#include <assert.h>
#include <errno.h>
#include <fcntl.h>
#include <stdlib.h>
#include <stralloc.h>
#include <string.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <unistd.h>

const char* cmdname;

/* print error message and exit */
void die(const char* reason) {
   stralloc msg = {0};
   if (stralloc_copys(&msg, cmdname) &&
	 stralloc_cats(&msg, ": ") &&
	 stralloc_cats(&msg, strerror(errno)) &&
	 stralloc_cats(&msg, ": ") &&
	 stralloc_cats(&msg, reason) &&
	 stralloc_cats(&msg, "\n")) {
      write(2, msg.s, msg.len);
   }
   exit(1);
}

/* print usage message and exit */
void usage() {
   stralloc usage = {0};
   if (stralloc_copys(&usage, "Usage: ") &&
	 stralloc_cats(&usage, cmdname) &&
	 stralloc_cats(&usage, " source\n")) {
      write(2, usage.s, usage.len);
   }
   exit(1);
}

/* print interval [cp, endp) to standard output
   and append a newline if none is included
   at the end of the comment */
void print(const char* cp, const char* endp) {
   char lastch = endp[-1];
   while (cp < endp) {
      ssize_t nbytes = write(1, cp, endp - cp);
      if (nbytes < 0) {
	 die("write error");
      }
      cp += nbytes;
   }
   if (lastch != '\n') {
      write(1, "\n", 1);
   }
}

/* invoke print for each comment found in [cp, endp) */
void extract_comments(const char* cp, const char* endp) {
   enum {
      START,  /* initial state */
      SLASH1, /* '/' seen, could start comment */
      SLASH2, /* "//" seen, within comment */
      STAR1,  /* '/', '*' seen, within comment */
      STAR2,  /* '/', '*', ..., '*' seen */
   } state = START;
   const char* comment = 0;
   while (cp < endp) {
      char ch = *cp;
      switch (state) {
	 case START:
	    if (ch == '/') state = SLASH1;
	    break;
	 case SLASH1:
	    if (ch == '/') {
	       state = SLASH2;
	       comment = cp - 1;
	    } else if (ch == '*') {
	       state = STAR1;
	       comment = cp - 1;
	    } else {
	       state = START;
	    }
	    break;
	 case SLASH2:
	    if (ch == '\n') {
	       print(comment, cp+1);
	       state = START;
	    }
	    break;
	 case STAR1:
	    if (ch == '*') {
	       state = STAR2;
	    }
	    break;
	 case STAR2:
	    if (ch == '/') {
	       print(comment, cp+1);
	       state = START;
	    } else {
	       state = STAR1;
	    }
	    break;
      }
      ++cp;
   }
   if (state == SLASH2 || state == STAR1 || state == STAR2) {
      /* print unfinished comment */
      print(comment, cp);
   }
}

int main(int argc, char* argv[]) {
   cmdname = *argv++; --argc;
   if (argc != 1) usage();
   const char* file = *argv++; --argc;

   int fd = open(file, O_RDONLY);
   if (fd < 0) die(file);

   struct stat statbuf; if (fstat(fd, &statbuf) < 0) die(file);
   off_t nbytes = statbuf.st_size;
   char* buf = (char*) mmap(0, nbytes, PROT_READ, MAP_SHARED, fd, 0);
   if (buf == MAP_FAILED) die(file);

   extract_comments(buf, buf + nbytes);

   if (munmap((caddr_t) buf, nbytes) < 0) die(file);
   if (close(fd) < 0) die(file);
}