// Written in the D programming language.

/**
 * Implements algorithms for generating random numbers drawn from
 * different statistical distributions.  Where possible, each random
 * _distribution is provided in two different forms:
 *
 * $(UL
 *   $(LI as a function, which takes as input the _distribution
 *        parameters and a uniform RNG, and returns a single value
 *        drawn from the distribution, and)
 *   $(LI as a range object, which wraps a uniform RNG instance and
 *        transforms its output into numbers drawn from the specified
 *        _distribution.)
 * )
 *
 * Typical reasons for rejecting a function implementation include
 * the function needing to hold state between calls to achieve
 * adequate performance, or the function needing to allocate memory
 * with each call.
 *
 * As with random number generators, the random _distribution range
 * objects implemented here are final classes in order to ensure
 * reference semantics.  They also assume reference type semantics on
 * the part of the RNGs that they wrap: user-supplied value-type RNGs
 * may produce unexpected and incorrect behaviour when combined with
 * these objects.
 *
 * Note: $(D hap._random._distribution.dice) uses a different algorithm
 * to its $(D std.random) counterpart and so will produce different
 * results.
 *
 * Copyright: © 2008-2011 Andrei Alexandrescu,
 *              2013      Chris Cain,
 *              2013      Andrej Mitrović,
 *              2013-2014 Joseph Rushton Wakeling
 *
 * License: $(WEB boost.org/LICENSE_1_0.txt, Boost License 1.0).
 *
 * Authors: $(WEB erdani.org, Andrei Alexandrescu),
 *          Chris Cain (discrete _distribution and integral-based uniform),
 *          Andrej Mitrović (enum-based uniform),
 *          $(WEB braingam.es, Joseph Rushton Wakeling)
 *
 * Source: $(HAPSRC hap/random/_distribution.d)
 */
module hap.random.distribution;

import hap.random.generator, hap.random.traits;

import std.range, std.traits;

// dice
/**
 * Rolls a random die with relative probabilities stored in $(D proportions).
 * Returns the index in $(D proportions) that was chosen.
 *
 * Example:
 * ----
 * auto x = dice(0.5, 0.5);   // x is 0 or 1 in equal proportions
 * auto y = dice(50, 50);     // y is 0 or 1 in equal proportions
 * auto z = dice(70, 20, 10); // z is 0 70% of the time, 1 20% of the time,
 *                            // and 2 10% of the time
 * ----
 *
 * The range counterpart of $(D dice) is the $(D DiscreteDistribution) class.
 *
 * Note: given an identically-seeded RNG as input, $(D hap.random.distribution._dice)
 * will produce different values to $(D std.random._dice).
 */
size_t dice(UniformRNG, Num)(UniformRNG rng, Num[] proportions...)
    if (isNumeric!Num && isForwardRange!UniformRNG)
{
    return diceImpl(rng, proportions);
}

/// ditto
size_t dice(UniformRNG, Range)(UniformRNG rng, Range proportions)
    if (isUniformRNG!UniformRNG && isForwardRange!Range &&
        isNumeric!(ElementType!Range) && !isArray!Range)
{
    return diceImpl(rng, proportions);
}

/// ditto
size_t dice(Range)(Range proportions)
    if (isForwardRange!Range && isNumeric!(ElementType!Range) && !isArray!Range)
{
    return diceImpl(rndGen, proportions);
}

/// ditto
size_t dice(Num)(Num[] proportions...)
    if (isNumeric!Num)
{
    return diceImpl(rndGen, proportions);
}

private size_t diceImpl(UniformRNG, Range)(UniformRNG rng, Range proportions)
    if (isUniformRNG!UniformRNG && isForwardRange!Range && isNumeric!(ElementType!Range))
{
    import std.algorithm, std.exception, hap.random.distribution;

    alias T = DiceType!Range;

    T sum = reduce!((a, b) { assert(b >= 0); return a + b; })(cast(T) 0, proportions.save);
    enforce(sum > 0, "Proportions in a dice cannot sum to zero");

    immutable point = uniform!("[)", T, T)(0, sum, rng);
    assert(point < sum);
    T mass = 0;

    size_t i = 0;
    foreach (e; proportions)
    {
        mass += e;
        if (point < mass)
        {
            return i;
        }
        i++;
    }
    // this point should not be reached
    assert(false);
}

unittest
{
    foreach (UniformRNG; UniformRNGTypes)
    {
        auto rng = new UniformRNG(unpredictableSeed);

        foreach (immutable _; 0 .. 100)
        {
            auto i = dice(rng, 0.0, 100.0);
            assert(i == 1);

            i = dice(rng, 100.0, 0.0);
            assert(i == 0);

            i = dice(100U, 0U);
            assert(i == 0);
        }

        import std.typetuple;

        foreach(T; TypeTuple!(byte, ubyte, short, ushort, int, uint,
                              long, ulong, float, double, real))
        {
            foreach (immutable l; 2 .. 100)
            {
                auto prop = new T[l];
                prop[] = 0;
                prop[0] = 10;
                prop[$-1] = 10;

                foreach (immutable _; 0 .. 100)
                {
                    auto i = dice(rng, prop);
                    assert(i == 0 || i == l - 1);
                }
            }
        }
    }
}

package template DiceType(Range)
    if (isInputRange!Range && isNumeric!(ElementType!Range))
{
    alias DiceType = DiceType!(ElementType!Range);
}

package template DiceType(T)
    if (isNumeric!T)
{
    static if (isIntegral!T)
    {
        static if (is(Largest!(ushort, Unsigned!T) == ushort))
        {
            alias DiceType = uint;
        }
        else static if (is(Largest!(ulong, Unsigned!T) == ulong))
        {
            alias DiceType = ulong;
        }
        else
        {
            static assert(false);
        }
    }
    else static if (isFloatingPoint!T)
    {
        alias DiceType = Largest!(double, T);
    }
}

unittest
{
    static assert(is(DiceType!byte == uint));
    static assert(is(DiceType!ubyte == uint));
    static assert(is(DiceType!short == uint));
    static assert(is(DiceType!ushort == uint));
    static assert(is(DiceType!int == ulong));
    static assert(is(DiceType!uint == ulong));
    static assert(is(DiceType!float == double));
    static assert(is(DiceType!double == double));
    static assert(is(DiceType!real == real));

    static assert(is(DiceType!(byte[]) == uint));
    static assert(is(DiceType!(ubyte[]) == uint));
    static assert(is(DiceType!(short[]) == uint));
    static assert(is(DiceType!(ushort[]) == uint));
    static assert(is(DiceType!(int[]) == ulong));
    static assert(is(DiceType!(uint[]) == ulong));
    static assert(is(DiceType!(float[]) == double));
    static assert(is(DiceType!(double[]) == double));
    static assert(is(DiceType!(real[]) == real));
}

/**
 * The range equivalent of $(D dice), each element of which is the
 * result of the roll of a random die with relative probabilities
 * stored in the range $(D proportions).  Each successive value of
 * $(D front) reflects the index in $(D proportions) that was chosen.
 * If no random number generator is specified, the default $(D rndGen)
 * will be used as the source of randomness.
 *
 * This offers a superior option in the event of making many rolls
 * of the same die, as the sum and CDF of $(D proportions) only needs
 * to be calculated upon construction and not with each call.
 */
final class DiscreteDistribution(SearchPolicy search, T, UniformRNG)
{
  private:
    SortedRange!(immutable(T)[]) _cumulativeProportions;
    UniformRNG _rng;
    size_t _value;

  public:
    enum bool isRandomDistribution = true;

    this(Range)(UniformRNG rng, Range proportions)
        if (isInputRange!Range && isNumeric!(ElementType!Range))
    in
    {
        assert(!proportions.empty, "Proportions of discrete distribution cannot be empty.");
    }
    body
    {
        import std.exception;

        _rng = rng;

        static if (isImplicitlyConvertible!(typeof(proportions.array), T[]))
        {
            T[] prop = proportions.array;
        }
        else
        {
            import std.algorithm, std.conv;
            T[] prop = proportions.map!(to!T).array;
        }

        alias A = Select!(isFloatingPoint!T, real, T);

        A accumulator = 0;

        foreach(ref e; prop)
        {
            assert(e >= 0, "Proportions of discrete distribution cannot be negative.");

            debug
            {
                A preAccumulation = accumulator;
            }

            accumulator += e;

            debug
            {
                static if (isIntegral!T)
                {
                    enforce(preAccumulation <= accumulator, "Integer overflow detected!");
                }
                else static if (isFloatingPoint!T)
                {
                    if (e > 0)
                    {
                        enforce(accumulator > preAccumulation, "Floating point rounding error detected!");
                    }
                }
                else
                {
                    static assert(0);
                }
            }

            e = accumulator;
        }

        _cumulativeProportions = assumeSorted(assumeUnique(prop)[]);

        popFront();
    }

    this(typeof(this) that)
    {
        this._cumulativeProportions = that._cumulativeProportions.save;
        this._rng = that._rng;
        this._value = that._value;
    }

    /// Range primitives
    enum bool empty = false;

    /// ditto
    size_t front() @property @safe const nothrow pure
    {
        return _value;
    }

    /// ditto
    void popFront()
    {
        immutable sum = _cumulativeProportions.back;
        immutable point = uniform!"[)"(0, sum, _rng);
        assert(point < sum);
        _value = _cumulativeProportions.length - _cumulativeProportions.upperBound!search(point).length;
    }

    /// ditto
    static if (isForwardRange!UniformRNG)
    {
        typeof(this) save() @property
        {
            auto ret = new typeof(this)(this);
            ret._rng = this._rng.save;
            return ret;
        }
    }
}

/// ditto
auto discreteDistribution(SearchPolicy search = SearchPolicy.binarySearch, UniformRNG, Range)
                         (UniformRNG rng, Range proportions)
    if (isInputRange!Range && isNumeric!(ElementType!Range) && isUniformRNG!UniformRNG)
{
    return new DiscreteDistribution!(search, DiceType!Range, UniformRNG)(rng, proportions);
}

/// ditto
auto discreteDistribution(SearchPolicy search = SearchPolicy.binarySearch, Range)
                         (Range proportions)
    if (isInputRange!Range && isNumeric!(ElementType!Range))
{
    return discreteDistribution(rndGen, proportions);
}

/// ditto
auto discreteDistribution(SearchPolicy search = SearchPolicy.binarySearch, UniformRNG, Num)
                         (UniformRNG rng, Num[] proportions...)
    if (isUniformRNG!UniformRNG && isNumeric!Num)
{
    return discreteDistribution(rng, proportions);
}

/// ditto
auto discreteDistribution(SearchPolicy search = SearchPolicy.binarySearch, Num)
                         (Num[] proportions...)
    if (isNumeric!Num)
{
    return discreteDistribution(rndGen, proportions);
}

unittest
{
    foreach (UniformRNG; UniformRNGTypes)
    {
        auto rng = new UniformRNG(unpredictableSeed);

        foreach (immutable d; discreteDistribution(rng, 100.0, 0.0).take(100))
        {
            assert(d == 0);
        }

        foreach (immutable d; discreteDistribution(rng, 0.0, 100.0).take(100))
        {
            assert(d == 1);
        }

        foreach (immutable d; discreteDistribution(100, 0).take(100))
        {
            assert(d == 0);
        }

        foreach (immutable l; 2 .. 100)
        {
            auto prop = new uint[l];
            prop[] = 0;
            prop[0] = prop[$-1] = 20;

            foreach (immutable d; discreteDistribution(rng, prop).take(100))
            {
                assert(d == 0 || d == l - 1);
            }
        }

        import std.typetuple;

        foreach(T; TypeTuple!(byte, ubyte, short, ushort, int, uint,
                              long, ulong, float, double, real))
        {
            foreach (immutable l; 2 .. 100)
            {
                auto prop = uniformDistribution!("[]", T, T)(1, 10, rng).take(10).array;

                foreach (immutable d; discreteDistribution(rng.save, prop).take(100))
                {
                    assert(d == dice(rng, prop));
                }
            }
        }

        // Check .save works
        {
            auto ddist1 = discreteDistribution(rng, 10.0, 3.0, 9.0);
            auto ddist2 = ddist1.save;

            assert(ddist1 !is ddist2);
            assert(ddist1._rng !is ddist2._rng);

            foreach (d1, d2; lockstep(ddist1.take(100), ddist2.take(100)))
            {
                assert(d1 == d2);
            }
        }
    }
}

/**
 * Returns a floating-point number drawn from a _normal (Gaussian)
 * distribution with mean $(D mu) and standard deviation $(D sigma).
 * If no random number generator is specified, the default $(D rndGen)
 * will be used as the source of randomness.
 *
 * Note that this function uses two variates from the uniform random
 * number generator to generate a single normally-distributed variate.
 * It is therefore an inefficient means of generating a large number of
 * normally-distributed variates.  If you wish to draw many variates
 * from the _normal distribution, it is better to use the range-based
 * $(D normalDistribution) instead.
 */
auto normal(T1, T2)(T1 mu, T2 sigma)
    if (isNumeric!T1 && isNumeric!T2)
{
    return normal!(T1, T2, Random)(mu, sigma, rndGen);
}

/// ditto
auto normal(T1, T2, UniformRNG)(T1 mu, T2 sigma, UniformRNG rng)
    if (isNumeric!T1 && isNumeric!T2 && isUniformRNG!UniformRNG)
{
    import std.math;

    static if (isFloatingPoint!(CommonType!(T1, T2)))
    {
        alias T = CommonType!(T1, T2);
    }
    else
    {
        alias T = double;
    }

    immutable T _r1 = uniform01!T(rng);
    immutable T _r2 = uniform01!T(rng);

    return sqrt(-2 * log(1 - _r2)) * cos(2 * PI * _r1) * sigma + mu;
}

unittest
{
    // Compare to behaviour of NormalDistribution
    foreach (UniformRNG; UniformRNGTypes)
    {
        auto rng = new UniformRNG(unpredictableSeed);
        auto ndist = normalDistribution(3.29, 7.64, rng.save);

        static assert(is(CommonType!(double, double) == double));

        foreach (immutable _; 0 .. 100)
        {
            import std.math;
            auto val = normal(3.29, 7.64, rng);
            assert(approxEqual(val, ndist.front));
            ndist.popFront();
            ndist.popFront();
        }
    }
}

/**
 * Provides an infinite range of random numbers distributed according to the
 * normal (Gaussian) distribution with mean $(D mu) and standard deviation
 * $(D sigma).  If no random number generator is specified, the default
 * $(D rndGen) will be used as the source of randomness.
 */
final class NormalDistribution(T, UniformRNG)
    if (isFloatingPoint!T && isUniformRNG!UniformRNG)
{
  private:
    alias NormalEngine = NormalEngineBoxMuller;
    NormalEngine!T _engine;
    UniformRNG _rng;
    T _value;

  public:
    enum bool isRandomDistribution = true;
    immutable T mean;
    immutable T stdev;

    this(T mu, T sigma, UniformRNG rng)
    {
        import std.exception;
        enforce(sigma > 0);
        mean = mu;
        stdev = sigma;
        _rng = rng;
        popFront();
    }

    this(typeof(this) that)
    {
        this.mean = that.mean;
        this.stdev = that.stdev;
        this._engine = that._engine;
        this._rng = that._rng;
        this._value = that._value;
    }

    /// Range primitives.
    enum bool empty = false;

    /// ditto
    T front() @property @safe const nothrow pure
    {
        return _value;
    }

    /// ditto
    void popFront()
    {
        _value = _engine(this.mean, this.stdev, _rng);
    }

    /// ditto
    static if (isForwardRange!UniformRNG)
    {
        typeof(this) save() @property
        {
            auto ret = new typeof(this)(this);
            ret._rng = this._rng.save;
            return ret;
        }
    }
}

/// ditto
auto normalDistribution(T1, T2, UniformRNG)
                       (T1 mu, T2 sigma, UniformRNG rng)
    if (isNumeric!T1 && isNumeric!T2 && isUniformRNG!UniformRNG)
{
    static if (isFloatingPoint!(CommonType!(T1, T2)))
    {
        alias T = CommonType!(T1, T2);
    }
    else
    {
        alias T = double;
    }

    return new NormalDistribution!(T, UniformRNG)(mu, sigma, rng);
}

/// ditto
auto normalDistribution(T1, T2)
                       (T1 mu, T2 sigma)
    if (isNumeric!T1 && isNumeric!T2)
{
    return normalDistribution!(T1, T2, Random)(mu, sigma, rndGen);
}

unittest
{
    // check type rules for NormalDistribution
    {
        auto ndist = normalDistribution(0, 1);
        static assert(is(typeof(ndist.front) == double));
    }
    {
        auto ndist = normalDistribution(0.0f, 1);
        static assert(is(typeof(ndist.front) == float));
    }
    {
        auto ndist = normalDistribution(0.0, 1);
        static assert(is(typeof(ndist.front) == double));
    }
    {
        auto ndist = normalDistribution(0.0L, 1);
        static assert(is(typeof(ndist.front) == real));
    }

    // check save works effectively
    foreach (UniformRNG; UniformRNGTypes)
    {
        auto rng = new UniformRNG(unpredictableSeed);
        auto ndist = normalDistribution(3.29, 7.64, rng);
        /* Box-Muller generates normal variates a pair at a time.
         * advancing to the second of these helps verify that the
         * .save method is truly copying the source distribution.
         */
        ndist.popFront();
        auto ndist2 = ndist.save;
        assert(ndist2 !is ndist);

        ndist.popFrontN(10);
        assert(ndist2.front != ndist.front);
        ndist2.popFrontN(10);
        assert(ndist2.front == ndist.front);
    }
}

/**
 * Generates random numbers drawn from a normal (Gaussian) distribution, using
 * the Box-Muller Transform method.
 *
 * This implementation of Box-Muller closely follows that of its counterpart
 * in the Boost.Random C++ library and should produce matching results aside
 * from discrepancies that arise out of differences in floating-point precision.
 */
private struct NormalEngineBoxMuller(T)
    if (isFloatingPoint!T)
{
  private:
    bool _valid = false;
    T _rho, _r1, _r2;

  public:
    /**
     * Generates a single random number drawn from a normal distribution with
     * mean $(D mu) and standard deviation $(D sigma), using $(D rng) as the
     * source of randomness.
     */
    T opCall(UniformRNG)(in T mu, in T sigma, UniformRNG rng)
        if (isUniformRNG!UniformRNG)
    {
        import std.math;

        _valid = !_valid;

        if (_valid)
        {
            /* N.B. Traditional Box-Muller asks for random numbers
             * in (0, 1], which uniform() can readily supply.  We
             * instead generate numbers in [0, 1) and use 1 - num
             * to match the output of Boost.Random.
             */
            _r1 = uniform01!T(rng);
            _r2 = uniform01!T(rng);
            _rho = sqrt(-2 * log(1 - _r2));

            return _rho * cos(2 * PI * _r1) * sigma + mu;
        }
        else
        {
            return _rho * sin(2 * PI * _r1) * sigma + mu;
        }
    }
}

unittest
{
    NormalEngineBoxMuller!double engine;

    foreach (UniformRNG; UniformRNGTypes)
    {
        auto rng1 = new UniformRNG(unpredictableSeed);
        auto rng2 = rng1.save;
        auto rng3 = rng1.save;
        double mu = 6.5, sigma = 3.2;

        /* The Box-Muller engine produces variates a pair at
         * a time.  We verify this is true by using a pair of
         * pseudo-random number generators sharing the same
         * initial state.
         */
        auto a1 = engine(mu, sigma, rng1);
        auto b2 = engine(mu, sigma, rng2);

        // verify that 1st RNG has been called but 2nd has not
        assert(rng3.front != rng1.front);
        assert(rng3.front == rng2.front);

        /* Now, calling with the RNG order reversed should
         * produce the same results: only rng2 will get called
         * this time.
         */
        auto a2 = engine(mu, sigma, rng2);
        auto b1 = engine(mu, sigma, rng1);

        assert(a1 == a2);
        assert(b1 == b2);
        assert(rng2.front == rng1.front);
        assert(rng3.front != rng2.front);

        // verify that the RNGs have each been called twice
        rng3.popFrontN(2);
        assert(rng3.front == rng2.front);
    }
}

/**
 * Generates a number between $(D a) and $(D b). The $(D boundaries)
 * parameter controls the shape of the interval (open vs. closed on
 * either side). Valid values for $(D boundaries) are $(D "[]"), $(D
 * "$(LPAREN)]"), $(D "[$(RPAREN)"), and $(D "()"). The default interval
 * is closed to the left and open to the right.  If no random number
 * generator is specified, the default $(D rndGen) will be used as the
 * source of randomness.
 *
 * Example:
 *
 * ----
 * auto gen = Random(unpredictableSeed);
 * // Generate an integer in [0, 1023]
 * auto a = uniform(0, 1024, gen);
 * // Generate a float in [0, 1$(RPAREN)
 * auto a = uniform(0.0f, 1.0f, gen);
 * ----
 */
auto uniform(string boundaries = "[)", T1, T2)
            (T1 a, T2 b)
    if (!is(CommonType!(T1, T2) == void))
{
    return uniform!(boundaries, T1, T2, Random)(a, b, rndGen);
}

unittest
{
    foreach (UniformRNG; UniformRNGTypes)
    {
        auto rng = new UniformRNG(unpredictableSeed);

        foreach (i; 0 .. 20)
        {
            auto x = uniform(0.0, 15.0, rng);
            assert(0 <= x && x < 15);
        }
        foreach (i; 0 .. 20)
        {
            auto x = uniform!"[]"('a', 'z', rng);
            assert('a' <= x && x <= 'z');
        }

        foreach (i; 0 .. 20)
        {
            auto x = uniform('a', 'z', rng);
            assert('a' <= x && x < 'z');
        }

        foreach(i; 0 .. 20)
        {
            immutable ubyte a = 0;
            immutable ubyte b = 15;
            auto x = uniform(a, b, rng);
            assert(a <= x && x < b);
        }
    }
}

// Implementation of uniform for floating-point types
/// ditto
auto uniform(string boundaries = "[)", T1, T2, UniformRNG)
            (T1 a, T2 b, UniformRNG rng)
    if (isFloatingPoint!(CommonType!(T1, T2)) && isUniformRNG!UniformRNG)
out (result)
{
    // We assume "[)" as the common case
    static if (boundaries[0] == '(')
    {
        assert(a < result);
    }
    else
    {
        assert(a <= result);
    }

    static if (boundaries[1] == ']')
    {
        assert(result <= b);
    }
    else
    {
        assert(result < b);
    }
}
body
{
    import std.exception, std.math, std.string : format;
    alias NumberType = Unqual!(CommonType!(T1, T2));
    static if (boundaries[0] == '(')
    {
        NumberType _a = nextafter(cast(NumberType) a, NumberType.infinity);
    }
    else
    {
        NumberType _a = a;
    }
    static if (boundaries[1] == ')')
    {
        NumberType _b = nextafter(cast(NumberType) b, -NumberType.infinity);
    }
    else
    {
        NumberType _b = b;
    }
    enforce(_a <= _b,
            format("hap.random.distribution.uniform(): invalid bounding interval %s%s, %s%s",
                   boundaries[0], a, b, boundaries[1]));
    NumberType result =
        _a + (_b - _a) * cast(NumberType) (rng.front - rng.min)
        / (rng.max - rng.min);
    rng.popFront();
    return result;
}

/* Implementation of uniform for integral types.
 *
 * Description of algorithm and suggestion of correctness:
 *
 * The modulus operator maps an integer to a small, finite space. For instance, `x
 * % 3` will map whatever x is into the range [0 .. 3). 0 maps to 0, 1 maps to 1, 2
 * maps to 2, 3 maps to 0, and so on infinitely. As long as the integer is
 * uniformly chosen from the infinite space of all non-negative integers then `x %
 * 3` will uniformly fall into that range.
 *
 * (Non-negative is important in this case because some definitions of modulus,
 * namely the one used in computers generally, map negative numbers differently to
 * (-3 .. 0]. `uniform` does not use negative number modulus, thus we can safely
 * ignore that fact.)
 *
 * The issue with computers is that integers have a finite space they must fit in,
 * and our uniformly chosen random number is picked in that finite space. So, that
 * method is not sufficient. You can look at it as the integer space being divided
 * into "buckets" and every bucket after the first bucket maps directly into that
 * first bucket. `[0, 1, 2]`, `[3, 4, 5]`, ... When integers are finite, then the
 * last bucket has the chance to be "incomplete": `[uint.max - 3, uint.max - 2,
 * uint.max - 1]`, `[uint.max]` ... (the last bucket only has 1!). The issue here
 * is that _every_ bucket maps _completely_ to the first bucket except for that
 * last one. The last one doesn't have corresponding mappings to 1 or 2, in this
 * case, which makes it unfair.
 *
 * So, the answer is to simply "reroll" if you're in that last bucket, since it's
 * the only unfair one. Eventually you'll roll into a fair bucket. Simply, instead
 * of the meaning of the last bucket being "maps to `[0]`", it changes to "maps to
 * `[0, 1, 2]`", which is precisely what we want.
 *
 * To generalize, `upperDist` represents the size of our buckets (and, thus, the
 * exclusive upper bound for our desired uniform number). `rnum` is a uniformly
 * random number picked from the space of integers that a computer can hold (we'll
 * say `UpperType` represents that type).
 *
 * We'll first try to do the mapping into the first bucket by doing `offset = rnum
 * % upperDist`. We can figure out the position of the front of the bucket we're in
 * by `bucketFront = rnum - offset`.
 *
 * If we start at `UpperType.max` and walk backwards `upperDist - 1` spaces, then
 * the space we land on is the last acceptable position where a full bucket can
 * fit:
 *
 * ```
 *    bucketFront     UpperType.max
 *       v                 v
 * [..., 0, 1, 2, ..., upperDist - 1]
 *       ^~~ upperDist - 1 ~~^
 * ```
 *
 * If the bucket starts any later, then it must have lost at least one number and
 * at least that number won't be represented fairly.
 *
 * ```
 *                 bucketFront     UpperType.max
 *                      v                v
 * [..., upperDist - 1, 0, 1, 2, ..., upperDist - 2]
 *           ^~~~~~~~ upperDist - 1 ~~~~~~~^
 * ```
 *
 * Hence, our condition to reroll is
 * `bucketFront > (UpperType.max - (upperDist - 1))`
 */
/// ditto
auto uniform(string boundaries = "[)", T1, T2, UniformRNG)
            (T1 a, T2 b, UniformRNG rng)
    if ((isIntegral!(CommonType!(T1, T2)) || isSomeChar!(CommonType!(T1, T2)))
        && isUniformRNG!UniformRNG)
out (result)
{
    // We assume "[)" as the common case
    static if (boundaries[0] == '(')
    {
        assert(a < result);
    }
    else
    {
        assert(a <= result);
    }

    static if (boundaries[1] == ']')
    {
        assert(result <= b);
    }
    else
    {
        assert(result < b);
    }
}
body
{
    import std.conv, std.exception;
    alias ResultType = Unqual!(CommonType!(T1, T2));
    // We handle the case "[)' as the common case, and we adjust all
    // other cases to fit it.
    static if (boundaries[0] == '(')
    {
        enforce(a < ResultType.max,
                text("hap.random.distribution.uniform(): invalid left bound ", a));
        ResultType lower = cast(ResultType) (a + 1);
    }
    else
    {
        ResultType lower = a;
    }

    static if (boundaries[1] == ']')
    {
        enforce(lower <= b,
                text("hap.random.distribution.uniform(): invalid bounding interval ",
                        boundaries[0], a, ", ", b, boundaries[1]));
        /* Cannot use this next optimization with dchar, as dchar
         * only partially uses its full bit range
         */
        static if (!is(ResultType == dchar))
        {
            if (b == ResultType.max && lower == ResultType.min)
            {
                // Special case - all bits are occupied
                return uniform!ResultType(rng);
            }
        }
        auto upperDist = unsigned(b - lower) + 1u;
    }
    else
    {
        enforce(lower < b,
                text("hap.random.distribution.uniform(): invalid bounding interval ",
                        boundaries[0], a, ", ", b, boundaries[1]));
        auto upperDist = unsigned(b - lower);
    }

    assert(upperDist != 0);

    alias UpperType = typeof(upperDist);
    static assert(UpperType.min == 0);

    UpperType offset, rnum, bucketFront;
    do
    {
        rnum = uniform!UpperType(rng);
        offset = rnum % upperDist;
        bucketFront = rnum - offset;
    } // while we're in an unfair bucket...
    while (bucketFront > (UpperType.max - (upperDist - 1)));

    return cast(ResultType)(lower + offset);
}

unittest
{
    import std.conv, std.typetuple;

    foreach (UniformRNG; UniformRNGTypes)
    {
        auto rng = new UniformRNG(unpredictableSeed);

        auto a = uniform(0, 1024, rng);
        assert(0 <= a && a <= 1024);
        auto b = uniform(0.0f, 1.0f, rng);
        assert(0 <= b && b < 1, to!string(b));
        auto c = uniform(0.0, 1.0);
        assert(0 <= c && c < 1);

        foreach (T; TypeTuple!(char, wchar, dchar, byte, ubyte, short, ushort,
                               int, uint, long, ulong, float, double, real))
        {
            foreach (boundaries; TypeTuple!("[)", "(]", "[]", "()"))
            {
                T lo = 0, hi = 100;
                T init = uniform!boundaries(lo, hi, rng);
                size_t i = 50;
                while (--i && uniform!boundaries(lo, hi, rng) == init) {}
                assert(i > 0);

                foreach (immutable _; 0 .. 100)
                {
                    auto u1 = uniform!boundaries(lo, hi, rng);
                    auto u2 = uniform!boundaries(lo, hi);

                    static if (boundaries[0] == '(')
                    {
                        assert(lo < u1);
                        assert(lo < u2);
                    }
                    else
                    {
                        assert(lo <= u1);
                        assert(lo <= u2);
                    }

                    static if (boundaries[1] == ']')
                    {
                        assert(u1 <= hi);
                        assert(u2 <= hi);
                    }
                    else
                    {
                        assert(u1 < hi);
                        assert(u2 < hi);
                    }
                }
            }

            /* test case with closed boundaries covering whole range
             * of integral type
             */
            static if (isIntegral!T || isSomeChar!T)
            {
                foreach (immutable _; 0 .. 100)
                {
                    auto u = uniform!"[]"(T.min, T.max, rng);
                    static assert(is(typeof(u) == T));
                    assert(T.min <= u, "Lower bound violation for uniform!\"[]\"(" ~ T.stringof ~ ")");
                    assert(u <= T.max, "Upper bound violation for uniform!\"[]\"(" ~ T.stringof ~ ")");
                }
            }
        }
    }

    auto reproRng = new Xorshift(239842);

    foreach (T; TypeTuple!(char, wchar, dchar, byte, ubyte, short,
                          ushort, int, uint, long, ulong))
    {
        foreach (boundaries; TypeTuple!("[)", "(]", "[]", "()"))
        {
            T lo = T.min + 10, hi = T.max - 10;
            T init = uniform!boundaries(lo, hi, reproRng);
            size_t i = 50;
            while (--i && uniform!boundaries(lo, hi, reproRng) == init) {}
            assert(i > 0);
        }
    }

    {
        bool sawLB = false, sawUB = false;
        foreach (i; 0 .. 50)
        {
            auto x = uniform!"[]"('a', 'd', reproRng);
            if (x == 'a') sawLB = true;
            if (x == 'd') sawUB = true;
            assert('a' <= x && x <= 'd');
        }
        assert(sawLB && sawUB);
    }

    {
        bool sawLB = false, sawUB = false;
        foreach (i; 0 .. 50)
        {
            auto x = uniform('a', 'd', reproRng);
            if (x == 'a') sawLB = true;
            if (x == 'c') sawUB = true;
            assert('a' <= x && x < 'd');
        }
        assert(sawLB && sawUB);
    }

    {
        bool sawLB = false, sawUB = false;
        foreach (i; 0 .. 50)
        {
            immutable int lo = -2, hi = 2;
            auto x = uniform!"()"(lo, hi, reproRng);
            if (x == (lo+1)) sawLB = true;
            if (x == (hi-1)) sawUB = true;
            assert(lo < x && x < hi);
        }
        assert(sawLB && sawUB);
    }

    {
        bool sawLB = false, sawUB = false;
        foreach (i; 0 .. 50)
        {
            immutable ubyte lo = 0, hi = 5;
            auto x = uniform(lo, hi, reproRng);
            if (x == lo) sawLB = true;
            if (x == (hi-1)) sawUB = true;
            assert(lo <= x && x < hi);
        }
        assert(sawLB && sawUB);
    }

    {
        foreach (i; 0 .. 30)
        {
            assert(i == uniform(i, i+1, reproRng));
        }
    }
}

/**
 * Generates a uniformly-distributed number in the range $(D [T.min, T.max])
 * for any integral type $(D T). If no random number generator is passed,
 * uses the default $(D rndGen).
 */
auto uniform(T, UniformRNG)(UniformRNG rng)
    if (!is(T == enum) && (isIntegral!T || isSomeChar!T)
        && isUniformRNG!UniformRNG)
{
    /* dchar does not use its full bit range, so we must
     * revert to using uniform with specified bounds
     */
    static if (is(T == dchar))
    {
        return uniform!"[]"(T.min, T.max, rng);
    }
    else
    {
        auto r = rng.front;
        rng.popFront();
        static if (T.sizeof <= r.sizeof)
        {
            return cast(T) r;
        }
        else
        {
            static assert(T.sizeof == 8 && r.sizeof == 4);
            T r1 = rng.front | (cast(T)r << 32);
            rng.popFront();
            return r1;
        }
    }
}

/// ditto
auto uniform(T)()
    if (!is(T == enum) && (isIntegral!T || isSomeChar!T))
{
    return uniform!T(rndGen);
}

unittest
{
    import std.typetuple;
    foreach(T; TypeTuple!(char, wchar, dchar, byte, ubyte, short, ushort,
                          int, uint, long, ulong))
    {
        T init = uniform!T();
        size_t i = 50;
        while (--i && uniform!T() == init) {}
        assert(i > 0);
        assert(i < 50);

        foreach (UniformRNG; UniformRNGTypes)
        {
            auto rng = new UniformRNG(unpredictableSeed);
            init = uniform!T(rng);
            i = 50;
            while (--i && uniform!T(rng) == init) {}
            assert(i > 0);
            assert(i < 50);

            foreach (immutable _; 0 .. 100)
            {
                auto u = uniform!T(rng);
                static assert(is(typeof(u) == T));
                assert(T.min <= u, "Lower bound violation for uniform!" ~ T.stringof);
                assert(u <= T.max, "Upper bound violation for uniform!" ~ T.stringof);
            }
        }
    }
}

/**
 * Returns a uniformly selected member of enum $(D E). If no random number
 * generator is passed, uses the default $(D rndGen).
 */
auto uniform(E, UniformRNG)
            (UniformRNG rng)
    if (is(E == enum) && isUniformRNG!UniformRNG)
{
    static immutable E[EnumMembers!E.length] members = [EnumMembers!E];
    return members[uniform!"[)"(0, members.length, rng)];
}

/// Ditto
auto uniform(E)()
    if (is(E == enum))
{
    return uniform!E(rndGen);
}

///
unittest
{
    enum Fruit { Apple = 12, Mango = 29, Pear = 72 }
    foreach (immutable _; 0 .. 100)
    {
        foreach (immutable f; [uniform!Fruit(), rndGen.uniform!Fruit()])
        {
            assert(f == Fruit.Apple || f == Fruit.Mango || f == Fruit.Pear);
        }
    }
}

unittest
{
    enum Fruit { Apple = 12, Mango = 29, Pear = 72 }
    foreach (UniformRNG; UniformRNGTypes)
    {
        auto rng = new UniformRNG(unpredictableSeed);
        foreach (immutable _; 0 .. 100)
        {
            foreach (immutable f; [uniform!Fruit, rng.uniform!Fruit])
            {
                assert(f == Fruit.Apple || f == Fruit.Mango || f == Fruit.Pear);
            }
        }
    }
}

/**
 * Provides an infinite sequence of random numbers uniformly distributed between
 * $(D a) and $(D b).  The $(D boundaries) parameter controls the shape of the
 * interval (open vs. closed on either side).  Valid values for $(D boundaries)
 * are $(D "[]"), $(D "$(LPAREN)]"), $(D "[$(RPAREN)"), and $(D "()"). The
 * default interval is closed to the left and open to the right. If no random
 * number generator is specified, the default $(D rndGen) will be used as the
 * source of randomness.
 */
final class UniformDistribution(string boundaries = "[)", T, UniformRNG)
    if ((isNumeric!T || isSomeChar!T) && isUniformRNG!UniformRNG)
{
  private:
    UniformRNG _rng;
    T _value;

  public:
    enum bool isRandomDistribution = true;

    immutable T min;
    immutable T max;

    this(T a, T b, UniformRNG rng)
    {
        import std.exception;
        enforce(a < b);
        min = a;
        max = b;
        _rng = rng;
        popFront();
    }

    this(typeof(this) that)
    {
        this(that.min, that.max, that._rng);
    }

    /// Range primitives.
    enum bool empty = false;

    /// ditto
    T front() @property @safe const nothrow pure
    {
        return _value;
    }

    /// ditto
    void popFront()
    {
        _value = uniform!(boundaries, T, T, UniformRNG)(min, max, _rng);
    }

    /// ditto
    static if (isForwardRange!UniformRNG)
    {
        typeof(this) save() @property
        {
            auto ret = new typeof(this)(this);
            ret._rng = this._rng.save;
            return ret;
        }
    }
}

/// ditto
auto uniformDistribution(string boundaries = "[)", T1, T2, UniformRNG)
                        (T1 a, T2 b, UniformRNG rng)
    if ((isNumeric!(CommonType!(T1, T2)) || isSomeChar!(CommonType!(T1, T2)))
        && isUniformRNG!UniformRNG)
{
    alias T = CommonType!(T1, T2);
    return new UniformDistribution!(boundaries, T, UniformRNG)(a, b, rng);
}

/// ditto
auto uniformDistribution(string boundaries = "[)", T1, T2)
                        (T1 a, T2 b)
    if (isNumeric!(CommonType!(T1, T2)) || isSomeChar!(CommonType!(T1, T2)))
{
    alias T = CommonType!(T1, T2);
    return new UniformDistribution!(boundaries, T, Random)(a, b, rndGen);
}

unittest
{
    // General tests
    foreach (UniformRNG; UniformRNGTypes)
    {
        import std.typetuple;

        foreach (T; TypeTuple!(char, wchar, dchar, byte, ubyte, short, ushort,
                               int, uint, long, ulong, float, double, real))
        {
            foreach (boundaries; TypeTuple!("[)", "(]", "[]", "()"))
            {
                static assert(isRandomDistribution!(UniformDistribution!(boundaries, T, UniformRNG)));

                T min = 0, max = 10;
                auto rng = new UniformRNG(unpredictableSeed);

                foreach (u; uniformDistribution!boundaries(min, max, rng.save).take(100))
                {
                    assert(u == uniform!boundaries(min, max, rng));
                }
            }
        }
    }

    // check distribution boundaries function OK for integers
    {
        auto udist = uniformDistribution(0, 10);
        size_t eqMin = 0;
        foreach (u; udist.take(100))
        {
            assert(udist.min <= u);
            assert(u < udist.max);
            if (u == udist.min) ++eqMin;
        }
        assert(eqMin > 0);
    }
    {
        auto udist = uniformDistribution!"()"(0, 10);
        foreach (u; udist.take(100))
        {
            assert(udist.min < u);
            assert(u < udist.max);
        }
    }
    {
        auto udist = uniformDistribution!"(]"(0, 10);
        size_t eqMax = 0;
        foreach (u; udist.take(100))
        {
            assert(udist.min < u);
            assert(u <= udist.max);
            if (u == udist.max) ++eqMax;
        }
        assert(eqMax > 0);
    }
    {
        auto udist = uniformDistribution!"[]"(0, 10);
        size_t eqMin = 0, eqMax = 0;
        foreach (u; udist.take(100))
        {
            assert(udist.min <= u);
            assert(u <= udist.max);
            if (u == udist.min) ++eqMin;
            if (u == udist.max) ++eqMax;
        }
        assert(eqMin > 0);
        assert(eqMax > 0);
    }

    // check that save works properly
    {
        auto udist = uniformDistribution(1.1, 3.3);
        auto udist2 = udist.save;
        assert(udist2 !is udist);
        udist.popFrontN(20);
        assert(udist2.front != udist.front);
        udist2.popFrontN(20);
        assert(udist2.front == udist.front);
    }
}

/**
 * Generates an infinite sequence of uniformly-distributed numbers in the
 * range $(D [T.min, T.max]) for any integral type $(D T).  If no random
 * number generator is specified, the default $(D rndGen) will be used as
 * the source of randomness.
 */
final class UniformDistribution(T, UniformRNG)
    if (!is(T == enum) && (isIntegral!T || isSomeChar!T)
        && isUniformRNG!UniformRNG)
{
  private:
    UniformRNG _rng;
    T _value;

  public:
    enum bool isRandomDistribution = true;

    enum T min = T.min;
    enum T max = T.max;

    this(UniformRNG rng)
    {
        _rng = rng;
        popFront();
    }

    this(typeof(this) that)
    {
        this._rng = that._rng;
        this._value = that._value;
    }

    /// Range primitives.
    enum bool empty = false;

    /// ditto
    T front() @property @safe const nothrow pure
    {
        return _value;
    }

    /// ditto
    void popFront()
    {
        _value = uniform!(T, UniformRNG)(_rng);
    }

    /// ditto
    static if (isForwardRange!UniformRNG)
    {
        typeof(this) save() @property
        {
            auto ret = new typeof(this)(this);
            ret._rng = this._rng.save;
            return ret;
        }
    }
}

/// ditto
auto uniformDistribution(T, UniformRNG)
                        (UniformRNG rng)
    if (!is(T == enum) && (isIntegral!T || isSomeChar!T)
        && isUniformRNG!UniformRNG)
{
    return new UniformDistribution!(T, UniformRNG)(rng);
}

/// ditto
auto uniformDistribution(T)()
    if (!is(T == enum) && (isIntegral!T || isSomeChar!T))
{
    return new UniformDistribution!(T, Random)(rndGen);
}

unittest
{
    foreach (UniformRNG; UniformRNGTypes)
    {
        import std.typetuple;

        foreach (T; TypeTuple!(char, wchar, dchar, byte, ubyte, short, ushort,
                               int, uint, long, ulong))
        {
            static assert(isRandomDistribution!(UniformDistribution!(T, UniformRNG)));

            auto rng = new UniformRNG(unpredictableSeed);
            auto udist = uniformDistribution!T(rng.save);

            foreach (u; udist.take(100))
            {
                assert(uniform!T(rng) == u);
                assert(u <= T.max);
                assert(u >= T.min);
            }

            // check that .save works
            auto udist2 = udist.save;
            assert(udist2 !is udist);
            foreach (u1, u2; lockstep(udist.take(100), udist2.take(100)))
            {
                assert(u1 == u2);
            }
        }
    }
}

/**
 * Generates an infinite sequence of uniformly selected members of
 * enum $(D E). If no random number generator is specified, the
 * default $(D rndGen) will be used as the source of randomness.
 */
final class UniformDistribution(E, UniformRNG)
    if (is(E == enum) && isUniformRNG!UniformRNG)
{
  private:
    UniformRNG _rng;
    E _value;

  public:
    enum bool isRandomDistribution = true;

    enum E min = E.min;
    enum E max = E.max;

    this(UniformRNG rng)
    {
        _rng = rng;
        popFront();
    }

    this(typeof(this) that)
    {
        this._rng = that._rng;
        this._value = that._value;
    }

    /// Range primitives.
    enum bool empty = false;

    /// ditto
    E front() @property @safe const nothrow pure
    {
        return _value;
    }

    /// ditto
    void popFront()
    {
        _value = uniform!(E, UniformRNG)(_rng);
    }

    /// ditto
    static if (isForwardRange!UniformRNG)
    {
        typeof(this) save() @property
        {
            auto ret = new typeof(this)(this);
            ret._rng = this._rng.save;
            return ret;
        }
    }
}

/// ditto
auto uniformDistribution(E, UniformRNG)
                        (UniformRNG rng)
    if (is(E == enum) && isUniformRNG!UniformRNG)
{
    return new UniformDistribution!(E, UniformRNG)(rng);
}

/// ditto
auto uniformDistribution(E)()
    if (is(E == enum))
{
    return new UniformDistribution!(E, Random)(rndGen);
}

///
unittest
{
    enum Fruit { Apple = 12, Mango = 29, Pear = 72 }

    foreach (immutable f; uniformDistribution!Fruit().take(100))
    {
        assert(f == Fruit.Apple || f == Fruit.Mango || f == Fruit.Pear);
    }
}

unittest
{
    enum Fruit { Apple = 12, Mango = 29, Pear = 72 }

    foreach (UniformRNG; UniformRNGTypes)
    {
        auto rng = new UniformRNG(unpredictableSeed);
        foreach (immutable f; uniformDistribution!Fruit(rng).take(100))
        {
            assert(f == Fruit.Apple || f == Fruit.Mango || f == Fruit.Pear);
        }

        // check that .save works
        auto udist1 = uniformDistribution!Fruit(rng);
        auto udist2 = udist1.save;
        assert(udist2 !is udist1);
        foreach (u1, u2; lockstep(udist1.take(100), udist2.take(100)))
        {
            assert(u1 == u2);
        }
    }
}

/**
 * Generates a uniformly-distributed floating point number of type
 * $(D T) in the range [0, 1).  If no random number generator is
 * specified, the default RNG $(D rndGen) will be used as the source
 * of randomness.
 *
 * $(D uniform01) offers a faster generation of random variates than
 * the equivalent $(D uniform!"[&#41;"(0.0, 1.0)) and so may be preferred
 * for some applications.
 */
T uniform01(T = double)()
    if (isFloatingPoint!T)
{
    return uniform01!T(rndGen);
}

/// ditto
T uniform01(T = double, UniformRNG)(UniformRNG rng)
    if (isFloatingPoint!T && isUniformRNG!UniformRNG)
out (result)
{
    assert(0 <= result);
    assert(result < 1);
}
body
{
    alias R = typeof(rng.front);
    static if (isIntegral!R)
    {
        enum T factor = 1 / (cast(T) 1 + rng.max - rng.min);
    }
    else static if (isFloatingPoint!R)
    {
        enum T factor = 1 / (rng.max - rng.min);
    }
    else
    {
        static assert(false);
    }

    while (true)
    {
        immutable T u = (rng.front - rng.min) * factor;
        rng.popFront();
        if (u < 1)
        {
            return u;
        }
    }

    // Shouldn't ever get here.
    assert(false);
}

unittest
{
    foreach (UniformRNG; UniformRNGTypes)
    {
        import std.typetuple;

        foreach (T; TypeTuple!(float, double, real))
        {
            UniformRNG rng = new UniformRNG(unpredictableSeed);

            auto a = uniform01();
            assert(is(typeof(a) == double));
            assert(0 <= a && a < 1);

            auto b = uniform01(rng);
            assert(is(typeof(a) == double));
            assert(0 <= b && b < 1);

            auto c = uniform01!T();
            assert(is(typeof(c) == T));
            assert(0 <= c && c < 1);

            auto d = uniform01!T(rng);
            assert(is(typeof(d) == T));
            assert(0 <= d && d < 1);

            T init = uniform01!T(rng);
            size_t i = 50;
            while (--i && uniform01!T(rng) == init) {}
            assert(i > 0);
            assert(i < 50);
        }
    }
}

/**
 * Provides an infinite sequence of random numbers uniformly distributed in the
 * interval [0, 1).  If no RNG is specified, $(D uniformDistribution) will use
 * the default generator $(D rndGen).
 */
final class Uniform01Distribution(T, UniformRNG)
    if (isFloatingPoint!T && isUniformRNG!UniformRNG)
{
  private:
    UniformRNG _rng;
    T _value;

  public:
    enum T min = 0;
    enum T max = 1;
    enum bool isRandomDistribution = true;

    this(UniformRNG rng)
    {
        _rng = rng;
        popFront();
    }

    this(typeof(this) that)
    {
        this._rng = that._rng;
        this._value = that._value;
    }

    /// Range primitives.
    enum bool empty = false;

    /// ditto
    T front() @property @safe const nothrow pure
    {
        return _value;
    }

    /// ditto
    void popFront()
    {
        _value = uniform01!(T, UniformRNG)(_rng);
    }

    /// ditto
    static if (isForwardRange!UniformRNG)
    {
        typeof(this) save() @property
        {
            auto ret = new typeof(this)(this);
            ret._rng = this._rng.save;
            return ret;
        }
    }
}

/// ditto
auto uniform01Distribution(T = double, UniformRNG)(UniformRNG rng)
    if (isFloatingPoint!T && isUniformRNG!UniformRNG)
{
    return new Uniform01Distribution!(T, UniformRNG)(rng);
}

/// ditto
auto uniform01Distribution(T = double)()
    if (isFloatingPoint!T)
{
    return new Uniform01Distribution!(T, Random)(rndGen);
}

unittest
{
    foreach (immutable u; uniform01Distribution().take(1_000_000))
    {
        assert(0 <= u);
        assert(u < 1);
    }

    foreach (UniformRNG; UniformRNGTypes)
    {
        import std.typetuple;

        foreach (T; TypeTuple!(float, double, real))
        {
            auto rng = new UniformRNG;

            foreach (immutable u; uniform01Distribution!T(rng.save).take(100))
            {
                assert(u == uniform01!T(rng));
                assert(0 <= u);
                assert(u < 1);
            }

            // check that .save works
            auto udist1 = uniform01Distribution(rng);
            auto udist2 = udist1.save;
            assert(udist2 !is udist1);
            foreach (u1, u2; lockstep(udist1.take(100), udist2.take(100)))
            {
                assert(u1 == u2);
            }
        }
    }
}