// For nanosleep(), pread()
#define _POSIX_C_SOURCE 200809L

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>
#include <time.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <sys/wait.h>
#include <assert.h>


// Warning when battery percentage is <= this
static const int WARN_PERC = 10;

// Time to sleep between checks
static const struct timespec SLEEP_SPEC = {60, 0};

// Format string with percentage %d
static const char *MESSAGE_FORMAT = "Battery at %d%%!";


static inline void parse(const char *buffer, bool *discharging, int *charge, int *full) {
	while (true) {
		if (memcmp(buffer, "POWER_SUPPLY_STATUS", 19) == 0) {
			*discharging = memcmp(buffer + 20, "Discharging", 11) == 0;
		} else if (memcmp(buffer, "POWER_SUPPLY_CHARGE_NOW", 23) == 0) {
			*charge = strtol(buffer + 24, NULL, 10);
		} else if (memcmp(buffer, "POWER_SUPPLY_CHARGE_FULL_DESIGN", 31) == 0) {
			*full = strtol(buffer + 32, NULL, 10);
		}
		buffer = strchr(buffer, '\n');
		if (buffer != NULL) buffer++;
		else break;
	}
}

int main(void) {
	int fd = open("/sys/class/power_supply/BAT1/uevent", O_RDONLY);
	if (fd < 0) {
		perror("open");
		return 1;
	}

	char buffer[1024];

	while (true) {
		ssize_t nr = pread(fd, buffer, sizeof buffer, 0);
		if (nr < 0) {
			perror("pread");
			break;
		}

		bool discharging = false;
		int charge = 1, full = 1;
		parse(buffer, &discharging, &charge, &full);

		if (full > 0 && discharging && charge * 100 / full <= WARN_PERC) {
			pid_t pid = fork();
			if (pid < 0) perror("fork");
			else if (pid == 0) {
				char msg[strlen(MESSAGE_FORMAT) + 16];
				snprintf(msg, sizeof msg, MESSAGE_FORMAT, charge * 100 / full);
				char *argv[4] = {"i3-nagbar", "-m", msg, NULL};
				execv("/usr/bin/i3-nagbar", argv);
				perror("execv");
				exit(255);
			} else {
				int status;
				do if (waitpid(pid, &status, 0) < 0 && errno == ECHILD) break;
				while (!WIFEXITED(status) && !WIFSIGNALED(status));
			}
		}

		(void)nanosleep(&SLEEP_SPEC, NULL);
	}

	if (close(fd) < 0) {
		perror("close");
		return 1;
	}
}