Provide BitField64 utility class.

src/utils.h

Index: src/utils.h
diff --git a/src/utils.h b/src/utils.h
index b2c2ff109847ee5bedec0d1848df61e01fcdc277..90d7f3cdc6eec3fb18e015e1c90b1f06e43bf256 100644
--- a/src/utils.h
+++ b/src/utils.h
@@ -242,42 +242,52 @@ inline int StrLength(const char* string) {
 // ----------------------------------------------------------------------------
 // BitField is a help template for encoding and decode bitfield with
 // unsigned content.
-template<class T, int shift, int size>
-class BitField {
+
+template<class T, int shift, int size, class U>
+class BitFieldBase {
  public:
-  // A uint32_t mask of bit field.  To use all bits of a uint32 in a
-  // bitfield without compiler warnings we have to compute 2^32 without
-  // using a shift count of 32.
-  static const uint32_t kMask = ((1U << shift) << size) - (1U << shift);
-  static const uint32_t kShift = shift;
-  static const uint32_t kSize = size;
+  // A type U mask of bit field.  To use all bits of a type U of x bits
+  // in a bitfield without compiler warnings we have to compute 2^x
+  // without using a shift count of x in the computation.
+  static const U kOne = static_cast<U>(1U);
+  static const U kMask = ((kOne << shift) << size) - (kOne << shift);
+  static const U kShift = shift;
+  static const U kSize = size;
 
   // Value for the field with all bits set.
   static const T kMax = static_cast<T>((1U << size) - 1);
 
   // Tells whether the provided value fits into the bit field.
   static bool is_valid(T value) {
-    return (static_cast<uint32_t>(value) & ~static_cast<uint32_t>(kMax)) == 0;
+    return (static_cast<U>(value) & ~static_cast<U>(kMax)) == 0;
   }
 
-  // Returns a uint32_t with the bit field value encoded.
-  static uint32_t encode(T value) {
+  // Returns a type U with the bit field value encoded.
+  static U encode(T value) {
     ASSERT(is_valid(value));
-    return static_cast<uint32_t>(value) << shift;
+    return static_cast<U>(value) << shift;
   }
 
-  // Returns a uint32_t with the bit field value updated.
-  static uint32_t update(uint32_t previous, T value) {
+  // Returns a type U with the bit field value updated.
+  static U update(U previous, T value) {
     return (previous & ~kMask) | encode(value);
   }
 
   // Extracts the bit field from the value.
-  static T decode(uint32_t value) {
+  static T decode(U value) {
     return static_cast<T>((value & kMask) >> shift);
   }
 };
 
 
+template<class T, int shift, int size>
+class BitField : public BitFieldBase<T, shift, size, uint32_t> { };
+
+
+template<class T, int shift, int size>
+class BitField64 : public BitFieldBase<T, shift, size, uint64_t> { };
+
+
 // ----------------------------------------------------------------------------
 // Hash function.