Update from chromium 62675d9fb31fb8cedc40f68e78e8445a74f362e7

sandbox/linux/bpf_dsl/cons.h

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef SANDBOX_LINUX_BPF_DSL_CONS_H_
+#define SANDBOX_LINUX_BPF_DSL_CONS_H_
+
+#include "base/memory/ref_counted.h"
+#include "sandbox/sandbox_export.h"
+
+namespace sandbox {
+namespace cons {
+
+// Namespace cons provides an abstraction for immutable "cons list"
+// data structures as commonly provided in functional programming
+// languages like Lisp or Haskell.
+//
+// A cons list is a linked list consisting of "cells", each of which
+// have a "head" and a "tail" element. A cell's head element contains
+// a user specified value, while the tail element contains a (possibly
+// null) pointer to another cell.
+//
+// An empty list (idiomatically referred to as "nil") can be
+// constructed as "cons::List<Foo>()" or simply as "nullptr" if Foo
+// can be inferred from context (e.g., calling a function that has a
+// "cons::List<Foo>" parameter).
+//
+// Existing lists (including empty lists) can be extended by
+// prepending new values to the front using the "Cons(head, tail)"
+// function, which will allocate a new cons cell. Notably, cons lists
+// support creating multiple lists that share a common tail sequence.
+//
+// Lastly, lists support iteration via C++11's range-based for loop
+// construct.
+//
+// Examples:
+//
+//   // basic construction
+//   const cons::List<char> kNil = nullptr;
+//   cons::List<char> ba = Cons('b', Cons('a', kNil));
+//
+//   // common tail sequence
+//   cons::List<char> cba = Cons('c', ba);
+//   cons::List<char> dba = Cons('d', ba);
+//
+//   // iteration
+//   for (const char& ch : cba) {
+//     // iterates 'c', 'b', 'a'
+//   }
+//   for (const char& ch : dba) {
+//     // iterates 'd', 'b', 'a'
+//   }
+
+// Forward declarations.
+template <typename T>
+class Cell;
+template <typename T>
+class ListIterator;
+
+// List represents a (possibly null) pointer to a cons cell.
+template <typename T>
+using List = scoped_refptr<const Cell<T>>;
+
+// Cons extends a cons list by prepending a new value to the front.
+template <typename T>
+List<T> Cons(const T& head, const List<T>& tail) {
+  return List<T>(new const Cell<T>(head, tail));
+}
+
+// Cell represents an individual "cons cell" within a cons list.
+template <typename T>
+class Cell : public base::RefCounted<Cell<T>> {
+ public:
+  Cell(const T& head, const List<T>& tail) : head_(head), tail_(tail) {}
+
+  // Head returns this cell's head element.
+  const T& head() const { return head_; }
+
+  // Tail returns this cell's tail element.
+  const List<T>& tail() const { return tail_; }
+
+ private:
+  virtual ~Cell() {}
+
+  T head_;
+  List<T> tail_;
+
+  friend class base::RefCounted<Cell<T>>;
+  DISALLOW_COPY_AND_ASSIGN(Cell);
+};
+
+// Begin returns a list iterator pointing to the first element of the
+// cons list. It's provided to support range-based for loops.
+template <typename T>
+ListIterator<T> begin(const List<T>& list) {
+  return ListIterator<T>(list);
+}
+
+// End returns a list iterator pointing to the "past-the-end" element
+// of the cons list (i.e., nil). It's provided to support range-based
+// for loops.
+template <typename T>
+ListIterator<T> end(const List<T>& list) {
+  return ListIterator<T>();
+}
+
+// ListIterator provides C++ forward iterator semantics for traversing
+// a cons list.
+template <typename T>
+class ListIterator {
+ public:
+  ListIterator() : list_() {}
+  explicit ListIterator(const List<T>& list) : list_(list) {}
+
+  const T& operator*() const { return list_->head(); }
+
+  ListIterator& operator++() {
+    list_ = list_->tail();
+    return *this;
+  }
+
+  friend bool operator==(const ListIterator& lhs, const ListIterator& rhs) {
+    return lhs.list_ == rhs.list_;
+  }
+
+ private:
+  List<T> list_;
+};
+
+template <typename T>
+bool operator!=(const ListIterator<T>& lhs, const ListIterator<T>& rhs) {
+  return !(lhs == rhs);
+}
+
+}  // namespace cons
+}  // namespace sandbox
+
+#endif  // SANDBOX_LINUX_BPF_DSL_CONS_H_