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The Indic shaper, from HarfBuzz 10.2's source: syllables found by the
Indic machine; each syllable's base consonant, its pre-base matras and
reph moved around it and masked for the basic features, applied one at
a time within the syllable; then matras, reph and pre-base-reordering
consonants placed again by what the features made. Vowel constraints,
Indic decompositions and compositions, dotted circles for broken
syllables, and per-syllable matching come with it.
Plans carry GSUB pauses (steps where the shaper does its part) and the
stages of the features the shaper asks whether they would substitute;
the runtime answers with each subtable's would_apply, as HarfBuzz does.
Masks are now the common ones and each shaper's own.
text/syllabic.py reads HarfBuzz's Ragel grammars (Indic, Khmer,
Myanmar) and compiles each to a minimal DFA with Ragel's scanner
semantics; beside the machines Ragel generated from the same grammars,
100,000 random runs each find no syllable that differs. It also reads
HarfBuzz's Indic table (Unicode 16) cell for cell.
Every installed font shapes as HarfBuzz does (213 fonts, 1,688 plans);
the Indic ones at 20,000 runs a plan (FreeSerif's eight scripts,
FreeSans, Unifont's nine without GSUB), and nine new-spec Noto Sans
fonts at 5,000.
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HarfBuzz reorders marks by a modified combining class; for U+0C55 and
U+0C56 (canonical classes 84 and 91) it is 4 and 5, so they sort before
the virama. Vega had 0, which rustybuzz 0.20 uses; HarfBuzz 10.2's
hb-unicode.hh says otherwise, and Unifont's Telugu showed it.
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HarfBuzz sorts each stage's lookups by index with hb_qsort, its copy of
Isaac Turner's quicksort, and then merges entries of the same lookup:
masks and joiner flags combine, but the per-syllable and random flags are
the first entry's. Which entry comes first is the quicksort's doing, so
the planner now sorts the same way instead of stably. Noto Sans
Malayalam, whose psts and rlig share a lookup, needed it: ma with a
combining dot joined across syllables in HarfBuzz and not here.
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HarfBuzz does
The planner now picks who positions as HarfBuzz 10.2 does: GPOS, the old
kern table when GPOS has no kern feature, and marks placed by combining
class and ink boxes when there is no GPOS; the Hangul shaper composes
syllables the font has whole, takes apart those it lacks into jamo for
ljmo, vjmo and tjmo, and moves tone marks before their syllable.
The section grows to an 80-byte header (the kern table, and each glyph's
ink box when a plan places marks without GPOS) and 88-byte plans (the
kern mask, the jamo masks). Kern tables are applied pair subtable by
pair subtable with HarfBuzz's own binary search; state machines, class
subtables and cross-stream kerning, which no installed font has, are
refused. The check spells Hangul syllables every way text does.
Every installed font Vega accepts shapes as HarfBuzz does (209 fonts,
1,520 plans, a thousand runs each), and the fonts refused before at
20,000 runs a plan: Noto Sans CJK KR and Baekmuk for Hangul, IPA Gothic
and Noto Mono for marks without GPOS, FreeSans, FreeSerif, Liberation
and DejaVu Sans ExtraLight for kern tables, Unifont's Latin, Greek,
Cyrillic, Hebrew and Hangul.
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HarfBuzz's Hebrew presentation forms join U+05F2 and patah into U+FB1F;
Vega had U+05D9 there. Only fonts without a GPOS mark feature take the
rule, so no font Vega accepted showed it; Unifont's Hebrew did.
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order
text/layout.c puts the checked parts together: a paragraph's levels,
scripts resolved for Common and Inherited characters, break
opportunities, one shaping to measure, lines broken at the last
opportunity that fits (trailing spaces hang, and a line that shapes
wider than measured backs off), then per line L1, runs of one level and
one script shaped again, the runs in L2's order and their glyphs placed.
test-layout checks 30,000 random mixed-script paragraphs: lines cover
the text, mandatory breaks end lines, lines fit when they can, and
glyphs come in the order UAX #9 gives the characters. vega-symbols
paragraph draws a layout through the symbol file's glyphs.
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UAX #14 line breaking (LB1-LB31, numbers tailored as Unicode's tests
are) and UAX #29's grapheme cluster, word and sentence boundaries, each
from generated property tables, the rules taken in order, in linear time.
They pass all of Unicode 15.1's LineBreakTest (10,274 cases),
GraphemeBreakTest (1,187), WordBreakTest (1,826) and SentenceBreakTest
(512).
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The build tool compiles a font's shaping plans the way HarfBuzz's planner
does (features, stages, masks, lookups per script and direction) and
carries them with the font's character map, advances and GDEF, GSUB and
GPOS tables in a section of the symbol file (VEGASYM4); glyphs shaping
reaches become symbols of their own. A small engine ports HarfBuzz 10's
default, Arabic and Hebrew shaping step for step and reads the tables in
place, every read bounds-checked. The build refuses a file whose shaping
differs from HarfBuzz's on random runs: DejaVu Sans agrees on 600,000
runs in six scripts both ways, and every installed font the tool accepts
agrees too, at 1.05-1.18 times HarfBuzz's time.
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Joining types, general categories and scripts (with their ISO 15924
codes) as two-stage tables, canonical decompositions one step at a time,
and Extended_Pictographic, from the UCD by text/unicode.py.
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