package middleware

import (
	"time"

	"github.com/gin-gonic/gin"
)

const (
	// TimezoneKey is the key used to store the user's timezone in the context
	TimezoneKey = "user_timezone"
	// UserNowKey is the key used to store the timezone-aware "now" time in the context
	UserNowKey = "user_now"
	// TimezoneHeader is the HTTP header name for the user's timezone
	TimezoneHeader = "X-Timezone"
)

// TimezoneMiddleware extracts the user's timezone from the request header
// and stores a timezone-aware "now" time in the context.
//
// The timezone should be an IANA timezone name (e.g., "America/Los_Angeles", "Europe/London")
// or a UTC offset (e.g., "-08:00", "+05:30").
//
// If no timezone is provided or it's invalid, UTC is used as the default.
func TimezoneMiddleware() gin.HandlerFunc {
	return func(c *gin.Context) {
		tzName := c.GetHeader(TimezoneHeader)
		loc := parseTimezone(tzName)

		// Store the location and the current time in that timezone
		c.Set(TimezoneKey, loc)

		// Calculate "now" in the user's timezone, then get start of day
		// For date comparisons, we want to compare against the START of the user's current day
		userNow := time.Now().In(loc)
		startOfDay := time.Date(userNow.Year(), userNow.Month(), userNow.Day(), 0, 0, 0, 0, loc)
		c.Set(UserNowKey, startOfDay)

		c.Next()
	}
}

// parseTimezone parses a timezone string and returns a *time.Location.
// Supports IANA timezone names (e.g., "America/Los_Angeles") and
// UTC offsets (e.g., "-08:00", "+05:30").
// Returns UTC if the timezone is invalid or empty.
func parseTimezone(tz string) *time.Location {
	if tz == "" {
		return time.UTC
	}

	// Try parsing as IANA timezone name first
	loc, err := time.LoadLocation(tz)
	if err == nil {
		return loc
	}

	// Try parsing as UTC offset (e.g., "-08:00", "+05:30")
	// We parse a reference time with the given offset to extract the offset value
	t, err := time.Parse("-07:00", tz)
	if err == nil {
		// time.Parse returns a time, we need to extract the offset
		// The parsed time will have the offset embedded
		_, offset := t.Zone()
		return time.FixedZone(tz, offset)
	}

	// Also try without colon (e.g., "-0800")
	t, err = time.Parse("-0700", tz)
	if err == nil {
		_, offset := t.Zone()
		return time.FixedZone(tz, offset)
	}

	// Default to UTC
	return time.UTC
}

// GetUserTimezone retrieves the user's timezone from the Gin context.
// Returns UTC if not set.
func GetUserTimezone(c *gin.Context) *time.Location {
	loc, exists := c.Get(TimezoneKey)
	if !exists {
		return time.UTC
	}
	return loc.(*time.Location)
}

// GetUserNow retrieves the timezone-aware "now" time from the Gin context.
// This represents the start of the current day in the user's timezone.
// Returns time.Now().UTC() if not set.
func GetUserNow(c *gin.Context) time.Time {
	now, exists := c.Get(UserNowKey)
	if !exists {
		return time.Now().UTC()
	}
	return now.(time.Time)
}