Add snappy::CompressFromIOVec.

This reads from an `iovec` array rather than from a `char` array as in `snappy::Compress`.

PiperOrigin-RevId: 476930623

snappy.h

@@ -71,14 +71,21 @@ namespace snappy {
   // Higher-level string based routines (should be sufficient for most users)
   // ------------------------------------------------------------------------
 
-  // Sets "*compressed" to the compressed version of "input[0,input_length-1]".
+  // Sets "*compressed" to the compressed version of "input[0..input_length-1]".
   // Original contents of *compressed are lost.
   //
   // REQUIRES: "input[]" is not an alias of "*compressed".
   size_t Compress(const char* input, size_t input_length,
                   std::string* compressed);
 
-  // Decompresses "compressed[0,compressed_length-1]" to "*uncompressed".
+  // Same as `Compress` above but taking an `iovec` array as input. Note that
+  // this function preprocesses the inputs to compute the sum of
+  // `iov[0..iov_cnt-1].iov_len` before reading. To avoid this, use
+  // `RawCompressFromIOVec` below.
+  size_t CompressFromIOVec(const struct iovec* iov, size_t iov_cnt,
+                           std::string* compressed);
+
+  // Decompresses "compressed[0..compressed_length-1]" to "*uncompressed".
   // Original contents of "*uncompressed" are lost.
   //
   // REQUIRES: "compressed[]" is not an alias of "*uncompressed".
@@ -124,6 +131,12 @@ namespace snappy {
                    char* compressed,
                    size_t* compressed_length);
 
+  // Same as `RawCompress` above but taking an `iovec` array as input. Note that
+  // `uncompressed_length` is the total number of bytes to be read from the
+  // elements of `iov` (_not_ the number of elements in `iov`).
+  void RawCompressFromIOVec(const struct iovec* iov, size_t uncompressed_length,
+                            char* compressed, size_t* compressed_length);
+
   // Given data in "compressed[0..compressed_length-1]" generated by
   // calling the Snappy::Compress routine, this routine
   // stores the uncompressed data to