dcsctp: Make Sequence Number API more consistent

* `AddTo` and `Difference` are made into static methods, as one may have
  believed that these modified the current object previously. The
  `Increment` method is kept, as it's obvious that it modifies the
  current object as it doesn't have a return value, and `next_value` is
  kept, as its naming (lower-case, snake) indicates that it's a simple
  accessor.
* Difference will return the absolute difference. This is actually the
  only reasonable choice, as the return value was unsigned and any
  negative value would just wrap.

Bug: webrtc:12614
Change-Id: If14a71636e67fc612d12759dc80a9c2518c85281
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/215069
Reviewed-by: Tommi <tommi@webrtc.org>
Commit-Queue: Victor Boivie <boivie@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33714}

net/dcsctp/common/sequence_numbers_test.cc

diff --git a/net/dcsctp/common/sequence_numbers_test.cc b/net/dcsctp/common/sequence_numbers_test.cc
index d1d1959..f5fa788 100644
--- a/net/dcsctp/common/sequence_numbers_test.cc
+++ b/net/dcsctp/common/sequence_numbers_test.cc
@@ -32,9 +32,9 @@
   EXPECT_LT(s1, s3);
   EXPECT_LT(s2, s3);
 
-  EXPECT_EQ(s1.Difference(s0), 1);
-  EXPECT_EQ(s2.Difference(s0), 2);
-  EXPECT_EQ(s3.Difference(s0), 3);
+  EXPECT_EQ(TestSequence::Difference(s1, s0), 1);
+  EXPECT_EQ(TestSequence::Difference(s2, s0), 2);
+  EXPECT_EQ(TestSequence::Difference(s3, s0), 3);
 
   EXPECT_GT(s1, s0);
   EXPECT_GT(s2, s0);
@@ -50,7 +50,7 @@
   s2.Increment();
   EXPECT_EQ(s2, s3);
 
-  EXPECT_EQ(s0.AddTo(2), s3);
+  EXPECT_EQ(TestSequence::AddTo(s0, 2), s3);
 }
 
 TEST(SequenceNumbersTest, MidValueUnwrapping) {
@@ -68,9 +68,9 @@
   EXPECT_LT(s1, s3);
   EXPECT_LT(s2, s3);
 
-  EXPECT_EQ(s1.Difference(s0), 1);
-  EXPECT_EQ(s2.Difference(s0), 2);
-  EXPECT_EQ(s3.Difference(s0), 3);
+  EXPECT_EQ(TestSequence::Difference(s1, s0), 1);
+  EXPECT_EQ(TestSequence::Difference(s2, s0), 2);
+  EXPECT_EQ(TestSequence::Difference(s3, s0), 3);
 
   EXPECT_GT(s1, s0);
   EXPECT_GT(s2, s0);
@@ -86,7 +86,7 @@
   s2.Increment();
   EXPECT_EQ(s2, s3);
 
-  EXPECT_EQ(s0.AddTo(2), s3);
+  EXPECT_EQ(TestSequence::AddTo(s0, 2), s3);
 }
 
 TEST(SequenceNumbersTest, WrappedUnwrapping) {
@@ -104,9 +104,9 @@
   EXPECT_LT(s1, s3);
   EXPECT_LT(s2, s3);
 
-  EXPECT_EQ(s1.Difference(s0), 1);
-  EXPECT_EQ(s2.Difference(s0), 2);
-  EXPECT_EQ(s3.Difference(s0), 3);
+  EXPECT_EQ(TestSequence::Difference(s1, s0), 1);
+  EXPECT_EQ(TestSequence::Difference(s2, s0), 2);
+  EXPECT_EQ(TestSequence::Difference(s3, s0), 3);
 
   EXPECT_GT(s1, s0);
   EXPECT_GT(s2, s0);
@@ -122,7 +122,7 @@
   s2.Increment();
   EXPECT_EQ(s2, s3);
 
-  EXPECT_EQ(s0.AddTo(2), s3);
+  EXPECT_EQ(TestSequence::AddTo(s0, 2), s3);
 }
 
 TEST(SequenceNumbersTest, WrapAroundAFewTimes) {
@@ -183,5 +183,20 @@
   }
 }
 
+TEST(SequenceNumbersTest, DifferenceIsAbsolute) {
+  TestSequence::Unwrapper unwrapper;
+
+  TestSequence this_value = unwrapper.Unwrap(Wrapped(10));
+  TestSequence other_value = TestSequence::AddTo(this_value, 100);
+
+  EXPECT_EQ(TestSequence::Difference(this_value, other_value), 100);
+  EXPECT_EQ(TestSequence::Difference(other_value, this_value), 100);
+
+  TestSequence minus_value = TestSequence::AddTo(this_value, -100);
+
+  EXPECT_EQ(TestSequence::Difference(this_value, minus_value), 100);
+  EXPECT_EQ(TestSequence::Difference(minus_value, this_value), 100);
+}
+
 }  // namespace
 }  // namespace dcsctp