Urban Hafner wrote:
Hej,
I need to create random bits and XOR them together. For creating random
numbers there is OS.rand so I at first figured I'd use that and then use
and xor function on them. But there doesn't seem to be such a function.
Then I thought I could use BitStrings. There is a bitwise or for
bitstrings, but I don't know if it is exclusive or inclusive. And then
there is the problem with generating random bitstrings.
BitString.disj implements inclusive or. Another undocumented feature is
that indices start at 0 (stupid choice IMHO).
Here is an idea for generating random bitstring. I assume that digits 0
and 1 have same probability. I determine the indices of digits 1 in the
bitstring by filtering the list of indices, randomly choosing to keep
them or not.
Cheers,
raph
%% returns true or false (each with probability .5)
fun {RandomCond _} {OS.rand} mod 2 > 0 end
%% returns a random bitstring of length I
fun {RandomBitString I}
{BitString.make I {Filter {List.number 0 I-1 1} RandomCond}}
end
%% bitwise XOR between bitstrings
fun {BitwiseXOR BS1 BS2}
%% a xor b == (a and (not b)) or ((not a) and b)
{BitString.disj
{BitString.conj BS1 {BitString.nega BS2}}
{BitString.conj {BitString.nega BS1} BS2}}
end
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