#define _POSIX_C_SOURCE 200809L

#include <assert.h>    /* assert */
#include <sys/stat.h>  /* mkdirat, openat */
#include <sys/types.h> /* mkdirat, openat */
#include <unistd.h>    /* unlinkat, linkat, close, write, … */
#include <fcntl.h>     /* mkdirat, unlinkat, openat, linkat, … */
#include <string.h>    /* strlen */
#include <stdbool.h>   /* bool */
#include <errno.h>
#include <err.h>

/* Napište podprogram ‹fcount›, kterému je předán popisovač otevřené
 * složky, a který spočítá, kolik je (přímo) v této odkazů na
 * «obyčejné» soubory (je-li tentýž soubor odkázán více než jednou,
 * počítá se každý odkaz). Nezáporná návratová hodnota určuje počet
 * nalezených souborů, -1 pak indikuje systémovou chybu. */

int fcount( int dir_fd );

/* ┄┄┄┄┄┄┄ %< ┄┄┄┄┄┄┄┄┄┄ následují testy ┄┄┄┄┄┄┄┄┄┄ %< ┄┄┄┄┄┄┄ */

static int create_and_open_dir( int base_dir, const char *name )
{
    if ( mkdirat( base_dir, name, 0777 ) == -1 )
        err( 2, "creating directory %s", name );

    int fd = openat( base_dir, name, O_RDONLY | O_DIRECTORY, 0777 );
    if ( fd == -1 )
        err( 2, "opening directory %s", name );

    return fd;
}

static void unlink_if_exists( int fd, const char *name, bool is_dir )
{
    if ( unlinkat( fd, name, is_dir ? AT_REMOVEDIR : 0 ) == -1 &&
         errno != ENOENT )
        err( 2, "unlinking %s", name );
}

static void close_or_warn( int fd, const char *name )
{
    if ( close( fd ) == -1 )
        warn( "closing %s", name );
}

static int create_file( int dir_fd, const char *name )
{
    int fd;

    if ( ( fd = openat( dir_fd, name,
                        O_CREAT | O_TRUNC | O_RDWR,
                        0666 ) ) == -1 )
        err( 2, "creating %s", name );

    return fd;
}

static void write_file( int dir, const char *name, const char *str )
{
    int fd = create_file( dir, name );

    if ( write( fd, str, strlen( str ) ) == -1 )
        err( 2, "writing file %s", name );

    close_or_warn( fd, name ); 
}

static void unlink_test_files()
{
    int dir1, dir2;

    dir1 = openat( AT_FDCWD, "zt.p1_test_dir1", O_DIRECTORY );

    if ( dir1 != -1 )
    {
        unlink_if_exists( dir1, "zt.p1_a", false );
        unlink_if_exists( dir1, "zt.p1_b", false );
        unlink_if_exists( dir1, "zt.p1_c", false );
        unlink_if_exists( dir1, "zt.p1_d", false );
        unlink_if_exists( dir1, "zt.p1_e", false );
        unlink_if_exists( dir1, "zt.p1_f", false );
        unlink_if_exists( dir1, "zt.p1_g", false );

        dir2 = openat( dir1, "zt.p1_test_dir2", O_DIRECTORY );

        if ( dir2 != -1 )
        {
            unlink_if_exists( dir2, "zt.p1_h", false );
            close( dir2 );
        }

        unlink_if_exists( dir1, "zt.p1_test_dir2", true );
        close( dir1 );
    }

    unlink_if_exists( AT_FDCWD, "zt.p1_test_dir1", true );
}

int main( void )
{
    int dir1, dir2;
    int dir = open( ".", O_RDONLY );

    if ( dir == -1 )
       err( 2, "opening working directory" );

    close_or_warn( dir, "working directory" );
    assert( fcount( dir ) == -1 );

    unlink_test_files();

    dir1 = create_and_open_dir( AT_FDCWD, "zt.p1_test_dir1" );

    assert( fcount( dir1 ) == 0 );

    write_file( dir1, "zt.p1_a", "All I ever wanted" );
    write_file( dir1, "zt.p1_b", "All I ever needed" );
    write_file( dir1, "zt.p1_c", "Is here in my arms" );
    write_file( dir1, "zt.p1_d", "Words are very unnecessary" );
    write_file( dir1, "zt.p1_e", "They can only do harm" );

    assert( fcount( dir1 ) == 5 );
    assert( fcntl( dir1, F_GETFD ) != -1 );

    dir2 = create_and_open_dir( dir1, "zt.p1_test_dir2" );
    assert( fcount( dir1 ) == 5 );

    if ( linkat( dir1, "zt.p1_a", dir1, "zt.p1_f", 0 ) == -1 )
        err( 2, "creating a link to %s", "zt.p1_a" );

    assert( fcount( dir1 ) == 6 );

    if ( symlinkat( "zt.p1_b", dir1, "zt.p1_g" ) == -1 )
        err( 2, "creating a symlink to %s", "zt.p1_b" );

    assert( fcount( dir1 ) == 6 );
    assert( fcount( dir2 ) == 0 );

    write_file( dir2, "zt.p1_h", "Life is a fight." );

    assert( fcount( dir2 ) == 1 );

    close_or_warn( dir2, "zt.p1_test_dir2" ); 
    close_or_warn( dir1, "zt.p1_test_dir1" ); 

    unlink_test_files();

    return 0;
}