"Random-looking" sequence generator

Question

I can't think of a good way to do this, and would appreciate some help, if possible!

I'm afraid I don't have any code to post yet as I haven't got that far.

I need to generate a sequence of values from 3 (or possible more) parameters in the range 0-999.

The value must always be the same for the given inputs but with a fair distribution between upper and lower boundaries so as to appear random.

For example:

function (1, 1, 1) = 423 function (1, 1, 2) = 716 function (1, 2, 1) = 112

These must be reasonably fast to produce, by which I mean I should be able to generate 100-200 during web page load with no noticeable delay.

The method must be do-able in C# but also in JavaScript, otherwise I'd probably use a CRC32 or MD5 hash algorithm.

If it helps this will be used as part of a procedural generation routine.

I had a go at asking this previously, but I think the poor quality of my explanation let me down.

I apologise if this is worded badly. Please just let me know if so and I'll try to explain further.

Thanks very much for any help.

You could probably use the Random class whereby you supply a specific seed value which you would base off of the numbers coming in. Perhaps build a hash out of the 3 integer inputs to produce a new integer which you would then feed into your new Random(seed) instance. EDIT: ohhh JavaScript too... nevermind then! :) — Chris Sinclair
– Chris Sinclair, Commented Aug 12, 2013 at 13:30
Do you need the C# and JS to produce the same output for the same input? — Tim S.
– Tim S., Commented Aug 12, 2013 at 13:31
I understood your question... you need to do some research and actually try something though. Look into seeded random numbers. See here and an example for random numbers in a range — tnw
– tnw, Commented Aug 12, 2013 at 13:31
If the use you want to make is something like @ChrisSinclair explained, you can calculate for each number : number ^ positionand sum all the results. — m_vdbeek
– m_vdbeek, Commented Aug 12, 2013 at 13:33

Joni · Accepted Answer · 2013-08-12 13:35:51Z

5

Here's one:

function sequence(x, y, z) { return Math.abs(441*x-311*y+293*z) % 1000; }

It even produces the output from your example!

answered Aug 12, 2013 at 13:35

Joni

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2 Comments

Joni Over a year ago

Hehe, it's probably not the best hash function, I just wanted to match the example given ;)

CompanyDroneFromSector7G Over a year ago

Agreed, but I like it :)

xanatos · Accepted Answer · 2013-08-29 11:48:58Z

Using the Marsaglia generator from the Wiki

public class SimpleMarsagliaRandom { private const uint original_w = 1023; private uint m_w = original_w; /* must not be zero */ private uint m_z = 0; /* must not be zero, initialized by the constructor */ public SimpleMarsagliaRandom() { this.init(666); } public void init(uint z) { this.m_w = original_w; this.m_z = z; } public uint get_random() { this.m_z = 36969 * (this.m_z & 65535) + (this.m_z >> 16); this.m_w = 18000 * (this.m_w & 65535) + (this.m_w >> 16); return (this.m_z << 16) + this.m_w; /* 32-bit result */ } public uint get_random(uint min, uint max) { // max excluded uint num = max - min; return (this.get_random() % num) + min; } }

and

simpleMarsagliaRandom = function() { var original_w = 1023 >>> 0; var m_w = 0, m_z = 0; this.init = function(z) { m_w = original_w; m_z = z >>> 0; }; this.init(666); var internalRandom = function() { m_z = (36969 * (m_z & 65535) + (m_z >>> 16)) >>> 0; m_w = (18000 * (m_w & 65535) + (m_w >>> 16)) >>> 0; return (((m_z << 16) >>> 0) + m_w) >>> 0; /* 32-bit result */ }; this.get_random = function(min, max) { if (arguments.length < 2) { return internalRandom(); } var num = ((max >>> 0) - (min >>> 0)) >>> 0; return ((internalRandom() % num) + min) >>> 0; } };

In Javascript all the >>> are to coerce the number to uint

Totally untested

Be aware that what is done in get_random to make numbers from x to y is wrong. Low numbers will happen a little more times than higher numbers. To make an example: let's say you have a standard 6 faces dice. You roll it, you get 1-6. Now let's say you print on it the numbers 0-5. You roll it, you get 0-5. No problems. But you need the numbers in the range 0-3. So you do roll % 3... So we have:

rolled => rolled % 3 0 => 0, 1 => 1, 2 => 2, 3 => 0, 4 => 1, 5 => 2, 6 => 0.

The 0 result is more common.

Ideone for C# version: http://ideone.com/VQudcV
JSFiddle for Javascript version: http://jsfiddle.net/dqayk/

Tim S. · Accepted Answer · 2013-08-12 13:40:30Z

You should be able to use MD5 hashing in both C# and JS.

In C#:

int Hash(params int[] values) { System.Security.Cryptography.MD5 hasher = MD5.Create(); string valuesAsString = string.Join(",", values); var hash = hasher.ComputeHash(Encoding.UTF8.GetBytes(valuesAsString)); var hashAsInt = BitConverter.ToInt32(hash, 0); return Math.Abs(hashAsInt % 1000); }

In JS, implement the same method using some MD5 algorithm (e.g. jshash)

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