#! perl -slw
use strict; # use Config; print $Config{ ccflags };
use Inline C => Config => BUILD_NOISY => 1; #, CCFLAGS => $Config{ ccflags } . "/link /FAs";
use Inline C => <<'END_C',  NAME => '_deBruijn', CLEAN_AFTER_BUILD =>0;

#define PERL_NO_GET_CONTEXT 1

int n, iseq;
STRLEN k;
char *seq, *a;

void dbc( int t, int p ) {
    int i;
    if( t > n ) {
        if( n % p == 0 )
            for( i = 1; i <= p; ++i )
                seq[ iseq++ ] = a[ i ];
    }
    else {
        a[ t ] = a[ t - p ];
        dbc( t+1, p );
        for( i = a[ (t - p) ] + 1; i < k; ++i ) {
            a[ t ] = i;
            dbc( t+1, t );
        }
    }
}

SV *deBruijnC( SV *svAlphabet, SV *len ) {
    int i;

    char *alphabet = SvPV( svAlphabet, k );
    n = (int)SvIV( len );

    iseq = 0;
    Newxz( seq, (int)pow( (double)k, (double)n), char );
    Newxz( a, k * n, char );

    dbc( 1, 1 );

    for( i = 0; i < iseq ; ++i ) {
        seq[ i ] = alphabet[ seq[ i ] ];
    }
    return newSVpv( seq, iseq );
}

END_C