Convert hex to binary faster than xxd part 2/2

   static int hexdigit(char x)
   {
     if (x >= '0' && x <= '9') return x - '0';
     if (x >= 'a' && x <= 'f') return 10 + (x - 'a');
     if (x >= 'A' && x <= 'F') return 10 + (x - 'A');
     return -1;
   }
   int hexparse(unsigned char *y,long long len,const char *x)
   {
     if (!x) return 0;
     while (len > 0) {
       int digit0;
       int digit1;
       digit0 = hexdigit(x[0]); if (digit0 == -1) return 0;
       digit1 = hexdigit(x[1]); if (digit1 == -1) return 0;
       *y++ = digit1 + 16 * digit0;
       --len;
       x += 2;
     }
     if (x[0]) return 0;
     return 1;
   }
   #define BLOCK 4096
   static unsigned char buf[BLOCK + 1];
   int main(int argc, char **argv) {
       long long r,w;
       for (;;) {
           r = readblock(0, buf, BLOCK);
           if (r == -1) _exit(0);
           for (;;) {
               if (r <= 0) goto end;
               if (buf[r - 1] == '\n') { --r; continue; }
               if (buf[r - 1] == '\r') { --r; continue; }
               break;
           }
           buf[r] = 0;
           if (r % 2) _exit(0);
           w = r / 2;
           if (!hexparse(buf, w, (char *)buf)) break;
           if (writeall(1, buf, w) == -1) break;
           if (r != BLOCK) break;
       }
   end:
       if (fsyncfd(1) == -1) _exit(0);
       _exit(0);
   }