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https://issues.apache.org/jira/browse/FOP-3346?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18123942#comment-18123942
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Joao Goncalves commented on FOP-3346:
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Can you add an FO to replicate the issue?
> ToUnicode selectors are off by one after every supplementary-plane character,
> so the text after an emoji or a mathematical letter extracts wrongly
> --------------------------------------------------------------------------------------------------------------------------------------------------
>
> Key: FOP-3346
> URL: https://issues.apache.org/jira/browse/FOP-3346
> Project: FOP
> Issue Type: Bug
> Components: renderer/pdf
> Affects Versions: 2.11
> Reporter: Jason Harrop
> Priority: Minor
>
> CIDSubset.getChars builds a char\[] with StringBuilder.appendCodePoint, so a
> supplementary-plane character occupies two slots. PDFToUnicodeCMap then
> derives the character selector from the array position. Every selector after
> the pair is therefore written one too high.
> Measured with the 2.11 command line, DejaVu Math TeX Gyre, the text "A𝐀BZ":
> the content stream uses selectors 3 4 5 6, and the CMap says
> {noformat}
> <0003> <0041> <0004> <d835dc00> <0006> <0042> <0007> <005a>
> {noformat}
> Selector 5, the B, has no entry, and selector 6, the Z, is published as B.
> pdftotext extracts "A𝐀 B"; pdf.js and PDFium give the missing selector as
> U+0005.
> PDFToUnicodeCMapTestCase.surrogatePairTest pins the drift: it expects the
> entry after the pair at 0x63 to be 0x65.
> h3. Fix
> Build the CMap from one destination per selector (a String, so a surrogate
> pair is one entry of length two) rather than from a positional char\[]. The
> range logic then needs no surrogate special cases: an entry may join a
> bfrange when it is one code point, and two entries are consecutive when their
> code points are and their selectors share a 256 block. Expectations in
> surrogatePairTest, surrogatePairRangeTest, surrogatePairsRangeTest and
> rangeSizeSurrogateTest change accordingly; the last also used low surrogates
> that ran past U+DFFF and now starts at U+DC00.
> After the fix the same file extracts as "A𝐀BZ" in pdftotext, mupdf, pdf.js
> and PDFium.
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