#if !defined HAVE_COMPOSITION_EX_COLEX_H__
#define      HAVE_COMPOSITION_EX_COLEX_H__
// This file is part of the FXT library.
// Copyright (C) 2010 Joerg Arndt
// License: GNU General Public License version 3 or later,
// see the file COPYING.txt in the main directory.


#include "fxttypes.h"


class composition_ex_colex
// Compositions of n into exactly k parts (k-compositions of n),
// co-lexicographic (colex) order.
// Must have:  n>=k.
{
public:
    ulong n_, k_;  // composition of n into exactly k parts
    ulong *x_;     // data (k elements)
    ulong nk1_;    // ==n-k+1

public:
    composition_ex_colex(ulong n, ulong k)
    // Must have n>=k
    {
        n_ = n;
        k_ = k;
        nk1_ = n - k + 1;  // must be >= 1
        if ( (long)nk1_ < 1 )  nk1_ = 1;  // avoid hang with invalid pair n,k
        x_ = new ulong[k_ + 1];
        x_[k] = 0;  // not one
        first();
    }

    ~composition_ex_colex()  { delete [] x_; }

    const ulong * data()  const  { return x_; }

    void first()
    {
        x_[0] = nk1_;  // all in first position
        for (ulong k=1; k<k_; ++k)  x_[k] = 1;
    }

    void last()
    {
        for (ulong k=0; k<k_; ++k)  x_[k] = 1;
        x_[k_-1] = nk1_;  // all in last position
    }

    ulong next()
    // Return position of rightmost change, return k with last composition.
    {
        ulong j = 0;
        while ( 1==x_[j] )  ++j;  // find first greater than one

        if ( j==k_ )  return k_;  // current composition is last

        ulong v = x_[j];  // value of first greater one
        x_[j] = 1;        // set to 1
        x_[0] = v - 1;    // value-1 to first position
        ++j;
        ++x_[j];          // increment next position

        return  j;
    }

    ulong prev()
    // Return position of rightmost change, return k with last composition.
    {
        const ulong v = x_[0];   // value at first position

        if ( nk1_==v )  return k_; // current composition is first

        x_[0] = 1;          // set first position to 1
        ulong j = 1;
        while ( 1==x_[j] )  ++j;  // find next greater than one
        --x_[j];                  // decrement value
        x_[j-1] = 1 + v;          // set previous position

        return  j;
    }
};
// -------------------------

#endif  // !defined HAVE_COMPOSITION_EX_COLEX_H__
