[voltLib] parse REVERSAL for reverse chaining context singl substitution

Lib/fontTools/voltLib/ast.py

@@ -62,7 +62,7 @@ class FeatureDefinition(ast.Statement):
 
 class LookupDefinition(ast.Statement):
     def __init__(self, location, name, base, marks, process_marks, all_flag,
-                 direction, comments, context, sub, pos):
+                 direction, reversal, comments, context, sub, pos):
         ast.Statement.__init__(self, location)
         self.name = name
         self.base = base
@@ -70,6 +70,7 @@ class LookupDefinition(ast.Statement):
         self.process_marks = process_marks
         self.all = all_flag
         self.direction = direction
+        self.reversal = reversal
         self.comments = comments
         self.context = context
         self.sub = sub

Lib/fontTools/voltLib/parser.py

@@ -177,6 +177,10 @@ class Parser(object):
             self.expect_keyword_("DIRECTION")
             direction = self.expect_name_()
             assert direction in ("LTR", "RTL")
+        reversal = None
+        if self.next_token_ == "REVERSAL":
+            self.expect_keyword_("REVERSAL")
+            reversal = True
         comments = None
         if self.next_token_ == "COMMENTS":
             self.expect_keyword_("COMMENTS")
@@ -193,7 +197,7 @@ class Parser(object):
             pos = self.parse_position_()
         def_lookup = ast.LookupDefinition(
             location, name, base, marks, process_marks, all_flag, direction,
-            comments, context, sub, pos)
+            reversal, comments, context, sub, pos)
         return def_lookup
 
     def parse_context_(self):

Lib/fontTools/voltLib/parser_test.py

@@ -429,6 +429,42 @@ class ParserTest(unittest.TestCase):
             (lookup.name, process_marks, all_marks),
             ("SomeSub", "SomeMarks", False))
 
+    def test_substitution_no_reversal(self):
+        [lookup] = self.parse(
+            'DEF_LOOKUP "Lookup" PROCESS_BASE PROCESS_MARKS ALL '
+            'DIRECTION LTR\n'
+            'IN_CONTEXT\n'
+            'RIGHT ENUM GLYPH "a" GLYPH "b" END_ENUM\n'
+            'END_CONTEXT\n'
+            'AS_SUBSTITUTION\n'
+            'SUB GLYPH "a"\n'
+            'WITH GLYPH "a.alt"\n'
+            'END_SUB\n'
+            'END_SUBSTITUTION'
+        ).statements
+        self.assertEqual(
+            (lookup.name, lookup.reversal),
+            ("Lookup", None)
+        )
+
+    def test_substitution_reversal(self):
+        [lookup] = self.parse(
+            'DEF_LOOKUP "RevLookup" PROCESS_BASE PROCESS_MARKS ALL '
+            'DIRECTION LTR REVERSAL\n'
+            'IN_CONTEXT\n'
+            'RIGHT ENUM GLYPH "a" GLYPH "b" END_ENUM\n'
+            'END_CONTEXT\n'
+            'AS_SUBSTITUTION\n'
+            'SUB GROUP "DFLT_Num_standardFigures"\n'
+            'WITH GROUP "DFLT_Num_numerators"\n'
+            'END_SUB\n'
+            'END_SUBSTITUTION'
+        ).statements
+        self.assertEqual(
+            (lookup.name, lookup.reversal),
+            ("RevLookup", True)
+        )
+
     # GPOS
     #  ATTACH_CURSIVE
     #  ATTACH