Replace uses of raw uint32's with a type-checked RtpTimeTicks data type.

media/cast/common/expanded_value_base.h

+// Copyright 2015 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 MEDIA_CAST_COMMON_EXPANDED_VALUE_BASE_H_
+#define MEDIA_CAST_COMMON_EXPANDED_VALUE_BASE_H_
+
+#include <stdint.h>
+
+#include <limits>
+
+namespace media {
+namespace cast {
+
+// Abstract base template class for common "sequence value" data types such as
+// RtpTimeTicks, FrameId, or PacketId which generally increment/decrement in
+// predictable amounts as media is streamed, and which often need to be reliably
+// truncated and re-expanded for over-the-wire transmission.
+//
+// FullPrecisionInteger should be a signed integer POD type that is of
+// sufficiently high precision to never wrap-around in the system.  Subclass is
+// the class inheriting the common functionality provided in this template, and
+// is used to provide operator overloads.  The Subclass must friend this class
+// to enable these operator overloads.
+//
+// Please see RtpTimeTicks and unit test code for examples of how to define
+// Subclasses and add features specific to their concrete data type, and how to
+// use data types derived from ExpandedValueBase.  For example, a RtpTimeTicks
+// adds math operators consisting of the meaningful and valid set of operations
+// allowed for doing "time math."  On the other hand, FrameId only adds math
+// operators for incrementing/decrementing since multiplication and division are
+// meaningless.
+template <typename FullPrecisionInteger, class Subclass>
+class ExpandedValueBase {
+  static_assert(std::numeric_limits<FullPrecisionInteger>::is_signed,
+                "FullPrecisionInteger must be a signed integer.");
+  static_assert(std::numeric_limits<FullPrecisionInteger>::is_integer,
+                "FullPrecisionInteger must be a signed integer.");
+
+ public:
+  // Methods that return the lower bits of this value.  This should only be used
+  // for serializing/wire-formatting, and not to subvert the restricted set of
+  // operators allowed on this data type.
+  uint8_t lower_8_bits() const { return static_cast<uint8_t>(value_); }
+  uint16_t lower_16_bits() const { return static_cast<uint16_t>(value_); }
+  uint32_t lower_32_bits() const { return static_cast<uint32_t>(value_); }
+
+  // Compute the value closest to |this| value whose lower bits are those of
+  // |x|.  The result is always within |max_distance_for_expansion()| of |this|
+  // value.
+  //
+  // The purpose of this method is to re-instantiate an original value from its
+  // truncated form, usually when deserializing off-the-wire.  Therefore, it is
+  // always important to call this method on an instance known to be close in
+  // distance to |x|.
+  template <typename ShortUnsigned>
+  Subclass Expand(ShortUnsigned x) const {
+    static_assert(!std::numeric_limits<ShortUnsigned>::is_signed,
+                  "|x| must be an unsigned integer.");
+    static_assert(std::numeric_limits<ShortUnsigned>::is_integer,
+                  "|x| must be an unsigned integer.");
+    static_assert(sizeof(ShortUnsigned) <= sizeof(FullPrecisionInteger),
+                  "|x| must fit within the FullPrecisionInteger.");
+
+    if (sizeof(ShortUnsigned) < sizeof(FullPrecisionInteger)) {
+      // Initially, the |result| is composed of upper bits from |value_| and
+      // lower bits from |x|.
+      const FullPrecisionInteger short_max =
+          std::numeric_limits<ShortUnsigned>::max();
+      FullPrecisionInteger result = (value_ & ~short_max) | x;
+
+      // Determine whether the shorter integer type encountered wrap-around, and
+      // increment/decrement the upper bits by one to account for that.
+      const FullPrecisionInteger diff = result - value_;
+      const FullPrecisionInteger pivot =
+          max_distance_for_expansion<ShortUnsigned>();
+      if (diff > pivot)
+        result -= short_max + 1;
+      else if (diff < -(pivot + 1))
+        result += short_max + 1;
+      return Subclass(result);
+    } else {
+      return Subclass(x);
+    }
+  }
+
+  // Comparison operators.
+  bool operator==(Subclass rhs) const { return value_ == rhs.value_; }
+  bool operator!=(Subclass rhs) const { return value_ != rhs.value_; }
+  bool operator<(Subclass rhs) const { return value_ < rhs.value_; }
+  bool operator>(Subclass rhs) const { return value_ > rhs.value_; }
+  bool operator<=(Subclass rhs) const { return value_ <= rhs.value_; }
+  bool operator>=(Subclass rhs) const { return value_ >= rhs.value_; }
+
+  // (De)Serialize for transmission over IPC.  Do not use these to subvert the
+  // valid set of operators allowed by this class or its Subclass.
+  uint64_t SerializeForIPC() const {
+    static_assert(sizeof(uint64_t) >= sizeof(FullPrecisionInteger),
+                  "Cannot serialize FullPrecisionInteger into an uint64_t.");
+    return static_cast<uint64_t>(value_);
+  }
+  static Subclass DeserializeForIPC(uint64_t serialized) {
+    return Subclass(static_cast<FullPrecisionInteger>(serialized));
+  }
+
+  // Design limit: Values that are truncated to the ShortUnsigned type must be
+  // no more than this maximum distance from each other in order to ensure the
+  // original value can be determined correctly.
+  template <typename ShortUnsigned>
+  static FullPrecisionInteger max_distance_for_expansion() {
+    return std::numeric_limits<ShortUnsigned>::max() / 2;
+  }
+
+ protected:
+  // Only subclasses are permitted to instantiate directly.
+  explicit ExpandedValueBase(FullPrecisionInteger value) : value_(value) {}
+
+  FullPrecisionInteger value_;
+};
+
+}  // namespace cast
+}  // namespace media
+
+#endif  // MEDIA_CAST_COMMON_EXPANDED_VALUE_BASE_H_