Refactor internal time zone database (#10572)

Summary:
Pull Request resolved: #10572

Refactoring the internal time zone database to prepare for the next
PR. Moving map from ID to string to vector, to remove the map access
for the hot path. Also re-organizing the code to allow them to
reference each other.

Reviewed By: mbasmanova

Differential Revision: D60211961

velox/type/tz/TimeZoneDatabase.cpp

@@ -30,12 +30,12 @@
 
 namespace facebook::velox::tz {
 
-const std::unordered_map<int64_t, std::string>& getTimeZoneDB() {
-  static auto* tzDB = new std::unordered_map<int64_t, std::string>([] {
+const std::vector<std::pair<int16_t, std::string>>& getTimeZoneEntries() {
+  static auto* tzDB = new std::vector<std::pair<int16_t, std::string>>([] {
     // Work around clang compiler bug causing multi-hour compilation
     // with -fsanitize=fuzzer
     // https://github.com/llvm/llvm-project/issues/75666
-    std::vector<std::pair<int64_t, std::string>> entries = {
+    return std::vector<std::pair<int16_t, std::string>>{
         {0, "+00:00"},
         {1, "-14:00"},
         {2, "-13:59"},
@@ -2266,8 +2266,6 @@ const std::unordered_map<int64_t, std::string>& getTimeZoneDB() {
         {2232, "Europe/Kyiv"},
         {2233, "America/Ciudad_Juarez"},
     };
-    return std::unordered_map<int64_t, std::string>(
-        entries.begin(), entries.end());
   }());
   return *tzDB;
 }

velox/type/tz/TimeZoneMap.cpp

@@ -26,23 +26,56 @@
 namespace facebook::velox::tz {
 
 // Defined in TimeZoneDatabase.cpp
-extern const std::unordered_map<int64_t, std::string>& getTimeZoneDB();
+extern const std::vector<std::pair<int16_t, std::string>>& getTimeZoneEntries();
+
+// TODO: The string will be moved to TimeZone in the next PR.
+using TTimeZoneDatabase = std::vector<std::unique_ptr<std::string>>;
+using TTimeZoneIndex = folly::F14FastMap<std::string, int16_t>;
 
 namespace {
 
-folly::F14FastMap<std::string, int64_t> makeReverseMap(
-    const std::unordered_map<int64_t, std::string>& map) {
-  folly::F14FastMap<std::string, int64_t> reversed;
-  reversed.reserve(map.size() + 1);
+// Flattens the input vector of pairs into a vector, assuming that the
+// timezoneIDs are (mostly) sequential. Note that since they are "mostly"
+// senquential, the vector can have holes. But it is still more efficient than
+// looking up on a map.
+TTimeZoneDatabase buildTimeZoneDatabase(
+    const std::vector<std::pair<int16_t, std::string>>& dbInput) {
+  TTimeZoneDatabase tzDatabase;
+  tzDatabase.resize(dbInput.back().first + 1);
+
+  for (const auto& entry : dbInput) {
+    tzDatabase[entry.first] = std::make_unique<std::string>(entry.second);
+  }
+  return tzDatabase;
+}
+
+const TTimeZoneDatabase& getTimeZoneDatabase() {
+  static TTimeZoneDatabase timeZoneDatabase =
+      buildTimeZoneDatabase(getTimeZoneEntries());
+  return timeZoneDatabase;
+}
+
+// Reverses the vector of pairs into a map key'ed by the timezone name for
+// reverse look ups.
+TTimeZoneIndex buildTimeZoneIndex(const TTimeZoneDatabase& tzDatabase) {
+  TTimeZoneIndex reversed;
+  reversed.reserve(tzDatabase.size() + 1);
 
-  for (const auto& entry : map) {
-    reversed.emplace(
-        boost::algorithm::to_lower_copy(entry.second), entry.first);
+  for (int16_t i = 0; i < tzDatabase.size(); ++i) {
+    if (tzDatabase[i] != nullptr) {
+      reversed.emplace(boost::algorithm::to_lower_copy(*tzDatabase[i]), i);
+    }
   }
   reversed.emplace("utc", 0);
   return reversed;
 }
 
+const TTimeZoneIndex& getTimeZoneIndex() {
+  static TTimeZoneIndex timeZoneIndex =
+      buildTimeZoneIndex(getTimeZoneDatabase());
+  return timeZoneIndex;
+}
+
 inline bool isDigit(char c) {
   return c >= '0' && c <= '9';
 }
@@ -111,28 +144,37 @@ std::string normalizeTimeZone(const std::string& originalZoneId) {
 } // namespace
 
 std::string getTimeZoneName(int64_t timeZoneID) {
-  const auto& tzDB = getTimeZoneDB();
-  auto it = tzDB.find(timeZoneID);
-  VELOX_CHECK(
-      it != tzDB.end(), "Unable to resolve timeZoneID '{}'", timeZoneID);
-  return it->second;
+  const auto& timeZoneDatabase = getTimeZoneDatabase();
+
+  VELOX_CHECK_LT(
+      timeZoneID,
+      timeZoneDatabase.size(),
+      "Unable to resolve timeZoneID '{}'",
+      timeZoneID);
+
+  // Check if timeZoneID is not one of the "holes".
+  VELOX_CHECK_NOT_NULL(
+      timeZoneDatabase[timeZoneID],
+      "Unable to resolve timeZoneID '{}'",
+      timeZoneID);
+  return *timeZoneDatabase[timeZoneID];
 }
 
 int16_t getTimeZoneID(std::string_view timeZone, bool failOnError) {
-  static folly::F14FastMap<std::string, int64_t> nameToIdMap =
-      makeReverseMap(getTimeZoneDB());
+  const auto& timeZoneIndex = getTimeZoneIndex();
+
   std::string timeZoneLowered;
   boost::algorithm::to_lower_copy(
       std::back_inserter(timeZoneLowered), timeZone);
 
-  auto it = nameToIdMap.find(timeZoneLowered);
-  if (it != nameToIdMap.end()) {
+  auto it = timeZoneIndex.find(timeZoneLowered);
+  if (it != timeZoneIndex.end()) {
     return it->second;
   }
 
   // If an exact match wasn't found, try to normalize the timezone name.
-  it = nameToIdMap.find(normalizeTimeZone(timeZoneLowered));
-  if (it != nameToIdMap.end()) {
+  it = timeZoneIndex.find(normalizeTimeZone(timeZoneLowered));
+  if (it != timeZoneIndex.end()) {
     return it->second;
   }
   if (failOnError) {

velox/type/tz/gen_timezone_database.py

@@ -52,16 +52,14 @@
 
 namespace facebook::velox::util {
 
-const std::unordered_map<int64_t, std::string>& getTimeZoneDB() {
-  static auto* tzDB = new std::unordered_map<int64_t, std::string>([] {
+const std::vector<std::pair<int16_t, std::string>>& getTimeZoneEntries() {
+  static auto* tzDB = new std::vector<std::pair<int16_t, std::string>>([] {
     // Work around clang compiler bug causing multi-hour compilation
     // with -fsanitize=fuzzer
     // https://github.com/llvm/llvm-project/issues/75666
-    std::vector<std::pair<int64_t, std::string>> entries = {
+    return std::vector<std::pair<int16_t, std::string>>{
 $entries
     };
-    return std::unordered_map<int64_t, std::string>(
-        entries.begin(), entries.end());
   }());
   return *tzDB;
 }

velox/type/tz/tests/TimeZoneMapTest.cpp

@@ -41,6 +41,7 @@ TEST(TimeZoneMapTest, getTimeZoneID) {
   EXPECT_EQ(0, getTimeZoneID("UTC"));
   EXPECT_EQ(0, getTimeZoneID("GMT"));
   EXPECT_EQ(0, getTimeZoneID("Z"));
+  EXPECT_EQ(0, getTimeZoneID("z"));
   EXPECT_EQ(0, getTimeZoneID("greenwich"));
   EXPECT_EQ(0, getTimeZoneID("ETC/GMT"));
   EXPECT_EQ(0, getTimeZoneID("ETC/GMT0"));