Question

Note: For brevity’s sake, the following will not discern between randomness and pseudo-randomness. Also, in this context, constrained means between given min and max values)

The System.Random class provides random generation of integers, doubles and byte arrays.
Using Random.Next, one can easily generate random constrained values of type Boolean, Char, (S)Byte, (U)Int16, (U)Int32. Using Random.NextDouble(), one can similarly generate constrained values of types Double and Single (as far as my understanding of this type goes). Random string generation (of a given length and alphabet) has also been tackled before.

Consider the remaining primitive data types (excluding Object): Decimal and (U)Int64. Their random generation has been tackled as well (Decimal, (U)Int64 using Random.NextBytes()), but not when constrained. Rejection sampling (i.e. looping until the generated value is the desired range) could theoretically be used, but it is obviously not a practical solution. Normalizing NextDouble() won’t work because it doesn’t have enough significant digits.

In short, I am asking for the proper implementation of the following functions:

long NextLong(long min, long max)
long NextDecimal(decimal min, decimal max)

Note that, since System.DateTime is based on a ulong, the first function would allow for random constrained generation of such structs as well (similar to here, only in ticks instead of minutes).

Answer 1

Let’s assume you know how to generate N random bits. This is pretty easily done either using NextBytes or repeated calls to Random.Next with appropriate limits.

To generate a long/ulong in the right range, work out how large the range is, and how many bits it takes to represent it. You can then use rejection sampling which will at worst reject half the generated values (e.g. if you want a value in the range [0, 128], which means you’ll generate [0, 255] multiple times). If you want a non-zero based range, just work out the size of the range, generate a random value in [0, size) and then add the base.

Generating a random decimal is signficantly harder, I believe – aside from anything else, you’d have to specify the distribution you wanted.