Reland "Move webrtc/{base => rtc_base}" (https://codereview.webrtc.org/2877023002)

Reland the base->rtc_base without adding stub headers (will be
done in follow-up CL). This preserves git blame history of all files.

BUG=webrtc:7634
NOTRY=True
TBR=kwiberg@webrtc.org

Change-Id: Iea3bb6f3f67b8374c96337b63e8f5aa3e6181012
Reviewed-on: https://chromium-review.googlesource.com/554611
Reviewed-by: Henrik Kjellander <kjellander@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#18821}

webrtc/rtc_base/mod_ops.h

@@ -0,0 +1,135 @@
+/*
+ *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef WEBRTC_RTC_BASE_MOD_OPS_H_
+#define WEBRTC_RTC_BASE_MOD_OPS_H_
+
+#include <limits>
+#include <type_traits>
+
+#include "webrtc/base/checks.h"
+
+namespace webrtc {
+
+template <unsigned long M>                                    // NOLINT
+inline unsigned long Add(unsigned long a, unsigned long b) {  // NOLINT
+  RTC_DCHECK_LT(a, M);
+  unsigned long t = M - b % M;  // NOLINT
+  unsigned long res = a - t;    // NOLINT
+  if (t > a)
+    return res + M;
+  return res;
+}
+
+template <unsigned long M>                                         // NOLINT
+inline unsigned long Subtract(unsigned long a, unsigned long b) {  // NOLINT
+  RTC_DCHECK_LT(a, M);
+  unsigned long sub = b % M;  // NOLINT
+  if (a < sub)
+    return M - (sub - a);
+  return a - sub;
+}
+
+// Calculates the forward difference between two wrapping numbers.
+//
+// Example:
+// uint8_t x = 253;
+// uint8_t y = 2;
+//
+// ForwardDiff(x, y) == 5
+//
+//   252   253   254   255    0     1     2     3
+// #################################################
+// |     |  x  |     |     |     |     |  y  |     |
+// #################################################
+//          |----->----->----->----->----->
+//
+// ForwardDiff(y, x) == 251
+//
+//   252   253   254   255    0     1     2     3
+// #################################################
+// |     |  x  |     |     |     |     |  y  |     |
+// #################################################
+// -->----->                              |----->---
+//
+template <typename T, T M>
+inline T ForwardDiff(T a, T b) {
+  static_assert(std::is_unsigned<T>::value,
+                "Type must be an unsigned integer.");
+  RTC_DCHECK_LT(a, M);
+  RTC_DCHECK_LT(b, M);
+  return a <= b ? b - a : M - (a - b);
+}
+
+template <typename T>
+inline T ForwardDiff(T a, T b) {
+  static_assert(std::is_unsigned<T>::value,
+                "Type must be an unsigned integer.");
+  return b - a;
+}
+
+// Calculates the reverse difference between two wrapping numbers.
+//
+// Example:
+// uint8_t x = 253;
+// uint8_t y = 2;
+//
+// ReverseDiff(y, x) == 5
+//
+//   252   253   254   255    0     1     2     3
+// #################################################
+// |     |  x  |     |     |     |     |  y  |     |
+// #################################################
+//          <-----<-----<-----<-----<-----|
+//
+// ReverseDiff(x, y) == 251
+//
+//   252   253   254   255    0     1     2     3
+// #################################################
+// |     |  x  |     |     |     |     |  y  |     |
+// #################################################
+// ---<-----|                             |<-----<--
+//
+template <typename T, T M>
+inline T ReverseDiff(T a, T b) {
+  static_assert(std::is_unsigned<T>::value,
+                "Type must be an unsigned integer.");
+  RTC_DCHECK_LT(a, M);
+  RTC_DCHECK_LT(b, M);
+  return b <= a ? a - b : M - (b - a);
+}
+
+template <typename T>
+inline T ReverseDiff(T a, T b) {
+  static_assert(std::is_unsigned<T>::value,
+                "Type must be an unsigned integer.");
+  return a - b;
+}
+
+// Calculates the minimum distance between to wrapping numbers.
+//
+// The minimum distance is defined as min(ForwardDiff(a, b), ReverseDiff(a, b))
+template <typename T, T M>
+inline T MinDiff(T a, T b) {
+  static_assert(std::is_unsigned<T>::value,
+                "Type must be an unsigned integer.");
+  return std::min(ForwardDiff<T, M>(a, b), ReverseDiff<T, M>(a, b));
+}
+
+template <typename T>
+inline T MinDiff(T a, T b) {
+  static_assert(std::is_unsigned<T>::value,
+                "Type must be an unsigned integer.");
+  return std::min(ForwardDiff(a, b), ReverseDiff(a, b));
+}
+
+}  // namespace webrtc
+
+#endif  // WEBRTC_RTC_BASE_MOD_OPS_H_