[GitHub] [arrow] emkornfield commented on a change in pull request #10461: ARROW-12096: [C++]: Allows users to define arrow timestamp unit for Parquet INT96 timestamp

[GitBox <gi...@apache.org>]

emkornfield commented on a change in pull request #10461:
URL: https://github.com/apache/arrow/pull/10461#discussion_r646777158



##########
File path: cpp/src/parquet/types.h
##########
@@ -602,6 +602,49 @@ static inline int64_t Int96GetNanoSeconds(const parquet::Int96& i96) {
   return static_cast<int64_t>(days_since_epoch * kNanosecondsPerDay + nanoseconds);
 }
 
+// ARROW-12096
+static inline int64_t Int96GetMicroSeconds(const parquet::Int96& i96) {
+  // We do the computations in the unsigned domain to avoid unsigned behaviour
+  // on overflow.
+  uint64_t days_since_epoch =
+      i96.value[2] - static_cast<uint64_t>(kJulianToUnixEpochDays);
+  uint64_t nanoseconds = 0;
+  memcpy(&nanoseconds, &i96.value, sizeof(uint64_t));
+
+  uint64_t microseconds = nanoseconds/static_cast<uint64_t>(1000);
+
+  return static_cast<int64_t>(days_since_epoch * kMicrosecondsPerDay + microseconds);
+}
+
+// ARROW-12096
+static inline int64_t Int96GetMilliSeconds(const parquet::Int96& i96) {
+  // We do the computations in the unsigned domain to avoid unsigned behaviour
+  // on overflow.
+  uint64_t days_since_epoch =
+      i96.value[2] - static_cast<uint64_t>(kJulianToUnixEpochDays);
+  uint64_t nanoseconds = 0;
+  memcpy(&nanoseconds, &i96.value, sizeof(uint64_t));
+
+  uint64_t milliseconds = nanoseconds/static_cast<uint64_t>(1000000);
+
+  return static_cast<int64_t>(days_since_epoch * kMillisecondsPerDay + milliseconds);
+}
+
+// ARROW-12096
+static inline int64_t Int96GetSeconds(const parquet::Int96& i96) {
+  // We do the computations in the unsigned domain to avoid unsigned behaviour

Review comment:
       ```suggestion
     // We do the computations in the unsigned domain to avoid undefined behaviour
   ```




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