add MPP sender with payment loop

web/src/main/java/de/cotto/lndmanagej/controller/PaymentStatusStream.java

@@ -0,0 +1,58 @@
+package de.cotto.lndmanagej.controller;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import de.cotto.lndmanagej.pickhardtpayments.model.PaymentStatus;
+import de.cotto.lndmanagej.pickhardtpayments.model.PaymentStatus.InstantWithString;
+import org.springframework.stereotype.Component;
+import org.springframework.web.servlet.mvc.method.annotation.StreamingResponseBody;
+
+import java.nio.charset.StandardCharsets;
+import java.util.List;
+
+@Component
+public class PaymentStatusStream {
+    private final ObjectMapper objectMapper;
+
+    public PaymentStatusStream() {
+        this.objectMapper = new ObjectMapper();
+    }
+
+    public StreamingResponseBody getFor(PaymentStatus paymentStatus) {
+        return response -> {
+            int seenMessages = 0;
+            do {
+                List<InstantWithString> messages = paymentStatus.getMessages();
+                int oldMessages = seenMessages;
+                seenMessages = messages.size();
+                for (int i = oldMessages; i < messages.size(); i++) {
+                    InstantWithString instantWithString = messages.get(i);
+                    Object messageToWrite = new Message(
+                            instantWithString.instant().toString(),
+                            instantWithString.string()
+                    );
+                    String json = objectMapper.writeValueAsString(messageToWrite);
+                    response.write(json.getBytes(StandardCharsets.UTF_8));
+                    response.write('\n');
+                    response.flush();
+                }
+            } while (notDone(paymentStatus, seenMessages));
+        };
+    }
+
+    private boolean notDone(PaymentStatus paymentStatus, int seenMessages) {
+        sleep();
+        return paymentStatus.getMessages().size() > seenMessages || paymentStatus.isPending();
+    }
+
+    private void sleep() {
+        try {
+            Thread.sleep(100);
+        } catch (InterruptedException ignored) {
+            // ignored
+        }
+    }
+
+    @SuppressWarnings("UnusedVariable")
+    private record Message(String timestamp, String message) {
+    }
+}