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SDIFF used an algorithm that was O(N) where N is the total number of elements of all the sets involved in the operation. The algorithm worked like that: ALGORITHM 1: 1) For the first set, add all the members to an auxiliary set. 2) For all the other sets, remove all the members of the set from the auxiliary set. So it is an O(N) algorithm where N is the total number of elements in all the sets involved in the diff operation. Cristobal Viedma suggested to modify the algorithm to the following: ALGORITHM 2: 1) Iterate all the elements of the first set. 2) For every element, check if the element also exists in all the other remaining sets. 3) Add the element to the auxiliary set only if it does not exist in any of the other sets. The complexity of this algorithm on the worst case is O(N*M) where N is the size of the first set and M the total number of sets involved in the operation. However when there are elements in common, with this algorithm we stop the computation for a given element as long as we find a duplicated element into another set. I (antirez) added an additional step to algorithm 2 to make it faster, that is to sort the set to subtract from the biggest to the smallest, so that it is more likely to find a duplicate in a larger sets that are checked before the smaller ones. WHAT IS BETTER? None of course, for instance if the first set is much larger than the other sets the second algorithm does a lot more work compared to the first algorithm. Similarly if the first set is much smaller than the other sets, the original algorithm will less work. So this commit makes Redis able to guess the number of operations required by each algorithm, and select the best at runtime according to the input received. However, since the second algorithm has better constant times and can do less work if there are duplicated elements, an advantage is given to the second algorithm. |
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| .. | ||
| type | ||
| aofrw.tcl | ||
| auth.tcl | ||
| basic.tcl | ||
| bitops.tcl | ||
| dump.tcl | ||
| expire.tcl | ||
| introspection.tcl | ||
| limits.tcl | ||
| maxmemory.tcl | ||
| multi.tcl | ||
| obuf-limits.tcl | ||
| other.tcl | ||
| printver.tcl | ||
| protocol.tcl | ||
| pubsub.tcl | ||
| quit.tcl | ||
| scripting.tcl | ||
| slowlog.tcl | ||
| sort.tcl | ||