ক্যাপস্টোন — “দোকানদার”: একটা আসল Java সিস্টেম

record Money(long paisa) {
    static final Money ZERO = new Money(0);
    Money { if (paisa < 0) throw new IllegalArgumentException("negative money"); }
    static Money tk(long taka) { return new Money(taka * 100); }
    Money plus(Money o)      { return new Money(paisa + o.paisa); }
    Money times(int n)       { return new Money(paisa * n); }
    Money percent(int p)     { return new Money((paisa * p + 50) / 100); }  // half-up
}

sealed interface PaymentResult permits Paid, Declined {}
record Paid(String txId) implements PaymentResult {}
record Declined(String reason) implements PaymentResult {}

enum OrderStatus {
    PENDING, PAID, DELIVERED, CANCELLED;
    boolean canMoveTo(OrderStatus n) {
        return switch (this) {
            case PENDING -> n == PAID || n == CANCELLED;
            case PAID -> n == DELIVERED || n == CANCELLED;
            case DELIVERED, CANCELLED -> false;          // শেষ অবস্থা
        };
    }
}

import java.time.*;
import java.util.*;
import java.util.stream.Collectors;
record Money(long paisa) {                       // ===== domain core: no framework =====
    static final Money ZERO = new Money(0);
    Money { if (paisa < 0) throw new IllegalArgumentException("negative money"); }
    static Money tk(long taka) { return new Money(taka * 100); }
    Money plus(Money o) { return new Money(paisa + o.paisa); }
    Money times(int n) { return new Money(paisa * n); }
    Money percent(int p) { return new Money((paisa * p + 50) / 100); }   // half-up
    public String toString() { return String.format("%d.%02d", paisa / 100, paisa % 100); }
}
enum PayMethod { BKASH, NAGAD, CASH }
enum OrderStatus {
    PENDING, PAID, DELIVERED, CANCELLED;
    boolean canMoveTo(OrderStatus n) {
        return switch (this) {
            case PENDING -> n == PAID || n == CANCELLED;
            case PAID -> n == DELIVERED || n == CANCELLED;
            case DELIVERED, CANCELLED -> false;          // final states
        };
    }
}
record Product(String sku, Money price) {}
record Line(String sku, int qty) { Line { if (qty <= 0) throw new IllegalArgumentException("bad qty"); } }
record Order(int id, String phone, List<Line> lines, Money total,
             PayMethod method, OrderStatus status, LocalDate day) {
    Order { lines = List.copyOf(lines); }
    Order moveTo(OrderStatus next) {
        if (!status.canMoveTo(next)) throw new IllegalStateException(status + " -> " + next);
        return new Order(id, phone, lines, total, method, next, day);
    }
}
sealed interface PaymentResult permits Paid, Declined {}
record Paid(String txId) implements PaymentResult {}
record Declined(String reason) implements PaymentResult {}
sealed interface PlaceResult permits Placed, Rejected {}
record Placed(Order order) implements PlaceResult {
    public String toString() { return "OK   #" + order.id() + " " + order.method() + " " + order.total(); }
}
record Rejected(String reason) implements PlaceResult { public String toString() { return "FAIL " + reason; } }
interface ProductRepository {                    // ===== ports =====
    Optional<Product> find(String sku); boolean reserve(String sku, int qty); void release(String sku, int qty);
}
interface OrderRepository { void save(Order o); Order get(int id); List<Order> all(); }
interface PaymentGateway { PaymentResult charge(PayMethod m, String phone, Money amount); }
interface SmsSender { void send(String phone, String text); }
class OrderService {                             // ===== core service =====
    static final int VAT_PERCENT = 15;
    private final ProductRepository products; private final OrderRepository orders;
    private final PaymentGateway gateway; private final SmsSender sms; private final Clock clock;
    OrderService(ProductRepository p, OrderRepository o, PaymentGateway g, SmsSender s, Clock c) {
        products = p; orders = o; gateway = g; sms = s; clock = c;
    }
    PlaceResult place(String phone, PayMethod method, List<Line> lines) {
        Money subtotal = Money.ZERO;
        List<Line> reserved = new ArrayList<>();
        for (Line l : lines) {                                   // 1. validate + reserve
            Optional<Product> p = products.find(l.sku());
            if (p.isEmpty() || !products.reserve(l.sku(), l.qty())) {
                releaseAll(reserved);
                return new Rejected((p.isEmpty() ? "unknown product " : "out of stock ") + l.sku());
            }
            reserved.add(l);
            subtotal = subtotal.plus(p.get().price().times(l.qty()));
        }
        Money total = subtotal.plus(subtotal.percent(VAT_PERCENT)); // 2. total + VAT
        var order = new Order(orders.all().size() + 1, phone, lines, total, method,
                              OrderStatus.PENDING, LocalDate.now(clock));
        return switch (gateway.charge(method, phone, total)) {     // 3. charge
            case Paid ok -> {
                orders.save(order.moveTo(OrderStatus.PAID));
                sms.send(phone, "অর্ডার #" + order.id() + " নিশ্চিত, মোট " + total + " টাকা");
                yield new Placed(orders.get(order.id()));
            }
            case Declined no -> {                                  // 4. compensate
                releaseAll(reserved);
                orders.save(order.moveTo(OrderStatus.CANCELLED));
                yield new Rejected("payment declined: " + no.reason());
            }
        };
    }
    void deliver(int id) { orders.save(orders.get(id).moveTo(OrderStatus.DELIVERED)); }
    void cancel(int id) {
        Order c = orders.get(id).moveTo(OrderStatus.CANCELLED);  // throws if forbidden
        releaseAll(c.lines());
        orders.save(c);
        sms.send(c.phone(), "অর্ডার #" + id + " বাতিল হয়েছে");
    }
    private void releaseAll(List<Line> ls) { ls.forEach(l -> products.release(l.sku(), l.qty())); }
}
class InMemoryProducts implements ProductRepository {   // ===== adapters =====
    final Map<String, Product> items = new TreeMap<>();
    final Map<String, Integer> stock = new TreeMap<>();
    void add(String sku, long taka, int qty) { items.put(sku, new Product(sku, Money.tk(taka))); stock.put(sku, qty); }
    public Optional<Product> find(String sku) { return Optional.ofNullable(items.get(sku)); }
    public synchronized boolean reserve(String sku, int qty) {
        int have = stock.getOrDefault(sku, 0);
        if (have < qty) return false;
        stock.put(sku, have - qty); return true;
    }
    public synchronized void release(String sku, int qty) { stock.merge(sku, qty, Integer::sum); }
}
class InMemoryOrders implements OrderRepository {
    private final Map<Integer, Order> byId = new TreeMap<>();
    public void save(Order o) { byId.put(o.id(), o); }
    public Order get(int id) { return Optional.ofNullable(byId.get(id)).orElseThrow(); }
    public List<Order> all() { return List.copyOf(byId.values()); }
}
class FakeWallets implements PaymentGateway {            // balance in paisa
    private final Map<String, Long> balance = new TreeMap<>(Map.of("01711", 500000L, "01811", 50000L));
    public PaymentResult charge(PayMethod m, String phone, Money amount) {
        if (m == PayMethod.CASH) return new Paid("cash");
        long have = balance.getOrDefault(phone, 0L);
        if (have < amount.paisa()) return new Declined("insufficient balance");
        balance.put(phone, have - amount.paisa()); return new Paid(m + "-" + phone);
    }
}
class Main {                                             // ===== scripted scenario =====
    public static void main(String[] args) {
        var products = new InMemoryProducts();
        products.add("RICE", 420, 10); products.add("OIL", 185, 5);
        products.add("DAL", 135, 8);   products.add("TEA", 260, 1);
        var clock = Clock.fixed(Instant.parse("2026-10-18T04:30:00Z"), ZoneId.of("Asia/Dhaka"));
        var orders = new InMemoryOrders();
        List<String> outbox = new ArrayList<>();
        SmsSender sms = (phone, text) -> outbox.add(phone + " : " + text);
        var shop = new OrderService(products, orders, new FakeWallets(), sms, clock);

        show(shop.place("01711", PayMethod.BKASH, List.of(new Line("RICE", 2), new Line("OIL", 1))));
        show(shop.place("01811", PayMethod.NAGAD, List.of(new Line("DAL", 3), new Line("TEA", 1))));
        show(shop.place("01911", PayMethod.CASH, List.of(new Line("TEA", 1), new Line("DAL", 2))));
        show(shop.place("01711", PayMethod.BKASH, List.of(new Line("TEA", 1))));
        show(shop.place("01811", PayMethod.NAGAD, List.of(new Line("OIL", 2))));
        shop.deliver(3);
        shop.cancel(4);
        try { shop.cancel(3); } catch (IllegalStateException e) { System.out.println("cancel #3 refused: " + e.getMessage()); }

        System.out.println("-- orders --");
        orders.all().forEach(o -> System.out.printf("#%d %-6s %-9s %8s%n", o.id(), o.method(), o.status(), o.total()));
        System.out.println("-- stock -- " + products.stock);
        LocalDate today = LocalDate.now(clock);
        Map<PayMethod, Money> sales = orders.all().stream()
            .filter(o -> o.day().equals(today) && o.status() != OrderStatus.CANCELLED)
            .collect(Collectors.groupingBy(Order::method, TreeMap::new,
                     Collectors.reducing(Money.ZERO, Order::total, Money::plus)));
        System.out.println("-- sales " + today + " --");
        for (PayMethod m : PayMethod.values()) System.out.printf("%-6s %8s%n", m, sales.getOrDefault(m, Money.ZERO));
        System.out.printf("%-6s %8s%n", "TOTAL", sales.values().stream().reduce(Money.ZERO, Money::plus));
        System.out.println("-- SMS outbox --");
        outbox.forEach(System.out::println);
    }
    static void show(PlaceResult r) { System.out.println(r); }
}

record Money(long paisa) {
    Money { if (paisa < 0) throw new IllegalArgumentException("negative money"); }
    Money percent(int p) { return new Money((paisa * p + 50) / 100); }   // half-up
}
enum OrderStatus {
    PENDING, PAID, DELIVERED, CANCELLED;
    boolean canMoveTo(OrderStatus n) {
        return switch (this) {
            case PENDING -> n == PAID || n == CANCELLED;
            case PAID -> n == DELIVERED || n == CANCELLED;
            case DELIVERED, CANCELLED -> false;
        };
    }
}
class Main {
    static int passed = 0, failed = 0;
    static void check(String name, boolean ok) {
        if (ok) passed++; else failed++;
        System.out.println((ok ? "PASS " : "FAIL ") + name);
    }
    static boolean rejects(Runnable r) {
        try { r.run(); return false; } catch (IllegalArgumentException e) { return true; }
    }
    public static void main(String[] args) {
        check("VAT 15% of 1025.00 = 153.75", new Money(102500).percent(15).paisa() == 15375);
        check("half-up: 15% of 0.10 = 0.02", new Money(10).percent(15).paisa() == 2);
        check("negative money rejected", rejects(() -> new Money(-1)));
        check("DELIVERED -> CANCELLED forbidden", !OrderStatus.DELIVERED.canMoveTo(OrderStatus.CANCELLED));
        int allowed = 0;
        System.out.println("          PEND PAID DELV CANC");
        for (OrderStatus from : OrderStatus.values()) {
            System.out.printf("%-9s", from);
            for (OrderStatus to : OrderStatus.values()) {
                if (from.canMoveTo(to)) allowed++;
                System.out.print(from.canMoveTo(to) ? "   Y " : "   . ");
            }
            System.out.println();
        }
        check("exactly 4 of 16 moves allowed", allowed == 4);
        System.out.printf("%d passed, %d failed%n", passed, failed);
    }
}

class OrderServiceTest {
    static final Clock FIXED = Clock.fixed(Instant.parse("2026-10-18T04:30:00Z"), ZoneId.of("Asia/Dhaka"));

    @Test
    void declinedPaymentReleasesReservedStock() {
        var products = new InMemoryProducts();
        products.add("TEA", 260, 1);
        PaymentGateway alwaysDecline = (m, phone, amount) -> new Declined("insufficient balance");
        var service = new OrderService(products, new InMemoryOrders(), alwaysDecline, (p, t) -> {}, FIXED);

        var result = service.place("01811", PayMethod.NAGAD, List.of(new Line("TEA", 1)));

        assertInstanceOf(Rejected.class, result);
        assertEquals(1, products.stock.get("TEA"));          // compensation কাজ করেছে
    }
}

CREATE TABLE product (
    sku         VARCHAR(20) PRIMARY KEY,
    name        VARCHAR(100) NOT NULL,
    price_paisa BIGINT NOT NULL CHECK (price_paisa >= 0),
    stock       INT    NOT NULL CHECK (stock >= 0)      -- শেষ প্রতিরক্ষা
);

public final class JdbcProductRepository implements ProductRepository {
    private final DataSource ds;
    public JdbcProductRepository(DataSource ds) { this.ds = ds; }

    @Override
    public boolean reserve(String sku, int qty) {
        String sql = "UPDATE product SET stock = stock - ? WHERE sku = ? AND stock >= ?";
        try (Connection c = ds.getConnection();
             PreparedStatement ps = c.prepareStatement(sql)) {
            ps.setInt(1, qty); ps.setString(2, sku); ps.setInt(3, qty);
            return ps.executeUpdate() == 1;            // 0 সারি = যথেষ্ট stock নেই
        } catch (SQLException e) {
            throw new IllegalStateException("reserve failed for " + sku, e);
        }
    }
    // find আর release একই ছকে; release: SET stock = stock + ?
}

@RestController
@RequestMapping("/api/orders")
class OrderController {
    private final OrderService service;
    OrderController(OrderService service) { this.service = service; }   // constructor injection

    record LineDto(@NotBlank String sku, @Positive int qty) {}
    record PlaceOrderRequest(@Pattern(regexp = "01[3-9]\\d{8}") String phone,
                             @NotNull PayMethod method,
                             @NotEmpty List<@Valid LineDto> lines) {}

    @PostMapping
    ResponseEntity<?> place(@Valid @RequestBody PlaceOrderRequest req) {
        var lines = req.lines().stream().map(l -> new Line(l.sku(), l.qty())).toList();
        return switch (service.place(req.phone(), req.method(), lines)) {
            case Placed p   -> ResponseEntity.status(HttpStatus.CREATED).body(p.order());
            case Rejected r -> ResponseEntity.unprocessableEntity().body(Map.of("message", r.reason()));
        };
    }

    @PostMapping("/{id}/cancel")
    void cancel(@PathVariable int id) { service.cancel(id); }
}

@RestControllerAdvice
class ApiErrors {
    private static final Logger log = LoggerFactory.getLogger(ApiErrors.class);

    @ExceptionHandler(IllegalStateException.class)
    ResponseEntity<Map<String, String>> conflict(IllegalStateException e) {
        log.warn("illegal order transition: {}", e.getMessage());          // ডেভেলপারের জন্য
        return ResponseEntity.status(HttpStatus.CONFLICT)
                .body(Map.of("message", "এই অবস্থায় অর্ডারটি বাতিল করা যায় না"));  // গ্রাহকের জন্য
    }
}

@Configuration
class Wiring {
    @Bean OrderService orderService(ProductRepository p, OrderRepository o,
                                    PaymentGateway g, SmsSender s) {
        return new OrderService(p, o, g, s, Clock.system(ZoneId.of("Asia/Dhaka")));
    }
    @Bean @Profile("dev")  PaymentGateway fakeWallets() { return new FakeWallets(); }
    @Bean @Profile("prod") PaymentGateway realGateway(@Value("${payment.api-key}") String key) {
        return new BkashGateway(key);
    }
}

spring:
  profiles:
    active: dev
---
spring:
  config:
    activate:
      on-profile: prod
  datasource:
    url: jdbc:postgresql://db:5432/dokandar
    username: dokandar_app
    password: ${DB_PASSWORD}        # গোপন তথ্য environment variable থেকে
payment:
  api-key: ${PAYMENT_API_KEY}

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>3.3.5</version>   <!-- লেখার সময়ের একটি 3.x; নতুনটা দেখে নিন -->
</parent>
<properties><java.version>21</java.version></properties>
<dependencies>
    <dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-web</artifactId></dependency>
    <dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-validation</artifactId></dependency>
    <dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-jdbc</artifactId></dependency>
    <dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-actuator</artifactId></dependency>
    <dependency><groupId>org.postgresql</groupId><artifactId>postgresql</artifactId><scope>runtime</scope></dependency>
    <dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-test</artifactId><scope>test</scope></dependency>
</dependencies>

mvn clean verify                          # compile + সব টেস্ট
java -jar target/dokandar-0.1.0.jar --spring.profiles.active=dev