[varLib.merger] set ClassDef{1,2}.Format using same logic as otTables.ClassDef.preWrite

Lib/fontTools/ttLib/tables/otTables.py

@@ -418,19 +418,19 @@ class ClassDef(FormatSwitchingBaseTable):
 			assert 0, "unknown format: %s" % self.Format
 		self.classDefs = classDefs
 
-	def preWrite(self, font):
+	def _getClassRanges(self, font):
 		classDefs = getattr(self, "classDefs", None)
 		if classDefs is None:
-			classDefs = self.classDefs = {}
-		format = 2
-		rawTable = {"ClassRangeRecord": []}
+			self.classDefs = {}
+			return
 		getGlyphID = font.getGlyphID
 		items = []
 		for glyphName, cls in classDefs.items():
-			if not cls: continue
+			if not cls:
+				continue
 			items.append((getGlyphID(glyphName), glyphName, cls))
-		items.sort()
 		if items:
+			items.sort()
 			last, lastName, lastCls = items[0]
 			ranges = [[lastCls, last, lastName]]
 			for glyphID, glyphName, cls in items[1:]:
@@ -441,7 +441,13 @@ class ClassDef(FormatSwitchingBaseTable):
 				lastName = glyphName
 				lastCls = cls
 			ranges[-1].extend([last, lastName])
+			return ranges
 
+	def preWrite(self, font):
+		format = 2
+		rawTable = {"ClassRangeRecord": []}
+		ranges = self._getClassRanges(font)
+		if ranges:
 			startGlyph = ranges[0][1]
 			endGlyph = ranges[-1][3]
 			glyphCount = endGlyph - startGlyph + 1

Lib/fontTools/varLib/merger.py

@@ -342,6 +342,18 @@ def _ClassDef_merge_classify(lst, allGlyphs=None):
 
 	return self, classes
 
+def _ClassDef_calculate_Format(self, font):
+	fmt = 2
+	ranges = self._getClassRanges(font)
+	if ranges:
+		startGlyph = ranges[0][1]
+		endGlyph = ranges[-1][3]
+		glyphCount = endGlyph - startGlyph + 1
+		if len(ranges) * 3 >= glyphCount + 1:
+			# Format 1 is more compact
+			fmt = 1
+	self.Format = fmt
+
 def _PairPosFormat2_merge(self, lst, merger):
 	merger.mergeObjects(self, lst,
 			    exclude=('Coverage',
@@ -368,6 +380,7 @@ def _PairPosFormat2_merge(self, lst, merger):
 
 	# Align first classes
 	self.ClassDef1, classes = _ClassDef_merge_classify([l.ClassDef1 for l in lst], allGlyphs=glyphSet)
+	_ClassDef_calculate_Format(self.ClassDef1, merger.font)
 	self.Class1Count = len(classes)
 	new_matrices = []
 	for l,matrix in zip(lst, matrices):
@@ -397,6 +410,7 @@ def _PairPosFormat2_merge(self, lst, merger):
 
 	# Align second classes
 	self.ClassDef2, classes = _ClassDef_merge_classify([l.ClassDef2 for l in lst])
+	_ClassDef_calculate_Format(self.ClassDef2, merger.font)
 	self.Class2Count = len(classes)
 	new_matrices = []
 	for l,matrix in zip(lst, matrices):