apollo_network_benchmark: add MessageIndexTracker struct

crates/apollo_network_benchmark/src/bin/broadcast_network_stress_test_node/main.rs

@@ -13,6 +13,7 @@ mod message_test;
 
 mod handlers;
 mod message;
+mod message_index_detector;
 pub mod metrics;
 mod protocol;
 mod stress_test_node;

crates/apollo_network_benchmark/src/bin/broadcast_network_stress_test_node/message_index_detector.rs

@@ -0,0 +1,31 @@
+// TODO(AndrewL): remove this once the struct is used
+#![allow(dead_code)]
+
+#[derive(Default, Clone, Copy)]
+pub struct MessageIndexTracker {
+    seen_messages_count: u64,
+    max_message_index: Option<u64>,
+    min_message_index: Option<u64>,
+}
+
+impl MessageIndexTracker {
+    pub fn seen_message(&mut self, message_index: u64) {
+        self.seen_messages_count += 1;
+        if self.max_message_index.is_none() || self.max_message_index.unwrap() < message_index {
+            self.max_message_index = Some(message_index);
+        }
+        if self.min_message_index.is_none() || self.min_message_index.unwrap() > message_index {
+            self.min_message_index = Some(message_index);
+        }
+    }
+
+    pub fn pending_messages_count(&self) -> u64 {
+        if self.seen_messages_count == 0 {
+            return 0;
+        }
+
+        let min_message_index = self.min_message_index.unwrap();
+        let max_message_index = self.max_message_index.unwrap();
+        max_message_index - min_message_index + 1 - self.seen_messages_count
+    }
+}