package core

import (
	"fmt"
	"strconv"
	"strings"
)

// CronExplain returns a short human-readable description of a cron expression.
// Supports 5-field standard cron and @hourly/@daily/@weekly/@monthly shortcuts.
// Returns the raw expression if the pattern is not recognized.
func CronExplain(expr string) string {
	expr = strings.TrimSpace(expr)

	// Handle shortcuts
	switch expr {
	case "@hourly":
		return "hourly"
	case "@daily", "@midnight":
		return "daily"
	case "@weekly":
		return "weekly"
	case "@monthly":
		return "monthly"
	case "@yearly", "@annually":
		return "yearly"
	}

	fields := strings.Fields(expr)
	if len(fields) < 5 {
		return expr
	}

	// Use first 5 fields (ignore optional 6th seconds/year field)
	minute := fields[0]
	hour := fields[1]
	dom := fields[2]   // day of month
	month := fields[3]
	dow := fields[4]   // day of week

	// "every N minutes": */N in minute field, rest are *
	if strings.HasPrefix(minute, "*/") && dom == "*" && month == "*" && dow == "*" {
		n := minute[2:]
		if n == "1" {
			return "every minute"
		}
		return fmt.Sprintf("every %s minutes", n)
	}

	// "every N hours": plain minute "0", */N in hour, rest are *
	if strings.HasPrefix(hour, "*/") && minute == "0" && dom == "*" && month == "*" && dow == "*" {
		n := hour[2:]
		if n == "1" {
			return "every hour"
		}
		return fmt.Sprintf("every %s hours", n)
	}

	// "daily at HH:MM": plain numbers for minute and hour, rest are *
	if dom == "*" && month == "*" && dow == "*" {
		m, errM := strconv.Atoi(minute)
		h, errH := strconv.Atoi(hour)
		if errM == nil && errH == nil {
			return fmt.Sprintf("daily at %02d:%02d", h, m)
		}
	}

	// "weekdays at HH:MM": plain minute/hour, dow == "1-5", dom/month are *
	if dom == "*" && month == "*" && dow == "1-5" {
		m, errM := strconv.Atoi(minute)
		h, errH := strconv.Atoi(hour)
		if errM == nil && errH == nil {
			return fmt.Sprintf("weekdays at %02d:%02d", h, m)
		}
	}

	// Not recognized — return raw expression
	return expr
}