Refactor utility function to manually replace a tag's value in a raw message

Author: davidsteiner

crates/hotfix/tests/common/test_messages.rs

@@ -147,3 +147,23 @@ impl FixMessage for TestMessage {
         }
     }
 }
+
+/// Replaces the value of a field in a raw FIX message.
+pub fn replace_field_value(raw_message: &mut Vec<u8>, tag: u32, new_value: &[u8]) {
+    let tag_bytes = format!("{}=", tag).into_bytes();
+
+    if let Some(field_start) = raw_message
+        .windows(tag_bytes.len())
+        .position(|window| window == tag_bytes)
+    {
+        let value_start = field_start + tag_bytes.len();
+        if let Some(field_end) = raw_message[value_start..]
+            .iter()
+            .position(|&b| b == b'\x01')
+        {
+            let value_end = value_start + field_end;
+
+            raw_message.splice(value_start..value_end, new_value.iter().cloned());
+        }
+    }
+}

crates/hotfix/tests/session_test_cases/invalid_message_tests.rs

@@ -1,7 +1,7 @@
 use crate::common::actions::when;
 use crate::common::assertions::then;
 use crate::common::setup::given_an_active_session;
-use crate::common::test_messages::TestMessage;
+use crate::common::test_messages::{TestMessage, replace_field_value};
 use hotfix::message::{FixMessage, generate_message};
 use hotfix::session::Status;
 use hotfix_message::dict::{FieldLocation, FixDatatype};
@@ -126,23 +126,7 @@ fn build_execution_report_with_incorrect_body_length(
     let mut raw_message =
         generate_message(sender_comp_id, target_comp_id, msg_seq_num, report).unwrap();
 
-    let message_str = String::from_utf8(raw_message.clone()).unwrap();
-
-    let body_length_start = message_str.find("9=").unwrap();
-    let body_length_end =
-        body_length_start + message_str[body_length_start..].find('\x01').unwrap();
-
-    let original_body_length = &message_str[body_length_start + 2..body_length_end];
-
-    // parse the original body length and make it incorrect (add 10 to it)
-    if let Ok(original_length) = original_body_length.parse::<u32>() {
-        let incorrect_length = original_length + 10;
-        let corrupted_message = message_str.replace(
-            &format!("9={}", original_length),
-            &format!("9={}", incorrect_length),
-        );
-        raw_message = corrupted_message.into_bytes();
-    }
+    replace_field_value(&mut raw_message, 9, b"999");
 
     raw_message
 }