[feaLib] Parse substitute from statements

Lib/fontTools/feaLib/ast.py

@@ -28,6 +28,12 @@ class GlyphClassDefinition(object):
         self.glyphs = glyphs
 
 
+class AlternateSubstitution(object):
+    def __init__(self, location, glyph, from_class):
+        self.location = location
+        self.glyph, self.from_class = (glyph, from_class)
+
+
 class AnchorDefinition(object):
     def __init__(self, location, name, x, y, contourpoint):
         self.location = location

Lib/fontTools/feaLib/parser.py

@@ -130,7 +130,7 @@ class Parser(object):
 
     def parse_glyph_pattern_(self):
         prefix, glyphs, lookups, suffix = ([], [], [], [])
-        while self.next_token_ not in {"by", ";"}:
+        while self.next_token_ not in {"by", "from", ";"}:
             gc = self.parse_glyphclass_(accept_glyphname=True)
             marked = False
             if self.next_token_ == "'":
@@ -218,15 +218,31 @@ class Parser(object):
         assert self.cur_token_ in {"substitute", "sub"}
         location = self.cur_token_location_
         old_prefix, old, lookups, old_suffix = self.parse_glyph_pattern_()
+
         if self.next_token_ == "by":
-            self.expect_keyword_("by")
+            keyword = self.expect_keyword_("by")
             new = [self.parse_glyphclass_(accept_glyphname=True)]
+        elif self.next_token_ == "from":
+            keyword = self.expect_keyword_("from")
+            new = [self.parse_glyphclass_(accept_glyphname=False)]
         else:
+            keyword = None
             new = []
         self.expect_symbol_(";")
         if len(new) is 0 and not any(lookups):
-            raise ParserError('Expected "by" or explicit lookup references',
-                              self.cur_token_location_)
+            raise ParserError(
+                'Expected "by", "from" or explicit lookup references',
+                self.cur_token_location_)
+
+        if keyword == "from":
+            if len(old) != 1 or len(old[0]) != 1:
+                raise ParserError('Expected a single glyph before "from"',
+                                  location)
+            if len(new) != 1:
+                raise ParserError('Expected a single glyphclass after "from"',
+                                  location)
+            return ast.AlternateSubstitution(location, list(old[0])[0], new[0])
+
         rule = ast.SubstitutionRule(location, old, new)
         rule.old_prefix, rule.old_suffix = old_prefix, old_suffix
         rule.lookups = lookups

Lib/fontTools/feaLib/parser_test.py

@@ -289,6 +289,25 @@ class ParserTest(unittest.TestCase):
         self.assertEqual(sub.new, [{"A.sc", "B.sc", "C.sc", "D.sc"}])
         self.assertEqual(sub.lookups, [None])
 
+    def test_substitute_from(self):  # GSUB LookupType 3
+        doc = self.parse("feature test {"
+                         "  substitute a from [a.1 a.2 a.3];"
+                         "} test;")
+        sub = doc.statements[0].statements[0]
+        self.assertEqual(type(sub), ast.AlternateSubstitution)
+        self.assertEqual(sub.glyph, "a")
+        self.assertEqual(sub.from_class, {"a.1", "a.2", "a.3"})
+
+    def test_substitute_from_glyphclass(self):  # GSUB LookupType 3
+        doc = self.parse("feature test {"
+                         "  @Ampersands = [ampersand.1 ampersand.2];"
+                         "  substitute ampersand from @Ampersands;"
+                         "} test;")
+        [glyphclass, sub] = doc.statements[0].statements
+        self.assertEqual(type(sub), ast.AlternateSubstitution)
+        self.assertEqual(sub.glyph, "ampersand")
+        self.assertEqual(sub.from_class, {"ampersand.1", "ampersand.2"})
+
     def test_substitute_ligature(self):  # GSUB LookupType 4
         doc = self.parse("feature liga {substitute f f i by f_f_i;} liga;")
         sub = doc.statements[0].statements[0]
@@ -306,7 +325,7 @@ class ParserTest(unittest.TestCase):
 
     def test_substitute_missing_by(self):
         self.assertRaisesRegex(
-            ParserError, 'Expected "by" or explicit lookup references',
+            ParserError, 'Expected "by", "from" or explicit lookup references',
             self.parse, "feature liga {substitute f f i;} liga;")
 
     def test_subtable(self):