revset: introduce an abstractsmartset class

This class documents all methods required by a smartset. This makes it easier
for people to respect the API and ensure we fail loudly when something does
not. It will later also contain common default implementations for multiple
methods, making it easier to have smartset classes with minimal work.

mercurial/revset.py

@@ -2195,7 +2195,93 b' def funcsused(tree):'
             funcs.add(tree[1][1])
         return funcs
 
-class baseset(list):
+class abstractsmartset(object):
+
+    def __nonzero__(self):
+        """True if the smartset is not empty"""
+        raise NotImplementedError()
+
+    def __contains__(self, rev):
+        """provide fast membership testing"""
+        raise NotImplementedError()
+
+    def __set__(self):
+        """Returns a set or a smartset containing all the elements.
+
+        The returned structure should be the fastest option for membership
+        testing.
+
+        This is part of the mandatory API for smartset."""
+        raise NotImplementedError()
+
+    def __iter__(self):
+        """iterate the set in the order it is supposed to be iterated"""
+        raise NotImplementedError()
+
+    def isascending(self):
+        """True if the set will iterate in ascending order"""
+        raise NotImplementedError()
+
+    def ascending(self):
+        """Sorts the set in ascending order (in place).
+
+        This is part of the mandatory API for smartset."""
+        raise NotImplementedError()
+
+    def isdescending(self):
+        """True if the set will iterate in descending order"""
+        raise NotImplementedError()
+
+    def descending(self):
+        """Sorts the set in descending order (in place).
+
+        This is part of the mandatory API for smartset."""
+        raise NotImplementedError()
+
+    def min(self):
+        """return the minimum element in the set"""
+        raise NotImplementedError()
+
+    def max(self):
+        """return the maximum element in the set"""
+        raise NotImplementedError()
+
+    def reverse(self):
+        """reverse the expected iteration order"""
+        raise NotImplementedError()
+
+    def sort(self, reverse=True):
+        """get the set to iterate in an ascending or descending order"""
+        raise NotImplementedError()
+
+    def __and__(self, other):
+        """Returns a new object with the intersection of the two collections.
+
+        This is part of the mandatory API for smartset."""
+        raise NotImplementedError()
+
+    def __add__(self, other):
+        """Returns a new object with the union of the two collections.
+
+        This is part of the mandatory API for smartset."""
+        raise NotImplementedError()
+
+    def __sub__(self, other):
+        """Returns a new object with the substraction of the two collections.
+
+        This is part of the mandatory API for smartset."""
+        raise NotImplementedError()
+
+    def filter(self, condition):
+        """Returns this smartset filtered by condition as a new smartset.
+
+        `condition` is a callable which takes a revision number and returns a
+        boolean.
+
+        This is part of the mandatory API for smartset."""
+        raise NotImplementedError()
+
+class baseset(list, abstractsmartset):
     """Basic data structure that represents a revset and contains the basic
     operation that it should be able to perform.
 
@@ -2320,7 +2406,7 b' class _orderedsetmixin(object):'
             return self._last()
         return self._first()
 
-class lazyset(object):
+class lazyset(abstractsmartset):
     """Duck type for baseset class which iterates lazily over the revisions in
     the subset and contains a function which tests for membership in the
     revset
@@ -2754,7 +2840,7 b' def spanset(repo, start=None, end=None):'
     return _spanset(repo, start, end)
 
 
-class _spanset(_orderedsetmixin):
+class _spanset(_orderedsetmixin, abstractsmartset):
     """Duck type for baseset class which represents a range of revisions and
     can work lazily and without having all the range in memory