Removes unused methods CRC::Empty() and CRC::Concat() from the internal

implementation.

PiperOrigin-RevId: 539749773
Change-Id: Iec83431ffd360a077b153cea00427580ae287d1f

absl/crc/internal/crc.cc

@@ -176,9 +176,6 @@ CRCImpl* CRCImpl::NewInternal() {
   return result;
 }
 
-// The CRC of the empty string is always the CRC polynomial itself.
-void CRCImpl::Empty(uint32_t* crc) const { *crc = kCrc32cPoly; }
-
 //  The 32-bit implementation
 
 void CRC32::InitTables() {
@@ -435,34 +432,6 @@ CRC* CRC::Crc32c() {
   return singleton;
 }
 
-// This Concat implementation works for arbitrary polynomials.
-void CRC::Concat(uint32_t* px, uint32_t y, size_t ylen) {
-  // https://en.wikipedia.org/wiki/Mathematics_of_cyclic_redundancy_checks
-  // The CRC of a message M is the remainder of polynomial division modulo G,
-  // where the coefficient arithmetic is performed modulo 2 (so +/- are XOR):
-  //   R(x) = M(x) x**n (mod G)
-  // (n is the degree of G)
-  // In practice, we use an initial value A and a bitmask B to get
-  //   R = (A ^ B)x**|M| ^ Mx**n ^ B (mod G)
-  // If M is the concatenation of two strings S and T, and Z is the string of
-  // len(T) 0s, then the remainder CRC(ST) can be expressed as:
-  //   R = (A ^ B)x**|ST| ^ STx**n ^ B
-  //     = (A ^ B)x**|SZ| ^ SZx**n ^ B ^ Tx**n
-  //     = CRC(SZ) ^ Tx**n
-  // CRC(Z) = (A ^ B)x**|T| ^ B
-  // CRC(T) = (A ^ B)x**|T| ^ Tx**n ^ B
-  // So R = CRC(SZ) ^ CRC(Z) ^ CRC(T)
-  //
-  // And further, since CRC(SZ) = Extend(CRC(S), Z),
-  //  CRC(SZ) ^ CRC(Z) = Extend(CRC(S) ^ CRC(''), Z).
-  uint32_t z;
-  uint32_t t;
-  Empty(&z);
-  t = *px ^ z;
-  ExtendByZeroes(&t, ylen);
-  *px = t ^ y;
-}
-
 }  // namespace crc_internal
 ABSL_NAMESPACE_END
 }  // namespace absl

absl/crc/internal/crc.h

@@ -40,9 +40,6 @@ class CRC {
  public:
   virtual ~CRC();
 
-  // Place the CRC of the empty string in "*crc"
-  virtual void Empty(uint32_t* crc) const = 0;
-
   // If "*crc" is the CRC of bytestring A, place the CRC of
   // the bytestring formed from the concatenation of A and the "length"
   // bytes at "bytes" into "*crc".
@@ -58,11 +55,6 @@ class CRC {
   // with those zero bytes removed.
   virtual void UnextendByZeroes(uint32_t* crc, size_t length) const = 0;
 
-  // If *px is the CRC (as defined by *crc) of some string X,
-  // and y is the CRC of some string Y that is ylen bytes long, set
-  // *px to the CRC of the concatenation of X followed by Y.
-  virtual void Concat(uint32_t* px, uint32_t y, size_t ylen);
-
   // Apply a non-linear transformation to "*crc" so that
   // it is safe to CRC the result with the same polynomial without
   // any reduction of error-detection ability in the outer CRC.

absl/crc/internal/crc_internal.h

@@ -70,8 +70,6 @@ class CRCImpl : public CRC {  // Implementation of the abstract class CRC
   // The internal version of CRC::New().
   static CRCImpl* NewInternal();
 
-  void Empty(uint32_t* crc) const override;
-
   // Fill in a table for updating a CRC by one word of 'word_size' bytes
   // [last_lo, last_hi] contains the answer if the last bit in the word
   // is set.