Bugfix: ferite hamt did not handle hash collision#5
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cention-nazri wants to merge 9 commits intodarkrock:masterfrom
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Bugfix: ferite hamt did not handle hash collision#5cention-nazri wants to merge 9 commits intodarkrock:masterfrom
cention-nazri wants to merge 9 commits intodarkrock:masterfrom
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added 9 commits
August 13, 2014 10:45
The goal is to be able to do something like this: $ make check If something breaks $ prove test
If the need arises the we should choose a more mature tap producer [1]. [1] http://en.wikipedia.org/wiki/Test_Anything_Protocol#List_of_TAP_producers
Buggy behavior is shown by ferite_amt-hash-index-from-hash.c
The buggy code was:
return (((index << (shiftAmount - AMT_SHIFT_AMOUNT)) >> 27) & 0x1F);
where shiftAmount is unsigned int and AMT_SHIFT_AMOUNT is 5.
The function was called successively with shiftAmount set to 32, 27, 22,
17, 12, 7, 2 (minus AMT_SHIFT_AMOUNT each time).
When shiftAmount is less than AMT_SHIFT_AMOUNT,
shiftAmount - AMT_SHIFT_AMOUNT
becomes:
(unsigned int)(2) - 5
which is a small unsigned integer that underflow to a very large
unsigned integer hence the buggy behavior.
Note that this bug has no effect on the behavior of the AMT as the value
is expected to be any value within 0-31 - the only visible effect is in
the actual placement of the hash items in memory.
This function is for when the hash bits are exhausted - the case when the hashes of two different keys collides.
Upon hash bits exhaustion we use the bits from the key. While this is not an optimal source of bits due to the non-random nature of the ascii characters in the keys which results in non optimal use of RAM, it should be good enough for use here under the assumption that hash collisions are rare.
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The bug is reproducible as follows:
The hamt code tries to cache the two functions using the following keys (note the colliding uint32 hashes):