The attached source code shows how you can save / retrieve “clovers” in / from a TTree (note: if you want a “non-zero-suppressed” TTree, add -D_ZERO_SUPPRESSION_LEVEL_=0 when compiling “canSort_new.cxx”).
It seems that ROOT 5.34 (using the ZLIB level 1 by default) achieves about three times better compression than ROOT 6.14 (using the LZ4 level 4 by default) without any significant difference in execution times so, with ROOT 6 one should always remember to enforce the compression algorithm and its level (which is usually not needed with ROOT 5), e.g.:
f = new TFile(hisfile, "RECREATE", "", 101); // 101 = 100 * ROOT::kZLIB + 1
One can further improve the compression by another factor two if one enforces LZMA (unfortunately, this will also double the execution time but, it may really be worth to bear it), e.g.:
f = new TFile(hisfile, "RECREATE", "", 201); // 201 = 100 * ROOT::kLZMA + 1
Note: The “man lzma” says that the “compression preset level” can be “-0 … -9”. It seems to me that ROOT is unable to use the “-0” (i.e. the “–fast”) level. When one tries “compress = 200” (= 100 * ROOT::kLZMA + 0), one gets uncompressed ROOT files. Manual trials show that ROOT files compressed with “-0” are almost the same size as with “-1” but the time needed is around 20% shorter (at least). So, it would really make sense to modify ROOT so that it recognizes “compress = 200” as “LZMA @ -0” (or maybe one could set the “actual LZMA level” = “ROOT LZMA compression - 1”, so that “201” would be “-0”, “202” would be “-1” and so on).
canSort_new.cxx (45.6 KB)
canSortTTreeDump.cxx (2.8 KB)