diff options
Diffstat (limited to 'Lib/fontTools/ttLib/tables/otTables.py')
| -rw-r--r-- | Lib/fontTools/ttLib/tables/otTables.py | 35 |
1 files changed, 31 insertions, 4 deletions
diff --git a/Lib/fontTools/ttLib/tables/otTables.py b/Lib/fontTools/ttLib/tables/otTables.py index 262f8d41..3505f423 100644 --- a/Lib/fontTools/ttLib/tables/otTables.py +++ b/Lib/fontTools/ttLib/tables/otTables.py @@ -1123,6 +1123,35 @@ class LigatureSubst(FormatSwitchingBaseTable): self.ligatures = ligatures del self.Format # Don't need this anymore + @staticmethod + def _getLigatureSortKey(components): + # Computes a key for ordering ligatures in a GSUB Type-4 lookup. + + # When building the OpenType lookup, we need to make sure that + # the longest sequence of components is listed first, so we + # use the negative length as the key for sorting. + # Note, we no longer need to worry about deterministic order because the + # ligature mapping `dict` remembers the insertion order, and this in + # turn depends on the order in which the ligatures are written in the FEA. + # Since python sort algorithm is stable, the ligatures of equal length + # will keep the relative order in which they appear in the feature file. + # For example, given the following ligatures (all starting with 'f' and + # thus belonging to the same LigatureSet): + # + # feature liga { + # sub f i by f_i; + # sub f f f by f_f_f; + # sub f f by f_f; + # sub f f i by f_f_i; + # } liga; + # + # this should sort to: f_f_f, f_f_i, f_i, f_f + # This is also what fea-rs does, see: + # https://github.com/adobe-type-tools/afdko/issues/1727 + # https://github.com/fonttools/fonttools/issues/3428 + # https://github.com/googlefonts/fontc/pull/680 + return -len(components) + def preWrite(self, font): self.Format = 1 ligatures = getattr(self, "ligatures", None) @@ -1135,13 +1164,11 @@ class LigatureSubst(FormatSwitchingBaseTable): # ligatures is map from components-sequence to lig-glyph newLigatures = dict() - for comps, lig in sorted( - ligatures.items(), key=lambda item: (-len(item[0]), item[0]) - ): + for comps in sorted(ligatures.keys(), key=self._getLigatureSortKey): ligature = Ligature() ligature.Component = comps[1:] ligature.CompCount = len(comps) - ligature.LigGlyph = lig + ligature.LigGlyph = ligatures[comps] newLigatures.setdefault(comps[0], []).append(ligature) ligatures = newLigatures |