[feaLib] Support multiple chain contexts after ignore sub

Although this construct is in violation of the `ignore sub` grammar
given by the current OpenType Feature File syntax specification,
the very same specification document illustrates (in example 3
of section 5.f.ii) the `ignore sub` statement with a comma-separated
list of backgrack/input/lookahead triples.

See https://github.com/adobe-type-tools/afdko/issues/105 for a request
to amend the OpenType Feature File syntax specification.

After this code change, feaLib can now parse testdata/spec5f_ii_3.fea;
the output is identical to what is generated by Adobe's makeotf tool.

https://github.com/behdad/fonttools/issues/503

Lib/fontTools/feaLib/ast.py

@@ -273,6 +273,20 @@ class FeatureReferenceStatement(Statement):
         builder.add_feature_reference(self.location, self.featureName)
 
 
+class IgnoreSubstStatement(Statement):
+    def __init__(self, location, chainContexts):
+        Statement.__init__(self, location)
+        self.chainContexts = chainContexts
+
+    def build(self, builder):
+        for prefix, glyphs, suffix in self.chainContexts:
+            prefix = [p.glyphSet() for p in prefix]
+            glyphs = [g.glyphSet() for g in glyphs]
+            suffix = [s.glyphSet() for s in suffix]
+            builder.add_chain_context_subst(
+                self.location, prefix, glyphs, suffix, [])
+
+
 class LanguageStatement(Statement):
     def __init__(self, location, language, include_default, required):
         Statement.__init__(self, location)

Lib/fontTools/feaLib/builder_test.py

@@ -31,7 +31,7 @@ def makeTTFont():
         "a.alt1 a.alt2 a.alt3 b.alt c.mid d.alt d.mid e.begin e.mid "
         "n.end s.end Eng Eng.alt1 Eng.alt2 Eng.alt3 "
         "f_l c_h c_k c_s c_t f_f f_f_i f_f_l f_i o_f_f_i s_t f_i.begin "
-        "ydieresis yacute breve "
+        "a_n_d ydieresis yacute breve "
         "grave acute dieresis macron circumflex cedilla umlaut ogonek caron "
         "damma hamza sukun kasratan lam_meem_jeem noon.final noon.initial "
     ).split()
@@ -49,7 +49,7 @@ class BuilderTest(unittest.TestCase):
         GPOS_1 GPOS_1_zero GPOS_2 GPOS_2b GPOS_3 GPOS_4 GPOS_5 GPOS_6 GPOS_8
         GSUB_2 GSUB_3 GSUB_6 GSUB_8
         spec4h1 spec5d1 spec5d2 spec5fi1 spec5fi2 spec5fi3 spec5fi4
-        spec5f_ii_1 spec5f_ii_2
+        spec5f_ii_1 spec5f_ii_2 spec5f_ii_3
         spec5h1 spec6b_ii spec6d2 spec6e spec6f spec6h_ii spec6h_iii_1 spec8a
         spec9b spec9c1 spec9c2 spec9c3
         bug463 bug501 bug502 bug505 bug506 bug509

Lib/fontTools/feaLib/parser.py

@@ -242,7 +242,7 @@ class Parser(object):
     def parse_glyph_pattern_(self, vertical):
         prefix, glyphs, lookups, values, suffix = ([], [], [], [], [])
         hasMarks = False
-        while self.next_token_ not in {"by", "from", ";"}:
+        while self.next_token_ not in {"by", "from", ";", ","}:
             gc = self.parse_glyphclass_(accept_glyphname=True)
             marked = False
             if self.next_token_ == "'":
@@ -291,13 +291,20 @@ class Parser(object):
         if self.cur_token_ in ["substitute", "sub"]:
             prefix, glyphs, lookups, values, suffix, hasMarks = \
                 self.parse_glyph_pattern_(vertical=False)
+            chainContext = [(prefix, glyphs, suffix)]
+            hasLookups = any(lookups)
+            while self.next_token_ == ",":
+                self.expect_symbol_(",")
+                prefix, glyphs, lookups, values, suffix, hasMarks = \
+                    self.parse_glyph_pattern_(vertical=False)
+                chainContext.append((prefix, glyphs, suffix))
+                hasLookups = hasLookups or any(lookups)
             self.expect_symbol_(";")
-            if any(lookups):
+            if hasLookups:
                 raise FeatureLibError(
                     "No lookups can be specified for \"ignore sub\"",
                     location)
-            return ast.ChainContextSubstStatement(
+            return ast.IgnoreSubstStatement(location, chainContext)
-                location, prefix, glyphs, suffix, [])
         raise FeatureLibError(
             "Expected \"substitute\"", self.next_token_location_)
 

Lib/fontTools/feaLib/parser_test.py

@@ -290,13 +290,16 @@ class ParserTest(unittest.TestCase):
         self.assertIsNone(s.componentGlyphs)
 
     def test_ignore_sub(self):
-        doc = self.parse("feature test {ignore sub e t' c;} test;")
+        doc = self.parse("feature test {ignore sub e t' c, q u' u' x;} test;")
         sub = doc.statements[0].statements[0]
-        self.assertIsInstance(sub, ast.ChainContextSubstStatement)
+        self.assertIsInstance(sub, ast.IgnoreSubstStatement)
-        self.assertEqual(glyphstr(sub.prefix), "e")
+        [(pref1, glyphs1, suff1), (pref2, glyphs2, suff2)] = sub.chainContexts
-        self.assertEqual(glyphstr(sub.glyphs), "t")
+        self.assertEqual(glyphstr(pref1), "e")
-        self.assertEqual(glyphstr(sub.suffix), "c")
+        self.assertEqual(glyphstr(glyphs1), "t")
-        self.assertEqual(sub.lookups, [])
+        self.assertEqual(glyphstr(suff1), "c")
+        self.assertEqual(glyphstr(pref2), "q")
+        self.assertEqual(glyphstr(glyphs2), "u u")
+        self.assertEqual(glyphstr(suff2), "x")
 
     def test_ignore_substitute(self):
         doc = self.parse(
@@ -304,11 +307,19 @@ class ParserTest(unittest.TestCase):
             "    ignore substitute f [a e] d' [a u]' [e y];"
             "} test;")
         sub = doc.statements[0].statements[0]
-        self.assertIsInstance(sub, ast.ChainContextSubstStatement)
+        self.assertIsInstance(sub, ast.IgnoreSubstStatement)
-        self.assertEqual(glyphstr(sub.prefix), "f [a e]")
+        [(prefix, glyphs, suffix)] = sub.chainContexts
-        self.assertEqual(glyphstr(sub.glyphs), "d [a u]")
+        self.assertEqual(glyphstr(prefix), "f [a e]")
-        self.assertEqual(glyphstr(sub.suffix), "[e y]")
+        self.assertEqual(glyphstr(glyphs), "d [a u]")
-        self.assertEqual(sub.lookups, [])
+        self.assertEqual(glyphstr(suffix), "[e y]")
+
+    def test_ignore_substitute_with_lookup(self):
+        self.assertRaisesRegex(
+            FeatureLibError,
+            'No lookups can be specified for "ignore sub"',
+            self.parse,
+            "lookup L { sub [A A.sc] by a; } L;"
+            "feature test { ignore sub f' i', A' lookup L; } test;")
 
     def test_language(self):
         doc = self.parse("feature test {language DEU;} test;")

Lib/fontTools/feaLib/testdata/spec5f_ii_3.fea

@@ -0,0 +1,9 @@
+# OpenType Feature File specification, section 5.f.ii, example 3
+# "Specifying exceptions to the Chain Sub rule"
+# http://www.adobe.com/devnet/opentype/afdko/topic_feature_file_syntax.html
+
+feature test {
+    @LETTER = [a-z];
+    ignore substitute @LETTER a' n' d', a' n' d' @LETTER;
+    substitute a' n' d' by a_n_d;
+} test;

Lib/fontTools/feaLib/testdata/spec5f_ii_3.ttx

@@ -0,0 +1,155 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ttFont sfntVersion="true" ttLibVersion="3.0">
+
+  <GSUB>
+    <Version value="1.0"/>
+    <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="0"/>
+        </Feature>
+      </FeatureRecord>
+    </FeatureList>
+    <LookupList>
+      <!-- LookupCount=2 -->
+      <Lookup index="0">
+        <!-- LookupType=6 -->
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=3 -->
+        <ChainContextSubst index="0" Format="3">
+          <!-- BacktrackGlyphCount=1 -->
+          <BacktrackCoverage index="0">
+            <Glyph value="a"/>
+            <Glyph value="b"/>
+            <Glyph value="c"/>
+            <Glyph value="d"/>
+            <Glyph value="e"/>
+            <Glyph value="f"/>
+            <Glyph value="g"/>
+            <Glyph value="h"/>
+            <Glyph value="i"/>
+            <Glyph value="j"/>
+            <Glyph value="k"/>
+            <Glyph value="l"/>
+            <Glyph value="m"/>
+            <Glyph value="n"/>
+            <Glyph value="o"/>
+            <Glyph value="p"/>
+            <Glyph value="q"/>
+            <Glyph value="r"/>
+            <Glyph value="s"/>
+            <Glyph value="t"/>
+            <Glyph value="u"/>
+            <Glyph value="v"/>
+            <Glyph value="w"/>
+            <Glyph value="x"/>
+            <Glyph value="y"/>
+            <Glyph value="z"/>
+          </BacktrackCoverage>
+          <!-- InputGlyphCount=3 -->
+          <InputCoverage index="0">
+            <Glyph value="a"/>
+          </InputCoverage>
+          <InputCoverage index="1">
+            <Glyph value="n"/>
+          </InputCoverage>
+          <InputCoverage index="2">
+            <Glyph value="d"/>
+          </InputCoverage>
+          <!-- LookAheadGlyphCount=0 -->
+          <!-- SubstCount=0 -->
+        </ChainContextSubst>
+        <ChainContextSubst index="1" Format="3">
+          <!-- BacktrackGlyphCount=0 -->
+          <!-- InputGlyphCount=3 -->
+          <InputCoverage index="0">
+            <Glyph value="a"/>
+          </InputCoverage>
+          <InputCoverage index="1">
+            <Glyph value="n"/>
+          </InputCoverage>
+          <InputCoverage index="2">
+            <Glyph value="d"/>
+          </InputCoverage>
+          <!-- LookAheadGlyphCount=1 -->
+          <LookAheadCoverage index="0">
+            <Glyph value="a"/>
+            <Glyph value="b"/>
+            <Glyph value="c"/>
+            <Glyph value="d"/>
+            <Glyph value="e"/>
+            <Glyph value="f"/>
+            <Glyph value="g"/>
+            <Glyph value="h"/>
+            <Glyph value="i"/>
+            <Glyph value="j"/>
+            <Glyph value="k"/>
+            <Glyph value="l"/>
+            <Glyph value="m"/>
+            <Glyph value="n"/>
+            <Glyph value="o"/>
+            <Glyph value="p"/>
+            <Glyph value="q"/>
+            <Glyph value="r"/>
+            <Glyph value="s"/>
+            <Glyph value="t"/>
+            <Glyph value="u"/>
+            <Glyph value="v"/>
+            <Glyph value="w"/>
+            <Glyph value="x"/>
+            <Glyph value="y"/>
+            <Glyph value="z"/>
+          </LookAheadCoverage>
+          <!-- SubstCount=0 -->
+        </ChainContextSubst>
+        <ChainContextSubst index="2" Format="3">
+          <!-- BacktrackGlyphCount=0 -->
+          <!-- InputGlyphCount=3 -->
+          <InputCoverage index="0">
+            <Glyph value="a"/>
+          </InputCoverage>
+          <InputCoverage index="1">
+            <Glyph value="n"/>
+          </InputCoverage>
+          <InputCoverage index="2">
+            <Glyph value="d"/>
+          </InputCoverage>
+          <!-- LookAheadGlyphCount=0 -->
+          <!-- SubstCount=1 -->
+          <SubstLookupRecord index="0">
+            <SequenceIndex value="0"/>
+            <LookupListIndex value="1"/>
+          </SubstLookupRecord>
+        </ChainContextSubst>
+      </Lookup>
+      <Lookup index="1">
+        <!-- LookupType=4 -->
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <LigatureSubst index="0">
+          <LigatureSet glyph="a">
+            <Ligature components="n,d" glyph="a_n_d"/>
+          </LigatureSet>
+        </LigatureSubst>
+      </Lookup>
+    </LookupList>
+  </GSUB>
+
+</ttFont>