Random number |
A Black Box algorithm supposes that natural
number sequence
is sorted
in non-descending order,
and for each p
(
)
an inequality
is valid.
Making tests for this algorithm we have met with the following problem.
For setting a random sequence
a usual random data generator did not fit. As the sequence
itself had been imposed certain
restrictions, the method of choosing the next random element
(in the interval defined by
restrictions) did not give the random sequence as a whole.
We have come to a conclusion that the problem can be solved in the following
way. If we arrange
all possible sequences in certain order (for example, in lexicographical
order) and assign each
sequence its number, after choice of the random number it is possible to
take the correspondent
sequence for the random one. At the first glance it seems enough to
make up a program generating
all these sequences in such order. Alas! Even having not great values
of M and N it would have
taken any powerful modern computer centuries to enumerate all such
sequences. It turned out it was
possible to avoid generating all sequences if we managed to create
required sequence according to
its number immediately. But even this statement does not cover all.
As the amount of sequences is
quite large, the number can be a long one, composed of hundreds decimal
digits, though our
random data generator could give only normal numbers. We decided to
produce a long random
number from a real random number distributed in [0,1]. Namely, present
the number in binary
notation:
,
where all b(i) = 0 or 1. Let us set a
regulation to associate such real number
to an integer from [A,B] segment:
Here we suppose, that ,
,
and ``div 2" is an integer
division by 2.
Let M, N (
)
and a binary real number
(
)
be given.
Write a program to find out the corresponding
sequence, i.e. to find a sequence
with
number in lexicographical order of all
possible
for the given
M and N (T is the quantity of such sequences). Numeration begins with 1.
Keep in mind that in
lexicographical order
proceeds
if after omitting equal
beginnings, the first number of
tail is smaller than the first number or
tail.
Following example illustrates the list of all possible
sequences for M = 4 and N = 3 in lexicographical order.
1, 2, 3 1, 2, 4 1, 3, 3 1, 3, 4 1, 4, 4 2, 2, 3 2, 2, 4 2, 3, 3 2, 3, 4 2, 4, 4 3, 3, 3 3, 3, 4 3, 4, 4 4, 4, 4
(here T=14)
4 3 0.01101101011110010001101010001011010
2 2 4