package reports

import (
	"fmt"
	"regexp"
	"strconv"
	"strings"
	"time"
)

type DateRange struct {
	Start time.Time
	End   time.Time
}

var monthNames = map[string]time.Month{
	"january":   time.January,
	"february":  time.February,
	"march":     time.March,
	"april":     time.April,
	"may":       time.May,
	"june":      time.June,
	"july":      time.July,
	"august":    time.August,
	"september": time.September,
	"october":   time.October,
	"november":  time.November,
	"december":  time.December,
	"jan":       time.January,
	"feb":       time.February,
	"mar":       time.March,
	"apr":       time.April,
	"jun":       time.June,
	"jul":       time.July,
	"aug":       time.August,
	"sep":       time.September,
	"sept":      time.September,
	"oct":       time.October,
	"nov":       time.November,
	"dec":       time.December,
}

func ParseDateRange(dateStr string) (DateRange, error) {
	dateStr = strings.TrimSpace(dateStr)
	now := time.Now().UTC()

	// Check for predefined ranges (case-insensitive)
	lowerDateStr := strings.ToLower(dateStr)
	switch lowerDateStr {
	case "last week":
		return getLastWeek(now), nil
	case "this week":
		return getThisWeek(now), nil
	case "last month":
		return getLastMonth(now), nil
	case "this month":
		return getThisMonth(now), nil
	}

	// Check for month name patterns (case-insensitive)
	if dateRange, err := parseMonthName(dateStr, now); err == nil {
		return dateRange, nil
	}

	// Check for "month year" format
	if dateRange, err := parseMonthYear(dateStr); err == nil {
		return dateRange, nil
	}

	// Check for "since YYYY-MM-DD" format
	if strings.HasPrefix(lowerDateStr, "since ") {
		return parseSinceDateRange(dateStr)
	}

	// Check for custom date range format: "YYYY-MM-DD to YYYY-MM-DD"
	if strings.Contains(lowerDateStr, " to ") {
		return parseCustomDateRange(dateStr)
	}

	return DateRange{}, fmt.Errorf("unsupported date range: %s (supported: 'this week', 'this month', 'last week', 'last month', month names like 'february', 'month year' like 'july 2023', 'since YYYY-MM-DD', or 'YYYY-MM-DD to YYYY-MM-DD')", dateStr)
}

func parseCustomDateRange(dateStr string) (DateRange, error) {
	lowerDateStr := strings.ToLower(dateStr)
	parts := strings.Split(lowerDateStr, " to ")
	if len(parts) != 2 {
		return DateRange{}, fmt.Errorf("invalid date range format: expected 'YYYY-MM-DD to YYYY-MM-DD'")
	}

	startStr := strings.TrimSpace(parts[0])
	endStr := strings.TrimSpace(parts[1])

	// Parse start date
	startDate, err := time.Parse("2006-01-02", startStr)
	if err != nil {
		return DateRange{}, fmt.Errorf("invalid start date '%s': expected YYYY-MM-DD format", startStr)
	}

	// Parse end date
	endDate, err := time.Parse("2006-01-02", endStr)
	if err != nil {
		return DateRange{}, fmt.Errorf("invalid end date '%s': expected YYYY-MM-DD format", endStr)
	}

	// Validate that start date is before or equal to end date
	if startDate.After(endDate) {
		return DateRange{}, fmt.Errorf("start date '%s' must be before or equal to end date '%s'", startStr, endStr)
	}

	// Convert to UTC and set times appropriately
	// Start date: beginning of day (00:00:00)
	startUTC := time.Date(startDate.Year(), startDate.Month(), startDate.Day(), 0, 0, 0, 0, time.UTC)

	// End date: end of day (23:59:59.999999999)
	endUTC := time.Date(endDate.Year(), endDate.Month(), endDate.Day(), 23, 59, 59, 999999999, time.UTC)

	return DateRange{
		Start: startUTC,
		End:   endUTC,
	}, nil
}

func getLastWeek(now time.Time) DateRange {
	// Find the start of current week (Monday)
	weekday := int(now.Weekday())
	if weekday == 0 { // Sunday
		weekday = 7
	}

	// Start of current week
	currentWeekStart := now.AddDate(0, 0, -(weekday - 1)).Truncate(24 * time.Hour)

	// Last week is the week before current week
	lastWeekStart := currentWeekStart.AddDate(0, 0, -7)
	lastWeekEnd := currentWeekStart.Add(-time.Nanosecond)

	return DateRange{
		Start: lastWeekStart,
		End:   lastWeekEnd,
	}
}

func getLastMonth(now time.Time) DateRange {
	// Start of current month
	currentMonthStart := time.Date(now.Year(), now.Month(), 1, 0, 0, 0, 0, time.UTC)

	// Last month start
	lastMonthStart := currentMonthStart.AddDate(0, -1, 0)

	// Last month end (last nanosecond of last month)
	lastMonthEnd := currentMonthStart.Add(-time.Nanosecond)

	return DateRange{
		Start: lastMonthStart,
		End:   lastMonthEnd,
	}
}

func getThisWeek(now time.Time) DateRange {
	// Find the start of current week (Monday)
	weekday := int(now.Weekday())
	if weekday == 0 { // Sunday
		weekday = 7
	}

	// Start of current week
	weekStart := now.AddDate(0, 0, -(weekday - 1)).Truncate(24 * time.Hour)

	// End of current week (Sunday 23:59:59.999999999)
	weekEnd := weekStart.AddDate(0, 0, 7).Add(-time.Nanosecond)

	return DateRange{
		Start: weekStart,
		End:   weekEnd,
	}
}

func getThisMonth(now time.Time) DateRange {
	// Start of current month
	monthStart := time.Date(now.Year(), now.Month(), 1, 0, 0, 0, 0, time.UTC)

	// End of current month (last nanosecond)
	nextMonthStart := monthStart.AddDate(0, 1, 0)
	monthEnd := nextMonthStart.Add(-time.Nanosecond)

	return DateRange{
		Start: monthStart,
		End:   monthEnd,
	}
}

func parseMonthName(dateStr string, now time.Time) (DateRange, error) {
	lowerDateStr := strings.ToLower(dateStr)
	month, exists := monthNames[lowerDateStr]
	if !exists {
		return DateRange{}, fmt.Errorf("not a valid month name")
	}

	// Find the most recent occurrence of this month
	var targetYear int
	currentMonth := now.Month()

	if month == currentMonth {
		// If it's the same month as now, use this year
		targetYear = now.Year()
	} else if month < currentMonth {
		// If the month has already passed this year, use this year
		targetYear = now.Year()
	} else {
		// If the month hasn't come yet this year, use last year
		targetYear = now.Year() - 1
	}

	return getMonthRange(targetYear, month), nil
}

func parseMonthYear(dateStr string) (DateRange, error) {
	// Match patterns like "july 2023", "feb 2022", etc.
	re := regexp.MustCompile(`^(\w+)\s+(\d{4})$`)
	matches := re.FindStringSubmatch(strings.ToLower(dateStr))
	if len(matches) != 3 {
		return DateRange{}, fmt.Errorf("invalid month year format")
	}

	monthStr := matches[1]
	yearStr := matches[2]

	// Parse year
	year, err := strconv.Atoi(yearStr)
	if err != nil {
		return DateRange{}, fmt.Errorf("invalid year: %s", yearStr)
	}

	// Parse month
	month, exists := monthNames[monthStr]
	if !exists {
		return DateRange{}, fmt.Errorf("invalid month name: %s", monthStr)
	}

	return getMonthRange(year, month), nil
}

func getMonthRange(year int, month time.Month) DateRange {
	// Start of the specified month
	monthStart := time.Date(year, month, 1, 0, 0, 0, 0, time.UTC)

	// End of the specified month (last nanosecond)
	nextMonthStart := monthStart.AddDate(0, 1, 0)
	monthEnd := nextMonthStart.Add(-time.Nanosecond)

	return DateRange{
		Start: monthStart,
		End:   monthEnd,
	}
}

func parseSinceDateRange(dateStr string) (DateRange, error) {
	// Remove "since " prefix (case-insensitive)
	lowerDateStr := strings.ToLower(dateStr)
	if !strings.HasPrefix(lowerDateStr, "since ") {
		return DateRange{}, fmt.Errorf("invalid since format: expected 'since YYYY-MM-DD'")
	}
	
	dateOnly := strings.TrimSpace(dateStr[6:]) // Remove "since " prefix
	
	// Parse start date
	startDate, err := time.Parse("2006-01-02", dateOnly)
	if err != nil {
		return DateRange{}, fmt.Errorf("invalid start date '%s' in since format: expected YYYY-MM-DD format", dateOnly)
	}
	
	// Convert to UTC and set start time to beginning of day (00:00:00)
	startUTC := time.Date(startDate.Year(), startDate.Month(), startDate.Day(), 0, 0, 0, 0, time.UTC)
	
	// No end date for "since" format - return zero time for End to indicate open-ended
	return DateRange{
		Start: startUTC,
		End:   time.Time{}, // Zero time indicates no end date
	}, nil
}