[feaLib] Implement GSUB chain substitution rules

Lib/fontTools/feaLib/ast.py

@@ -169,6 +169,12 @@ class SubstitutionRule(Statement):
         self.old_suffix = []
         self.lookups = [None] * len(old)
 
+    def build(self, builder):
+        builder.add_substitution(
+            self.location,
+            self.old_prefix, self.old, self.old_suffix,
+            self.new, self.lookups)
+
 
 class ValueRecord(Statement):
     def __init__(self, location, xPlacement, yPlacement, xAdvance, yAdvance):

Lib/fontTools/feaLib/builder.py

@@ -247,6 +247,19 @@ class Builder(object):
         self.set_language(location, "dflt",
                           include_default=True, required=False)
 
+    def add_substitution(self, location, old_prefix, old, old_suffix, new,
+                         lookups):
+        assert len(new) == 0, new
+        lookup_builders = []
+        for lookup in lookups:
+            if lookup is not None:
+                lookup_builders.append(self.named_lookups_.get(lookup.name))
+            else:
+                lookup_builders.append(None)
+        lookup = self.get_lookup_(location, ChainContextSubstBuilder)
+        lookup.substitutions.append((old_prefix, old, old_suffix,
+                                     lookup_builders))
+
     def add_alternate_substitution(self, location, glyph, from_class):
         lookup = self.get_lookup_(location, AlternateSubstBuilder)
         if glyph in lookup.alternates:
@@ -314,6 +327,59 @@ class AlternateSubstBuilder(LookupBuilder):
         return lookup
 
 
+class ChainContextSubstBuilder(LookupBuilder):
+    def __init__(self, location, lookup_flag):
+        LookupBuilder.__init__(self, location, 'GSUB', 6, lookup_flag)
+        self.substitutions = []  # (prefix, input, suffix, lookups)
+
+    def equals(self, other):
+        return (LookupBuilder.equals(self, other) and
+                self.substitutions == other.substitutions)
+
+    def build(self):
+        lookup = otTables.Lookup()
+        lookup.SubTable = []
+        for (prefix, input, suffix, lookups) in self.substitutions:
+            st = otTables.ChainContextSubst()
+            lookup.SubTable.append(st)
+            st.Format = 3
+
+            st.BacktrackGlyphCount = len(prefix)
+            st.BacktrackCoverage = []
+            for p in reversed(prefix):
+                coverage = otTables.BacktrackCoverage()
+                coverage.glyphs = sorted(list(p))
+                st.BacktrackCoverage.append(coverage)
+
+            st.InputGlyphCount = len(input)
+            st.InputCoverage = []
+            for i in input:
+                coverage = otTables.InputCoverage()
+                coverage.glyphs = sorted(list(i))
+                st.InputCoverage.append(coverage)
+
+            st.LookAheadGlyphCount = len(suffix)
+            st.LookAheadCoverage = []
+            for s in suffix:
+                coverage = otTables.LookAheadCoverage()
+                coverage.glyphs = sorted(list(s))
+                st.LookAheadCoverage.append(coverage)
+
+            st.SubstCount = len([l for l in lookups if l is not None])
+            st.SubstLookupRecord = []
+            for sequenceIndex, l in enumerate(lookups):
+                if l is not None:
+                    rec = otTables.SubstLookupRecord()
+                    rec.SequenceIndex = sequenceIndex
+                    rec.LookupListIndex = l.lookup_index
+                    st.SubstLookupRecord.append(rec)
+
+        lookup.LookupFlag = self.lookup_flag
+        lookup.LookupType = self.lookup_type
+        lookup.SubTableCount = len(lookup.SubTable)
+        return lookup
+
+
 class LigatureSubstBuilder(LookupBuilder):
     def __init__(self, location, lookup_flag):
         LookupBuilder.__init__(self, location, 'GSUB', 4, lookup_flag)

Lib/fontTools/feaLib/builder_test.py

@@ -136,8 +136,7 @@ class BuilderTest(unittest.TestCase):
         # OpenType Feature File specification, section 5.f.i, example 1.
         font = TTFont()
         addOpenTypeFeatures(self.getpath("spec5fi1.fea"), font)
-        # TODO: Fix the implementation until the test case passes.
+        self.expect_ttx(font, self.getpath("spec5fi1.ttx"))
-        # self.expect_ttx(font, self.getpath("spec5fi1.ttx"))
 
     def test_languagesystem(self):
         builder = Builder(None, TTFont())

Lib/fontTools/feaLib/parser_test.py

@@ -342,7 +342,7 @@ class ParserTest(unittest.TestCase):
 
     def test_substitute_lookups(self):
         doc = Parser(self.getpath("spec5fi1.fea")).parse()
-        [ligs, sub, feature] = doc.statements
+        [langsys, ligs, sub, feature] = doc.statements
         self.assertEqual(feature.statements[0].lookups, [ligs, None, sub])
         self.assertEqual(feature.statements[1].lookups, [ligs, None, sub])
 

Lib/fontTools/feaLib/testdata/spec5fi1.fea

@@ -2,6 +2,8 @@
 # "Specifying a Chain Sub rule and marking sub-runs"
 # http://www.adobe.com/devnet/opentype/afdko/topic_feature_file_syntax.html
 
+languagesystem latn dflt;
+
 lookup CNTXT_LIGS {
     substitute f i by f_i;
     substitute c t by c_t;

Lib/fontTools/feaLib/testdata/spec5fi1.ttx

@@ -4,10 +4,28 @@
   <GSUB>
     <Version value="1.0"/>
     <ScriptList>
-      <!-- ScriptCount=0 -->
+      <!-- ScriptCount=1 -->
+      <ScriptRecord index="0">
+        <ScriptTag value="latn"/>
+        <Script>
+          <DefaultLangSys>
+            <ReqFeatureIndex value="65535"/>
+            <!-- FeatureCount=1 -->
+            <FeatureIndex index="0" value="0"/>
+          </DefaultLangSys>
+          <!-- LangSysCount=0 -->
+        </Script>
+      </ScriptRecord>
     </ScriptList>
     <FeatureList>
-      <!-- FeatureCount=0 -->
+      <!-- FeatureCount=1 -->
+      <FeatureRecord index="0">
+        <FeatureTag value="test"/>
+        <Feature>
+          <!-- LookupCount=1 -->
+          <LookupListIndex index="0" value="2"/>
+        </Feature>
+      </FeatureRecord>
     </FeatureList>
     <LookupList>
       <!-- LookupCount=3 -->