package callwindow

import (
	"errors"
	"fmt"
	"time"
)

// WeeklySchedule is a Shanghai-local, half-open weekly timetable. A period
// crossing midnight must be represented by two windows on adjacent days.
type WeeklySchedule struct {
	TimeZone      string              `json:"time_zone"`
	WeeklyWindows map[string][]Window `json:"weekly_windows"`
}

type Window struct {
	Start string `json:"start"`
	End   string `json:"end"`
}

type TaskSchedule struct {
	WeeklySchedule
	StartsAt      string   `json:"starts_at"`
	EndsAt        string   `json:"ends_at"`
	ExcludedDates []string `json:"excluded_dates"`
}

var weekdays = [...]string{"sunday", "monday", "tuesday", "wednesday", "thursday", "friday", "saturday"}

var ErrWindowClosed = errors.New("outside the allowed calling window")

// Evaluate returns the earliest time at which the task/line intersection
// closes. A rejected command must not wait for the next window or choose a
// different line after selection.
func Evaluate(task TaskSchedule, line WeeklySchedule, at time.Time) (time.Time, error) {
	loc, err := time.LoadLocation("Asia/Shanghai")
	if err != nil {
		return time.Time{}, fmt.Errorf("load outbound time zone: %w", err)
	}
	if err := task.WeeklySchedule.validate(); err != nil {
		return time.Time{}, fmt.Errorf("task schedule: %w", err)
	}
	if err := line.validate(); err != nil {
		return time.Time{}, fmt.Errorf("line schedule: %w", err)
	}
	var end time.Time
	if task.StartsAt != "" {
		start, err := time.Parse(time.RFC3339, task.StartsAt)
		if err != nil {
			return time.Time{}, fmt.Errorf("parse task starts_at: %w", err)
		}
		if at.Before(start) {
			return time.Time{}, fmt.Errorf("%w: task is before its validity period", ErrWindowClosed)
		}
		if task.EndsAt != "" {
			end, err = time.Parse(time.RFC3339, task.EndsAt)
			if err != nil {
				return time.Time{}, fmt.Errorf("parse task ends_at: %w", err)
			}
			if !start.Before(end) {
				return time.Time{}, errors.New("task schedule starts_at must precede ends_at")
			}
		}
	} else if task.EndsAt != "" {
		var err error
		end, err = time.Parse(time.RFC3339, task.EndsAt)
		if err != nil {
			return time.Time{}, fmt.Errorf("parse task ends_at: %w", err)
		}
	}
	if !end.IsZero() && !at.Before(end) {
		return time.Time{}, fmt.Errorf("%w: task is past its validity period", ErrWindowClosed)
	}
	seen := make(map[string]struct{}, len(task.ExcludedDates))
	for _, date := range task.ExcludedDates {
		parsed, err := time.Parse("2006-01-02", date)
		if err != nil || parsed.Format("2006-01-02") != date {
			return time.Time{}, fmt.Errorf("invalid excluded date %q", date)
		}
		if _, duplicate := seen[date]; duplicate {
			return time.Time{}, fmt.Errorf("duplicate excluded date %q", date)
		}
		seen[date] = struct{}{}
	}
	if _, excluded := seen[at.In(loc).Format("2006-01-02")]; excluded {
		return time.Time{}, fmt.Errorf("%w: task excludes the Shanghai calendar date", ErrWindowClosed)
	}
	taskEnd, err := task.WeeklySchedule.endAt(at, loc)
	if err != nil {
		return time.Time{}, fmt.Errorf("task schedule: %w", err)
	}
	lineEnd, err := line.endAt(at, loc)
	if err != nil {
		return time.Time{}, fmt.Errorf("line schedule: %w", err)
	}
	if lineEnd.Before(taskEnd) {
		taskEnd = lineEnd
	}
	if !end.IsZero() && end.Before(taskEnd) {
		taskEnd = end
	}
	return taskEnd, nil
}

func (schedule WeeklySchedule) validate() error {
	if schedule.TimeZone != "Asia/Shanghai" || len(schedule.WeeklyWindows) != len(weekdays) {
		return errors.New("schedule must define all seven weekdays in Asia/Shanghai")
	}
	for _, name := range weekdays {
		windows, exists := schedule.WeeklyWindows[name]
		if !exists {
			return fmt.Errorf("missing weekday %s", name)
		}
		previousEnd := 0
		for _, window := range windows {
			start, err := minuteOfDay(window.Start, false)
			if err != nil {
				return fmt.Errorf("%s start: %w", name, err)
			}
			end, err := minuteOfDay(window.End, true)
			if err != nil {
				return fmt.Errorf("%s end: %w", name, err)
			}
			if start >= end || start < previousEnd {
				return fmt.Errorf("%s windows must be ordered, disjoint, and split at midnight", name)
			}
			previousEnd = end
		}
	}
	return nil
}

func minuteOfDay(clock string, allowEndOfDay bool) (int, error) {
	if len(clock) != 5 || clock[2] != ':' || clock[0] < '0' || clock[0] > '9' || clock[1] < '0' || clock[1] > '9' || clock[3] < '0' || clock[3] > '9' || clock[4] < '0' || clock[4] > '9' {
		return 0, fmt.Errorf("invalid clock time %q", clock)
	}
	hours := int(clock[0]-'0')*10 + int(clock[1]-'0')
	minutes := int(clock[3]-'0')*10 + int(clock[4]-'0')
	if minutes > 59 || hours > 24 || hours == 24 && (minutes != 0 || !allowEndOfDay) {
		return 0, fmt.Errorf("invalid clock time %q", clock)
	}
	return hours*60 + minutes, nil
}

func (schedule WeeklySchedule) endAt(at time.Time, loc *time.Location) (time.Time, error) {
	local := at.In(loc)
	windows := schedule.WeeklyWindows[weekdays[local.Weekday()]]
	minute := local.Hour()*60 + local.Minute()
	for _, window := range windows {
		start, _ := minuteOfDay(window.Start, false) // validated above
		end, _ := minuteOfDay(window.End, true)
		if minute >= start && minute < end {
			midnight := time.Date(local.Year(), local.Month(), local.Day(), 0, 0, 0, 0, loc)
			return midnight.Add(time.Duration(end) * time.Minute), nil
		}
	}
	return time.Time{}, ErrWindowClosed
}