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fix: remove unnecessary hash calculations #8

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jrandolf wants to merge 1 commit from patch-1 into master
pull from: patch-1
merge into: lezer:master
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jrandolf commented 2021-03-19 00:12:57 +01:00 (Migrated from github.com)
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marijnh commented 2021-03-19 07:53:44 +01:00 (Migrated from github.com)
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What's the reasoning behind this change? The extra shift and add won't actually be expensive, and I've often seen this pattern (with a bitshift on both values) in simple hash functions.

What's the reasoning behind this change? The extra shift and add won't actually be expensive, and I've often seen this pattern (with a bitshift on both values) in simple hash functions.
jrandolf commented 2021-03-19 09:53:38 +01:00 (Migrated from github.com)
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I don't see the reason why both values are bit shifted. The depth is already bit shifted (<< 0) and the parent just bit shifts to allocate for the maximal depth (in this case, 2^8).

[Edit]: We also need to modulo the parent hash with some prime to truncate for later, but this hash holds for at least 16 or so indents without overflowing.

I don't see the reason why both values are bit shifted. The depth is already bit shifted (<< 0) and the parent just bit shifts to allocate for the maximal depth (in this case, 2^8). [Edit]: We also need to modulo the parent hash with some prime to truncate for later, but this hash holds for at least 16 or so indents without overflowing.
marijnh commented 2021-03-19 10:31:00 +01:00 (Migrated from github.com)
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I don't know a lot about hash functions, but running a simulation on hashing groups of small integers this change increases the amounts of clashes by about 30%, so I don't think it's a good idea.

I don't know a lot about hash functions, but running a simulation on hashing groups of small integers this change increases the amounts of clashes by about 30%, so I don't think it's a good idea.

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