package main

import (
	"log"
	"os"
	"strconv"
	"time"
	"unicode/utf8"

	"github.com/gdamore/tcell/v2"
)

const (
	dayWidth       = 2
	smallGap       = 1
	largeGap       = 3
	maxMonthWidth  = 7*dayWidth + 6*smallGap
	maxMonthHeight = (31+6+(7-1))/7 + 2
	/* maxMonthHeight is the height of a 31-day month starting on Sunday (month
	name and weekdays included) */

	titleHeight = smallGap + 1
)

var (
	months = [...]string{"", "Јануар", "Фебруар", "Март", "Април", "Мај", "Јун",
		"Јул", "Август", "Септембар", "Октобар", "Новембар", "Децембар"}
	weekdays = [...]string{"Недеља", "Понедељак", "Уторак", "Среда", "Четвртак",
		"Петак", "Субота"}
)

var (
	wall       tcell.Screen
	defStyle   tcell.Style
	todayStyle tcell.Style
	selStyle   tcell.Style

	monthsWide  int
	monthsHigh  int
	monthsDrawn int
)

var (
	selTime time.Time
	today   time.Time
)

func max(a, b int) int {
	if a > b {
		return a
	}
	return b
}

func min(a, b int) int {
	if a < b {
		return a
	}
	return b
}

func date(year int, month time.Month, day int) time.Time {
	return time.Date(year, month, day, 0, 0, 0, 0, time.Local)
}

func lastDay(m time.Month) time.Time {
	return date(selTime.Year(), m+1, 0)
}

func monthHeight(m time.Month) int {
	return (offset(m) + lastDay(m).Day() - 1) / 7
}

func centeredText(s string, x, y, width int, scr tcell.Screen, style tcell.Style) {
	start := x + max((width-utf8.RuneCountInString(s))/2, 0)
	i := 0
	for _, char := range s {
		if i >= width {
			break
		}

		scr.SetContent(start+i, y, char, nil, style)
		i++
	}
}

func wrappedText(s string, scr tcell.Screen, style tcell.Style) {
	x := 0
	y := 0
	w, h := scr.Size()
	for _, char := range s {
		if x >= w {
			if y >= h {
				break
			}
			x = 0
			y++
		}
		scr.SetContent(x, y, char, nil, style)
		x++
	}
}

func bottomOfMonth(m time.Month, w time.Weekday) (time time.Time) {
	last := lastDay(m)
	day := last.Day()

	if w > last.Weekday() {
		day -= 7
	}

	day += int(w - last.Weekday())

	time = date(selTime.Year(), m, day)
	return
}

func topOfMonth(m time.Month, w time.Weekday) (time time.Time) {
	first := date(selTime.Year(), m, 1)
	day := 1

	if w < first.Weekday() {
		day += 7
	}

	day += int(w - first.Weekday())

	time = date(selTime.Year(), m, day)
	return
}

func moveUp() {
	weekday := selTime.Weekday()
	newTime := selTime.AddDate(0, 0, -7)
	if newTime.Month() == selTime.Month() {
		selTime = newTime
	} else if int(selTime.Month()) > monthsWide {
		newMonth := selTime.Month() - time.Month(monthsWide)
		selTime = bottomOfMonth(newMonth, weekday)
	}
}

func moveDown() {
	weekday := selTime.Weekday()
	newTime := selTime.AddDate(0, 0, +7)
	if newTime.Month() == selTime.Month() {
		selTime = newTime
	} else if int(selTime.Month()) <= (12-1)/monthsWide*monthsWide {
		newMonth := selTime.Month() + time.Month(monthsWide)
		selTime = topOfMonth(newMonth, weekday)
	}
}

func offset(m time.Month) int {
	return (int(date(selTime.Year(), m, 1).Weekday()) + 6) % 7

}

func rowInMonth(m time.Month, day int) int {
	return (day + offset(m) - 1) / 7
}

func leftmostInRow(m time.Month, row int) time.Time {
	if row == 0 {
		return date(selTime.Year(), m, 1)
	}
	if row > monthHeight(m) {
		row = monthHeight(m)
	}

	day := row*7 + 1 - offset(m)

	return date(selTime.Year(), m, day)
}

func rightmostInRow(m time.Month, row int) time.Time {
	if row >= monthHeight(m) {
		return lastDay(m)
	}

	day := row*7 + 7 - offset(m)

	return date(selTime.Year(), m, day)
}

func moveRight() {
	if selTime.Weekday() != time.Sunday && selTime.Day() != lastDay(selTime.Month()).Day() {
		selTime = selTime.AddDate(0, 0, 1)
	} else if int(selTime.Month())%monthsWide != 0 {
		selTime = leftmostInRow(selTime.Month()+1, rowInMonth(selTime.Month(), selTime.Day()))
	}
}

func moveLeft() {
	if selTime.Weekday() != time.Monday && selTime.Day() != 1 {
		selTime = selTime.AddDate(0, 0, -1)
	} else if int(selTime.Month())%monthsWide != 1 && monthsWide != 1 { // special case, if monthsWide
		// is 1, (x%monthsWide) is 0
		selTime = rightmostInRow(selTime.Month()-1, rowInMonth(selTime.Month(), selTime.Day()))
	}
}

func drawMonth(m time.Month, x, y int) {
	centeredText(months[m], x, y, maxMonthWidth, wall, defStyle)

	for i := 1; i <= 7; i++ {
		centeredText(weekdays[i%7], x+(i-1)*(dayWidth+smallGap), y+1, dayWidth, wall, defStyle)
	}

	dayNum := date(selTime.Year(), m+1, 0).Day()
	for i := 1; i <= dayNum; i++ {
		weekday := int(date(selTime.Year(), m, i).Weekday())
		style := defStyle
		if today.Day() == i && today.Month() == m {
			style = todayStyle
		} else if selTime.Day() == i && selTime.Month() == m {
			style = selStyle
		}
		centeredText(
			strconv.Itoa(i),
			x+((weekday-1+7)%7)*(dayWidth+smallGap),
			y+2,
			2,
			wall,
			style)
		if weekday == 0 {
			y++
		}
	}
}

func drawWall() {
	wall.Clear()
	w, h := wall.Size()
	if w < maxMonthWidth+2*smallGap || h < maxMonthHeight+2*smallGap+titleHeight {
		wrappedText("Екран је премали за календар.", wall, defStyle)
		return
	}
	monthsWide = (w - 2*smallGap + largeGap) / (maxMonthWidth + largeGap)
	monthsHigh = (h - smallGap - titleHeight) / (maxMonthHeight + smallGap)
	monthsDrawn = min(monthsHigh*monthsWide, 12)

	startingMonth := time.Month((int(selTime.Month())-1)/monthsDrawn*monthsDrawn + 1)
	endingMonth := time.Month(min(12, int(startingMonth)+monthsDrawn-1))

	centerVert := (w - monthsWide*maxMonthWidth - (monthsWide-1)*largeGap) / 2
	for i, m := 0, startingMonth; i < monthsHigh; i++ {
		for j := 0; j < monthsWide; j++ {
			drawMonth(m, centerVert+j*(maxMonthWidth+largeGap), titleHeight+smallGap+i*(maxMonthHeight+smallGap))
			if m >= endingMonth {
				return
			}
			m++
		}
	}
}

func main() {
	today = time.Now()
	selTime = today
	var err error
	wall, err = tcell.NewScreen()
	if err != nil {
		log.Fatalf("%+v", err)
	}
	if err := wall.Init(); err != nil {
		log.Fatalf("%+v", err)
	}
	defStyle = tcell.StyleDefault.Background(tcell.ColorReset).Foreground(tcell.ColorReset)
	todayStyle = tcell.StyleDefault.Background(tcell.ColorReset).Foreground(tcell.GetColor("blue")).Bold(true).Reverse(true)
	selStyle = tcell.StyleDefault.Background(tcell.ColorReset).Foreground(tcell.GetColor("green")).Bold(true)

	wall.SetStyle(defStyle)
	wall.Clear()
	drawWall()

	quit := func() {
		wall.Fini()
		os.Exit(0)
	}
	for {
		// Update screen
		wall.Show()

		// Poll event
		ev := wall.PollEvent()

		// Process event
		switch ev := ev.(type) {
		case *tcell.EventResize:
		case *tcell.EventKey:
			if ev.Key() == tcell.KeyEscape || ev.Key() == tcell.KeyCtrlC ||
				ev.Rune() == 'Q' || ev.Rune() == 'q' {
				quit()
			}
			switch ev.Rune() {
			case 'h':
				moveLeft()
			case 'j':
				moveDown()
			case 'k':
				moveUp()
			case 'l':
				moveRight()
			}
		}

		drawWall()
		wall.Sync()
	}
}