[erlang-questions] Pattern matching of literal small/big/bignum integers and binary search
Sun Jun 16 10:27:58 CEST 2019
I have some questions about how pattern matches on integer literals are
evaluated by the emulator.
As an example, say I have the following function which expects an integer
that's a power of 2 and returns corresponding position of the lone "on"
single_cardinalty_to_pos(2#1) -> 1;
single_cardinalty_to_pos(2#10) -> 2;
single_cardinalty_to_pos(2#100) -> 3;
single_cardinalty_to_pos(2#100...0) -> 128.
I'm trying to gauge whether or not I've correctly understood the resultant
select_val BEAM assembly instruction w.r.t gen_select_val  in
beam_load.c (without even being at all certain that gen_select_val is even
I could use some help to determine if the following assumptions are
A) That rather than brute forcing, the emulator will perform a binary
search  for the function argument against all the literals from all the
clauses which is the purpose of quick-sorting the select_val list, and as
B) If I want to increase the upper limit that the function can handle,
I can simply add more clauses rather than trying to implement a binary
search for the "on" bit myself using various masks and bit shifts.
C) That the emulator will be smart about whether on not the function
argument is an 8 bit, 32/64 bit integer versus a bignum integer and thus
know what candidates can be excluded from the binary search so there's no
benefit to splitting the function to handle different ranges.
D) That it's not unreasonable in this case to have even say, 1024
clauses in order to handle bignum integers of up to 1024 bits. The only
downside will be longer compile times and possibly longer loading of the
If these assumptions are correct, then I could have the emulator do a lot
of work for me with a number of similar functions and avoid writing a lot
Thanks in advance.
- Edmond -
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