import std::io;
import std::io::file;
import std::thread;

// import some functions from the C standard library
extern fn long strtol(char *buffer, char **endptr, int base);

const String LUMINANCE_DEV = "/sys/bus/iio/devices/iio:device0/in_illuminance_raw";
const String MAX_BRIGHTNESS_DEV = "/sys/class/backlight/intel_backlight/max_brightness";
const String BRIGHTNESS_DEV = "/sys/class/backlight/intel_backlight/brightness";
const String ACTUAL_BRIGHTNESS_DEV = "/sys/class/backlight/intel_backlight/actual_brightness";

char[128] str_buffer;

// max luminance that the display can output
const int MAX_LUMINANCE = 700;

// minimum display brightness in percent
const float MIN_BRIGHTNESS_PERCENT = 0.05;

const int SLEEP_DELAY = 3000;

macro abs(x) { return x < 0 ? -x : x; }
macro clamp(x, l, h) { return max(min(x, h), l); }

/**
 * read a string to the buffer up until the end of the buffer or the first '\n'
 * @require fp != null
**/
fn void! io::File.gets(&self, char[] buffer)
{
    usz i = 0;
    char !c;
    for (; i < buffer.len-1; i++) {
        c = self.read_byte();
        if (catch excuse = c) {
            if (excuse == io::IoError.EOF) {
                break;
            } else {
                return excuse?;
            }
        }
        buffer[i] = c;
    }
    buffer[i] = '\0';
}

/**
 * read a string from a file and convert it to int, should only be used for
 * positive numbers since negative numbers mean an error
 * @ensure return > 0, return < 1000000
**/
fn int! read_int(String path)
{
    static io::File! fp;
	fp = io::file::open((String)path, "r")!;
	defer (void)fp.close();

	fp.gets(str_buffer[..])!;
	int r = (int)strtol(&str_buffer, null, 10);
	return r;
}

// write an intger value to a file as a string
fn void! write_int(String path, int value)
{
    static io::File! fp;
    fp = io::file::open(path, "w")!;
    defer (void)fp.close();

    char[] b = io::bprintf(str_buffer[..], "%d\n", value)!;
	fp.write(b)!;
}

// returns the luminance value in percentage between 0 and 1
macro get_luminance_raw() { return read_int(LUMINANCE_DEV); }

fn float bezier(float t)
{
	const float B = 0.66;
	const float D = 0.28;
	return 3*B*t+(3*D-6*B)*t*t+(1+3*B-3*D)*t*t*t;
}

// TODO: add some command line options like:
//       -v: verbose/logging
//       -c file; specify a config file
fn int main()
{
	// open the luminance device and try reading from it
	if (catch excuse = get_luminance_raw()) {
		io::eprintfn("Error on initial opening of %s: %s", LUMINANCE_DEV, excuse);
		thread::exit(1);
	}

	// get the max brightness
	int! _x = read_int(MAX_BRIGHTNESS_DEV);
	if (catch excuse = _x) {
		io::eprintfn("Error reading max brightness on %s: %s", MAX_BRIGHTNESS_DEV, excuse);
		thread::exit(1);
	}
	int max_brightness = _x!!;
	int min_brightness = (int)(max_brightness * MIN_BRIGHTNESS_PERCENT);

	// try writing to brightness device
	_x = read_int(BRIGHTNESS_DEV);
	if (catch excuse = _x) {
		io::eprintfn("Error reading from %s: %s", BRIGHTNESS_DEV, excuse);
		thread::exit(1);
	}
	if (catch excuse = write_int(BRIGHTNESS_DEV, _x)) {
	   io::eprintfn("Error writing to brightness device %s: %s", BRIGHTNESS_DEV, excuse);
		thread::exit(1);
	}

	// try reading from the actual brightness device
	if (catch excuse = read_int(ACTUAL_BRIGHTNESS_DEV)) {
		io::eprintfn("Error reading from %s: %s", ACTUAL_BRIGHTNESS_DEV, excuse);
		thread::exit(1);
	}

	int x = get_luminance_raw() ?? MAX_LUMINANCE;
	int[<4>] luminance_samples = x;
	usz idx;
	int user_offset;
	int brightness_target = min_brightness;
	int brightness_old = min_brightness;
	int brightness_adjusted = min_brightness;
	int brightness_delta;
	int luminance_median;
	float luminance_percent;
	while (1) {
		int! tmp = get_luminance_raw();
		if (catch excuse = tmp) {
			io::eprintfn("Error reading luminance: %s", excuse);
		} else {
			luminance_samples[idx] = tmp;
			idx = (idx + 1) % luminance_samples.len;
		}

		luminance_median = (float)luminance_samples.sum() / luminance_samples.len;
		luminance_percent = bezier(clamp(luminance_median/MAX_LUMINANCE, 0.0f, 1.0f));

		brightness_old = brightness_target;
		brightness_target = max((int)(luminance_percent * max_brightness), min_brightness);
		brightness_delta = (int)abs((float)(brightness_target - brightness_old)/brightness_old);

		// if the brightness changes by more than 10% reduce the user offset, scaling
		// with the delta, this accounts for large changes in brightness no longer
		// corresponding to the adjustments that the user made to the brightness
		if (brightness_delta > 0.1) {
			user_offset = (int)(user_offset * clamp(1-(int)(brightness_delta*1.2), 0, 1));
		}

		brightness_adjusted = clamp(brightness_target + user_offset, min_brightness, max_brightness);
		if (catch excuse = write_int(BRIGHTNESS_DEV, brightness_adjusted)) {
		  io::eprintfn("Error writing brightness: %s", excuse);
		}

		thread::sleep_ms(SLEEP_DELAY);

		// read actual brightness to get the user offset
		int! actual_brightness;
		int off;
		actual_brightness = read_int(ACTUAL_BRIGHTNESS_DEV);
		if (catch excuse = actual_brightness) {
			io::eprintfn("Error reading actual brightness: %s", excuse);
			actual_brightness = brightness_adjusted;
		}

		// if the actual brightness is zero, it means that the screen is turned
		// off for some reason, usually DPMS. As such it should not count for user
		// offset. Look at the TODO file.
		if (actual_brightness!! != 0) {
		off = actual_brightness!! - brightness_target;
			if (brightness_target != max_brightness || off < 0){
				user_offset = off;
			}
		}

		// TODO: option to turn off logging
		io::printfn("brightness target: %d;  user offset: %d; brightness adjusted: %d", brightness_target, user_offset, brightness_adjusted);
	}
}