This fixes a discrepancy between the C++ and Java version of the ToFourDigitString() function, given byte array inputs that include negative signed byte values.

Added a unit test to verify that the output with negative signed bytes in the input will result in the same output as the Java version.

PiperOrigin-RevId: 712709020

internal/platform/BUILD

@@ -403,6 +403,7 @@ cc_test(
         ":base",
         ":util",
         "@com_github_protobuf_matchers//protobuf-matchers",
+        "@com_google_absl//absl/strings:string_view",
         "@com_google_googletest//:gtest_main",
     ],
 )

internal/platform/byte_utils.cc

@@ -14,6 +14,7 @@
 
 #include "internal/platform/byte_utils.h"
 
+#include <cstdint>
 #include <cstdlib>
 #include <string>
 
@@ -29,7 +30,7 @@ std::string ByteUtils::ToFourDigitString(ByteArray& bytes) {
 
   BaseInputStream base_input_stream{bytes};
   while (base_input_stream.IsAvailable(1)) {
-    auto byte = static_cast<int>(base_input_stream.ReadUint8());
+    auto byte = static_cast<int8_t>(base_input_stream.ReadUint8());
     hashCode = (hashCode + byte * multiplier) % kHashBasePrime;
     multiplier = multiplier * kHashBaseMultiplier % kHashBasePrime;
   }

internal/platform/byte_utils_test.cc

@@ -14,7 +14,10 @@
 
 #include "internal/platform/byte_utils.h"
 
+#include <string>
+
 #include "gtest/gtest.h"
+#include "absl/strings/string_view.h"
 #include "internal/platform/byte_array.h"
 
 namespace nearby {
@@ -22,6 +25,8 @@ namespace nearby {
 constexpr absl::string_view kFooBytes{"rawABCDE"};
 constexpr absl::string_view kFooFourDigitsToken{"0392"};
 constexpr absl::string_view kEmptyFourDigitsToken{"0000"};
+constexpr absl::string_view kNegativeBytes{"raw\xd5\x01\xe4\x03\x81"};
+constexpr absl::string_view kNegativeFourDigitsToken{"9084"};
 
 TEST(ByteUtilsTest, ToFourDigitStringCorrect) {
   ByteArray bytes{std::string(kFooBytes)};
@@ -31,6 +36,14 @@ TEST(ByteUtilsTest, ToFourDigitStringCorrect) {
   EXPECT_EQ(std::string(kFooFourDigitsToken), four_digit_string);
 }
 
+TEST(ByteUtilsTest, ToFourDigitStringNegativeCorrect) {
+  ByteArray bytes{std::string(kNegativeBytes)};
+
+  auto four_digit_string = ByteUtils::ToFourDigitString(bytes);
+
+  EXPECT_EQ(std::string(kNegativeFourDigitsToken), kNegativeFourDigitsToken);
+}
+
 TEST(ByteUtilsTest, TestEmptyByteArrayCorrect) {
   ByteArray bytes;