Chunk filters #122
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Chunk filters #122
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This implements a reasonable efficient way to avoid reading chunks that do not contain messages that the reader is not interested in. For each chunk where there is at least one write that includes a filter value a bloom filter will be added to the chunk immediately after the chunk header. The chunk filter is calculated from all the filter values in the chunk. If there are writes that contain no filter value the chunk filters first bit will also be set to indicate there are "unfiltered" entries in the chunk. At read time an offset reader can supply one or more filter values that are used to match against the bloom filter. If none of the read filter values matches the chunk body is not read and the reader moves on to the next chunk and so on. There is an option also to include "unfiltered" entries in the match evaluation. The bloom filter can be in the size range of 16-255 bytes with the default the smallest value at 16 bytes. This should be good for distinguishing between ~50 unique filter values. The bloom filter size is recorded in the first of the spare 4 bytes at the end of the chunk header.
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Optimise bloom filter hashing to hash only once and bit mangle to get two values out of it.