package services

import (
	"crypto/sha256"
	"encoding/binary"
	"fmt"
	"log"
	"math/rand"
	"strings"
	"time"

	"fotbal-club/internal/models"
	"fotbal-club/pkg/email"

	"gorm.io/gorm"
	"gorm.io/gorm/clause"
)

// SweepstakesService encapsulates business logic for sweepstakes
type SweepstakesService struct {
	DB    *gorm.DB
	Email email.EmailService
}

func NewSweepstakesService(db *gorm.DB, es email.EmailService) *SweepstakesService {
	return &SweepstakesService{DB: db, Email: es}
}

// StartSweepstakesScheduler periodically finalizes ended sweepstakes and picks winners
func StartSweepstakesScheduler(db *gorm.DB, es email.EmailService) {
	go func() {
		ticker := time.NewTicker(1 * time.Minute)
		defer ticker.Stop()
		for {
			if err := finalizeDueSweepstakes(db, es); err != nil {
				log.Printf("[sweepstakes] finalize cycle error: %v", err)
			}
			<-ticker.C
		}
	}()
}

func finalizeDueSweepstakes(db *gorm.DB, es email.EmailService) error {
	// Find sweepstakes that ended and have not selected winners yet
	var list []models.Sweepstake
	now := time.Now()
	if err := db.Where("end_at <= ? AND (winners_selected_at IS NULL) AND status <> ?", now, "finalized").Find(&list).Error; err != nil {
		return err
	}
	if len(list) == 0 {
		return nil
	}
	svc := NewSweepstakesService(db, es)
	for _, s := range list {
		if err := svc.FinalizeSweepstake(&s, ""); err != nil {
			log.Printf("[sweepstakes] finalize %d failed: %v", s.ID, err)
		}
	}
	return nil
}

// FinalizeSweepstake selects winners deterministically (if DrawSeed exists) and updates records.
// If seed is empty and sweepstake.DrawSeed empty, a new seed is generated from time.
func (s *SweepstakesService) FinalizeSweepstake(sw *models.Sweepstake, seed string) error {
	return s.DB.Transaction(func(tx *gorm.DB) error {
		// Reload for update to avoid races
		var cur models.Sweepstake
		if err := tx.Clauses(clause.Locking{Strength: "UPDATE"}).First(&cur, sw.ID).Error; err != nil {
			return err
		}
		if cur.WinnersSelectedAt != nil {
			return nil // already finalized
		}
		// Load entries
		var entries []models.SweepstakeEntry
		if err := tx.Where("sweepstake_id = ? AND status = ?", cur.ID, "valid").Find(&entries).Error; err != nil {
			return err
		}
		// Load prizes
		var prizes []models.SweepstakePrize
		if err := tx.Where("sweepstake_id = ?", cur.ID).Order("display_order ASC, id ASC").Find(&prizes).Error; err != nil {
			return err
		}
		// Determine number of winners
		nWinners := 0
		for _, p := range prizes {
			nWinners += max(0, p.Quantity)
		}
		if nWinners == 0 {
			if cur.TotalPrizes > 0 {
				nWinners = cur.TotalPrizes
			}
		}
		// Cap winners to a safe maximum
		if nWinners > 100 {
			nWinners = 100
		}
		if nWinners > len(entries) {
			nWinners = len(entries)
		}

		// Build seed
		effSeed := strings.TrimSpace(seed)
		if effSeed == "" {
			effSeed = strings.TrimSpace(cur.DrawSeed)
		}
		if effSeed == "" {
			effSeed = fmt.Sprintf("%d-%d", cur.ID, time.Now().UnixNano())
		}
		// Deterministic RNG from SHA-256
		h := sha256.Sum256([]byte(effSeed))
		base := binary.LittleEndian.Uint64(h[:8])
		rng := rand.New(rand.NewSource(int64(base)))

		// Shuffle indices
		idx := rng.Perm(len(entries))
		picked := make(map[uint]bool)
		winners := make([]models.SweepstakeWinner, 0, nWinners)
		assign := func(userID uint, entryID uint, prize *models.SweepstakePrize) {
			w := models.SweepstakeWinner{
				SweepstakeID: cur.ID,
				EntryID:      entryID,
				UserID:       userID,
				ClaimStatus:  "pending",
			}
			if prize != nil {
				w.PrizeID = &prize.ID
				w.PrizeName = prize.Name
			}
			winners = append(winners, w)
		}

		pos := 0
		// Assign by prizes first
		for i := range prizes {
			q := max(0, prizes[i].Quantity)
			for j := 0; j < q && pos < len(idx); j++ {
				cand := entries[idx[pos]]
				pos++
				if picked[cand.UserID] {
					j--
					continue
				}
				picked[cand.UserID] = true
				assign(cand.UserID, cand.ID, &prizes[i])
				if len(winners) >= nWinners {
					break
				}
			}
			if len(winners) >= nWinners {
				break
			}
		}
		// If still need more (when TotalPrizes used)
		for len(winners) < nWinners && pos < len(idx) {
			cand := entries[idx[pos]]
			pos++
			if picked[cand.UserID] {
				continue
			}
			picked[cand.UserID] = true
			assign(cand.UserID, cand.ID, nil)
		}

		// Persist winners
		for i := range winners {
			if err := tx.Create(&winners[i]).Error; err != nil {
				return err
			}
		}
		now := time.Now()
		vis := cur.EndAt.Add(72 * time.Hour)
		if err := tx.Model(&models.Sweepstake{}).Where("id = ?", cur.ID).Updates(map[string]interface{}{
			"winners_selected_at": now,
			"visibility_until":    vis,
			"draw_seed":           effSeed,
			"status":              "finalized",
		}).Error; err != nil {
			return err
		}

		// Send emails (best-effort)
		if s.Email != nil && len(winners) > 0 {
			for _, w := range winners {
				var user models.User
				_ = tx.First(&user, w.UserID).Error
				if strings.TrimSpace(user.Email) == "" {
					continue
				}
				// Localize end date to Czech format in Europe/Prague timezone
				loc, _ := time.LoadLocation("Europe/Prague")
				endsLocal := cur.EndAt.In(loc)
				endsAtCz := endsLocal.Format("02. 01. 2006 15:04")
				_ = s.Email.SendEmail(&email.EmailData{
					Subject:  "Vyhráli jste v soutěži!",
					To:       []string{strings.TrimSpace(user.Email)},
					Template: "sweepstake_winner_user",
					Data: map[string]interface{}{
						"Title":     cur.Title,
						"PrizeName": w.PrizeName,
						"EndsAt":    endsAtCz,
					},
				})
			}
			// Admin summary
			var set models.Settings
			_ = tx.First(&set).Error
			adminTo := strings.TrimSpace(set.ContactEmail)
			if adminTo == "" {
				adminTo = strings.TrimSpace(set.SMTPFrom)
			}
			if adminTo != "" {
				_ = s.Email.SendEmail(&email.EmailData{
					Subject:  "Soutěž – vybraní výherci",
					To:       []string{adminTo},
					Template: "sweepstake_winner_admin",
					Data: map[string]interface{}{
						"Title":        cur.Title,
						"WinnersCount": len(winners),
					},
				})
			}
		}
		return nil
	})
}

func max(a, b int) int {
	if a > b {
		return a
	}
	return b
}