super fast print powerset

powerset.c

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

#define BUF_SIZE 4096

void print_powerset(uint32_t n) {
  char buf[BUF_SIZE];
  int offset = 0;
  for (uint32_t i = 0; i < (1 << n); i++) {
    for (uint32_t j = 0; (i >> j) != 0; j++) {
      if ((i >> j) & 1) {
	if (j+1 < 10) {
	  buf[offset++] = j+1 + 48;
	} else {
	  buf[offset++] = ((j+1)%100)/10 + 48;
	  buf[offset++] = (j+1)%10 + 48;	  
	}
	buf[offset++] = ' ';
	if (BUF_SIZE - offset < 13) {
	  write(1, buf, offset);
	  offset = 0;
	}
      }
    }
    buf[offset++] = '\n';
  }
  write(1, buf, offset);
}

int main(int argc, char** argv) {
  if (argc != 2) {
    fprintf(stderr, "expected one argument\n");
    return 1;
  }
  long i = strtol(argv[1], NULL, 10);
  if (i <= 0) {
    fprintf(stderr, "expected positive integer\n");
    return 1;
  }
  print_powerset((uint32_t)i);
}

not all of the #includes are necessary