Zippy level 2 for denser compression and faster decompression

We also increased the hashtable size by 1 bit as it significantly degraded the ratio. Thus even level 1 might slightly improve.

PiperOrigin-RevId: 621456036

snappy.h

@@ -50,13 +50,33 @@ namespace snappy {
   class Source;
   class Sink;
 
+  struct CompressionOptions {
+    // Compression level.
+    // Level 1 is the fastest
+    // Level 2 is a little slower but provides better compression. Level 2 is
+    // **EXPERIMENTAL** for the time being. It might happen that we decide to
+    // fall back to level 1 in the future.
+    // Levels 3+ are currently not supported. We plan to support levels up to
+    // 9 in the future.
+    // If you played with other compression algorithms, level 1 is equivalent to
+    // fast mode (level 1) of LZ4, level 2 is equivalent to LZ4's level 2 mode
+    // and compresses somewhere around zstd:-3 and zstd:-2 but generally with
+    // faster decompression speeds than snappy:1 and zstd:-3.
+    int level = DefaultCompressionLevel();
+
+    static constexpr int MinCompressionLevel() { return 1; }
+    static constexpr int MaxCompressionLevel() { return 2; }
+    static constexpr int DefaultCompressionLevel() { return 1; }
+  };
+
   // ------------------------------------------------------------------------
   // Generic compression/decompression routines.
   // ------------------------------------------------------------------------
 
-  // Compress the bytes read from "*source" and append to "*sink". Return the
+  // Compress the bytes read from "*reader" and append to "*writer". Return the
   // number of bytes written.
-  size_t Compress(Source* source, Sink* sink);
+  size_t Compress(Source* reader, Sink* writer,
+                  CompressionOptions options = {});
 
   // Find the uncompressed length of the given stream, as given by the header.
   // Note that the true length could deviate from this; the stream could e.g.
@@ -76,14 +96,15 @@ namespace snappy {
   //
   // REQUIRES: "input[]" is not an alias of "*compressed".
   size_t Compress(const char* input, size_t input_length,
-                  std::string* compressed);
+                  std::string* compressed, CompressionOptions options = {});
 
   // 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);
+                           std::string* compressed,
+                           CompressionOptions options = {});
 
   // Decompresses "compressed[0..compressed_length-1]" to "*uncompressed".
   // Original contents of "*uncompressed" are lost.
@@ -126,16 +147,15 @@ namespace snappy {
   //    RawCompress(input, input_length, output, &output_length);
   //    ... Process(output, output_length) ...
   //    delete [] output;
-  void RawCompress(const char* input,
-                   size_t input_length,
-                   char* compressed,
-                   size_t* compressed_length);
+  void RawCompress(const char* input, size_t input_length, char* compressed,
+                   size_t* compressed_length, CompressionOptions options = {});
 
   // 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);
+                            char* compressed, size_t* compressed_length,
+                            CompressionOptions options = {});
 
   // Given data in "compressed[0..compressed_length-1]" generated by
   // calling the Snappy::Compress routine, this routine
@@ -215,7 +235,7 @@ namespace snappy {
   static constexpr int kMinHashTableBits = 8;
   static constexpr size_t kMinHashTableSize = 1 << kMinHashTableBits;
 
-  static constexpr int kMaxHashTableBits = 14;
+  static constexpr int kMaxHashTableBits = 15;
   static constexpr size_t kMaxHashTableSize = 1 << kMaxHashTableBits;
 }  // end namespace snappy