Add Documentation.md #370
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Ngoguey42
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Nov 15, 2021
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| Further, keys must be hashable (with the hash represented as an int), and the implementation even requires that the user specify the number of bits in the hash that are relevant. | ||
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| FIXME There is a worrying comment regarding the key hash: "underestimation [of the number of relevant bits] will result in undefined behavior"; most code that uses hashes should still work (albeit very slowly) even if all hashes have the same value. So, this comment is a bit unusual. What happens if hashes collide? What do we do about this? |
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You must be referring to that part of the doc:
Lines 17 to 22 in fe5e962
hash_size is used by the fan to select the bits in the short hash of a keys, depending on the size of the fan.
Overestimating imply that the fan will refer to bits that don't carry informations (typically zeroes).
Underestimating should be fine in my opinion.
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Underestimating causes an undefined behaviour because that means the bits selection drops the MSBs, which breaks the order. Having the bit selection preserving the hash order is an important invariant to the fan-out.
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High-level doc of Index functioning