fix: One Simple Trick to significantly improve the speed and memory usage of Occ #448
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Daniel-Liu-c0deb0t
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Occ wastes a lot of memory because it reserves memory for all alphabets up to the one with the max rank. Without compression, rank is based on the ASCII value, so this means that a small alphabet like DNA nucleotides would take ~10x more memory than necessary. The FMD index also does not support compressed alphabets, so there is no easy way to sidestep this issue. The fix is simple: transpose the Occ vector so that the first dimension is the alphabet and the second dimension is the sampled Occ values. Then, for characters not in the alphabet, the second dimension can simply be empty, and no space would be wasted. This should massively improve speed and memory usage.
This will need to be rebased after #447 is merged.