Fix a corner case in SpyHashState for exact boundaries.

AbslHash allows for piecewise chunks to be streamed incrementally into hash states and requires them to hash identically to one giant stream.  The exact size window for this is an internal details `PiecewiseChunkSize`.  There was an off by one error in this code.  Add tests and fix it.

PiperOrigin-RevId: 602463183
Change-Id: I159bbb5e7e745f55b2fe6eaf0d2735bd0a08aca9

absl/hash/internal/spy_hash_state.h

@@ -149,20 +149,20 @@ class SpyHashStateImpl : public HashStateBase<SpyHashStateImpl<T>> {
                                              const unsigned char* begin,
                                              size_t size) {
     const size_t large_chunk_stride = PiecewiseChunkSize();
-    if (size > large_chunk_stride) {
     // Combining a large contiguous buffer must have the same effect as
     // doing it piecewise by the stride length, followed by the (possibly
     // empty) remainder.
-      while (size >= large_chunk_stride) {
+    while (size > large_chunk_stride) {
       hash_state = SpyHashStateImpl::combine_contiguous(
           std::move(hash_state), begin, large_chunk_stride);
       begin += large_chunk_stride;
       size -= large_chunk_stride;
     }
-    }
 
+    if (size > 0) {
       hash_state.hash_representation_.emplace_back(
           reinterpret_cast<const char*>(begin), size);
+    }
     return hash_state;
   }
 

absl/strings/BUILD.bazel

@@ -918,6 +918,7 @@ cc_test(
         "//absl/container:fixed_array",
         "//absl/functional:function_ref",
         "//absl/hash",
+        "//absl/hash:hash_testing",
         "//absl/log",
         "//absl/log:check",
         "//absl/random",

absl/strings/CMakeLists.txt

@@ -1072,6 +1072,7 @@ absl_cc_test(
     absl::fixed_array
     absl::function_ref
     absl::hash
+    absl::hash_testing
     absl::log
     absl::optional
     absl::random_random

absl/strings/cord_test.cc

@@ -42,6 +42,7 @@
 #include "absl/container/fixed_array.h"
 #include "absl/functional/function_ref.h"
 #include "absl/hash/hash.h"
+#include "absl/hash/hash_testing.h"
 #include "absl/log/check.h"
 #include "absl/log/log.h"
 #include "absl/random/random.h"
@@ -2013,6 +2014,26 @@ TEST(CordTest, CordMemoryUsageBTree) {
                 rep2_size);
 }
 
+TEST(CordTest, TestHashFragmentation) {
+  // Make sure we hit these boundary cases precisely.
+  EXPECT_EQ(1024, absl::hash_internal::PiecewiseChunkSize());
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly({
+      absl::Cord(),
+      absl::MakeFragmentedCord({std::string(600, 'a'), std::string(600, 'a')}),
+      absl::MakeFragmentedCord({std::string(1200, 'a')}),
+      absl::MakeFragmentedCord({std::string(900, 'b'), std::string(900, 'b')}),
+      absl::MakeFragmentedCord({std::string(1800, 'b')}),
+      absl::MakeFragmentedCord(
+          {std::string(2000, 'c'), std::string(2000, 'c')}),
+      absl::MakeFragmentedCord({std::string(4000, 'c')}),
+      absl::MakeFragmentedCord({std::string(1024, 'd')}),
+      absl::MakeFragmentedCord({std::string(1023, 'd'), "d"}),
+      absl::MakeFragmentedCord({std::string(1025, 'e')}),
+      absl::MakeFragmentedCord({std::string(1024, 'e'), "e"}),
+      absl::MakeFragmentedCord({std::string(1023, 'e'), "e", "e"}),
+  }));
+}
+
 // Regtest for a change that had to be rolled back because it expanded out
 // of the InlineRep too soon, which was observable through MemoryUsage().
 TEST_P(CordTest, CordMemoryUsageInlineRep) {