in reply to Re^6: Odometer pattern iterator (in C).
in thread Odometer pattern iterator (in C). (Updated.)

I have now compared it to the solution you have posted and it probably amounts to almost the same algorithm...

  • Comment on Re^7: Odometer pattern iterator (in C).

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Re^8: Odometer pattern iterator (in C).
by BrowserUk (Patriarch) on May 29, 2015 at 14:34 UTC

    I attempted to adapt your Perl to C, but I made a mistake somewhere. I don't know if you can see the problem, but I can't and I need to move on:

    U8 *firstIdx(  U8 N, U8 M ) {
    U32 i;
    U8 *a = malloc( M );
    for( i = N - M; i < N; ++i ) a[ i - M + 1 ] = i;
    return a;
}

U8 *nextIdx( U8 N, U8 M, U8 *idx ) {
    U32 i = M - 1, j;

    if( idx[ i ] == i ) return NULL;

    while( i > 0 && idx[ i ] - idx[ i - 1 ] == 1 ) i--;

    --idx[ i ];

    for( j = i+1; j < M; ++j ) idx[ i ] += N - idx[ M - j + i ];

    return idx;
}

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      You cannot do this inplace: 

      for( j = i+1; j < M; ++j ) idx[ i ] += N - idx[ M - j + i ];

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      as you are overwriting values that are still needed. This corresponds to the "reversing" the string part of the regex. In addition, you misread the formula, there is no += involved.

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