encoder as implemented in zxing.

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Validated using https://play.google.com/store/apps/details?id=com.chaniotisalexandros.gs1barcodescanner and generating the barcodes below using our API.

content in human-readable format:

(01)05391538880109(17)261130(10)28967(21)1049931544602459836

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[makiuchi-d]

Thank you for the contribution.
Does this PR add functionality that is not present in the original ZXing?

[gijsbotje]

@makiuchi-d not for the datamatrix. I implemented the minimal encoder, like you suggested.

I couldn't find an actual prepend for the code128 in the original ZXing. Do you need me to remove it?

If anything else needs to change let me know!

[gijsbotje]

@makiuchi-d i removed the code128 changes as they are not mirroring ZXing's implementation.

[makiuchi-d]

@gijsbotje
Happy New Year!
Thank you for the great contribution.
I'm checking it now and will merge it after that.
Please wait for a while.

[makiuchi-d]

I added some tests and fixed a bug in how character encoding is handled.
Since string indexing in Go is not character-based, it needs to be converted to []rune. Also, encoded byte sequences shouldn’t be treated as strings or runes.

The HighLevelEncoder naming (which I wrote) wasn’t very consistent either, but I fixed some inconsistent naming in MinimalEncoder. Was that written by AI, by any chance?

I’ll add a few more tests and plan to merge if everything looks good.

[codecov]

Codecov Report

❌ Patch coverage is 82.44186% with 151 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.37%. Comparing base (95e256b) to head (0e62da5).

Files with missing lines	Patch %	Lines
datamatrix/encoder/minimal_encoder.go	79.96%	72 Missing and 32 partials ⚠️
datamatrix/datamatrix_writer.go	21.05%	27 Missing and 3 partials ⚠️
common/minimal_eci_input.go	92.16%	5 Missing and 8 partials ⚠️
datamatrix/encoder/high_level_encoder.go	86.95%	1 Missing and 2 partials ⚠️
common/eci_encoder_set.go	98.79%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #78      +/-   ##
==========================================
- Coverage   97.93%   96.37%   -1.56%     
==========================================
  Files         122      126       +4     
  Lines       12951    11352    -1599     
==========================================
- Hits        12684    10941    -1743     
- Misses        191      291     +100     
- Partials       76      120      +44     

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[gijsbotje]

@makiuchi-d anything I can do to help?

[makiuchi-d]

@gijsbotje
Sorry for the delay -- I haven't had much time recently, but I'm still working on the review.

If possible, it would really help if you could also port GS1 support for DataMatrixReader so we can cross-verify with the Writer.

[gijsbotje]

@makiuchi-d i've tried adding the support and added a test with the image from my initial comment. Hopefully it's all correct :)

[gijsbotje]

@makiuchi-d anything else i can do to help you out on this?