Support variable feature syntax (#2432)

Lib/fontTools/feaLib/ast.py

@@ -34,6 +34,7 @@
     "ChainContextPosStatement",
     "ChainContextSubstStatement",
     "CharacterStatement",
+    "ConditionsetStatement",
     "CursivePosStatement",
     "ElidedFallbackName",
     "ElidedFallbackNameID",
@@ -1261,11 +1262,21 @@ def build(self, builder):
         if not self.replacement and hasattr(self.glyph, "glyphSet"):
             for glyph in self.glyph.glyphSet():
                 builder.add_multiple_subst(
-                    self.location, prefix, glyph, suffix, self.replacement, self.forceChain
+                    self.location,
+                    prefix,
+                    glyph,
+                    suffix,
+                    self.replacement,
+                    self.forceChain,
                 )
         else:
             builder.add_multiple_subst(
-                self.location, prefix, self.glyph, suffix, self.replacement, self.forceChain
+                self.location,
+                prefix,
+                self.glyph,
+                suffix,
+                self.replacement,
+                self.forceChain,
             )
 
     def asFea(self, indent=""):
@@ -2033,3 +2044,79 @@ def asFea(self, res=""):
         res += f"location {self.tag} "
         res += f"{' '.join(str(i) for i in self.values)};\n"
         return res
+
+
+class ConditionsetStatement(Statement):
+    """
+    A variable layout conditionset
+
+    Args:
+        name (str): the name of this conditionset
+        conditions (dict): a dictionary mapping axis tags to a
+            tuple of (min,max) userspace coordinates.
+    """
+
+    def __init__(self, name, conditions, location=None):
+        Statement.__init__(self, location)
+        self.name = name
+        self.conditions = conditions
+
+    def build(self, builder):
+        builder.add_conditionset(self.name, self.conditions)
+
+    def asFea(self, res="", indent=""):
+        res += indent + f"conditionset {self.name} " + "{\n"
+        for tag, (minvalue, maxvalue) in self.conditions.items():
+            res += indent + SHIFT + f"{tag} {minvalue} {maxvalue};\n"
+        res += indent + "}" + f" {self.name};\n"
+        return res
+
+
+class VariationBlock(Block):
+    """A variation feature block, applicable in a given set of conditions."""
+
+    def __init__(self, name, conditionset, use_extension=False, location=None):
+        Block.__init__(self, location)
+        self.name, self.conditionset, self.use_extension = (
+            name,
+            conditionset,
+            use_extension,
+        )
+
+    def build(self, builder):
+        """Call the ``start_feature`` callback on the builder object, visit
+        all the statements in this feature, and then call ``end_feature``."""
+        builder.start_feature(self.location, self.name)
+        if (
+            self.conditionset != "NULL"
+            and self.conditionset not in builder.conditionsets_
+        ):
+            raise FeatureLibError(
+                f"variation block used undefined conditionset {self.conditionset}",
+                self.location,
+            )
+
+        # language exclude_dflt statements modify builder.features_
+        # limit them to this block with temporary builder.features_
+        features = builder.features_
+        builder.features_ = {}
+        Block.build(self, builder)
+        for key, value in builder.features_.items():
+            items = builder.feature_variations_.setdefault(key, {}).setdefault(
+                self.conditionset, []
+            )
+            items.extend(value)
+            if key not in features:
+                features[key] = []  # Ensure we make a feature record
+        builder.features_ = features
+        builder.end_feature()
+
+    def asFea(self, indent=""):
+        res = indent + "variation %s " % self.name.strip()
+        res += self.conditionset + " "
+        if self.use_extension:
+            res += "useExtension "
+        res += "{\n"
+        res += Block.asFea(self, indent=indent)
+        res += indent + "} %s;\n" % self.name.strip()
+        return res