Multi-key Quick Sort

multikey-quicksort.c

//! gcc -o multikeyquicksort.bin multikeyquicksort.c -W -Wall -O3 -std=gnu99 

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>

typedef char * string;

int multikey_qsort( string *strs, int len );
string string_create( char *str );

int main( void )
{
    int len = 5;
    string s[10];
    s[0] = string_create( "ccbb" );
    s[1] = string_create( "ccc"  );
    s[2] = string_create( "zzzzz" );
    s[3] = string_create( "aaaa" );
    s[4] = string_create( "aaa" );
    
    multikey_qsort( s, len );

    for ( int i = 0; i < len; i++ ) {
        printf( "%s\n", s[i] );
    }

    return 0;
}


/*** implementation ***/
string string_create( char *str )
{
    return str;
}

int string_cmp( string s1, string s2, int pos )
{
         if ( s1[pos] > s2[pos] ) return  1;
    else if ( s1[pos] < s2[pos] ) return -1;
    else                          return  0;
}

int string_null( string s, int pos )
{
    return s[pos] == '\0';
}

int _multikey_qsort( string *strs, int len, int pos )
{
    if ( len < 2 ) return 0;
    string  pivot    = strs[0];
    int     nnull    = 0;
    int     nsmaller = 0;
    int     nsame    = 0;
    int     nlarger  = 0;
    string  null   [len];
    string  smaller[len];
    string  same   [len];
    string  larger [len];

    for ( int i = 0; i < len; i++ ) {
        if ( string_null( strs[i], pos ) ) {
            null[nnull++] = strs[i];
            continue;
        }

        int result = string_cmp( strs[i], pivot, pos );
             if ( result < 0 ) smaller[nsmaller++] = strs[i];
        else if ( result > 0 ) larger [nlarger++]  = strs[i];
        else                   same   [nsame++]    = strs[i]; 

    }
    
    int head = 0;
    memcpy( &strs[head], null,    nnull    * sizeof(string) );
    head += nnull;
    memcpy( &strs[head], smaller, nsmaller * sizeof(string) );
    head += nsmaller;
    memcpy( &strs[head], same,    nsame    * sizeof(string) );
    head += nsame;
    memcpy( &strs[head], larger,  nlarger  * sizeof(string) );

    head = nnull;
    _multikey_qsort( &strs[head], nsmaller, pos );
    head += nsmaller;
    _multikey_qsort( &strs[head], nsame   , pos+1 );
    head += nsame;
    _multikey_qsort( &strs[head], nlarger , pos );
    
    return 0;
}

int multikey_qsort( string *strs, int len )
{
    _multikey_qsort( strs, len, 0 );
    return 0;
}