1 files changed, 88 insertions, 0 deletions
diff --git a/sphinx/util/tags.py b/sphinx/util/tags.py
new file mode 100644
index 0000000..73e1a83
--- /dev/null
+++ b/sphinx/util/tags.py
@@ -0,0 +1,88 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+from jinja2 import nodes
+from jinja2.environment import Environment
+from jinja2.parser import Parser
+
+if TYPE_CHECKING:
+    from collections.abc import Iterator
+
+    from jinja2.nodes import Node
+
+
+env = Environment()
+
+
+class BooleanParser(Parser):
+    """
+    Only allow condition exprs and/or/not operations.
+    """
+
+    def parse_compare(self) -> Node:
+        node: Node
+        token = self.stream.current
+        if token.type == 'name':
+            if token.value in ('true', 'false', 'True', 'False'):
+                node = nodes.Const(token.value in ('true', 'True'),
+                                   lineno=token.lineno)
+            elif token.value in ('none', 'None'):
+                node = nodes.Const(None, lineno=token.lineno)
+            else:
+                node = nodes.Name(token.value, 'load', lineno=token.lineno)
+            next(self.stream)
+        elif token.type == 'lparen':
+            next(self.stream)
+            node = self.parse_expression()
+            self.stream.expect('rparen')
+        else:
+            self.fail(f"unexpected token '{token}'", token.lineno)
+        return node
+
+
+class Tags:
+    def __init__(self, tags: list[str] | None = None) -> None:
+        self.tags = dict.fromkeys(tags or [], True)
+
+    def has(self, tag: str) -> bool:
+        return tag in self.tags
+
+    __contains__ = has
+
+    def __iter__(self) -> Iterator[str]:
+        return iter(self.tags)
+
+    def add(self, tag: str) -> None:
+        self.tags[tag] = True
+
+    def remove(self, tag: str) -> None:
+        self.tags.pop(tag, None)
+
+    def eval_condition(self, condition: str) -> bool:
+        # exceptions are handled by the caller
+        parser = BooleanParser(env, condition, state='variable')
+        expr = parser.parse_expression()
+        if not parser.stream.eos:
+            msg = 'chunk after expression'
+            raise ValueError(msg)
+
+        def eval_node(node: Node) -> bool:
+            if isinstance(node, nodes.CondExpr):
+                if eval_node(node.test):
+                    return eval_node(node.expr1)
+                else:
+                    return eval_node(node.expr2)
+            elif isinstance(node, nodes.And):
+                return eval_node(node.left) and eval_node(node.right)
+            elif isinstance(node, nodes.Or):
+                return eval_node(node.left) or eval_node(node.right)
+            elif isinstance(node, nodes.Not):
+                return not eval_node(node.node)
+            elif isinstance(node, nodes.Name):
+                return self.tags.get(node.name, False)
+            else:
+                msg = 'invalid node, check parsing'
+                raise ValueError(msg)
+
+        return eval_node(expr)