[Issue 5249] New: Strongly pure random generator

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=5249

           Summary: Strongly pure random generator
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: Phobos
        AssignedTo: nobody at puremagic.com
        ReportedBy: bearophile_hugs at eml.cc


--- Comment #0 from bearophile_hugs at eml.cc 2010-11-21 15:47:51 PST ---
As pure functions become more and more common in D2 programs, I'd like to
generate some random values inside them too. So I suggest to add to the
std.random module a strongly pure function that keeps no state and generates
random values.

This code shows that it's doable (but it's just for demonstration, because this
pseudo random generator is too much weak):


import std.stdio: writeln;

immutable struct rndPair {
    double seed, rnd;
}

// strongly pure
// Probably with DMD 2.050 a std.typecons.Tuple can't
// be used as return value here
pure nothrow rndPair nextRandom(const double seed, const double max) {
    enum int IA = 3_877, IC = 29_573, IM = 139_968;
    immutable double new_seed = (seed * IA + IC) % IM;
    return rndPair(new_seed, max * (new_seed * (1.0 / IM)));
}

// strongly pure
pure double[] foo(const int n, const double firstSeed=42) {
    double seed = firstSeed;
    auto res = new double[n];
    foreach (ref r; res) {
        auto seed_rnd = nextRandom(seed, 1.0);
        r = seed_rnd.rnd;
        seed = seed_rnd.seed;
    }
    return res;
}

void main() {
    writeln(foo(5));
    // Output:
    // [0.37465, 0.729024, 0.636467, 0.793481, 0.538545]
}


If you want two different strongly pure functions may be added, one good enough
generator and one better generator.


Once some unpacking syntax for tuples is present in DMD the foo() may become
more elegant, similar to (this uses what Andrei calls the 'banana syntax', but
other syntaxes are possible):


pure double[] foo(const int n, const double firstSeed=42) {
    double seed = firstSeed;
    auto res = new double[n];
    foreach (ref r; res)
        (|r, seed|) = nextRandom(seed, 1.0);
    return res;
}


See also bug 5124

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