Choose an Indic run's shaper on the script chosen for it, as HarfBuzz does (#199)

[jakejackson1]

Closes #199

What changes

Otl::applyOTL() picked a run's shaper from its Unicode script alone. It then sent the run to that shaper under any GSUB script, DFLT and latn included. When the font offered no script for the run, it skipped the run entirely: no reordering, no GSUB, no GPOS. HarfBuzz does the reverse: hb_ot_shaper_categorize() (src/hb-ot-shaper.hh, 14.3.1) decides on the GSUB script it chose. Under DFLT, or latn picked as the last fallback, the run gets the default shaper. With no script (HB_TAG_NONE), the script's own shaper still runs.

• shaperForScriptTag() makes that choice once the tags are known, shaper by shaper, and replaces the Myanmar "only under mym2" check.
• A run with no script now gets past the early continue, as long as its script has a shaper and the document opened it with useOTL 0x80. Every script with a shaper sits behind that bit, so a document that closed the script still gets nothing (testARunInAScriptTheDocumentHasNotOpenedToOpenTypeLayoutIsNotShaped).
• Such a run is shaped with no lookups. I checked every read on that path:
  • applyGSUB() goes on when there are no features but there is a shaper.
  • loadGsubDerivedData() still sets GSUBfont. That key has no cached derived data, so the reorderers get empty rphf/pref/blwf/pstf/rtlSUB/finals. loadGsubLookups(), which reads the GSUB table, coverage and lookup list, is only called where there are features; a font with no GSUB table has none (NotoSans-GPOS3-Synthetic in the shaping master is one).
  • The four GSUB rule walks, and applyOTL() itself, read features through features(), which returns none for an empty tag (the GPOS read beside it uses it too).
  • removeJoinControls(true), the word-boundary passes and applyGPOS() (gated on its own features) need no change.
  • No warnings under the suite's error handling.

Per shaper, as HarfBuzz 14.3.1 has it

mPDF shaper	Scripts	HarfBuzz	Under DFLT	Under latn	No script	mPDF before
I	Devanagari … Malayalam	Indic	default	default	Indic	I under any tag, skipped with none
S	Sinhala	USE	default	default	USE	S under any tag, skipped with none
E	New Tai Lue, Cham, Tai Tham	USE	default	default	USE	E under any tag, skipped with none
A	N'Ko, Mandaic	USE	default	default	USE	A under any tag, skipped with none
A	Syriac	Arabic unless DFLT	default	Arabic	Arabic	A under any tag, skipped with none
A	Arabic	Arabic always	Arabic	Arabic	Arabic	skipped with none
M	Myanmar	Myanmar; default under DFLT, latn, mymr	default	default	Myanmar	M only under mym2, skipped with none
K	Khmer	Khmer always	Khmer	Khmer	Khmer	skipped with none
T / L	Thai, Lao	Thai always	Thai	Thai	Thai	skipped with none

In each case mPDF now does what the HarfBuzz column says, using its own shaper where HarfBuzz has USE. Scripts mPDF has no shaper for are unchanged. mPDF never picks an *3 tag, so HarfBuzz's USE route for indy3 tags never comes up.

Original specification versus v2. HarfBuzz sets is_old_spec from the chosen tag not ending in 2 (hb-ot-shaper-indic.cc:324), so a run with no script is laid out to the original specification. With no lookups, mPDF's two reorderings draw the same thing. I shaped all 14,762 Bengali sequences of up to five of KA, SSA, RA, YA, VIRAMA, ZWJ, ZWNJ, I and NUKTA (starting with KA or RA) through the guru-only font with and without forcing the original specification, and nothing differed. So the dispatch keeps OtlTags' flag rather than adding a line no test could catch.

Measured

KA + I and KA + VIRAMA + SSA + I, drawn text via TextRecordingMpdf:

Font's GSUB scripts	Before	After	hb-shape 14.3.1
DFLT, deva (NotoSansBengali-DevaScript-Synthetic)	09BF 0997 / 0997 09CD 09BF 09B7	0997 09BF / 0997 09CD 09B7 09BF	uni0997 uni09BF / uni0997 uni09CD uni09B7 uni09BF
guru only (NotoSansBengali-GuruScript-Synthetic, new)	0995 09BF / 0995 09CD 09B7 09BF	09BF 0995 / 0995 09CD 09BF 09B7	uni09BF uni0995 / uni0995 uni09CD uni09BF uni09B7

ShaperChoiceTest pins both rows, the useOTL gate, and a Tai Tham run under Lanna Alif's latn (default shaper, no dotted circle; hb-shape draws gid144 = U+F001).

Fixture diff

Golden masters were regenerated cold: I deleted tests/Mpdf/tmp/mpdf, tmp/mpdf and tmp/ttfontdata, then ran fontcache, otldump, shaping and subset:update all. Only the shaping master moved, plus the four fixtures for each of the two new fonts. A script read every moved line and shaped the same text in the same font with hb-shape. It split runs by script as analyseCharacters() does, used --direction=ltr --cluster-level=2 so glyph order maps back to codepoints (.notdef taken from its cluster, other glyphs from mPDF's glyphIDtoUni), added --features=kern,-liga for that setting, and left scripts that 0x80 keeps closed unshaped.

	Lines
Moved (31 fonts, 96 runs × 3 settings)	288
Glyph order changed	270
Match hb-shape after	288
Matched hb-shape before	18
Matched before and not after	0

Run	Fonts	Before → after (0xFF)	hb-shape
bengali	8 with no Bengali script	0995 09CD 09B7 09BF → 0995 09CD 09BF 09B7	reordered
bengali	22 under DFLT/latn, 1 more (DevaScript) with its locl	0995 09CD 09BF 09B7 → 0995 09CD 09B7 09BF	not reordered
devanagari	8 / 12	reordered / no longer reordered, as above	same
gurmukhi	7 / 23	reordered / no longer reordered, as above	same
khmer	8 with no Khmer script	1780 17D2 1781 17C1 → 17C1 1780 17D2 1781	reordered
font characters 0	NotoSansBengali-DevaScript-Synthetic (DFLT)	… 09BF 09B7 … → … 09B7 09BF …	not reordered
arabic	6 with no Arabic script	same glyphs, gains gpos={"1":{"kashida":8}}	same glyphs

The 18 lines that matched before are the Arabic run. It now reaches the Arabic shaper, which has no forms to substitute, and it marks the tatweel as a kashida point, as it already did in every font that offers any script for Arabic. Glyphs are unchanged. NotoSansGurmukhiUI-GPOS71-Subset is one of the "reordered" fonts: its Devanagari and Bengali runs come back reordered, which is what the issue expects.

Snapshots

tai-tham-sakot is the only snapshot that moves; the other 78 are unchanged. The South East Asian shaper is now reached only under a Tai Tham script tag. Lanna Alif offers latn and Noto Sans Tai Tham offers DFLT and latn, so neither reaches it, and with Lanna Alif the snapshot and SeaTest would no longer reach the #104 path they pin (a cluster reaching the reorderer through a ccmp ligature). Both move to NotoSansTaiTham-LanaScript-Synthetic: a subset of Noto Sans Tai Tham 2.002 (OFL 1.1) with its script retagged lana, its Sakot ligatures moved from liga to ccmp, the rule swapping the ligature for a second form after a High Ka taken out (see #212), and a GDEF added because mPDF refuses useOTL without one.

• hb-shape draws every sample in the snapshot and every SeaTest expectation the same way.
• The page changes: the new font has no dotted circle, so the broken cluster draws the subscript form alone, hanging left of its zero-width advance as the font draws it. The broken-cluster assertion is now "one glyph, the same subscript form as the well-formed cluster".
• Mutation: with the E branches of Three unguarded reads warn under E_ALL when loading Noto Sans Arabic, Bengali and Devanagari, and when shaping Tai Tham #104's category copy in GSUBsubstitute() removed, SeaTest fails again with Undefined array key "sea_category".

Mutation check

Restored	Fails
DFLT/latn keep the script's shaper (and Myanmar only under mym2)	26: ShaperChoiceTest default-script Indic and Tai Tham tests, and 24 shaping masters
No script skips the run	10: both ShaperChoiceTest no-script tests, and 8 shaping masters
useOTL 0x80 gate dropped	testARunInAScriptTheDocumentHasNotOpenedToOpenTypeLayoutIsNotShaped
loadGsubLookups() called with no features	shaping master NotoSans-GPOS3-Synthetic (no GSUB table) errors
features() indexes the features directly	62, including both no-script tests

Verification

• vendor/bin/phpunit: 2103 tests, 5993 assertions, 1 skipped (base 2084 / 5955 / 1).
• --group=snapshot: 79 tests, 109 assertions (one assertion left the Tai Tham test with the dotted circle).
• composer cs: clean. phpstan analyse --memory-limit=2G: 33, none in files touched.
• git merge-tree --write-tree --name-only fix/199-indic-shaper-choice origin/fix/193-glyph-class-membership (Test a glyph's membership of a GDEF class by the glyph, not by its hex turning up in the class (#193) #205, head d9451ed): clean, no conflicts. The source diff is confined to applyOTL()'s dispatch, applyGSUB() and the GSUB loading it calls (split in two), two new private methods, and the one-line feature read in each GSUB rule walk. Re-run after the /simplify commit: still clean.
• Stack: Build GSUB match patterns and ignore strings in the parser only, and name what the dump skips (#160) #186, Read the dump's font through the parser's getMetrics, extractInfo and GDEF read (#160, #182) #188 and Walk GSUB lookups once for the parser and the dump, and build or report each rule through hooks (#160) #190 have been squash-merged into gravitypdf. This PR's base, Select ZHH, ZHS and ZHT for a Chinese region in the lang attribute (#191) #202 (fix/191-chinese-region-langsys), is still open at b38b6fd, which this branch sits on, so it was neither rebased nor retargeted.

Test fonts are built in fontTools 4.59.2: NotoSansBengali-GuruScript-Synthetic (Noto Sans Bengali 3.011, OFL 1.1, as #200's DevaScript font with a single guru script whose locl gives KA the glyph of KHA) and NotoSansTaiTham-LanaScript-Synthetic above.

Found on the way and not fixed here: #212, _applyGSUBrulesSingly() applies every feature it is given once per feature it is given.

Upstream

mpdf/mpdf development (744f75a) has the same dispatch inline in src/Otl.php:

• :233-268 selects the shaper from the script block.
• :311-321 skips the subchunk when all four tags are empty. Mirror: keep the skip only where the shaper is empty or !($useOTL & 0x80), after replacing :323-326 (the mym2 check) with the per-shaper choice above, made once $GSUBscriptTag is known.
• :364 gates all of GSUB on ($useOTL & 0xFF) && $GSUBscriptTag && $GSUBlangsys && $GSUBFeatures. Mirror: ($useOTL & 0xFF) && ($GSUBFeatures || $this->shaper), with the GSLuCoverage/GSUBLookups load at :391-398 done only when there are features.
• :1288, :1336, :1391 and :1448 read $this->mpdf->CurrentFont['GSUBFeatures'][$scriptTag][$langsys] unguarded. Mirror: an isset falling back to [].

🤖 Generated with Claude Code

[jakejackson1]

Rebased: #190 was squash-merged into gravitypdf (f9b3c0e), so this stack still carried its pre-squash commits. This branch is rebased onto the rebased fix/191-chinese-region-langsys; no conflicts. One follow-up commit, ae5339f: #175's NotoSansCoptic-IgnoreMarksClass-Synthetic reached gravitypdf after this PR was written, and under this PR's rule its Bengali and Gurmukhi runs take the default shaper under DFLT, so they are left in logical order. hb-shape 14.3.1 agrees: it returns those runs as two grapheme clusters (0 0 2 2), not one Indic syllable. That fixture is the only one to move on a cold regeneration. Suite (2132), snapshots and cs pass.

[jakejackson1]

This PR merged into a base branch that had already been squash-merged, so its change never reached gravitypdf. It's carried to gravitypdf by #222.