[feaLib] Support multiple lookups per glyph position (#1905)

This allows for more than one "lookup ..." chaining statements at each glyph position in a chaining contextual substitution or positioning rule: e.g.

    sub a b c' lookup lookup1 lookup lookup2 d;

The corresponding change in the Adobe OpenType Feature File Specification (and implementation in makeotf) happened in adobe-type-tools/afdko#1132.

Lib/fontTools/feaLib/ast.py

@@ -546,8 +546,9 @@ class ChainContextPosStatement(Statement):
                 res += " ".join(g.asFea() for g in self.prefix) + " "
             for i, g in enumerate(self.glyphs):
                 res += g.asFea() + "'"
-                if self.lookups[i] is not None:
-                    res += " lookup " + self.lookups[i].name
+                if self.lookups[i]:
+                    for lu in self.lookups[i]:
+                        res += " lookup " + lu.name
                 if i < len(self.glyphs) - 1:
                     res += " "
             if len(self.suffix):
@@ -578,8 +579,9 @@ class ChainContextSubstStatement(Statement):
                 res += " ".join(g.asFea() for g in self.prefix) + " "
             for i, g in enumerate(self.glyphs):
                 res += g.asFea() + "'"
-                if self.lookups[i] is not None:
-                    res += " lookup " + self.lookups[i].name
+                if self.lookups[i]:
+                    for lu in self.lookups[i]:
+                        res += " lookup " + lu.name
                 if i < len(self.glyphs) - 1:
                     res += " "
             if len(self.suffix):

Lib/fontTools/feaLib/builder.py

@@ -203,7 +203,10 @@ class Builder(object):
                 raise FeatureLibError("Feature %s has not been defined" % name,
                                       location)
             for script, lang, feature, lookups in feature:
-                for lookup in lookups:
+                for lookuplist in lookups:
+                    if not isinstance(lookuplist, list):
+                        lookuplist = [lookuplist]
+                    for lookup in lookuplist:
                         for glyph, alts in lookup.getAlternateGlyphs().items():
                             alternates.setdefault(glyph, set()).update(alts)
         single = {glyph: list(repl)[0] for glyph, repl in alternates.items()
@@ -797,9 +800,10 @@ class Builder(object):
         If an input name is None, it gets mapped to a None LookupBuilder.
         """
         lookup_builders = []
-        for lookup in lookups:
-            if lookup is not None:
-                lookup_builders.append(self.named_lookups_.get(lookup.name))
+        for lookuplist in lookups:
+            if lookuplist is not None:
+                lookup_builders.append([self.named_lookups_.get(l.name)
+                    for l in lookuplist])
             else:
                 lookup_builders.append(None)
         return lookup_builders
@@ -1259,10 +1263,15 @@ class ChainContextPosBuilder(LookupBuilder):
             self.setLookAheadCoverage_(suffix, st)
             self.setInputCoverage_(glyphs, st)
 
-            st.PosCount = len([l for l in lookups if l is not None])
+            st.PosCount = 0
             st.PosLookupRecord = []
-            for sequenceIndex, l in enumerate(lookups):
-                if l is not None:
+            for sequenceIndex, lookupList in enumerate(lookups):
+                if lookupList is not None:
+                    if not isinstance(lookupList, list):
+                        # Can happen with synthesised lookups
+                        lookupList = [ lookupList ]
+                    for l in lookupList:
+                        st.PosCount += 1
                         if l.lookup_index is None:
                             raise FeatureLibError('Missing index of the specified '
                                 'lookup, might be a substitution lookup',
@@ -1310,10 +1319,15 @@ class ChainContextSubstBuilder(LookupBuilder):
             self.setLookAheadCoverage_(suffix, st)
             self.setInputCoverage_(input, st)
 
-            st.SubstCount = len([l for l in lookups if l is not None])
+            st.SubstCount = 0
             st.SubstLookupRecord = []
-            for sequenceIndex, l in enumerate(lookups):
-                if l is not None:
+            for sequenceIndex, lookupList in enumerate(lookups):
+                if lookupList is not None:
+                    if not isinstance(lookupList, list):
+                        # Can happen with synthesised lookups
+                        lookupList = [ lookupList ]
+                    for l in lookupList:
+                        st.SubstCount += 1
                         if l.lookup_index is None:
                             raise FeatureLibError('Missing index of the specified '
                                 'lookup, might be a positioning lookup',
@@ -1326,9 +1340,12 @@ class ChainContextSubstBuilder(LookupBuilder):
 
     def getAlternateGlyphs(self):
         result = {}
-        for (_, _, _, lookups) in self.substitutions:
-            if lookups == self.SUBTABLE_BREAK_:
+        for (_, _, _, lookuplist) in self.substitutions:
+            if lookuplist == self.SUBTABLE_BREAK_:
                 continue
+            for lookups in lookuplist:
+                if not isinstance(lookups, list):
+                    lookups = [lookups]
                 for lookup in lookups:
                     if lookup is not None:
                         alts = lookup.getAlternateGlyphs()

Lib/fontTools/feaLib/parser.py

@@ -404,8 +404,10 @@ class Parser(object):
             else:
                 values.append(None)
 
-            lookup = None
-            if self.next_token_ == "lookup":
+            lookuplist = None
+            while self.next_token_ == "lookup":
+                if lookuplist is None:
+                    lookuplist = []
                 self.expect_keyword_("lookup")
                 if not marked:
                     raise FeatureLibError(
@@ -417,8 +419,9 @@ class Parser(object):
                     raise FeatureLibError(
                         'Unknown lookup "%s"' % lookup_name,
                         self.cur_token_location_)
+                lookuplist.append(lookup)
             if marked:
-                lookups.append(lookup)
+                lookups.append(lookuplist)
 
         if not glyphs and not suffix:  # eg., "sub f f i by"
             assert lookups == []

Tests/feaLib/builder_test.py

@@ -71,7 +71,8 @@ class BuilderTest(unittest.TestCase):
         ZeroValue_ChainSinglePos_horizontal ZeroValue_ChainSinglePos_vertical
         PairPosSubtable ChainSubstSubtable ChainPosSubtable LigatureSubtable
         AlternateSubtable MultipleSubstSubtable SingleSubstSubtable
-        aalt_chain_contextual_subst AlternateChained
+        aalt_chain_contextual_subst AlternateChained MultipleLookupsPerGlyph
+        MultipleLookupsPerGlyph2
     """.split()
 
     def __init__(self, methodName):

Tests/feaLib/data/MultipleLookupsPerGlyph.fea

@@ -0,0 +1,11 @@
+lookup a_to_bc {
+	sub a by b c;
+} a_to_bc;
+
+lookup b_to_d {
+	sub b by d;
+} b_to_d;
+
+feature test {
+	sub a' lookup a_to_bc lookup b_to_d b;
+} test;

Tests/feaLib/data/MultipleLookupsPerGlyph.ttx

@@ -0,0 +1,76 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ttFont>
+
+  <GSUB>
+    <Version value="0x00010000"/>
+    <ScriptList>
+      <!-- ScriptCount=1 -->
+      <ScriptRecord index="0">
+        <ScriptTag value="DFLT"/>
+        <Script>
+          <DefaultLangSys>
+            <ReqFeatureIndex value="65535"/>
+            <!-- FeatureCount=1 -->
+            <FeatureIndex index="0" value="0"/>
+          </DefaultLangSys>
+          <!-- LangSysCount=0 -->
+        </Script>
+      </ScriptRecord>
+    </ScriptList>
+    <FeatureList>
+      <!-- FeatureCount=1 -->
+      <FeatureRecord index="0">
+        <FeatureTag value="test"/>
+        <Feature>
+          <!-- LookupCount=1 -->
+          <LookupListIndex index="0" value="2"/>
+        </Feature>
+      </FeatureRecord>
+    </FeatureList>
+    <LookupList>
+      <!-- LookupCount=3 -->
+      <Lookup index="0">
+        <LookupType value="2"/>
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <MultipleSubst index="0">
+          <Substitution in="a" out="b,c"/>
+        </MultipleSubst>
+      </Lookup>
+      <Lookup index="1">
+        <LookupType value="1"/>
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <SingleSubst index="0">
+          <Substitution in="b" out="d"/>
+        </SingleSubst>
+      </Lookup>
+      <Lookup index="2">
+        <LookupType value="6"/>
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <ChainContextSubst index="0" Format="3">
+          <!-- BacktrackGlyphCount=0 -->
+          <!-- InputGlyphCount=1 -->
+          <InputCoverage index="0">
+            <Glyph value="a"/>
+          </InputCoverage>
+          <!-- LookAheadGlyphCount=1 -->
+          <LookAheadCoverage index="0">
+            <Glyph value="b"/>
+          </LookAheadCoverage>
+          <!-- SubstCount=2 -->
+          <SubstLookupRecord index="0">
+            <SequenceIndex value="0"/>
+            <LookupListIndex value="0"/>
+          </SubstLookupRecord>
+          <SubstLookupRecord index="1">
+            <SequenceIndex value="0"/>
+            <LookupListIndex value="1"/>
+          </SubstLookupRecord>
+        </ChainContextSubst>
+      </Lookup>
+    </LookupList>
+  </GSUB>
+
+</ttFont>

Tests/feaLib/data/MultipleLookupsPerGlyph2.fea

@@ -0,0 +1,11 @@
+lookup a_reduce_sb {
+       pos a <-80 0 -160 0>;
+} a_reduce_sb;
+
+lookup a_raise {
+       pos a <0 100 0 0>;
+} a_raise;
+
+feature test {
+       pos a' lookup a_reduce_sb lookup a_raise b;
+} test;

Tests/feaLib/data/MultipleLookupsPerGlyph2.ttx

@@ -0,0 +1,84 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ttFont>
+
+  <GPOS>
+    <Version value="0x00010000"/>
+    <ScriptList>
+      <!-- ScriptCount=1 -->
+      <ScriptRecord index="0">
+        <ScriptTag value="DFLT"/>
+        <Script>
+          <DefaultLangSys>
+            <ReqFeatureIndex value="65535"/>
+            <!-- FeatureCount=1 -->
+            <FeatureIndex index="0" value="0"/>
+          </DefaultLangSys>
+          <!-- LangSysCount=0 -->
+        </Script>
+      </ScriptRecord>
+    </ScriptList>
+    <FeatureList>
+      <!-- FeatureCount=1 -->
+      <FeatureRecord index="0">
+        <FeatureTag value="test"/>
+        <Feature>
+          <!-- LookupCount=1 -->
+          <LookupListIndex index="0" value="2"/>
+        </Feature>
+      </FeatureRecord>
+    </FeatureList>
+    <LookupList>
+      <!-- LookupCount=3 -->
+      <Lookup index="0">
+        <LookupType value="1"/>
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <SinglePos index="0" Format="1">
+          <Coverage>
+            <Glyph value="a"/>
+          </Coverage>
+          <ValueFormat value="5"/>
+          <Value XPlacement="-80" XAdvance="-160"/>
+        </SinglePos>
+      </Lookup>
+      <Lookup index="1">
+        <LookupType value="1"/>
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <SinglePos index="0" Format="1">
+          <Coverage>
+            <Glyph value="a"/>
+          </Coverage>
+          <ValueFormat value="2"/>
+          <Value YPlacement="100"/>
+        </SinglePos>
+      </Lookup>
+      <Lookup index="2">
+        <LookupType value="8"/>
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <ChainContextPos index="0" Format="3">
+          <!-- BacktrackGlyphCount=0 -->
+          <!-- InputGlyphCount=1 -->
+          <InputCoverage index="0">
+            <Glyph value="a"/>
+          </InputCoverage>
+          <!-- LookAheadGlyphCount=1 -->
+          <LookAheadCoverage index="0">
+            <Glyph value="b"/>
+          </LookAheadCoverage>
+          <!-- PosCount=2 -->
+          <PosLookupRecord index="0">
+            <SequenceIndex value="0"/>
+            <LookupListIndex value="0"/>
+          </PosLookupRecord>
+          <PosLookupRecord index="1">
+            <SequenceIndex value="0"/>
+            <LookupListIndex value="1"/>
+          </PosLookupRecord>
+        </ChainContextPos>
+      </Lookup>
+    </LookupList>
+  </GPOS>
+
+</ttFont>

Tests/feaLib/parser_test.py

@@ -1065,7 +1065,7 @@ class ParserTest(unittest.TestCase):
         self.assertEqual(glyphstr(pos.prefix), "[A a] [B b]")
         self.assertEqual(glyphstr(pos.glyphs), "I [N n] P")
         self.assertEqual(glyphstr(pos.suffix), "[Y y] [Z z]")
-        self.assertEqual(pos.lookups, [lookup1, lookup2, None])
+        self.assertEqual(pos.lookups, [[lookup1], [lookup2], None])
 
     def test_gpos_type_8_lookup_with_values(self):
         self.assertRaisesRegex(
@@ -1508,8 +1508,8 @@ class ParserTest(unittest.TestCase):
     def test_substitute_lookups(self):  # GSUB LookupType 6
         doc = Parser(self.getpath("spec5fi1.fea"), GLYPHNAMES).parse()
         [_, _, _, langsys, ligs, sub, feature] = doc.statements
-        self.assertEqual(feature.statements[0].lookups, [ligs, None, sub])
-        self.assertEqual(feature.statements[1].lookups, [ligs, None, sub])
+        self.assertEqual(feature.statements[0].lookups, [[ligs], None, [sub]])
+        self.assertEqual(feature.statements[1].lookups, [[ligs], None, [sub]])
 
     def test_substitute_missing_by(self):
         self.assertRaisesRegex(