template specialized Hash functions for 16-bit hash types (#4631)

added access to HashFunction for 16 bit

include/flatbuffers/hash.h

@@ -57,13 +57,28 @@ template<typename T> T HashFnv1a(const char *input) {
   return hash;
 }
 
-template<typename T> struct NamedHashFunction {
+template <> inline uint16_t HashFnv1<uint16_t>(const char *input) {
+  uint32_t hash = HashFnv1<uint32_t>(input);
+  return (hash >> 16) ^ (hash & 0xffff);
+}
+
+template <> inline uint16_t HashFnv1a<uint16_t>(const char *input) {
+  uint32_t hash = HashFnv1a<uint32_t>(input);
+  return (hash >> 16) ^ (hash & 0xffff);
+}
+
+template <typename T> struct NamedHashFunction {
   const char *name;
 
   typedef T (*HashFunction)(const char *);
   HashFunction function;
 };
 
+const NamedHashFunction<uint16_t> kHashFunctions16[] = {
+  { "fnv1_16",  HashFnv1<uint16_t> },
+  { "fnv1a_16", HashFnv1a<uint16_t> },
+};
+
 const NamedHashFunction<uint32_t> kHashFunctions32[] = {
   { "fnv1_32", HashFnv1<uint32_t> },
   { "fnv1a_32", HashFnv1a<uint32_t> },
@@ -74,6 +89,17 @@ const NamedHashFunction<uint64_t> kHashFunctions64[] = {
   { "fnv1a_64", HashFnv1a<uint64_t> },
 };
 
+inline NamedHashFunction<uint16_t>::HashFunction FindHashFunction16(
+    const char *name) {
+  std::size_t size = sizeof(kHashFunctions16) / sizeof(kHashFunctions16[0]);
+  for (std::size_t i = 0; i < size; ++i) {
+    if (std::strcmp(name, kHashFunctions16[i].name) == 0) {
+      return kHashFunctions16[i].function;
+    }
+  }
+  return nullptr;
+}
+
 inline NamedHashFunction<uint32_t>::HashFunction FindHashFunction32(
     const char *name) {
   std::size_t size = sizeof(kHashFunctions32) / sizeof(kHashFunctions32[0]);

src/flathash.cpp

@@ -26,8 +26,15 @@ int main(int argc, char *argv[]) {
   const char *name = argv[0];
   if (argc <= 1) {
     printf("%s HASH [OPTION]... STRING... [-- STRING...]\n", name);
-    printf("Available hashing algorithms:\n  32 bit:\n");
+    printf("Available hashing algorithms:\n");
-    size_t size = sizeof(flatbuffers::kHashFunctions32) /
+    printf("  16 bit:\n");
+    size_t size = sizeof(flatbuffers::kHashFunctions16) /
+                  sizeof(flatbuffers::kHashFunctions16[0]);
+    for (size_t i = 0; i < size; ++i) {
+      printf("    * %s\n", flatbuffers::kHashFunctions16[i].name);
+    }
+    printf("  32 bit:\n");
+    size = sizeof(flatbuffers::kHashFunctions32) /
                   sizeof(flatbuffers::kHashFunctions32[0]);
     for (size_t i = 0; i < size; ++i) {
       printf("    * %s\n", flatbuffers::kHashFunctions32[i].name);
@@ -48,12 +55,14 @@ int main(int argc, char *argv[]) {
 
   const char *hash_algorithm = argv[1];
 
+  flatbuffers::NamedHashFunction<uint16_t>::HashFunction hash_function16 =
+      flatbuffers::FindHashFunction16(hash_algorithm);
   flatbuffers::NamedHashFunction<uint32_t>::HashFunction hash_function32 =
       flatbuffers::FindHashFunction32(hash_algorithm);
   flatbuffers::NamedHashFunction<uint64_t>::HashFunction hash_function64 =
       flatbuffers::FindHashFunction64(hash_algorithm);
 
-  if (!hash_function32 && !hash_function64) {
+  if (!hash_function16 && !hash_function32 && !hash_function64) {
     printf("\"%s\" is not a known hash algorithm.\n", hash_algorithm);
     return 0;
   }
@@ -87,7 +96,9 @@ int main(int argc, char *argv[]) {
         ss << std::hex;
         ss << "0x";
       }
-      if (hash_function32)
+      if (hash_function16)
+        ss << hash_function16(arg);
+      else if (hash_function32)
         ss << hash_function32(arg);
       else if (hash_function64)
         ss << hash_function64(arg);

src/idl_parser.cpp

@@ -681,6 +681,13 @@ CheckedError Parser::ParseField(StructDef &struct_def) {
   auto hash_name = field->attributes.Lookup("hash");
   if (hash_name) {
     switch ((type.base_type == BASE_TYPE_VECTOR) ? type.element : type.base_type) {
+      case BASE_TYPE_SHORT:
+      case BASE_TYPE_USHORT: {
+        if (FindHashFunction16(hash_name->constant.c_str()) == nullptr)
+          return Error("Unknown hashing algorithm for 16 bit types: " +
+                       hash_name->constant);
+        break;
+      }
       case BASE_TYPE_INT:
       case BASE_TYPE_UINT: {
         if (FindHashFunction32(hash_name->constant.c_str()) == nullptr)
@@ -697,7 +704,7 @@ CheckedError Parser::ParseField(StructDef &struct_def) {
       }
       default:
         return Error(
-            "only int, uint, long and ulong data types support hashing.");
+            "only short, ushort, int, uint, long and ulong data types support hashing.");
     }
   }
   auto cpp_type = field->attributes.Lookup("cpp_type");
@@ -1278,6 +1285,18 @@ CheckedError Parser::ParseHash(Value &e, FieldDef *field) {
   assert(field);
   Value *hash_name = field->attributes.Lookup("hash");
   switch (e.type.base_type) {
+    case BASE_TYPE_SHORT: {
+      auto hash = FindHashFunction16(hash_name->constant.c_str());
+      int16_t hashed_value = static_cast<int16_t>(hash(attribute_.c_str()));
+      e.constant = NumToString(hashed_value);
+      break;
+    }
+    case BASE_TYPE_USHORT: {
+      auto hash = FindHashFunction16(hash_name->constant.c_str());
+      uint16_t hashed_value = hash(attribute_.c_str());
+      e.constant = NumToString(hashed_value);
+      break;
+    }
     case BASE_TYPE_INT: {
       auto hash = FindHashFunction32(hash_name->constant.c_str());
       int32_t hashed_value = static_cast<int32_t>(hash(attribute_.c_str()));