Add prefix-checking to message

We want to make sure the message doesn't match any of the station's listed prefixes.

Also, we give a finite number of tries to generate a valid message just in case prefixes somehow cover all messages.
Make it a setting so it can be increased if needed.

src/main/java/name/nathanmcrae/numbersstation/Main.java

@@ -176,7 +176,7 @@ public class Main extends Application {
             Path nextMessagePath = stationDirPath.resolve("next-message.txt");
             String messageText;
             if (!Files.exists(nextMessagePath)) {
-                messageText = loadedStation.generateMessage();
+                messageText = loadedStation.generateMessage(settings.getMessageGenerationAttempts());
             } else {
                 messageText = new String(Files.readAllBytes(nextMessagePath));
             }
@@ -202,9 +202,9 @@ public class Main extends Application {
                 System.exit(1);
             }
 
-            String newMessageText = loadedStation.generateMessage();
+            String newMessageText = loadedStation.generateMessage(settings.getMessageGenerationAttempts());
             Files.write(nextMessagePath, newMessageText.getBytes(StandardCharsets.UTF_8));
-        } catch (IOException e) {
+        } catch (IOException | StationSettings.MessageGenerationException e) {
             logger.log(Level.SEVERE, "Exception while posting message to station " + stationName, e);
             // TODO: Notification
             System.exit(1);

src/main/java/name/nathanmcrae/numbersstation/MainSettings.java

@@ -18,8 +18,8 @@ public class MainSettings {
     private static final Logger logger = Logger.getLogger(Main.class.getName());
 
     private int digitsPerGroup;
+    private int messageGenerationAttempts;
     private String username;
-    private int refreshInterval;
     private String selectedStationName;
 
     private ArrayList<StationSettings> stations;
@@ -27,6 +27,7 @@ public class MainSettings {
     public MainSettings() {
         stations = new ArrayList<>();
         stations.add(new StationSettings("Station 1"));
+        messageGenerationAttempts = 5;
     }
 
     public static Path getSettingsFilePath() {
@@ -47,6 +48,14 @@ public class MainSettings {
         return stations;
     }
 
+    public int getMessageGenerationAttempts() {
+        return messageGenerationAttempts;
+    }
+
+    public void setMessageGenerationAttempts(int messageGenerationAttempts) {
+        this.messageGenerationAttempts = messageGenerationAttempts;
+    }
+
     public String getUsername() {
         return username;
     }
@@ -55,14 +64,6 @@ public class MainSettings {
         this.username = username;
     }
 
-    public int getRefreshInterval() {
-        return refreshInterval;
-    }
-
-    public void setRefreshInterval(int refreshInterval) {
-        this.refreshInterval = refreshInterval;
-    }
-
     public String getSelectedStationName() {
         return selectedStationName;
     }

src/main/java/name/nathanmcrae/numbersstation/StationSettings.java

@@ -7,9 +7,12 @@ import java.security.SecureRandom;
 import java.time.LocalDate;
 import java.time.LocalTime;
 import java.util.ArrayList;
+import java.util.logging.Logger;
 import java.util.Random;
 
 public class StationSettings {
+    private static final Logger logger = Logger.getLogger(Main.class.getName());
+
     private String address;
     private int digitsPerGroup;
     private String externalProgramCommand;
@@ -76,18 +79,51 @@ public class StationSettings {
         return directoryPath.resolve(stationDirName);
     }
 
-    public String generateMessage() {
+    public String generateMessage(int generationAttempts) throws MessageGenerationException {
         SecureRandom random = new SecureRandom();
         StringBuilder messageBuilder = new StringBuilder();
 
+        int attemptsLeft = generationAttempts;
+        boolean noPrefixCollisions = false;
+
+        while (!noPrefixCollisions) {
+            messageBuilder = new StringBuilder();
+
             for (int i = 0; i < messageLength; i++) {
-            if (i > 0 && i % digitsPerGroup == 0) {
-                messageBuilder.append(" ");
-            }
                 messageBuilder.append(random.nextInt(10));
             }
 
-        return messageBuilder.toString();
+            String message = messageBuilder.toString();
+
+            boolean prefixCollision = false;
+
+            for (String prefix : prefixes) {
+                if (message.startsWith(prefix)) {
+                    prefixCollision = true;
+                }
+            }
+
+            if (!prefixCollision) {
+                noPrefixCollisions = true;
+            }
+
+            attemptsLeft--;
+            if (prefixCollision && attemptsLeft <= 0) {
+                throw new MessageGenerationException("Failed " + generationAttempts + " attempts to generate message without matching prefixes. Ensure prefixes are valid.");
+            }
+        }
+
+        String messageNoSpaces = messageBuilder.toString();
+        StringBuilder messageFormattedBuilder = new StringBuilder();
+
+        for (int i = 0; i < messageNoSpaces.length(); i++) {
+            if (i > 0 && i % digitsPerGroup == 0) {
+                messageFormattedBuilder.append(" ");
+            }
+            messageFormattedBuilder.append(messageNoSpaces.charAt(i));
+        }
+
+        return messageFormattedBuilder.toString();
     }
 
     public String getAddress() {
@@ -197,4 +233,10 @@ public class StationSettings {
     public String toString() {
         return name;
     }
+
+    public class MessageGenerationException extends Exception {
+        public MessageGenerationException(String message) {
+            super(message);
+        }
+    }
 }