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134388ba6b
this patch makes btultra do 2 passes on the first block, the first one being dedicated to collecting statistics so that the 2nd pass is more accurate. It translates into a very small compression ratio gain : enwik7, level 20: blocks 4K : 2.142 -> 2.153 blocks 16K : 2.447 -> 2.457 blocks 64K : 2.716 -> 2.726 On the other hand, the cpu cost is doubled. The trade off looks bad. Though, that's ultimately a price to pay to reach better compression ratio. So it's only enabled when setting btultra.