Simple Caesar cypher with negative shift support

/*
* Simple Caesar cypher algorithm
* Preserves case and non-alpha characters
* by The Lightning Stalker
*/

#include <stdio.h>
#include <ctype.h>

#define ALPHA 26                    // letters in alphabet
#define CONVBASE -7                 // conversion base
#if CONVBASE < 0
#define CONVBASE_IS_NEG
#endif
#ifdef CONVBASE_IS_NEG
#define COMPLEMENT ALPHA + CONVBASE // conversion complement
#define CHARLMIN *"a" - CONVBASE    // smallest char without rollover
#define CHARUMIN *"A" - CONVBASE    // . . .
#else
#define COMPLEMENT CONVBASE - ALPHA // conversion complement
#define CHARLMIN *"z" - CONVBASE    // largest character without
#define CHARUMIN *"Z" - CONVBASE    // rollover
#endif

static const char stringin[] = "Joyvtl Jvbuayf";  // string to convert
//~ static const char stringin[] = "Chrome Country";  // . . .

int main (int argc, char **argv)
{
    int c;
    char stringout[256];

    for (c = 0; c < sizeof(stringin); c++)   // Take a look at each
    {                                        // character, one at a time
        if (isalpha(stringin[c]))            // and apply the cypher.
            if (islower(stringin[c]))
#ifdef CONVBASE_IS_NEG
                if (stringin[c] >= CHARLMIN)
#else
                if (stringin[c] <= CHARLMIN)
#endif
                    stringout[c] = stringin[c] + CONVBASE;
                else
                    stringout[c] = stringin[c] + COMPLEMENT;
            else
#ifdef CONVBASE_IS_NEG
                if (stringin[c] >= CHARUMIN)
#else
                if (stringin[c] <= CHARUMIN)
#endif
                    stringout[c] = stringin[c] + CONVBASE;
                else
                    stringout[c] = stringin[c] + COMPLEMENT;
        else
        stringout[c] = stringin[c];
    }
    printf("%s\n", stringout);
    return(0);
}