pg_trgm Memory Allocation logic

In the pg_trgm module, within function generate_trgm, the memory for trigrams
is allocated as follows:

trg = (TRGM *) palloc(TRGMHDRSIZE + sizeof(trgm) * (slen / 2 + 1) *3);

I have been trying to understand why this is so because it seems to be
allocating more space than that is required.

The following table shows the palloced size [(slen / 2 + 1) *3] and the
actual trgm count for different string length.

slen    palloc size    actual trgm count
2    6        3
26    42        27
38    60        39

Can somebody please explain this to me.


I had tried changing the allocation to slen + 1 and it seemed to be working
without any problem.

trg = (TRGM *) palloc(TRGMHDRSIZE + sizeof(trgm) * (slen + 1)); 


Maybe I am missisng some scenarios. 

Any help would be appreciated.

Thank you,

Beena Emerson



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Beena Emerson

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