[core] Use the lz4 tool compression levels for LZ4

README/ReleaseNotes/v642/index.md

@@ -152,6 +152,7 @@ Replace `obj.Connect(signal, "TPyDispatcher", disp, "Dispatch()")` with
 ## I/O
 
 * Reading a collection without its dictionary no longer crashes when the elements hold a `std::string` or a `TString`. The emulated collection proxy relocated its elements with a raw memory copy when its buffer had to grow, which corrupts an object that points into itself, such as a `std::string` using the small string optimization; the invalid pointer was then freed when the object was destroyed. Such elements are now destroyed and reconstructed at the new location instead. This affected for instance a `std::vector<std::pair<std::string,double>>` read back without a dictionary.
+* The LZ4 compression levels now follow those of the `lz4` command line tool. Level 1 uses the fast compressor as before. Level 2 uses the faster high-compression mode introduced in LZ4 1.10.0, or the fast compressor with older LZ4 versions. Level 3 now uses the high-compression (HC) compressor, like levels 4 to 9. Previously, levels 1 to 3 all used the fast compressor and produced identical output. Files written with levels 2 and 3 can be read by older ROOT versions, since the on-disk format is unchanged.
 
 ## Core
 

core/lz4/src/ZipLZ4.cxx

@@ -77,7 +77,11 @@ void R__zipLZ4(int cxlevel, int *srcsize, const char *src, int *tgtsize, char *t
    if (cxlevel > 9) {
       cxlevel = 9;
    }
-   if (cxlevel >= 4) {
+   // Use the same compression levels as the lz4 command line tool: level 1 is the fast compressor, level 2 is the
+   // "medium" compressor added in LZ4 1.10.0 (or the fast compressor with older versions), and higher levels use
+   // the high-compression (HC) compressor with the given level.
+   const int minHCLevel = (LZ4_version >= 11000) ? 2 : 3;
+   if (cxlevel >= minHCLevel) {
       returnStatus = LZ4_compress_HC(src, &tgt[kHeaderSize], *srcsize, *tgtsize - kHeaderSize, cxlevel);
    } else {
       returnStatus = LZ4_compress_default(src, &tgt[kHeaderSize], *srcsize, *tgtsize - kHeaderSize);