quick and dirty branflakes interpreter in C

/*  qdb.c -- quick and dirty branflakes interpreter.
    Version 1.2
    Anonymous Pastebin User  */

/*  Make any use you like of this software. I can't stop you anyway. :)*/

/*  This interpreter uses byte cells, wraps around on overflow or
    underflow, has unpredictable misbehavior if the pointer is moved
    outside the array, and interprets '#' as a command to output the
    first 64 cells of the array, plus a '^' for the pointer. It
    should be fairly fast, but no match for a decent compiler or even
    for a really clever interpreter.  */

#include <stdio.h>
#include <stdlib.h>

#define S 655360

int t[S];
char a[16777216], p[S], bf[256], b;
int c, g, i, j, m, x;
FILE* prog;

int main(int argc, char** argv) {
    if (!(prog=fopen(argv[1], "r"))){
        fprintf(stderr, "can't open the file \"%s\".\n", argv[1]);
        exit(1);
    }
    g=fread(p, 1, S, prog);
    fclose(prog);
    bf['<']=bf['>']=bf['+']=bf['-']=bf[',']=bf['.']=bf['[']=bf[']']=bf['#']=1;
    m=S;
    while (i<g){
        for (b=0; i<g && (!bf[p[i]] || (b?b==p[i]?++c:0:(b=p[i],c=1)));) i++;
        switch (b) {
        case '+': if (c <= 4) p[j++]='a'-1+c; else p[j]='+', t[j++]=c; break;
        case '-': if (c<=4) p[j++]='d'+c; else p[j]='-', t[j++]=c; break;
        case '>': if (c<=4) p[j++]='h'+c; else p[j]='>', t[j++]=c; break;
        case '<': if (c<=4) p[j++]='l'+c; else p[j]='<', t[j++]=c; break;
        case ',': case '.': case '#': while (c--) p[j++]=b; break;
        case '[': while (c--) t[--m]=j; p[j++]='['; break;
        case ']': if (m+c>S) fprintf(stderr, "unbalanced ].\n"), exit(1);
            t[j]=t[m]+1; while (c--) t[t[m++]]=j+1; p[j++]=']'; break;
        }
    }
    p[j]=0;
    if (m!=S) fprintf(stderr, "unbalanced [.\n"), exit(1);
    j=0;
    while (1) {
        switch (p[j]) {
        case '+': a[x]+=t[j++]; break;
        case '-': a[x]-=t[j++]; break;
        case '>': x+=t[j++]; break;
        case '<': x-=t[j++]; break;
        case '[': if (!a[x]) j=t[j]; else j++; break;
        case ']': if (a[x]) j=t[j]; else j++; break;
#if '\n' == 10
        case ',': if ((c=getchar())!=EOF) a[x]=c; j++; break;
        case '.': putchar(a[x]); fflush(stdout); j++; break;
#else
        case ',': if ((c=getchar())!=EOF) a[x]=c=='\n'?10:c; j++; break;
        case '.': putchar(a[x]==10?'\n':a[x]); fflush(stdout); j++; break;
#endif
        case 'a': a[x]++; j++; break;
        case 'b': a[x]+=2; j++; break;
        case 'c': a[x]+=3; j++; break;
        case 'd': a[x]+=4; j++; break;
        case 'e': a[x]--; j++; break;
        case 'f': a[x]-=2; j++; break;
        case 'g': a[x]-=3; j++; break;
        case 'h': a[x]-=4; j++; break;
        case 'i': x++; j++; break;
        case 'j': x+=2; j++; break;
        case 'k': x+=3; j++; break;
        case 'l': x+=4; j++; break;
        case 'm': x--; j++; break;
        case 'n': x-=2; j++; break;
        case 'o': x-=3; j++; break;
        case 'p': x-=4; j++; break;
        case '#': printf("\n"); for(i=0;i<64;i++) printf("%4d",(signed char)a[i]);
            printf("\n%*s\n", x*4+4, "^"); j++; break;
        case 0: exit(0); break;
        }
    }
}