Export of internal Abseil changes

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2130ba98c8359b08d97fb16d84dfd05687005dcf by Abseil Team <absl-team@google.com>:

Tweaking the documentation of c_all_of to state the effect more directly.

PiperOrigin-RevId: 410557900

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4732289bf4b56123fed113e36be4710b55c6a6c7 by Greg Falcon <gfalcon@google.com>:

Improve the quality of absl::Hash<std::vector<bool>>.

This previously dispatched to std::hash<vector<bool>>, which suffers from trivial collisions on many platforms.  (They often hash the internal words but no size info, so that, e.g., {1, 1} and {1, 1, 0} collide.)

Also extended the unit test to exercise this.

PiperOrigin-RevId: 410329943

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1c5f3934230a7669f74c96b305251786a265e235 by Greg Falcon <gfalcon@google.com>:

Add broader testing of absl hash contracts in the hash unit test.

In particular, test that the hash erasure mechanism works.

PiperOrigin-RevId: 410312738

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5e1923f527ed3d02f6752a5b38d5e1c17a4a146f by Derek Mauro <dmauro@google.com>:

Internal change

PiperOrigin-RevId: 410290663

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8c74bc962b3b98a5908017c345efc592393048ea by Martijn Vels <mvels@google.com>:

Add Cord::CreateFlat() function

PiperOrigin-RevId: 410260776

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bd0de4e94c85620d3b8dd60fae367b730fc4cb34 by Evan Brown <ezb@google.com>:

Rename node_hash_policy to node_slot_policy.

Motivation: we can potentially reuse this code for node_btree_*.
PiperOrigin-RevId: 410082271
GitOrigin-RevId: 2130ba98c8359b08d97fb16d84dfd05687005dcf
Change-Id: Ie052084cf992dee250d8b2f388d39c4de0dcff40

1 files changed, 17 insertions, 2 deletions

diff --git a/absl/hash/internal/hash.h b/absl/hash/internal/hash.h
index b1e33caf..97b68bad 100644
--- a/absl/hash/internal/hash.h
+++ b/absl/hash/internal/hash.h
@@ -502,10 +502,9 @@ AbslHashValue(H hash_state, const std::vector<T, Allocator>& vector) {
                     vector.size());
 }
 
+// AbslHashValue special cases for hashing std::vector<bool>
 #if defined(ABSL_IS_BIG_ENDIAN) && \
     (defined(__GLIBCXX__) || defined(__GLIBCPP__))
-// AbslHashValue for hashing std::vector<bool>
-//
 // std::hash in libstdc++ does not work correctly with vector<bool> on Big
 // Endian platforms therefore we need to implement a custom AbslHashValue for
 // it. More details on the bug:
@@ -521,6 +520,22 @@ AbslHashValue(H hash_state, const std::vector<T, Allocator>& vector) {
   }
   return H::combine(combiner.finalize(std::move(hash_state)), vector.size());
 }
+#else
+// When not working around the libstdc++ bug above, we still have to contend
+// with the fact that std::hash<vector<bool>> is often poor quality, hashing
+// directly on the internal words and on no other state.  On these platforms,
+// vector<bool>{1, 1} and vector<bool>{1, 1, 0} hash to the same value.
+//
+// Mixing in the size (as we do in our other vector<> implementations) on top
+// of the library-provided hash implementation avoids this QOI issue.
+template <typename H, typename T, typename Allocator>
+typename std::enable_if<is_hashable<T>::value && std::is_same<T, bool>::value,
+                        H>::type
+AbslHashValue(H hash_state, const std::vector<T, Allocator>& vector) {
+  return H::combine(std::move(hash_state),
+                    std::hash<std::vector<T, Allocator>>{}(vector),
+                    vector.size());
+}
 #endif
 
 // -----------------------------------------------------------------------------