Merge pull request #1300 from anthrotype/explicit-subtable-break

support explicit "subtable" breaks in kerning lookups

Lib/fontTools/feaLib/ast.py

@@ -1077,6 +1077,9 @@ class SubtableStatement(Statement):
     def __init__(self, location=None):
         Statement.__init__(self, location)
 
+    def build(self, builder):
+        builder.add_subtable_break(self.location)
+
     def asFea(self, indent=""):
         return indent + "subtable;"
 

Lib/fontTools/feaLib/builder.py

@@ -999,6 +999,16 @@ class Builder(object):
         lookup = self.get_lookup_(location, PairPosBuilder)
         lookup.addClassPair(location, glyphclass1, value1, glyphclass2, value2)
 
+    def add_subtable_break(self, location):
+        if type(self.cur_lookup_) is not PairPosBuilder:
+            raise FeatureLibError(
+                'explicit "subtable" statement is intended for use with only '
+                "Pair Adjustment Positioning Format 2 (i.e. pair class kerning)",
+                location
+            )
+        lookup = self.get_lookup_(location, PairPosBuilder)
+        lookup.add_subtable_break(location)
+
     def add_specific_pair_pos(self, location, glyph1, value1, glyph2, value2):
         lookup = self.get_lookup_(location, PairPosBuilder)
         lookup.addGlyphPair(location, glyph1, value1, glyph2, value2)

Tests/feaLib/builder_test.py

@@ -512,6 +512,18 @@ class BuilderTest(unittest.TestCase):
         addOpenTypeFeatures(font, tree)
         assert "GSUB" in font
 
+    def test_unsupported_subtable_break(self):
+        self.assertRaisesRegex(
+            FeatureLibError,
+            'explicit "subtable" statement is intended for .* class kerning',
+            self.build,
+            "feature liga {"
+            "    sub f f by f_f;"
+            "    subtable;"
+            "    sub f i by f_i;"
+            "} liga;"
+        )
+
 
 def generate_feature_file_test(name):
     return lambda self: self.check_feature_file(name)

Tests/feaLib/data/PairPosSubtable.fea

@@ -1,12 +1,16 @@
 languagesystem DFLT dflt;
 languagesystem latn dflt;
 
+@group1 = [b o];
+@group2 = [c d];
+@group3 = [v w];
+
 lookup kernlookup {
     pos A V -34;
     subtable;
-    @group1 = [b o];
-    @group2 = [c d];
     pos @group1 @group2 -12;
+    subtable;
+    pos @group3 @group2 -20;
 } kernlookup;
 
 feature kern {

Tests/feaLib/data/PairPosSubtable.ttx

@@ -43,7 +43,7 @@
       <Lookup index="0">
         <LookupType value="2"/>
         <LookupFlag value="0"/>
-        <!-- SubTableCount=2 -->
+        <!-- SubTableCount=3 -->
         <PairPos index="0" Format="1">
           <Coverage>
             <Glyph value="A"/>
@@ -82,6 +82,29 @@
             </Class2Record>
           </Class1Record>
         </PairPos>
+        <PairPos index="2" Format="2">
+          <Coverage>
+            <Glyph value="v"/>
+            <Glyph value="w"/>
+          </Coverage>
+          <ValueFormat1 value="4"/>
+          <ValueFormat2 value="0"/>
+          <ClassDef1>
+          </ClassDef1>
+          <ClassDef2>
+            <ClassDef glyph="c" class="1"/>
+            <ClassDef glyph="d" class="1"/>
+          </ClassDef2>
+          <!-- Class1Count=1 -->
+          <!-- Class2Count=2 -->
+          <Class1Record index="0">
+            <Class2Record index="0">
+            </Class2Record>
+            <Class2Record index="1">
+              <Value1 XAdvance="-20"/>
+            </Class2Record>
+          </Class1Record>
+        </PairPos>
       </Lookup>
     </LookupList>
   </GPOS>