#include <stdlib.h>
#include <stdio.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <asm/types.h>
#include <fcntl.h>
#include <unistd.h>
#include <linux/hiddev.h>
#include <signal.h>

/*
 * https://www.elecom.co.jp/products/M-XGL20DLBK.html
 * このマウスの Fnボタンの状態を捕捉し表示します。
 */


/*
 *   0: 変化なし
 *   1: Down
 *   2: Up
 */
int button_state(char state, char state_diff, char target_bit) {
  if (state_diff & target_bit) {

    if (state & target_bit) {
      return 1;
    } else {
      return 2;
    };
  }
  return 0;
}


int fd = -1;

void abort_handler(int sig) {
  close(fd);
};


int main (int argc, char **argv) {

  char name[1024];
  char buf[128];
  int count = 0;
      
  /* ioctl() requires a file descriptor, so we check we got one, and then open it */
  if (argc != 2) {
    fprintf(stderr, "usage: %s [hiddevice]\nprobably /dev/usb/hiddev0\n", argv[0]);
    exit(1);
  }

  if ((fd = open(argv[1], O_RDONLY)) < 0) {
    fprintf(stderr, "hiddev %s open", argv[1]);
    exit(1);
  }
  signal(SIGINT, abort_handler);

  /* ioctl() accesses the underlying driver */
  ioctl(fd, HIDIOCGNAME(sizeof(name)), name);

  /* the HIDIOCGVERSION ioctl() returns an int */
  /* so we unpack it and display it */
  printf("hiddev device name is %s\n",name);

  char before_status = 0;
  for (;;) {

    int c = read(fd, buf + count, 8); /* 8以下だと何も返ってこない。この値はどう決まるか？ */

    if (0 > c) {
      printf ("hiddev %s: read error", argv[1]);
      break;
    }
      
    count += c;

    if (128 == count) {
      count = 0;
	
      char state = buf[20];
      char diff = buf[20] ^ before_status;
      before_status = state;
      const char fn1_flag = 0x20;
      const char fn2_flag = 0x40;
      const char fn3_flag = 0x80;

      switch(button_state(state,diff,fn1_flag)) {
      case 1: printf("  Fn1 Down\n"); break;
      case 2: printf("  Fn1 Up\n"); break;
      }

      switch(button_state(state,diff,fn2_flag)) {
      case 1: printf("    Fn2 Down\n"); break;
      case 2: printf("    Fn2 Up\n"); break;
      }

      switch(button_state(state,diff,fn3_flag)) {
      case 1: printf("      Fn3 Down\n"); break;
      case 2: printf("      Fn3 Up\n"); break;
      }

    }
  }
    
  close(fd);

  exit(0);
}