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+package store.domain;
+
+import camp.nextstep.edu.missionutils.DateTimes;
+import java.io.BufferedReader;
+import java.io.FileReader;
+import java.io.IOException;
+import java.time.LocalDate;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import store.common.ErrorMessages;
+import store.io.output.StoreOutput;
+
+public class Inventory {
+    private static final String PROMOTIONS_FILE_PATH = "src/main/resources/promotions.md";
+    private static final String PRODUCTS_FILE_PATH = "src/main/resources/products.md";
+    public static final String MD_FILE_DELIMITER = ",";
+
+    private static final int NAME_INDEX = 0;
+    private static final int BUY_QUANTITY_INDEX = 1;
+    private static final int FREE_QUANTITY_INDEX = 2;
+    private static final int START_DATE_INDEX = 3;
+    private static final int END_DATE_INDEX = 4;
+    private static final int PRICE_INDEX = 1;
+    private static final int STOCK_INDEX = 2;
+    private static final int PROMOTION_NAME_INDEX = 3;
+
+    private static final int DEFAULT_STOCK = 0;
+    private static final int EMPTY_PROMOTION = 0;
+
+    private final List<Product> products = new ArrayList<>();
+    private final Map<String, Promotion> promotions = new HashMap<>();
+
+    public Inventory() {
+        loadPromotions();
+        loadProducts();
+    }
+
+    private void loadPromotions() {
+        LocalDate currentDate = DateTimes.now().toLocalDate();
+        try (BufferedReader reader = new BufferedReader(new FileReader(PROMOTIONS_FILE_PATH))) {
+            reader.readLine();
+            String line;
+            while ((line = reader.readLine()) != null) {
+                Promotion promotion = parsePromotion(line);
+                addPromotionToMap(promotion, currentDate);
+            }
+        } catch (IOException e) {
+            System.out.println(ErrorMessages.ERROR_LOADING_PROMOTIONS_FILE);
+        }
+    }
+
+    private Promotion parsePromotion(String line) {
+        String[] fields = line.split(MD_FILE_DELIMITER);
+        String name = fields[NAME_INDEX];
+        int buyQuantity = Integer.parseInt(fields[BUY_QUANTITY_INDEX]);
+        int freeQuantity = Integer.parseInt(fields[FREE_QUANTITY_INDEX]);
+        LocalDate startDate = LocalDate.parse(fields[START_DATE_INDEX]);
+        LocalDate endDate = LocalDate.parse(fields[END_DATE_INDEX]);
+        return new Promotion(name, buyQuantity, freeQuantity, startDate, endDate);
+    }
+
+    private void addPromotionToMap(Promotion promotion, LocalDate currentDate) {
+        if (!promotion.isValid(currentDate)) {
+            promotions.put(promotion.getName(), null);
+            return;
+        }
+        promotions.put(promotion.getName(), promotion);
+    }
+
+    private void loadProducts() {
+        try (BufferedReader reader = new BufferedReader(new FileReader(PRODUCTS_FILE_PATH))) {
+            reader.readLine();
+            String line;
+            while ((line = reader.readLine()) != null) {
+                Product product = parseProduct(line);
+                addProductToInventory(product);
+            }
+        } catch (IOException e) {
+            System.out.println(ErrorMessages.ERROR_LOADING_PRODUCTS_FILE);
+        }
+    }
+
+    private Product parseProduct(String line) {
+        String[] fields = line.split(MD_FILE_DELIMITER);
+        String name = fields[NAME_INDEX];
+        int price = Integer.parseInt(fields[PRICE_INDEX]);
+        int stock = Integer.parseInt(fields[STOCK_INDEX]);
+
+        String promotionName = null;
+        if (fields.length > PROMOTION_NAME_INDEX) {
+            promotionName = fields[PROMOTION_NAME_INDEX];
+        }
+
+        Promotion promotion = promotions.get(promotionName);
+        return new Product(name, price, stock, promotion);
+    }
+
+    private void addProductToInventory(Product product) {
+        Optional<Product> existingProduct = findProductByNameAndPromotion(product.getName(), product.getPromotion());
+        if (existingProduct.isEmpty()) {
+            products.add(product);
+            return;
+        }
+        existingProduct.get().addStock(product.getStock());
+    }
+
+    private Optional<Product> findProductByNameAndPromotion(String name, Promotion promotion) {
+        return products.stream()
+                .filter(product -> isMatchingProduct(product, name, promotion))
+                .findFirst();
+    }
+
+    private boolean isMatchingProduct(Product product, String name, Promotion promotion) {
+        if (!product.getName().equals(name)) {
+            return false;
+        }
+        return isMatchingPromotion(product, promotion);
+    }
+
+    private boolean isMatchingPromotion(Product product, Promotion promotion) {
+        if (product.getPromotion() == null && promotion == null) {
+            return true;
+        }
+        if (product.getPromotion() != null) {
+            return product.getPromotion().equals(promotion);
+        }
+        return false;
+    }
+
+    public Optional<Product> getProductByNameAndPromotion(String productName, boolean hasPromotion) {
+        return products.stream()
+                .filter(product -> isMatchingProductByNameAndPromotion(product, productName, hasPromotion))
+                .findFirst();
+    }
+
+    private boolean isMatchingProductByNameAndPromotion(Product product, String productName, boolean hasPromotion) {
+        if (!product.getName().equalsIgnoreCase(productName)) {
+            return false;
+        }
+        if (hasPromotion) {
+            return product.getPromotion() != null;
+        }
+        return product.getPromotion() == null;
+    }
+
+    public void updateInventory(Cart cart) {
+        validateStock(cart);
+        reduceStock(cart);
+    }
+
+    private void validateStock(Cart cart) {
+        for (CartItem item : cart.getItems()) {
+            Product product = item.getProduct();
+            int requiredQuantity = item.getQuantity();
+            int totalAvailableStock = getTotalAvailableStock(product);
+
+            if (requiredQuantity > totalAvailableStock) {
+                throw new IllegalStateException(ErrorMessages.EXCEED_STOCK);
+            }
+        }
+    }
+
+    private int getTotalAvailableStock(Product product) {
+        int availablePromoStock = getAvailablePromoStock(product);
+        int availableRegularStock = getAvailableRegularStock(product);
+        return availablePromoStock + availableRegularStock;
+    }
+
+    private int getAvailablePromoStock(Product product) {
+        Optional<Product> promoProduct = getProductByNameAndPromotion(product.getName(), true);
+        if (promoProduct.isPresent()) {
+            return promoProduct.get().getStock();
+        }
+        return DEFAULT_STOCK;
+    }
+
+    private int getAvailableRegularStock(Product product) {
+        Optional<Product> regularProduct = getProductByNameAndPromotion(product.getName(), false);
+        if (regularProduct.isPresent()) {
+            return regularProduct.get().getStock();
+        }
+        return DEFAULT_STOCK;
+    }
+
+    private void reduceStock(Cart cart) {
+        for (CartItem item : cart.getItems()) {
+            Product product = item.getProduct();
+            int requiredQuantity = item.getQuantity();
+
+            int promoQuantity = calculatePromoQuantity(product, requiredQuantity);
+            int regularQuantity = requiredQuantity - promoQuantity;
+
+            reduceStock(product.getName(), promoQuantity, regularQuantity);
+        }
+    }
+
+    private int calculatePromoQuantity(Product product, int requiredQuantity) {
+        if (product.getPromotion() == null) {
+            return EMPTY_PROMOTION;
+        }
+        if (!product.getPromotion().isValid(DateTimes.now().toLocalDate())) {
+            return EMPTY_PROMOTION;
+        }
+        return Math.min(requiredQuantity, product.getStock());
+    }
+
+    private void reduceStock(String productName, int promoQuantity, int regularQuantity) {
+        reducePromotionStock(productName, promoQuantity);
+        reduceRegularStock(productName, regularQuantity);
+    }
+
+    private void reducePromotionStock(String productName, int promoQuantity) {
+        if (promoQuantity <= DEFAULT_STOCK) {
+            return;
+        }
+        Optional<Product> promoProduct = getProductByNameAndPromotion(productName, true);
+        if (promoProduct.isPresent()) {
+            promoProduct.get().reducePromotionStock(promoQuantity);
+        }
+    }
+
+    private void reduceRegularStock(String productName, int regularQuantity) {
+        if (regularQuantity <= DEFAULT_STOCK) {
+            return;
+        }
+        Optional<Product> regularProduct = getProductByNameAndPromotion(productName, false);
+        if (regularProduct.isPresent()) {
+            regularProduct.get().reduceRegularStock(regularQuantity);
+        }
+    }
+
+    public void printProductList(StoreOutput storeOutput) {
+        storeOutput.printProductList(products);
+    }
+}

리뷰: Inventory 상수를 위한 클래스를 따로 구현하시는건 어떨까요??