#include <GxEPD2_BW.h>
#include <U8g2_for_Adafruit_GFX.h>
#include <ctime>

#define CELL_SPACING 8
#define FONTSIZE 10

extern GxEPD2_GFX_BASE_CLASS &get_display();
extern U8G2_FOR_ADAFRUIT_GFX u8g2Fonts;

static const char *week_days[] {
	"Mo", "Di", "Mi", "Do", "Fr", "Sa", "So",
};

static const char *months[] {
	"Januar", "Februar", "März", "April", "Mai", "Juni", "Juli",
	"August", "September", "Oktober", "November", "Dezember",
};

enum month_t {
	JANUARY = 0,
	FEBRUARY,
	MARCH,
	APRIL,
	MAY,
	JUNE,
	JULY,
	AUGUST,
	SEPTEMBER,
	OCTOBER,
	NOVEMBER,
	DECEMBER,
};

static int days_in_month(int month, int year) {
	switch (month) {
		case JANUARY:
		case MARCH:
		case MAY:
		case JULY:
		case AUGUST:
		case OCTOBER:
		case DECEMBER:
			return 31;
		case APRIL:
		case JUNE:
		case SEPTEMBER:
		case NOVEMBER:
			return 30;
		case FEBRUARY:
			if (year % 400 == 0) return 29;
			if (year % 100 == 0) return 28;
			if (year %   4 == 0) return 29;
	}
	assert(month >= 0 && month < 12);
	return 0;
}

/*
 * @x0         absolute initial X coord
 * @y0         absolute initial Y coord
 * @w0         width of a cell
 * @h0         height of a cell
 * @xoff       X pos of cell
 * @yoff       Y pos of cell
 * @text       text to draw
 * @highlight  whether text should be highlighted
 */
static void draw_cell_content(int16_t x0, int16_t y0, uint16_t w0, uint16_t h0, uint8_t xoff, uint8_t yoff, String &text, bool highlight = false) {
	uint16_t w = u8g2Fonts.getUTF8Width(text.c_str());
	uint16_t h = FONTSIZE;

	int16_t w_diff = w0 - w;
	int x2 = x0 + xoff * w0 + w_diff/2;
	int y2 = y0 + yoff * h0;

	u8g2Fonts.setCursor(x2, y2);
	u8g2Fonts.print(text);
	if (highlight) {
		get_display().drawCircle(x2 + w/2, y2 - h/2 - 1, w0/2 - 3, GxEPD_BLACK);
	}
}

void draw_calendar(int16_t x0, int16_t y0) {
	struct tm now;
	if (!getLocalTime(&now)) {
		return;
	}

	uint8_t n_days = days_in_month(now.tm_mon, now.tm_year);

	/* on which weekday is the 1st? */
	int8_t day_of_1st = ((1 - 7 - (now.tm_mday - now.tm_wday)) % 7) + 7;

	/* remap days from So-Sa = 0..6 to Mo-So = 0..6 */
	day_of_1st = (day_of_1st - 1 + 7) % 7;

	/* determine sizes */
	u8g2Fonts.setFont(u8g2_font_helvB10_tf);
	uint16_t cell_width, cell_height;
	int w = u8g2Fonts.getUTF8Width(week_days[0]);
	int h = FONTSIZE;
	cell_width = w + CELL_SPACING;
	cell_height = h + CELL_SPACING + 5;

	/* draw title with month and year */
	u8g2Fonts.setFont(u8g2_font_helvB14_tf);
	String title_text = String(months[now.tm_mon]) + " " + String(1900 + now.tm_year);
	draw_cell_content(x0, y0 + 14, cell_width * 7, h, 0, 0, title_text);
	y0 += 14 + 2 * CELL_SPACING;

	/* draw table header with week days */
	u8g2Fonts.setFont(u8g2_font_helvB10_tf);
	for (int i=0; i<7; i++) {
		String week_day = String(week_days[i]);
		draw_cell_content(x0, y0 + FONTSIZE, cell_width, cell_height, i, 0, week_day);
	}

	/* draw days */
	u8g2Fonts.setFont(u8g2_font_helvR10_tf);
	int8_t wday = day_of_1st;
	uint8_t row = 1;
	for (uint8_t day = 1; day <= n_days; day++) {
		String day_str = String(day);
		draw_cell_content(x0, y0 + FONTSIZE, cell_width, cell_height, wday, row, day_str, day == now.tm_mday);

		wday++;
		if (wday > 6) {
			wday = 0;
			row++;
		}
	}
}