package service

import (
	"context"
	"crypto/rand"
	"crypto/sha256"
	"encoding/hex"
	"errors"
	"fmt"
	"strings"
	"time"

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

	"wx_service/internal/membership/model"
	usermodel "wx_service/internal/model"
)

var (
	ErrAdminTokenRequired = errors.New("admin api token is not configured")
	ErrInvalidAdminToken  = errors.New("invalid admin token")

	ErrRedeemCodeInvalid  = errors.New("redeem code is invalid")
	ErrRedeemCodeExpired  = errors.New("redeem code is expired")
	ErrRedeemCodeUsedUp   = errors.New("redeem code is already used")
	ErrRedeemCodeDisabled = errors.New("redeem code is disabled")
)

type RedeemCodeService struct {
	db         *gorm.DB
	adminToken string
}

func NewRedeemCodeService(db *gorm.DB, adminToken string) *RedeemCodeService {
	return &RedeemCodeService{
		db:         db,
		adminToken: adminToken,
	}
}

type GenerateRedeemCodesRequest struct {
	Count        int
	Plan         string
	DurationDays int
	ExpiresAt    *time.Time
	MaxUses      int
}

type GeneratedRedeemCode struct {
	Code string `json:"code"`
	Plan string `json:"plan"`
}

func (s *RedeemCodeService) ValidateAdminToken(token string) error {
	if s.adminToken == "" {
		return ErrAdminTokenRequired
	}
	if token == "" || token != s.adminToken {
		return ErrInvalidAdminToken
	}
	return nil
}

func (s *RedeemCodeService) Generate(ctx context.Context, req GenerateRedeemCodesRequest) ([]GeneratedRedeemCode, error) {
	count := req.Count
	if count <= 0 {
		count = 1
	}
	if count > 500 {
		count = 500
	}
	plan := strings.TrimSpace(req.Plan)
	if plan == "" {
		plan = "default"
	}
	if req.DurationDays <= 0 {
		return nil, fmt.Errorf("duration_days must be > 0")
	}
	maxUses := req.MaxUses
	if maxUses <= 0 {
		maxUses = 1
	}

	results := make([]GeneratedRedeemCode, 0, count)
	records := make([]model.MembershipRedeemCode, 0, count)

	// 生成时尽量保证唯一性；如遇碰撞（极低概率）则重试。
	for len(records) < count {
		code, err := generateCode(20)
		if err != nil {
			return nil, err
		}
		hash := hashCode(code)
		suffix := suffixOf(code, 6)

		records = append(records, model.MembershipRedeemCode{
			CodeHash:      hash,
			CodeSuffix:    suffix,
			Plan:          plan,
			DurationDays:  req.DurationDays,
			ExpiresAt:     req.ExpiresAt,
			MaxUses:       maxUses,
			UsedUses:      0,
			Status:        "active",
		})
		results = append(results, GeneratedRedeemCode{Code: code, Plan: plan})
	}

	if err := s.db.WithContext(ctx).Create(&records).Error; err != nil {
		return nil, fmt.Errorf("save redeem codes: %w", err)
	}
	return results, nil
}

type RedeemResult struct {
	Plan      string    `json:"plan"`
	StartsAt  time.Time `json:"starts_at"`
	EndsAt    time.Time `json:"ends_at"`
	Extended  bool      `json:"extended"`
	CodeSuffix string   `json:"code_suffix"`
}

func (s *RedeemCodeService) Redeem(ctx context.Context, user *usermodel.User, code string, clientIP string, userAgent string) (*RedeemResult, error) {
	normalized := normalizeCode(code)
	if normalized == "" {
		return nil, ErrRedeemCodeInvalid
	}
	hash := hashCode(normalized)

	now := time.Now()
	var result *RedeemResult

	err := s.db.WithContext(ctx).Transaction(func(tx *gorm.DB) error {
		var redeemCode model.MembershipRedeemCode
		if err := tx.Clauses(clause.Locking{Strength: "UPDATE"}).
			Where("code_hash = ? AND deleted_at IS NULL", hash).
			First(&redeemCode).Error; err != nil {
			if errors.Is(err, gorm.ErrRecordNotFound) {
				return ErrRedeemCodeInvalid
			}
			return fmt.Errorf("load redeem code: %w", err)
		}

		if redeemCode.Status != "" && redeemCode.Status != "active" {
			return ErrRedeemCodeDisabled
		}
		if redeemCode.ExpiresAt != nil && redeemCode.ExpiresAt.Before(now) {
			return ErrRedeemCodeExpired
		}
		if redeemCode.MaxUses <= 0 {
			redeemCode.MaxUses = 1
		}
		if redeemCode.UsedUses >= redeemCode.MaxUses {
			return ErrRedeemCodeUsedUp
		}
		if redeemCode.DurationDays <= 0 {
			return fmt.Errorf("redeem code duration_days invalid")
		}

		// 兑换：先创建/延长会员，再计数，最后写 redemption log。
		var membership usermodel.UserMembership
		var hasActive bool
		if err := tx.
			Where("mini_program_id = ? AND user_id = ? AND status = ? AND ends_at > ?",
				user.MiniProgramID, user.ID, "active", now).
			Order("ends_at DESC").
			First(&membership).Error; err == nil {
			hasActive = true
		} else if err != nil && !errors.Is(err, gorm.ErrRecordNotFound) {
			return fmt.Errorf("load membership: %w", err)
		}

		base := now
		if hasActive && membership.EndsAt.After(now) {
			base = membership.EndsAt
		}
		newEnds := base.AddDate(0, 0, redeemCode.DurationDays)

		if hasActive {
			if err := tx.Model(&membership).Updates(map[string]interface{}{
				"ends_at":    newEnds,
				"updated_at": now,
			}).Error; err != nil {
				return fmt.Errorf("extend membership: %w", err)
			}
		} else {
			membership = usermodel.UserMembership{
				MiniProgramID: user.MiniProgramID,
				UserID:        user.ID,
				Plan:          redeemCode.Plan,
				Status:        "active",
				StartsAt:      now,
				EndsAt:        newEnds,
			}
			if err := tx.Create(&membership).Error; err != nil {
				return fmt.Errorf("create membership: %w", err)
			}
		}

		if err := tx.Model(&redeemCode).UpdateColumn("used_uses", gorm.Expr("used_uses + 1")).Error; err != nil {
			return fmt.Errorf("update redeem usage: %w", err)
		}

		redemption := model.MembershipRedemption{
			MiniProgramID: user.MiniProgramID,
			UserID:        user.ID,
			RedeemCodeID:  redeemCode.ID,
			CodeSuffix:    redeemCode.CodeSuffix,
			MembershipID:  membership.ID,
			ClientIP:      truncateString(clientIP, 64),
			UserAgent:     truncateString(userAgent, 255),
		}
		if err := tx.Create(&redemption).Error; err != nil {
			return fmt.Errorf("create redemption log: %w", err)
		}

		result = &RedeemResult{
			Plan:       redeemCode.Plan,
			StartsAt:   membership.StartsAt,
			EndsAt:     newEnds,
			Extended:   hasActive,
			CodeSuffix: redeemCode.CodeSuffix,
		}
		return nil
	})
	if err != nil {
		return nil, err
	}
	return result, nil
}

func normalizeCode(code string) string {
	c := strings.TrimSpace(code)
	c = strings.ReplaceAll(c, "-", "")
	c = strings.ReplaceAll(c, " ", "")
	c = strings.ToUpper(c)
	return c
}

func hashCode(code string) string {
	sum := sha256.Sum256([]byte(code))
	return hex.EncodeToString(sum[:])
}

func suffixOf(code string, n int) string {
	if n <= 0 {
		return ""
	}
	if len(code) <= n {
		return code
	}
	return code[len(code)-n:]
}

func generateCode(length int) (string, error) {
	if length <= 0 {
		length = 16
	}
	// 去掉易混淆字符：0/O, 1/I/L
	const alphabet = "ABCDEFGHJKMNPQRSTUVWXYZ23456789"
	buf := make([]byte, length)
	if _, err := rand.Read(buf); err != nil {
		return "", fmt.Errorf("rand: %w", err)
	}
	out := make([]byte, length)
	for i, b := range buf {
		out[i] = alphabet[int(b)%len(alphabet)]
	}
	return string(out), nil
}

func truncateString(s string, max int) string {
	if max <= 0 || len(s) <= max {
		return s
	}
	return s[:max]
}