Enable motion with HJKL keys

This commit will enable the user of the program to navigate the dates
within a given year, using vi-like keybindings. The movement between
months should act intuitively across different sizes of the terminal.

Main.go

@@ -63,6 +63,14 @@ 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
@@ -93,6 +101,105 @@ func wrappedText(s string, scr tcell.Screen, style tcell.Style) {
 	}
 }
 
+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)
 
@@ -181,13 +288,24 @@ func main() {
 		// Process event
 		switch ev := ev.(type) {
 		case *tcell.EventResize:
-			drawWall()
-			wall.Sync()
 		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()
+	}
 }