[Issue 7863] New: randomShuffle doesn't work with a Xorshift

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=7863

           Summary: randomShuffle doesn't work with a Xorshift
           Product: D
           Version: D2
          Platform: x86
        OS/Version: Windows
            Status: NEW
          Keywords: rejects-valid
          Severity: normal
          Priority: P2
         Component: Phobos
        AssignedTo: nobody at puremagic.com
        ReportedBy: bearophile_hugs at eml.cc


--- Comment #0 from bearophile_hugs at eml.cc 2012-04-08 17:11:51 PDT ---
D2 code:

import std.random: Xorshift, randomShuffle;
void main() {
    Xorshift rndEng = Xorshift(1);
    int[] a = [10, 20, 30, 40];
    randomShuffle(a, rndEng);
}



DMD 2.058beta3 gives:

...\dmd2\src\phobos\std\random.d(1263): Error: cannot implicitly convert
expression (rndGen()) of type
MersenneTwisterEngine!(uint,32,624,397,31,-1727483681u,11,7,-1658038656u,15,-272236544u,18)
to XorshiftEngine!(uint,128,11,8,19)
test.d(5): Error: template instance
std.random.randomShuffle!(int[],XorshiftEngine!(uint,128,11,8,19)) error
instantiating



Currently randomShuffle is defined like this:

void randomShuffle(Range, RandomGen = Random)(Range r,
        ref RandomGen gen = rndGen)
{
    foreach (i; 0 .. r.length)
    {
        swapAt(r, i, i + uniform(0, r.length - i, gen));
    }
}



I think this alternative definitions avoid the problem:


import std.random : uniform, Xorshift, rndGen;
import std.algorithm: swapAt;

/**
Shuffles elements of $(D r) using $(D gen) as a shuffler. $(D r) must be
a random-access range with length.
 */
void randomShuffle(Range)(Range r)
{
    foreach (i; 0 .. r.length)
        swapAt(r, i, i + uniform(0, r.length - i));
}

/// ditto
void randomShuffle(Range, UniformRandomNumberGenerator)(Range r,
        ref UniformRandomNumberGenerator urng)
{
    foreach (i; 0 .. r.length)
        swapAt(r, i, i + uniform(0, r.length - i, urng));
}

void main() { // demo
    Xorshift rndEng = Xorshift(1);
    int[] a = [10, 20, 30, 40];
    randomShuffle(a);
    randomShuffle(a, rndEng);
}


See also Issue 4851

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