I am exploring options to Generate 9 digit unique number without using database
I have to run same application in multiple machines
if I use RNGCryptoServiceProvider to generate number what are the possibilities of generating same number in different machine?
Nine digit numbers range from 000000000 to 999999999. That is 1,000,000,000 possible values. Your chances of a duplicate will be 1 in 1,000,000,000 for the second number picked. 2 in 1,000,000,000 for the third number picked and so on. How many numbers do you intend to pick?
Related
Given a list of deals of Klondike Solitaire that are known to win, is there a way to store a reasonable amount of deals (say 10,000+) in a reasonable amount of space (say 5MB) to retrieve on command? (These numbers are arbitrary)
I thought of using a pseudo random generator where a given seed would generate a decimal string of numbers, where each two digits represents a card, and the index represents the location of the deal. In this case, you would only have to store the seed and the PRG code.
The only cons I can think of would be that A) the number of possible deals is 52!, and so the number of possible seeds would be at least 52!, and would be monstrous to store in the higher number range, and B) the generated number can't repeat a two digit number (though they can be ignored in the deck construction)
Given no prior information, the theoretical limit on how compactly you can represent an ordered deck of cards is 226 bits. Even the simple naive 6-bits-per card is only 312 bits, so you probably won't gain much by being clever.
If you're willing to sacrifice a large part of the state-space, you could use a 32- or 64-bit PRNG to generate the decks, and then you could reproduce them from the 32- or 64-bit initial PRNG state. But that limits you to 2^64 different decks out of the possible 2^225+.
If you are asking hypothetically, I would say that you would need at least 3.12 MB to store 10,000 possible deals. You need 6 bits to represent each card (assuming you number them 1-52) and then you would need to order them so 6 * 52 = 312. Take that and multiply it by the number of deals 312 * 10,000 and you get 3,120,000 bits or 3.12 MB.
I have long integers numbers like this: 5291658276538691055
How could I programmatically convert this number to a 4-6 capital letters only that is a unique combination that can also be reversed to get back to the number?
For example using OBJ-C.
There are 26 capital letters;
6 of them could represent 26 ^ 6 numbers (308915776);
So, no. You are trying to map a much larger range of numbers into a much smaller range, it cannot be reversible.
Also, log 5291658276538691055 / log 26 is less than 14, so if 14 letters is good for you, just transform the number into 26-based and map the digits to letters.
And one more thing - if the range of numbers is small enough, you could do some manipulation on the numbers (e.g., just subtract the min) and encode it, which will cost you less digits.
You will need to convert the numbers to Base 26 (Hexavigesimal - snappy name!)
The Wikipedia article on Hexavigesimal gives example code in Java - you should be able to adapt this pretty easily.
NB: You cannot get the long number you mentioned down to 4-6 capital letters only using a conversion algorithm (your example in Base 26 is BCKSATKEBRYBXJ). If you need conversion that short, you only have two options:
Lookup tables (store mappings, e.g. 5291658276538691055 = ABCDEF). Obviously only useful if you have a discrete set of numbers.
Including additional characters (e.g. lower case + numbers).
I need to generate unique numbers and I can think of the consecutive way, for example, I can have a counter starting from 0, every time a unique number is needed I return the counter and increase the counter by 1, this simply works until I may have a lot of unique numbers which go beyond the range of the data type(say int), also, the generated unique numbers, for example, the counter is 10 but 4 and 5 are not used any more so they can be re-used, how do I make use of the reusable numbers without keeping all of the in a data structure?
Thanks!
Are you able to substitute numbers you've already handed out? If so then as soon as any number is returned, substitute the most recently handed out one with it and decrement the allocation counter. If it's the most recent one that's been returned then skip the substitution.
Otherwise, I guess the best you're going to be able to do is keep a sorted array of ranges.
To allocate a new number:
If the array is empty, create a new range and return the only number in it.
Otherwise, get the first range in the array and increase its length by 1. Return that number. Check whether that makes the first two ranges join up. If so then merge them into a single range.
To return a number:
Find the range it falls within (eg, by binary search; see NSOrderedSet if your deployment plans allow it). If the returned number is at either end of the range then just shrink the range. Otherwise split the one into two with the returned number as the hole.
Please comment and critique the approach.
Scenario: I have a large dataset(200 million entries) in a flat file. Data is of the form - a 10 digit phone number followed by 5-6 binary fields.
Every week I will be getting a Delta files which will only contain changes to the data.
Problem : Given a list of items i need to figure out whether each item(which will be the 10 digit number) is present in the dataset.
The approach I have planned :
Will parse the dataset and put it a DB(To be done at the start of the
week) like MySQL or Postgres. The reason i want to have RDBMS in the
first step is I want to have full time series data.
Then generate some kind of Key Value store out of this database with
the latest valid data which supports operation to find out whether
each item is present in the dataset or not(Thinking some kind of a
NOSQL db, like Redis here optimised for search. Should have
persistence and be distributed). This datastructure will be read-only.
Query this key value store to find out whether each item is present
(if possible match a list of values all at once instead of matching
one item at a time). Want this to be blazing fast. Will be using this functionality as the back-end to a REST API
Sidenote: Language of my preference is Python.
A few considerations for the fast lookup:
If you want to check a set of numbers at a time, you could use the Redis SINTER which performs set intersection.
You might benefit from using a grid structure by distributing number ranges over some hash function such as the first digit of the phone number (there are probably better ones, you have to experiment), this would e.g. reduce the size per node, when using an optimal hash, to near 20 million entries when using 10 nodes.
If you expect duplicate requests, which is quite likely, you could cache the last n requested phone numbers in a smaller set and query that one first.
I would like to use newId to generate random numbers. Usually you would use it just once, but I might be generating up to 10 random numbers per newId.
Is it random enough?
Usually you would use it just once, but I might be generating up to 10 random numbers per newId. Is it random enough?
It depends on how you extract the numbers from the newid. You cannot treat it as 128 independently random bits.
For example if you use the first 8 bits for generating one random number between 0 and 255, use the next 8 bits to generate another number, etc. then you will see that your numbers will not be uniformly random.
v
E058D654-35A8-47F2-AE40-1C4EEBBDC549
01461481-ED8D-4B85-90FA-C08621D98DAE
AE861E4E-3469-4BDB-A38B-0031DACC8DAE
AF8905D0-E41B-4300-94F2-33BB45698CD1
003308A6-AE0A-4E20-9F24-047A6955E748
76F9B7ED-79AB-4EB1-B361-8C0AF5177CE3
B8F1CAC0-591D-436B-BB21-FAAD9EECA983
7FBEAEFD-2163-4315-A783-8106909E47D8
85E2FC60-E7B3-400F-B20A-CEFBECAEE4F9
17ED0A03-ADAD-4521-97EE-04815A867B32
^
|
always 4
You should also try to avoid reusing the same bits to generate different random numbers as your numbers will become related. If in doubt, don't reuse the same number.
Note that there is also a RAND function which you can call. This returns numbers from a uniform distribution.
Yes, it's statistically random. It's simply a GUID.
How do you plan on generating 10 numbers from one seed though? CHECKSUM(NEWID()) is normally how you'd do it for one value, perhaps with modulo and ABS
NewID generates a GUID. It is random enough.
Random enough for what? When you say use it to generate, are you just going to use it to seed a PNRG? I'm not sure it's any better than a timestamp for that. Or are you going to extract bits from the GUID - that's a bad idea.
http://www.random.org/randomness/