#include <stdio.h>
#include <time.h>

#define DEVICE "ppp0"
#define COUNT_VAR "ppp0_bytes"
#define UPDATE_VAR "ppp0_updated"

void read_procnet(char *device, int *bytesrx, int *bytestx) {
	FILE *file = fopen("/proc/net/dev", "r");
	char linebuf[256];
	
	while (fgets(linebuf, 256, file)) {
		char *pos = strstr(linebuf, ":");
		if (!pos)
			continue;
		
		if (memcmp(pos - strlen(device), device, strlen(device)) != 0)
			continue;
		
		*bytesrx = get_field(pos + 1, 0);
		*bytestx = get_field(pos + 1, 8);
		
		fclose(file);
		return;
	}
	
	fclose(file);
	return;
}

long long get_nvram(char *varname) {
	char command[64];
	snprintf(command, 64, "/usr/sbin/nvram get %s", varname);
	
	FILE *pipe;
	pipe = popen(command, "r");
	
	char value[64];
	fgets(value, 64, pipe);
	
	pclose(pipe);
	
	return atoll(value);
}

void set_nvram(char *varname, long long value) {
	char command[128];
	snprintf(command, 128, "/usr/sbin/nvram set %s=%lld", varname, value);
	system(command);
}

int period_changed(time_t oldtime, time_t newtime) {
	struct tm period_s;
	time_t period_t;
	
	/* Find the next period change after oldtime */
	
	localtime_r(&oldtime, &period_s);
	
	const int change_day = 24;
	if (period_s.tm_mday >= change_day)
		period_s.tm_mon++;
	
	period_s.tm_mday = change_day;
	period_s.tm_hour = period_s.tm_min = period_s.tm_sec = 0;
	period_t = mktime(&period_s);
	
	/* Is newtime after the change? */
	
	if (period_t < newtime)
		return 1;
	else
		return 0;
}

void update_counter() {
	static int last_count = 0;
	
	int now_ifrx = 0, now_iftx = 0;
	read_procnet(DEVICE, &now_ifrx, &now_iftx);
	int now_count = now_ifrx + now_iftx;
	
	if (now_count < last_count) // Interface counter has reset
		last_count = 0;
	
	long long total = get_nvram(COUNT_VAR);
	
	time_t old_update = get_nvram(UPDATE_VAR);
	if (period_changed(old_update, time(NULL))) // Period has changed
		total = 0;
	
	total += now_count - last_count;
	last_count = now_count;
	
	set_nvram(COUNT_VAR, total);
	set_nvram(UPDATE_VAR, time(NULL));
}

int main() {
	
}