From: Benjamin Barenblat <bbaren@google.com>
Subject: Don’t examine irrelevant destination buckets in DiscreteDistributionTest
Forwarded: yes
Applied-Upstream: https://github.com/abseil/abseil-cpp/commit/7ba826e50dff1878e6ecc6b9af44097c040c8968

Abseil generates discrete distributions using Walker’s aliasing
algorithm. This creates uniformly distributed buckets, each with a
probability of sending traffic to a different bucket. Abseil represents
a bucket as a pair

    (probability of retaining traffic ×
     alternate bucket if traffic is passed)

and a distribution as a vector of such pairs. For example, {(0.3, 1),
(1.0, 1)} represents a distribution with two buckets, the zeroth of
which passes 70% of its traffic to bucket 1 and the first of which holds
on to all its traffic.

This representation is not unique: When a bucket retains traffic with
probability 1, the alternate bucket is irrelevant. Continuing the
example above, {(0.3, 1), (1.0, 0)} _also_ represents a two-bucket
distribution where the zeroth bucket passes 70% of its traffic to the
first and the first hangs on to all traffic. Exactly what representation
Abseil generates for a given input is related to how much precision is
used in intermediate floating-point operations, which is an
architectural implementation detail. Remove sensitivity to that detail
by not examining the alternate bucket when the retention probability is
1.0.

The author works at Google. Upstream applied this patch as Piper
revision 372993410 and exported it to GitHub; the Applied-Upstream URL
above points to the exported commit.

--- a/absl/random/discrete_distribution_test.cc
+++ b/absl/random/discrete_distribution_test.cc
@@ -99,6 +99,7 @@
 }
 
 TEST(DiscreteDistributionTest, InitDiscreteDistribution) {
+  using testing::_;
   using testing::Pair;
 
   {
@@ -111,8 +112,8 @@ TEST(DiscreteDistributionTest, InitDiscreteDistribution) {
     // Each bucket is p=1/3, so bucket 0 will send half it's traffic
     // to bucket 2, while the rest will retain all of their traffic.
     EXPECT_THAT(q, testing::ElementsAre(Pair(0.5, 2),  //
-                                        Pair(1.0, 1),  //
-                                        Pair(1.0, 2)));
+                                        Pair(1.0, _),  //
+                                        Pair(1.0, _)));
   }
 
   {
@@ -135,7 +136,7 @@ TEST(DiscreteDistributionTest, InitDiscreteDistribution) {
 
     EXPECT_THAT(q, testing::ElementsAre(Pair(b0, 3),   //
                                         Pair(b1, 3),   //
-                                        Pair(1.0, 2),  //
+                                        Pair(1.0, _),  //
                                         Pair(b3, 2),   //
                                         Pair(b1, 3)));
   }