package stocklock

import (
	"context"
	"errors"

	"github.com/redis/go-redis/v9"
)

var (
	ErrSoldOut   = errors.New("库存不足")
	ErrDuplicate = errors.New("每人限购一件，已经抢过了")
)

// 原子地：检查是否已购买 → 检查库存 → 扣库存 → 记录用户
var reserveScript = redis.NewScript(`
if redis.call("SISMEMBER", KEYS[2], ARGV[1]) == 1 then
  return -2
end
local left = tonumber(redis.call("GET", KEYS[1]) or "0")
if left <= 0 then
  return -1
end
redis.call("DECR", KEYS[1])
redis.call("SADD", KEYS[2], ARGV[1])
return left - 1`)

// Reserve 预扣库存。成功后应把订单消息发到 MQ，由消费者异步落库。
func Reserve(ctx context.Context, rdb *redis.Client, sku, userID string) (left int, err error) {
	n, err := reserveScript.Run(ctx, rdb, []string{"stock:" + sku, "stock:" + sku + ":buyers"}, userID).Int()
	switch {
	case err != nil:
		return 0, err
	case n == -1:
		return 0, ErrSoldOut
	case n == -2:
		return 0, ErrDuplicate
	}
	return n, nil
}