[Issue 5249] Strongly pure random generator

[d-bugmail at puremagic.com]

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



--- Comment #3 from bearophile_hugs at eml.cc 2010-11-21 20:03:55 PST ---
A bit more realistic (but not complete, not commented, etc) test using one of
the rnd generator of the std.random module:


pure nothrow UIntType pureLinearCongruential(UIntType, UIntType a, UIntType c,
UIntType m)
                                            (UIntType x0) {
    // perform compile-time tests on a, c, m here...

    static if (m) {
        UIntType _x = x0 % m; // slow?

        static if (is(UIntType == uint) && m == uint.max) {
            immutable ulong x = (cast(ulong) a * _x + c);
            immutable ulong v = x >> 32;
            immutable ulong w = x & uint.max;
            immutable y = cast(uint)(v + w);
            _x = (y < v || y == uint.max) ? (y + 1) : y;
        } else static if (is(UIntType == uint) && m == int.max) {
            immutable ulong x = (cast(ulong) a * _x + c);
            immutable ulong v = x >> 31;
            immutable ulong w = x & int.max;
            immutable uint y = cast(uint)(v + w);
            _x = (y >= int.max) ? (y - int.max) : y;
        } else {
            _x = cast(UIntType) ((cast(ulong) a * _x + c) % m);
        }
    } else {
        UIntType _x = a * _x0 + c;
    }

    return _x;
}

alias pureLinearCongruential!(uint, 16807, 0, 2147483647) pureMinstdRand0;

import std.stdio: writeln;

void main() {
    uint rnd = 100000;
    foreach (_; 0 .. 10) {
        rnd = pureMinstdRand0(rnd);
        writeln(rnd);
    }
}

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