Add symbol dependency

All Symbol instances now have a set which points which
others it requires and which ones requires the given instance.

An symbol that requires another means that all the parents of the former
will also requires it.

Author: boriel

symbols/symbol_.py

@@ -10,13 +10,15 @@
 # ----------------------------------------------------------------------
 
 import re
+from functools import reduce
+from typing import Set, Optional
+
 from ast_ import Ast
 import api.global_
 
 
 class Symbol(Ast):
-    """ Symbol object to store everything related to
-    a symbol.
+    """ Symbol object to store everything related to a symbol.
     """
     def __init__(self, *children):
         super(Symbol, self).__init__()
@@ -25,6 +27,34 @@ def __init__(self, *children):
             assert isinstance(child, Symbol)
             self.appendChild(child)
 
+        self._required_by: Set['Symbol'] = set()  # Symbols that depends on this one
+        self._requires: Set['Symbol'] = set()  # Symbols this one depends on
+        self._cached_required_by: Optional[Set['Symbol']] = set()
+
+    @property
+    def required_by(self) -> Set['Symbol']:
+        if self._cached_required_by is not None:
+            return self._cached_required_by
+
+        self._cached_required_by = reduce(lambda x: x.union, (x.required_by for x in self.children),
+                                          set(self._required_by))
+        return self._cached_required_by
+
+    @property
+    def requires(self) -> Set['Symbol']:
+        return set(self._requires)
+
+    def mark_as_required_by(self, other: 'Symbol'):
+        self._required_by.add(other)
+        self._cached_required_by.add(other)
+        if self.parent is not None:
+            assert isinstance(self.parent, Symbol)
+            self.parent.mark_as_required_by(other)
+
+    def add_required_symbol(self, other: 'Symbol'):
+        self._requires.add(other)
+        other.mark_as_required_by(self)
+
     @property
     def token(self):
         """ token = AST Symbol class name, removing the 'Symbol' prefix.
@@ -63,3 +93,7 @@ def copy_attr(self, other):
             val = getattr(other, attr)
             if isinstance(val, str) or str(val)[0] != '<':  # Not a value
                 setattr(self, attr, val)
+
+    @property
+    def is_needed(self) -> bool:
+        return len(self.required_by) > 0