1 files changed, 31 insertions, 10 deletions

diff --git a/absl/strings/internal/cord_internal.h b/absl/strings/internal/cord_internal.h
index 0300ac4..bfe5564 100644
--- a/absl/strings/internal/cord_internal.h
+++ b/absl/strings/internal/cord_internal.h
@@ -87,6 +87,9 @@ class RefcountAndFlags {
   constexpr RefcountAndFlags() : count_{kRefIncrement} {}
   struct Immortal {};
   explicit constexpr RefcountAndFlags(Immortal) : count_(kImmortalFlag) {}
+  struct WithCrc {};
+  explicit constexpr RefcountAndFlags(WithCrc)
+      : count_(kCrcFlag | kRefIncrement) {}
 
   // Increments the reference count. Imposes no memory ordering.
   inline void Increment() {
@@ -122,14 +125,32 @@ class RefcountAndFlags {
     return count_.load(std::memory_order_acquire) >> kNumFlags;
   }
 
-  // Returns whether the atomic integer is 1.
-  // If the reference count is used in the conventional way, a
-  // reference count of 1 implies that the current thread owns the
-  // reference and no other thread shares it.
-  // This call performs the test for a reference count of one, and
-  // performs the memory barrier needed for the owning thread
-  // to act on the object, knowing that it has exclusive access to the
-  // object.  Always returns false when the immortal bit is set.
+  // Returns true if the referenced object carries a CRC value.
+  bool HasCrc() const {
+    return (count_.load(std::memory_order_relaxed) & kCrcFlag) != 0;
+  }
+
+  // Returns true iff the atomic integer is 1 and this node does not store
+  // a CRC.  When both these conditions are met, the current thread owns
+  // the reference and no other thread shares it, so its contents may be
+  // safely mutated.
+  //
+  // If the referenced item is shared, carries a CRC, or is immortal,
+  // it should not be modified in-place, and this function returns false.
+  //
+  // This call performs the memory barrier needed for the owning thread
+  // to act on the object, so that if it returns true, it may safely
+  // assume exclusive access to the object.
+  inline bool IsMutable() {
+    return (count_.load(std::memory_order_acquire)) == kRefIncrement;
+  }
+
+  // Returns whether the atomic integer is 1.  Similar to IsMutable(),
+  // but does not check for a stored CRC.  (An unshared node with a CRC is not
+  // mutable, because changing its data would invalidate the CRC.)
+  //
+  // When this returns true, there are no other references, and data sinks
+  // may safely adopt the children of the CordRep.
   inline bool IsOne() {
     return (count_.load(std::memory_order_acquire) & kRefcountMask) ==
            kRefIncrement;
@@ -149,14 +170,14 @@ class RefcountAndFlags {
     kNumFlags = 2,
 
     kImmortalFlag = 0x1,
-    kReservedFlag = 0x2,
+    kCrcFlag = 0x2,
     kRefIncrement = (1 << kNumFlags),
 
     // Bitmask to use when checking refcount by equality.  This masks out
     // all flags except kImmortalFlag, which is part of the refcount for
     // purposes of equality.  (A refcount of 0 or 1 does not count as 0 or 1
     // if the immortal bit is set.)
-    kRefcountMask = ~kReservedFlag,
+    kRefcountMask = ~kCrcFlag,
   };
 
   std::atomic<int32_t> count_;