I have a grid with 200 lines and 200 columns. I want to generate random pairs of coordinates i,j by using a numeric seed. This seed is a value i am incrementing each time I generate a pair of numbers.
After 40,000 values have been generated, all pairs of coordinates are unique amongst themselves, as there is no i,j and m,n where i=m and j=n.
For example:
seed 0: generates 43,12
seed 1: generates 154, 62
and so forth...
The seed implies the same input with the same function generates the same result, I am fine with that.
I am aware I need some sort of pseudorandom algorithm, as using the computer time or something might generate two identical pairs, but where do I start?
If you want every seed to return a random point, and all of those points to be unique, the easiest way to do it is to put the points in an array, shuffle the array, and then use integer seeds to index the shuffled array. For example, seed=0 would get whatever element happened to be shuffled into the first position.
It seems a bit easier to me to let integers represent the pairs, so make an array from 0 to 40000 (ie, 200x200), shuffle this, and then use seeds in the range 0 to 40000. To convert the integer, n, to a point pair use, i=n%200 and j=(n-i)/200.
Of course, since you want each seed to return a unique point, you must have equal or fewer seeds than the number of points,
You need a random number generator that you can set the seed value for. Seems like you're aware of that. You can't set the seed for Math.random() but there's plenty of pseudorandom number generators out there. I suggest you take a look at seedrandom.js.