Repo created

TMessagesProj/jni/voip/webrtc/base/containers/buffer_iterator.h

@@ -0,0 +1,145 @@
+// Copyright 2019 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 BASE_CONTAINERS_BUFFER_ITERATOR_H_
+#define BASE_CONTAINERS_BUFFER_ITERATOR_H_
+
+#include <type_traits>
+
+#include "base/bit_cast.h"
+#include "base/containers/span.h"
+#include "base/numerics/checked_math.h"
+
+namespace base {
+
+// BufferIterator is a bounds-checked container utility to access variable-
+// length, heterogeneous structures contained within a buffer. If the data are
+// homogeneous, use base::span<> instead.
+//
+// After being created with a weakly-owned buffer, BufferIterator returns
+// pointers to structured data within the buffer. After each method call that
+// returns data in the buffer, the iterator position is advanced by the byte
+// size of the object (or span of objects) returned. If there are not enough
+// bytes remaining in the buffer to return the requested object(s), a nullptr
+// or empty span is returned.
+//
+// This class is similar to base::Pickle, which should be preferred for
+// serializing to disk. Pickle versions its header and does not support writing
+// structures, which are problematic for serialization due to struct padding and
+// version shear concerns.
+//
+// Example usage:
+//
+//    std::vector<uint8_t> buffer(4096);
+//    if (!ReadSomeData(&buffer, buffer.size())) {
+//      LOG(ERROR) << "Failed to read data.";
+//      return false;
+//    }
+//
+//    BufferIterator<uint8_t> iterator(buffer);
+//    uint32_t* num_items = iterator.Object<uint32_t>();
+//    if (!num_items) {
+//      LOG(ERROR) << "No num_items field."
+//      return false;
+//    }
+//
+//    base::span<const item_struct> items =
+//        iterator.Span<item_struct>(*num_items);
+//    if (items.size() != *num_items) {
+//      LOG(ERROR) << "Not enough items.";
+//      return false;
+//    }
+//
+//    // ... validate the objects in |items|.
+template <typename B>
+class BufferIterator {
+ public:
+  static_assert(std::is_same<std::remove_const_t<B>, char>::value ||
+                    std::is_same<std::remove_const_t<B>, unsigned char>::value,
+                "Underlying buffer type must be char-type.");
+
+  BufferIterator() {}
+  BufferIterator(B* data, size_t size)
+      : BufferIterator(make_span(data, size)) {}
+  explicit BufferIterator(span<B> buffer)
+      : buffer_(buffer), remaining_(buffer) {}
+  ~BufferIterator() {}
+
+  // Returns a pointer to a mutable structure T in the buffer at the current
+  // position. On success, the iterator position is advanced by sizeof(T). If
+  // there are not sizeof(T) bytes remaining in the buffer, returns nullptr.
+  template <typename T,
+            typename =
+                typename std::enable_if_t<std::is_trivially_copyable<T>::value>>
+  T* MutableObject() {
+    size_t size = sizeof(T);
+    size_t next_position;
+    if (!CheckAdd(position(), size).AssignIfValid(&next_position))
+      return nullptr;
+    if (next_position > total_size())
+      return nullptr;
+    T* t = bit_cast<T*>(remaining_.data());
+    remaining_ = remaining_.subspan(size);
+    return t;
+  }
+
+  // Returns a const pointer to an object of type T in the buffer at the current
+  // position.
+  template <typename T,
+            typename =
+                typename std::enable_if_t<std::is_trivially_copyable<T>::value>>
+  const T* Object() {
+    return MutableObject<const T>();
+  }
+
+  // Returns a span of |count| T objects in the buffer at the current position.
+  // On success, the iterator position is advanced by |sizeof(T) * count|. If
+  // there are not enough bytes remaining in the buffer to fulfill the request,
+  // returns an empty span.
+  template <typename T,
+            typename =
+                typename std::enable_if_t<std::is_trivially_copyable<T>::value>>
+  span<T> MutableSpan(size_t count) {
+    size_t size;
+    if (!CheckMul(sizeof(T), count).AssignIfValid(&size))
+      return span<T>();
+    size_t next_position;
+    if (!CheckAdd(position(), size).AssignIfValid(&next_position))
+      return span<T>();
+    if (next_position > total_size())
+      return span<T>();
+    auto result = span<T>(bit_cast<T*>(remaining_.data()), count);
+    remaining_ = remaining_.subspan(size);
+    return result;
+  }
+
+  // Returns a span to |count| const objects of type T in the buffer at the
+  // current position.
+  template <typename T,
+            typename =
+                typename std::enable_if_t<std::is_trivially_copyable<T>::value>>
+  span<const T> Span(size_t count) {
+    return MutableSpan<const T>(count);
+  }
+
+  // Resets the iterator position to the absolute offset |to|.
+  void Seek(size_t to) { remaining_ = buffer_.subspan(to); }
+
+  // Returns the total size of the underlying buffer.
+  size_t total_size() { return buffer_.size(); }
+
+  // Returns the current position in the buffer.
+  size_t position() { return buffer_.size_bytes() - remaining_.size_bytes(); }
+
+ private:
+  // The original buffer that the iterator was constructed with.
+  const span<B> buffer_;
+  // A subspan of |buffer_| containing the remaining bytes to iterate over.
+  span<B> remaining_;
+  // Copy and assign allowed.
+};
+
+}  // namespace base
+
+#endif  // BASE_CONTAINERS_BUFFER_ITERATOR_H_