/**
This module contains various combinatorics algorithms.

Authors: Sebastian Wilzbach, Ilya Yaroshenko

License: $(HTTP www.apache.org/licenses/LICENSE-2.0, Apache-2.0)
*/
module mir.combinatorics;

import mir.primitives: hasLength;
import mir.qualifier;
import std.traits;

///
version(mir_test) unittest
{
    import mir.ndslice.fuse;

    assert(['a', 'b'].permutations.fuse == [['a', 'b'], ['b', 'a']]);
    assert(['a', 'b'].cartesianPower(2).fuse == [['a', 'a'], ['a', 'b'], ['b', 'a'], ['b', 'b']]);
    assert(['a', 'b'].combinations(2).fuse == [['a', 'b']]);
    assert(['a', 'b'].combinationsRepeat(2).fuse == [['a', 'a'], ['a', 'b'], ['b', 'b']]);

    assert(permutations!ushort(2).fuse == [[0, 1], [1, 0]]);
    assert(cartesianPower!ushort(2, 2).fuse == [[0, 0], [0, 1], [1, 0], [1, 1]]);
    assert(combinations!ushort(2, 2).fuse == [[0, 1]]);
    assert(combinationsRepeat!ushort(2, 2).fuse == [[0, 0], [0, 1], [1, 1]]);

    assert([3, 1].permutations!ubyte.fuse == [[3, 1], [1, 3]]);
    assert([3, 1].cartesianPower!ubyte(2).fuse == [[3, 3], [3, 1], [1, 3], [1, 1]]);
    assert([3, 1].combinations!ubyte(2).fuse == [[3, 1]]);
    assert([3, 1].combinationsRepeat!ubyte(2).fuse == [[3, 3], [3, 1], [1, 1]]);
}

/**
Checks whether we can do basic arithmetic operations, comparisons, modulo and
assign values to the type.
*/
private template isArithmetic(R)
{
    enum bool isArithmetic = is(typeof(
    (inout int = 0)
    {
        R r = 1;
        R test = (r * r / r + r - r) % r;
        if (r < r && r > r) {}
    }));
}

/**
Checks whether we can do basic arithmetic operations, comparison and modulo
between two types. R needs to support item assignment of S (it includes S).
Both R and S need to be arithmetic types themselves.
*/
private template isArithmetic(R, S)
{
    enum bool isArithmetic = is(typeof(
    (inout int = 0)
    {
        if (isArithmetic!R && isArithmetic!S) {}
        S s = 1;
        R r = 1;
        R test = r * s + r * s;
        R test2 = r / s + r / s;
        R test3 = r - s + r - s;
        R test4 = r % s + r % s;
        if (r < s && s > r) {}
        if (s < r && r > s) {}
    }));
}

/**
Computes the $(WEB en.wikipedia.org/wiki/Binomial_coefficient, binomial coefficient)
of n and k.
It is also known as "n choose k" or more formally as `_n!/_k!(_n-_k)`.
If a fixed-length integer type is used and an overflow happens, `0` is returned.

Uses the generalized binomial coefficient for negative integers and floating
point number

Params:
    n = arbitrary arithmetic type
    k = arbitrary arithmetic type

Returns:
    Binomial coefficient
*/
R binomial(R = ulong, T)(T n, T k)
    if (isArithmetic!(R, T) &&
        ((is(typeof(T.min < 0)) && is(typeof(T.init & 1))) || !is(typeof(T.min < 0))) )
{
    R result = 1;

    enum hasMinProperty = is(typeof(T.min < 0));
    // only add negative support if possible
    static if ((hasMinProperty && T.min < 0) || !hasMinProperty)
    {
        if (n < 0)
        {
            if (k >= 0)
            {
                return (k & 1 ? -1 : 1) * binomial!(R, T)(-n + k-1, k);
            }
            else if (k <= n)
            {
                return ((n-k) & 1 ? -1 : 1) * binomial!(R, T)(-k-1, n-k);
            }
        }
        if (k < 0)
        {
            result = 0;
            return result;
        }
    }

    if (k > n)
    {
        result = 0;
        return result;
    }
    if (k > n - k)
    {
        k = n - k;
    }
    // make a copy of n (could be a custom type)
    for (T i = 1, m = n; i <= k; i++, m--)
    {
        // check whether an overflow can happen
        // hasMember!(Result, "max") doesn't work with dmd2.068 and ldc 0.17
        static if (is(typeof(0 > R.max)))
        {
            if (result / i > R.max / m) return 0;
            result = result / i * m + result % i * m / i;
        }
        else
        {
            result = result * m / i;
        }
    }
    return result;
}

///
pure version(mir_test) unittest
{
    assert(binomial(5, 2) == 10);
    assert(binomial(6, 4) == 15);
    assert(binomial(3, 1) == 3);

    import std.bigint: BigInt;
    assert(binomial!BigInt(1000, 10) == BigInt("263409560461970212832400"));
}

pure nothrow @safe @nogc version(mir_test) unittest
{
    assert(binomial(5, 1) == 5);
    assert(binomial(5, 0) == 1);
    assert(binomial(1, 2) == 0);
    assert(binomial(1, 0) == 1);
    assert(binomial(1, 1) == 1);
    assert(binomial(2, 1) == 2);
    assert(binomial(2, 1) == 2);

    // negative
    assert(binomial!long(-5, 3) == -35);
    assert(binomial!long(5, -3) == 0);
}

version(mir_test) unittest
{
    import std.bigint;

    // test larger numbers
    assert(binomial(100, 10) == 17_310_309_456_440);
    assert(binomial(999, 5) == 82_09_039_793_949);
    assert(binomial(300, 10) == 1_398_320_233_241_701_770LU);
    assert(binomial(300LU, 10LU) == 1_398_320_233_241_701_770LU);

    // test overflow
    assert(binomial(500, 10) == 0);

    // all parameters as custom types
    BigInt n = 1010, k = 9;
    assert(binomial!BigInt(n, k) == BigInt("2908077120956865974260"));

    // negative
    assert(binomial!BigInt(-5, 3) == -35);
    assert(binomial!BigInt(5, -3) == 0);
    assert(binomial!BigInt(-5, -7) == 15);
}

/**
Creates a projection of a generalized `Collection` range for the numeric case
case starting from `0` onto a custom `range` of any type.

Params:
    collection = range to be projected from
    range = random access range to be projected to

Returns:
    Range with a projection to range for every element of collection

See_Also:
    $(LREF permutations), $(LREF cartesianPower), $(LREF combinations),
    $(LREF combinationsRepeat)
*/
IndexedRoR!(Collection, Range) indexedRoR(Collection, Range)(Collection collection, Range range)
    if (__traits(compiles, Range.init[size_t.init]))
{
    return IndexedRoR!(Collection, Range)(collection, range);
}

/// ditto
struct IndexedRoR(Collection, Range)
    if (__traits(compiles, Range.init[size_t.init]))
{
    private Collection c;
    private Range r;

    ///
    alias DeepElement = ForeachType!Range;

    ///
    this()(Collection collection, Range range)
    {
        this.c = collection;
        this.r = range;
    }

    ///
    auto lightScope()()
    {
        return IndexedRoR!(LightScopeOf!Collection, LightScopeOf!Range)(.lightScope(c), .lightScope(r));
    }

    ///
    auto lightScope()() const
    {
        return IndexedRoR!(LightConstOf!(LightScopeOf!Collection), LightConstOf!(LightScopeOf!Range))(.lightScope(c), .lightScope(r));
    }

    ///
    auto lightConst()() const
    {
        return IndexedRoR!(LightConstOf!Collection, LightConstOf!Range)(.lightConst(c), .lightConst(r));
    }

    /// Input range primitives
    auto front()() @property
    {
        import mir.ndslice.slice: isSlice, sliced;
        import mir.ndslice.topology: indexed;
        import std.traits: ForeachType;
        static if (isSlice!(ForeachType!Collection))
            return r.indexed(c.front);
        else
            return r.indexed(c.front.sliced);
    }

    /// ditto
    void popFront() scope
    {
        c.popFront;
    }

    /// ditto
    bool empty()() @property scope const
    {
        return c.empty;
    }

    static if (hasLength!Collection)
    {
        /// ditto
        @property size_t length()() scope const
        {
            return c.length;
        }

        /// 
        @property size_t[2] shape()() scope const
        {
            return c.shape;
        }
    }

    static if (__traits(hasMember, Collection, "save"))
    {
        /// Forward range primitive. Calls `collection.save`.
        typeof(this) save()() @property
        {
            return IndexedRoR!(Collection, Range)(c.save, r);
        }
    }
}

///
@safe pure nothrow version(mir_test) unittest
{
    import mir.ndslice.fuse;

    auto perms = 2.permutations;
    assert(perms.save.fuse == [[0, 1], [1, 0]]);

    auto projection = perms.indexedRoR([1, 2]);
    assert(projection.fuse == [[1, 2], [2, 1]]);
}

///
version(mir_test) unittest
{
    import mir.ndslice.fuse;
    // import mir.ndslice.topology: only;

    auto projectionD = 2.permutations.indexedRoR("ab"d);
    assert(projectionD.fuse == [['a', 'b'], ['b', 'a']]);

    // auto projectionC = 2.permutations.indexedRoR(only('a', 'b'));
    // assert(projectionC.fuse == [['a', 'b'], ['b', 'a']]);
}

@safe pure nothrow version(mir_test) unittest
{
    import mir.ndslice.fuse;
    import std.range: dropOne;

    auto perms = 2.permutations;
    auto projection = perms.indexedRoR([1, 2]);
    assert(projection.length == 2);

    // can save
    assert(projection.save.dropOne.front == [2, 1]);
    assert(projection.front == [1, 2]);
}

@safe nothrow @nogc version(mir_test) unittest
{
    import mir.algorithm.iteration: all;
    import mir.ndslice.slice: sliced;
    import mir.ndslice.fuse;
    static perms = 2.permutations;
    static immutable projectionArray = [1, 2];
    auto projection = perms.indexedRoR(projectionArray);

    static immutable result = [1, 2,
                               2, 1];
    assert(result.sliced(2, 2).all!"a == b"(projection));
}

/**
Lazily computes all _permutations of `r` using $(WEB
en.wikipedia.org/wiki/Heap%27s_algorithm, Heap's algorithm).

While generating a new item is in `O(k)` (amortized `O(1)`),
the number of permutations is `|n|!`.

Params:
    n = number of elements (`|r|`)
    r = random access field. A field may not have iteration primitivies.
    alloc = custom Allocator

Returns:
    Forward range, which yields the permutations

See_Also:
    $(LREF Permutations)
*/
Permutations!T permutations(T = uint)(size_t n) @safe pure nothrow
    if (isUnsigned!T && T.sizeof <= size_t.sizeof)
{
    assert(n, "must have at least one item");
    return Permutations!T(new T[n-1], new T[n]);
}

/// ditto
IndexedRoR!(Permutations!T, Range) permutations(T = uint, Range)(Range r) @safe pure nothrow
    if (__traits(compiles, Range.init[size_t.init]))
{
    return permutations!T(r.length).indexedRoR(r);
}

/// ditto
Permutations!T makePermutations(T = uint, Allocator)(auto ref Allocator alloc, size_t n)
    if (isUnsigned!T && T.sizeof <= size_t.sizeof)
{
    assert(n, "must have at least one item");
    import std.experimental.allocator: makeArray;
    auto state = alloc.makeArray!T(n - 1);
    auto indices = alloc.makeArray!T(n);
    return Permutations!T(state, indices);
}

/**
Lazy Forward range of permutations using $(WEB
en.wikipedia.org/wiki/Heap%27s_algorithm, Heap's algorithm).

It always generates the permutations from 0 to `n - 1`,
use $(LREF indexedRoR) to map it to your range.

Generating a new item is in `O(k)` (amortized `O(1)`),
the total number of elements is `n^k`.

See_Also:
    $(LREF permutations), $(LREF makePermutations)
*/
struct Permutations(T)
    if (isUnsigned!T && T.sizeof <= size_t.sizeof)
{
    import mir.ndslice.slice: sliced, Slice;

    private T[] indices, state;
    private bool _empty;
    private size_t _max_states = 1, _pos;

    ///
    alias DeepElement = const T;

    /**
    state should have the length of `n - 1`,
    whereas the length of indices should be `n`
    */
    this()(T[] state, T[] indices) @safe pure nothrow @nogc
    {
        assert(state.length + 1 == indices.length);
        // iota
        foreach (i, ref index; indices)
            index = cast(T)i;
        state[] = 0;

        this.indices = indices;
        this.state = state;

        _empty = indices.length == 0;

        // factorial
        foreach (i; 1..indices.length + 1)
            _max_states *= i;
    }

    /// Input range primitives
    @property Slice!(const(T)*) front()() @safe pure nothrow @nogc
    {
        import mir.ndslice.slice: sliced;
        return indices.sliced;
    }

    /// ditto
    void popFront()() scope @safe pure nothrow @nogc
    {
        import std.algorithm.mutation : swapAt;

        assert(!empty);
        _pos++;

        for (T h = 0;;h++)
        {
            if (h + 2 > indices.length)
            {
                _empty = true;
                break;
            }

            indices.swapAt((h & 1) ? 0 : state[h], h + 1);

            if (state[h] == h + 1)
            {
                state[h] = 0;
                continue;
            }
            state[h]++;
            break;
        }
    }

    /// ditto
    @property bool empty()() @safe pure nothrow @nogc scope const
    {
        return _empty;
    }

    /// ditto
    @property size_t length()() @safe pure nothrow @nogc scope const
    {
        return _max_states - _pos;
    }

    /// 
    @property size_t[2] shape()() scope const
    {
        return [length, indices.length];
    }

    /// Forward range primitive. Allocates using GC.
    @property Permutations save()() @safe pure nothrow
    {
        typeof(this) c = this;
        c.indices = indices.dup;
        c.state = state.dup;
        return c;
    }
}

///
pure @safe nothrow version(mir_test) unittest
{
    import mir.ndslice.fuse;
    import mir.ndslice.topology : iota;

    auto expectedRes = [[0, 1, 2],
         [1, 0, 2],
         [2, 0, 1],
         [0, 2, 1],
         [1, 2, 0],
         [2, 1, 0]];

    auto r = iota(3);
    auto rp = permutations(r.length).indexedRoR(r);
    assert(rp.fuse == expectedRes);

    // direct style
    auto rp2 = iota(3).permutations;
    assert(rp2.fuse == expectedRes);
}

///
@nogc version(mir_test) unittest
{
    import mir.algorithm.iteration: equal;
    import mir.ndslice.slice: sliced;
    import mir.ndslice.topology : iota;

    import std.experimental.allocator.mallocator;

    static immutable expected2 = [0, 1, 1, 0];
    auto r = iota(2);
    auto rp = makePermutations(Mallocator.instance, r.length);
    assert(expected2.sliced(2, 2).equal(rp.indexedRoR(r)));
    dispose(Mallocator.instance, rp);
}

pure @safe nothrow version(mir_test) unittest
{
    // is copyable?
    import mir.ndslice.fuse;
    import mir.ndslice.topology: iota;
    import std.range: dropOne;
    auto a = iota(2).permutations;
    assert(a.front == [0, 1]);
    assert(a.save.dropOne.front == [1, 0]);
    assert(a.front == [0, 1]);

    // length
    assert(1.permutations.length == 1);
    assert(2.permutations.length == 2);
    assert(3.permutations.length == 6);
    assert(4.permutations.length == 24);
    assert(10.permutations.length == 3_628_800);
}

version (assert)
version(mir_test) unittest
{
    // check invalid
    import std.exception: assertThrown;
    import core.exception: AssertError;
    import std.experimental.allocator.mallocator: Mallocator;

    assertThrown!AssertError(0.permutations);
    assertThrown!AssertError(Mallocator.instance.makePermutations(0));
}

/**
Disposes a Permutations object. It destroys and then deallocates the
Permutations object pointed to by a pointer.
It is assumed the respective entities had been allocated with the same allocator.

Params:
    alloc = Custom allocator
    perm = Permutations object

See_Also:
    $(LREF makePermutations)
*/
void dispose(T, Allocator)(auto ref Allocator alloc, auto ref Permutations!T perm)
{
    import std.experimental.allocator: dispose;
    dispose(alloc, perm.state);
    dispose(alloc, perm.indices);
}

/**
Lazily computes the Cartesian power of `r` with itself
for a number of repetitions `D repeat`.
If the input is sorted, the product is in lexicographic order.

While generating a new item is in `O(k)` (amortized `O(1)`),
the total number of elements is `n^k`.

Params:
    n = number of elements (`|r|`)
    r = random access field. A field may not have iteration primitivies.
    repeat = number of repetitions
    alloc = custom Allocator

Returns:
    Forward range, which yields the product items

See_Also:
    $(LREF CartesianPower)
*/
CartesianPower!T cartesianPower(T = uint)(size_t n, size_t repeat = 1) @safe pure nothrow
    if (isUnsigned!T && T.sizeof <= size_t.sizeof)
{
    assert(repeat >= 1, "Invalid number of repetitions");
    return CartesianPower!T(n, new T[repeat]);
}

/// ditto
IndexedRoR!(CartesianPower!T, Range) cartesianPower(T = uint, Range)(Range r, size_t repeat = 1)
if (isUnsigned!T && __traits(compiles, Range.init[size_t.init]))
{
    assert(repeat >= 1, "Invalid number of repetitions");
    return cartesianPower!T(r.length, repeat).indexedRoR(r);
}

/// ditto
CartesianPower!T makeCartesianPower(T = uint, Allocator)(auto ref Allocator alloc, size_t n, size_t repeat)
    if (isUnsigned!T && T.sizeof <= size_t.sizeof)
{
    assert(repeat >= 1, "Invalid number of repetitions");
    import std.experimental.allocator: makeArray;
    return CartesianPower!T(n, alloc.makeArray!T(repeat));
}

/**
Lazy Forward range of Cartesian Power.
It always generates Cartesian Power from 0 to `n - 1`,
use $(LREF indexedRoR) to map it to your range.

Generating a new item is in `O(k)` (amortized `O(1)`),
the total number of elements is `n^k`.

See_Also:
    $(LREF cartesianPower), $(LREF makeCartesianPower)
*/
struct CartesianPower(T)
    if (isUnsigned!T && T.sizeof <= size_t.sizeof)
{
    import mir.ndslice.slice: Slice;

    private T[] _state;
    private size_t n;
    private size_t _max_states, _pos;

    ///
    alias DeepElement = const T;

    /// state should have the length of `repeat`
    this()(size_t n, T[] state) @safe pure nothrow @nogc
    {
        assert(state.length >= 1, "Invalid number of repetitions");

        import std.math: pow;
        this.n = n;
        assert(n <= T.max);
        this._state = state;

        _max_states = pow(n, state.length);
    }

    /// Input range primitives
    @property Slice!(const(T)*) front()() @safe pure nothrow @nogc
    {
        import mir.ndslice.slice: sliced;
        return _state.sliced;
    }

    /// ditto
    void popFront()() scope @safe pure nothrow @nogc
    {
        assert(!empty);
        _pos++;

        /*
        * Bitwise increment - starting from back
        * It works like adding 1 in primary school arithmetic.
        * If a block has reached the number of elements, we reset it to
        * 0, and continue to increment, e.g. for n = 2:
        *
        * [0, 0, 0] -> [0, 0, 1]
        * [0, 1, 1] -> [1, 0, 0]
        */
        foreach_reverse (i, ref el; _state)
        {
            ++el;
            if (el < n)
                break;

            el = 0;
        }
    }

    /// ditto
    @property size_t length()() @safe pure nothrow @nogc scope const
    {
        return _max_states - _pos;
    }

    /// ditto
    @property bool empty()() @safe pure nothrow @nogc scope const
    {
        return _pos == _max_states;
    }

    /// 
    @property size_t[2] shape()() scope const
    {
        return [length, _state.length];
    }

    /// Forward range primitive. Allocates using GC.
    @property CartesianPower save()() @safe pure nothrow
    {
        typeof(this) c = this;
        c._state = _state.dup;
        return c;
    }
}

///
pure nothrow @safe version(mir_test) unittest
{
    import mir.ndslice.fuse;
    import mir.ndslice.topology: iota;
    assert(iota(2).cartesianPower.fuse == [[0], [1]]);
    assert(iota(2).cartesianPower(2).fuse == [[0, 0], [0, 1], [1, 0], [1, 1]]);

    auto three_nums_two_bins = [[0, 0], [0, 1], [0, 2], [1, 0], [1, 1], [1, 2], [2, 0], [2, 1], [2, 2]];
    assert(iota(3).cartesianPower(2).fuse == three_nums_two_bins);

    assert("AB"d.cartesianPower(2).fuse == ["AA"d, "AB"d, "BA"d, "BB"d]);
}

///
@nogc version(mir_test) unittest
{
    import mir.ndslice.topology: iota;
    import mir.algorithm.iteration: equal;
    import mir.ndslice.slice: sliced;

    import std.experimental.allocator.mallocator: Mallocator;
    auto alloc = Mallocator.instance;

    static immutable expected2r2 = [
        0, 0,
        0, 1,
        1, 0,
        1, 1];
    auto r = iota(2);
    auto rc = alloc.makeCartesianPower(r.length, 2);
    assert(expected2r2.sliced(4, 2).equal(rc.indexedRoR(r)));
    alloc.dispose(rc);
}

pure nothrow @safe version(mir_test) unittest
{
    import mir.ndslice.fuse;
    import mir.array.allocation: array;
    import mir.ndslice.topology: iota;
    import std.range: dropOne;

    assert(iota(0).cartesianPower.length == 0);
    assert("AB"d.cartesianPower(3).fuse == ["AAA"d, "AAB"d, "ABA"d, "ABB"d, "BAA"d, "BAB"d, "BBA"d, "BBB"d]);
    auto expected = ["AA"d, "AB"d, "AC"d, "AD"d,
                     "BA"d, "BB"d, "BC"d, "BD"d,
                     "CA"d, "CB"d, "CC"d, "CD"d,
                     "DA"d, "DB"d, "DC"d, "DD"d];
    assert("ABCD"d.cartesianPower(2).fuse == expected);
    // verify with array too
    assert("ABCD"d.cartesianPower(2).fuse == expected);

    assert(iota(2).cartesianPower.front == [0]);

    // is copyable?
    auto a = iota(2).cartesianPower;
    assert(a.front == [0]);
    assert(a.save.dropOne.front == [1]);
    assert(a.front == [0]);

    // test length shrinking
    auto d = iota(2).cartesianPower;
    assert(d.length == 2);
    d.popFront;
    assert(d.length == 1);
}

version(assert)
version(mir_test) unittest
{
    // check invalid
    import std.exception: assertThrown;
    import core.exception: AssertError;
    import std.experimental.allocator.mallocator : Mallocator;

    assertThrown!AssertError(0.cartesianPower(0));
    assertThrown!AssertError(Mallocator.instance.makeCartesianPower(0, 0));
}

// length
pure nothrow @safe version(mir_test) unittest
{
    assert(1.cartesianPower(1).length == 1);
    assert(1.cartesianPower(2).length == 1);
    assert(2.cartesianPower(1).length == 2);
    assert(2.cartesianPower(2).length == 4);
    assert(2.cartesianPower(3).length == 8);
    assert(3.cartesianPower(1).length == 3);
    assert(3.cartesianPower(2).length == 9);
    assert(3.cartesianPower(3).length == 27);
    assert(3.cartesianPower(4).length == 81);
    assert(4.cartesianPower(10).length == 1_048_576);
    assert(14.cartesianPower(7).length == 105_413_504);
}

/**
Disposes a CartesianPower object. It destroys and then deallocates the
CartesianPower object pointed to by a pointer.
It is assumed the respective entities had been allocated with the same allocator.

Params:
    alloc = Custom allocator
    cartesianPower = CartesianPower object

See_Also:
    $(LREF makeCartesianPower)
*/
void dispose(T = uint, Allocator)(auto ref Allocator alloc, auto ref CartesianPower!T cartesianPower)
{
    import std.experimental.allocator: dispose;
    dispose(alloc, cartesianPower._state);
}

/**
Lazily computes all k-combinations of `r`.
Imagine this as the $(LREF cartesianPower) filtered for only strictly ordered items.

While generating a new combination is in `O(k)`,
the number of combinations is `binomial(n, k)`.

Params:
    n = number of elements (`|r|`)
    r = random access field. A field may not have iteration primitivies.
    k = number of combinations
    alloc = custom Allocator

Returns:
    Forward range, which yields the k-combinations items

See_Also:
    $(LREF Combinations)
*/
Combinations!T combinations(T = uint)(size_t n, size_t k = 1) @safe pure nothrow
    if (isUnsigned!T && T.sizeof <= size_t.sizeof)
{
    assert(k >= 1, "Invalid number of combinations");
    return Combinations!T(n, new T[k]);
}

/// ditto
IndexedRoR!(Combinations!T, Range) combinations(T = uint, Range)(Range r, size_t k = 1)
if (isUnsigned!T && __traits(compiles, Range.init[size_t.init]))
{
    assert(k >= 1, "Invalid number of combinations");
    return combinations!T(r.length, k).indexedRoR(r);
}

/// ditto
Combinations!T makeCombinations(T = uint, Allocator)(auto ref Allocator alloc, size_t n, size_t repeat)
    if (isUnsigned!T && T.sizeof <= size_t.sizeof)
{
    assert(repeat >= 1, "Invalid number of repetitions");
    import std.experimental.allocator: makeArray;
    return Combinations!T(cast(T) n, alloc.makeArray!T(cast(T) repeat));
}

/**
Lazy Forward range of Combinations.
It always generates combinations from 0 to `n - 1`,
use $(LREF indexedRoR) to map it to your range.

Generating a new combination is in `O(k)`,
the number of combinations is `binomial(n, k)`.

See_Also:
    $(LREF combinations), $(LREF makeCombinations)
*/
struct Combinations(T)
    if (isUnsigned!T && T.sizeof <= size_t.sizeof)
{
    import mir.ndslice.slice: Slice;

    private T[] state;
    private size_t n;
    private size_t max_states, pos;

    ///
    alias DeepElement = const T;

    /// state should have the length of `repeat`
    this()(size_t n, T[] state) @safe pure nothrow @nogc
    {
        import mir.ndslice.topology: iota;

        assert(state.length <= T.max);
        this.n = n;
        assert(n <= T.max);
        this.max_states = cast(size_t) binomial(n, state.length);
        this.state = state;

        // set initial state and calculate max possibilities
        if (n > 0)
        {
            // skip first duplicate
            if (n > 1 && state.length > 1)
            {
                auto iotaResult = iota(state.length);
                foreach (i, ref el; state)
                {
                    el = cast(T) iotaResult[i];
                }
            }
        }
    }

    /// Input range primitives
    @property Slice!(const(T)*) front()() @safe pure nothrow @nogc
    {
        import mir.ndslice.slice: sliced;
        return state.sliced;
    }

    /// ditto
    void popFront()() scope @safe pure nothrow @nogc
    {
        assert(!empty);
        pos++;
        // we might have bumped into the end state now
        if (empty) return;

        // Behaves like: do _getNextState();  while (!_state.isStrictlySorted);
        size_t i = state.length - 1;
        /* Go from the back to next settable block
        * - A must block must be lower than it's previous
        * - A state i is not settable if it's maximum height is reached
        *
        * Think of it as a backwords search on state with
        * iota(_repeat + d, _repeat + d) as search mask.
        * (d = _nrElements -_repeat)
        *
        * As an example n = 3, r = 2, iota is [1, 2] and hence:
        * [0, 1] -> i = 2
        * [0, 2] -> i = 1
        */
        while (state[i] == n - state.length + i)
        {
            i--;
        }
        state[i] = cast(T)(state[i] + 1);

        /* Starting from our changed block, we need to take the change back
        * to the end of the state array and update them by their new diff.
        * [0, 1, 4] -> [0, 2, 3]
        * [0, 3, 4] -> [1, 2, 3]
        */
        foreach (j; i + 1 .. state.length)
        {
            state[j] = cast(T)(state[i] + j - i);
        }
    }

    /// ditto
    @property size_t length()() @safe pure nothrow @nogc scope const
    {
        return max_states - pos;
    }

    /// ditto
    @property bool empty()() @safe pure nothrow @nogc scope const
    {
        return pos == max_states;
    }

    /// 
    @property size_t[2] shape()() scope const
    {
        return [length, state.length];
    }

    /// Forward range primitive. Allocates using GC.
    @property Combinations save()() @safe pure nothrow
    {
        typeof(this) c = this;
        c.state = state.dup;
        return c;
    }
}

///
pure nothrow @safe version(mir_test) unittest
{
    import mir.ndslice.fuse;
    import mir.ndslice.topology: iota;
    assert(iota(3).combinations(2).fuse == [[0, 1], [0, 2], [1, 2]]);
    assert("AB"d.combinations(2).fuse == ["AB"d]);
    assert("ABC"d.combinations(2).fuse == ["AB"d, "AC"d, "BC"d]);
}

///
@nogc version(mir_test) unittest
{
    import mir.algorithm.iteration: equal;
    import mir.ndslice.slice: sliced;
    import mir.ndslice.topology: iota;

    import std.experimental.allocator.mallocator;
    auto alloc = Mallocator.instance;

    static immutable expected3r2 = [
        0, 1,
        0, 2,
        1, 2];
    auto r = iota(3);
    auto rc = alloc.makeCombinations(r.length, 2);
    assert(expected3r2.sliced(3, 2).equal(rc.indexedRoR(r)));
    alloc.dispose(rc);
}

pure nothrow @safe version(mir_test) unittest
{
    import mir.ndslice.fuse;
    import mir.array.allocation: array;
    import mir.ndslice.topology: iota;
    import std.range: dropOne;

    assert(iota(0).combinations.length == 0);
    assert(iota(2).combinations.fuse == [[0], [1]]);

    auto expected = ["AB"d, "AC"d, "AD"d, "BC"d, "BD"d, "CD"d];
    assert("ABCD"d.combinations(2).fuse == expected);
    // verify with array too
    assert("ABCD"d.combinations(2).fuse == expected);
    assert(iota(2).combinations.front == [0]);

    // is copyable?
    auto a = iota(2).combinations;
    assert(a.front == [0]);
    assert(a.save.dropOne.front == [1]);
    assert(a.front == [0]);

    // test length shrinking
    auto d = iota(2).combinations;
    assert(d.length == 2);
    d.popFront;
    assert(d.length == 1);

    // test larger combinations
    auto expected5 = [[0, 1, 2], [0, 1, 3], [0, 1, 4],
                      [0, 2, 3], [0, 2, 4], [0, 3, 4],
                      [1, 2, 3], [1, 2, 4], [1, 3, 4],
                      [2, 3, 4]];
    assert(iota(5).combinations(3).fuse == expected5);
    assert(iota(4).combinations(3).fuse == [[0, 1, 2], [0, 1, 3], [0, 2, 3], [1, 2, 3]]);
    assert(iota(3).combinations(3).fuse == [[0, 1, 2]]);
    assert(iota(2).combinations(3).length == 0);
    assert(iota(1).combinations(3).length == 0);

    assert(iota(3).combinations(2).fuse == [[0, 1], [0, 2], [1, 2]]);
    assert(iota(2).combinations(2).fuse == [[0, 1]]);
    assert(iota(1).combinations(2).length == 0);

    assert(iota(1).combinations(1).fuse == [[0]]);
}

pure nothrow @safe version(mir_test) unittest
{
    // test larger combinations
    import mir.ndslice.fuse;
    import mir.ndslice.topology: iota;

    auto expected6r4 = [[0, 1, 2, 3], [0, 1, 2, 4], [0, 1, 2, 5],
                        [0, 1, 3, 4], [0, 1, 3, 5], [0, 1, 4, 5],
                        [0, 2, 3, 4], [0, 2, 3, 5], [0, 2, 4, 5],
                        [0, 3, 4, 5], [1, 2, 3, 4], [1, 2, 3, 5],
                        [1, 2, 4, 5], [1, 3, 4, 5], [2, 3, 4, 5]];
    assert(iota(6).combinations(4).fuse == expected6r4);

    auto expected6r3 = [[0, 1, 2], [0, 1, 3], [0, 1, 4], [0, 1, 5],
                        [0, 2, 3], [0, 2, 4], [0, 2, 5], [0, 3, 4],
                        [0, 3, 5], [0, 4, 5], [1, 2, 3], [1, 2, 4],
                        [1, 2, 5], [1, 3, 4], [1, 3, 5], [1, 4, 5],
                        [2, 3, 4], [2, 3, 5], [2, 4, 5], [3, 4, 5]];
    assert(iota(6).combinations(3).fuse == expected6r3);

    auto expected6r2 = [[0, 1], [0, 2], [0, 3], [0, 4], [0, 5],
                        [1, 2], [1, 3], [1, 4], [1, 5], [2, 3],
                        [2, 4], [2, 5], [3, 4], [3, 5], [4, 5]];
    assert(iota(6).combinations(2).fuse == expected6r2);

    auto expected7r5 = [[0, 1, 2, 3, 4], [0, 1, 2, 3, 5], [0, 1, 2, 3, 6],
                        [0, 1, 2, 4, 5], [0, 1, 2, 4, 6], [0, 1, 2, 5, 6],
                        [0, 1, 3, 4, 5], [0, 1, 3, 4, 6], [0, 1, 3, 5, 6],
                        [0, 1, 4, 5, 6], [0, 2, 3, 4, 5], [0, 2, 3, 4, 6],
                        [0, 2, 3, 5, 6], [0, 2, 4, 5, 6], [0, 3, 4, 5, 6],
                        [1, 2, 3, 4, 5], [1, 2, 3, 4, 6], [1, 2, 3, 5, 6],
                        [1, 2, 4, 5, 6], [1, 3, 4, 5, 6], [2, 3, 4, 5, 6]];
    assert(iota(7).combinations(5).fuse == expected7r5);
}

// length
pure nothrow @safe version(mir_test) unittest
{
    assert(1.combinations(1).length == 1);
    assert(1.combinations(2).length == 0);
    assert(2.combinations(1).length == 2);
    assert(2.combinations(2).length == 1);
    assert(2.combinations(3).length == 0);
    assert(3.combinations(1).length == 3);
    assert(3.combinations(2).length == 3);
    assert(3.combinations(3).length == 1);
    assert(3.combinations(4).length == 0);
    assert(4.combinations(10).length == 0);
    assert(14.combinations(11).length == 364);
    assert(20.combinations(7).length == 77_520);
    assert(30.combinations(10).length == 30_045_015);
    assert(30.combinations(15).length == 155_117_520);
}

version(assert)
version(mir_test) unittest
{
    // check invalid
    import std.exception: assertThrown;
    import core.exception: AssertError;
    import std.experimental.allocator.mallocator: Mallocator;

    assertThrown!AssertError(0.combinations(0));
    assertThrown!AssertError(Mallocator.instance.makeCombinations(0, 0));
}

/**
Disposes a Combinations object. It destroys and then deallocates the
Combinations object pointed to by a pointer.
It is assumed the respective entities had been allocated with the same allocator.

Params:
    alloc = Custom allocator
    perm = Combinations object

See_Also:
    $(LREF makeCombinations)
*/
void dispose(T, Allocator)(auto ref Allocator alloc, auto ref Combinations!T perm)
{
    import std.experimental.allocator: dispose;
    dispose(alloc, perm.state);
}

/**
Lazily computes all k-combinations of `r` with repetitions.
A k-combination with repetitions, or k-multicombination,
or multisubset of size k from a set S is given by a sequence of k
not necessarily distinct elements of S, where order is not taken into account.
Imagine this as the cartesianPower filtered for only ordered items.

While generating a new combination with repeats is in `O(k)`,
the number of combinations with repeats is `binomial(n + k - 1, k)`.

Params:
    n = number of elements (`|r|`)
    r = random access field. A field may not have iteration primitivies.
    k = number of combinations
    alloc = custom Allocator

Returns:
    Forward range, which yields the k-multicombinations items

See_Also:
    $(LREF CombinationsRepeat)
*/
CombinationsRepeat!T combinationsRepeat(T = uint)(size_t n, size_t k = 1) @safe pure nothrow
    if (isUnsigned!T && T.sizeof <= size_t.sizeof)
{
    assert(k >= 1, "Invalid number of combinations");
    return CombinationsRepeat!T(n, new T[k]);
}

/// ditto
IndexedRoR!(CombinationsRepeat!T, Range) combinationsRepeat(T = uint, Range)(Range r, size_t k = 1)
    if (isUnsigned!T && __traits(compiles, Range.init[size_t.init]))
{
    assert(k >= 1, "Invalid number of combinations");
    return combinationsRepeat!T(r.length, k).indexedRoR(r);
}

/// ditto
CombinationsRepeat!T makeCombinationsRepeat(T = uint, Allocator)(auto ref Allocator alloc, size_t n, size_t repeat)
    if (isUnsigned!T && T.sizeof <= size_t.sizeof)
{
    assert(repeat >= 1, "Invalid number of repetitions");
    import std.experimental.allocator: makeArray;
    return CombinationsRepeat!T(n, alloc.makeArray!T(repeat));
}

/**
Lazy Forward range of combinations with repeats.
It always generates combinations with repeats from 0 to `n - 1`,
use $(LREF indexedRoR) to map it to your range.

Generating a new combination with repeats is in `O(k)`,
the number of combinations with repeats is `binomial(n, k)`.

See_Also:
    $(LREF combinationsRepeat), $(LREF makeCombinationsRepeat)
*/
struct CombinationsRepeat(T)
    if (isUnsigned!T && T.sizeof <= size_t.sizeof)
{
    import mir.ndslice.slice: Slice;

    private T[] state;
    private size_t n;
    private size_t max_states, pos;

    ///
    alias DeepElement = const T;

    /// state should have the length of `repeat`
    this()(size_t n, T[] state) @safe pure nothrow @nogc
    {
        this.n = n;
        assert(n <= T.max);
        this.state = state;
        size_t repeatLen = state.length;

        // set initial state and calculate max possibilities
        if (n > 0)
        {
            max_states = cast(size_t) binomial(n + repeatLen - 1, repeatLen);
        }
    }

    /// Input range primitives
    @property Slice!(const(T)*) front()() @safe pure nothrow @nogc
    {
        import mir.ndslice.slice: sliced;
        return state.sliced;
    }

    /// ditto
    void popFront()() scope @safe pure nothrow @nogc
    {
        assert(!empty);
        pos++;

        immutable repeat = state.length;

        // behaves like: do _getNextState();  while (!_state.isSorted);
        size_t i = repeat - 1;
        // go to next settable block
        // a block is settable if its not in the end state (=nrElements - 1)
        while (state[i] == n - 1 && i != 0)
        {
            i--;
        }
        state[i] = cast(T)(state[i] + 1);

        // if we aren't at the last block, we need to set all blocks
        // to equal the current one
        // e.g. [0, 2] -> (upper block: [1, 2]) -> [1, 1]
        if (i != repeat - 1)
        {
            for (size_t j = i + 1; j < repeat; j++)
                state[j] = state[i];
        }
    }

    /// ditto
    @property size_t length()() @safe pure nothrow @nogc scope const
    {
        return max_states - pos;
    }

    /// ditto
    @property bool empty()() @safe pure nothrow @nogc scope const
    {
        return pos == max_states;
    }

    /// 
    @property size_t[2] shape()() scope const
    {
        return [length, state.length];
    }

    /// Forward range primitive. Allocates using GC.
    @property CombinationsRepeat save()() @safe pure nothrow
    {
        typeof(this) c = this;
        c.state = state.dup;
        return c;
    }
}

///
pure nothrow @safe version(mir_test) unittest
{
    import mir.ndslice.fuse;
    import mir.ndslice.topology: iota;

    assert(iota(2).combinationsRepeat.fuse == [[0], [1]]);
    assert(iota(2).combinationsRepeat(2).fuse == [[0, 0], [0, 1], [1, 1]]);
    assert(iota(3).combinationsRepeat(2).fuse == [[0, 0], [0, 1], [0, 2], [1, 1], [1, 2], [2, 2]]);
    assert("AB"d.combinationsRepeat(2).fuse == ["AA"d, "AB"d,  "BB"d]);
}

///
@nogc version(mir_test) unittest
{
    import mir.algorithm.iteration: equal;
    import mir.ndslice.slice: sliced;
    import mir.ndslice.topology: iota;

    import std.experimental.allocator.mallocator;
    auto alloc = Mallocator.instance;

    static immutable expected3r1 = [
        0, 
        1, 
        2];
    auto r = iota(3);
    auto rc = alloc.makeCombinationsRepeat(r.length, 1);
    assert(expected3r1.sliced(3, 1).equal(rc.indexedRoR(r)));
    alloc.dispose(rc);
}

version(mir_test) unittest
{
    import mir.ndslice.fuse;
    import mir.array.allocation: array;
    import mir.ndslice.topology: iota;
    import std.range: dropOne;

    assert(iota(0).combinationsRepeat.length == 0);
    assert("AB"d.combinationsRepeat(3).fuse == ["AAA"d, "AAB"d, "ABB"d,"BBB"d]);

    auto expected = ["AA"d, "AB"d, "AC"d, "AD"d, "BB"d, "BC"d, "BD"d, "CC"d, "CD"d, "DD"d];
    assert("ABCD"d.combinationsRepeat(2).fuse == expected);
    // verify with array too
    assert("ABCD"d.combinationsRepeat(2).fuse == expected);

    assert(iota(2).combinationsRepeat.front == [0]);

    // is copyable?
    auto a = iota(2).combinationsRepeat;
    assert(a.front == [0]);
    assert(a.save.dropOne.front == [1]);
    assert(a.front == [0]);

    // test length shrinking
    auto d = iota(2).combinationsRepeat;
    assert(d.length == 2);
    d.popFront;
    assert(d.length == 1);
}

// length
pure nothrow @safe version(mir_test) unittest
{
    assert(1.combinationsRepeat(1).length == 1);
    assert(1.combinationsRepeat(2).length == 1);
    assert(2.combinationsRepeat(1).length == 2);
    assert(2.combinationsRepeat(2).length == 3);
    assert(2.combinationsRepeat(3).length == 4);
    assert(3.combinationsRepeat(1).length == 3);
    assert(3.combinationsRepeat(2).length == 6);
    assert(3.combinationsRepeat(3).length == 10);
    assert(3.combinationsRepeat(4).length == 15);
    assert(4.combinationsRepeat(10).length == 286);
    assert(11.combinationsRepeat(14).length == 1_961_256);
    assert(20.combinationsRepeat(7).length == 657_800);
    assert(20.combinationsRepeat(10).length == 20_030_010);
    assert(30.combinationsRepeat(10).length == 635_745_396);
}

pure nothrow @safe version(mir_test) unittest
{
    // test larger combinations
    import mir.ndslice.fuse;
    import mir.ndslice.topology: iota;

    auto expected3r1 = [[0], [1], [2]];
    assert(iota(3).combinationsRepeat(1).fuse == expected3r1);

    auto expected3r2 = [[0, 0], [0, 1], [0, 2], [1, 1], [1, 2], [2, 2]];
    assert(iota(3).combinationsRepeat(2).fuse == expected3r2);

    auto expected3r3 = [[0, 0, 0], [0, 0, 1], [0, 0, 2], [0, 1, 1],
                        [0, 1, 2], [0, 2, 2], [1, 1, 1], [1, 1, 2],
                        [1, 2, 2], [2, 2, 2]];
    assert(iota(3).combinationsRepeat(3).fuse == expected3r3);

    auto expected3r4 = [[0, 0, 0, 0], [0, 0, 0, 1], [0, 0, 0, 2],
                        [0, 0, 1, 1], [0, 0, 1, 2], [0, 0, 2, 2],
                        [0, 1, 1, 1], [0, 1, 1, 2], [0, 1, 2, 2],
                        [0, 2, 2, 2], [1, 1, 1, 1], [1, 1, 1, 2],
                        [1, 1, 2, 2], [1, 2, 2, 2], [2, 2, 2, 2]];
    assert(iota(3).combinationsRepeat(4).fuse == expected3r4);

    auto expected4r3 = [[0, 0, 0], [0, 0, 1], [0, 0, 2],
                        [0, 0, 3], [0, 1, 1], [0, 1, 2],
                        [0, 1, 3], [0, 2, 2], [0, 2, 3],
                        [0, 3, 3], [1, 1, 1], [1, 1, 2],
                        [1, 1, 3], [1, 2, 2], [1, 2, 3],
                        [1, 3, 3], [2, 2, 2], [2, 2, 3],
                        [2, 3, 3], [3, 3, 3]];
    assert(iota(4).combinationsRepeat(3).fuse == expected4r3);

    auto expected4r2 = [[0, 0], [0, 1], [0, 2], [0, 3],
                         [1, 1], [1, 2], [1, 3], [2, 2],
                         [2, 3], [3, 3]];
    assert(iota(4).combinationsRepeat(2).fuse == expected4r2);

    auto expected5r3 = [[0, 0, 0], [0, 0, 1], [0, 0, 2], [0, 0, 3], [0, 0, 4],
                        [0, 1, 1], [0, 1, 2], [0, 1, 3], [0, 1, 4], [0, 2, 2],
                        [0, 2, 3], [0, 2, 4], [0, 3, 3], [0, 3, 4], [0, 4, 4],
                        [1, 1, 1], [1, 1, 2], [1, 1, 3], [1, 1, 4], [1, 2, 2],
                        [1, 2, 3], [1, 2, 4], [1, 3, 3], [1, 3, 4], [1, 4, 4],
                        [2, 2, 2], [2, 2, 3], [2, 2, 4], [2, 3, 3], [2, 3, 4],
                        [2, 4, 4], [3, 3, 3], [3, 3, 4], [3, 4, 4], [4, 4, 4]];
    assert(iota(5).combinationsRepeat(3).fuse == expected5r3);

    auto expected5r2 = [[0, 0], [0, 1], [0, 2], [0, 3], [0, 4],
                        [1, 1], [1, 2], [1, 3], [1, 4], [2, 2],
                        [2, 3], [2, 4], [3, 3], [3, 4], [4, 4]];
    assert(iota(5).combinationsRepeat(2).fuse == expected5r2);
}

version(assert)
version(mir_test) unittest
{
    // check invalid
    import std.exception: assertThrown;
    import core.exception: AssertError;
    import std.experimental.allocator.mallocator: Mallocator;

    assertThrown!AssertError(0.combinationsRepeat(0));
    assertThrown!AssertError(Mallocator.instance.makeCombinationsRepeat(0, 0));
}

/**
Disposes a CombinationsRepeat object. It destroys and then deallocates the
CombinationsRepeat object pointed to by a pointer.
It is assumed the respective entities had been allocated with the same allocator.

Params:
    alloc = Custom allocator
    perm = CombinationsRepeat object

See_Also:
    $(LREF makeCombinationsRepeat)
*/
void dispose(T, Allocator)(auto ref Allocator alloc, auto ref CombinationsRepeat!T perm)
{
    import std.experimental.allocator: dispose;
    dispose(alloc, perm.state);
}