Avoid logboog to grow too much

app/src/main/java/it/danieleverducci/lunatracker/entities/Logbook.kt

@@ -1,5 +1,19 @@
 package it.danieleverducci.lunatracker.entities
 
 class Logbook {
+    companion object {
+        val MAX_SAFE_LOGBOOK_SIZE = 30000
+    }
     val logs = ArrayList<LunaEvent>()
+
+    fun isTooBig(): Boolean {
+        return logs.size > MAX_SAFE_LOGBOOK_SIZE
+    }
+
+    /**
+     * Halves the logbook to avoid the file being too big
+     */
+    fun trim() {
+        logs.subList(MAX_SAFE_LOGBOOK_SIZE/2, logs.size).clear()
+    }
 }