added expected return

src/message.rs

@@ -143,6 +143,23 @@ impl Protocol300Message {
         bytes_vec
 
     }
+
+    // TODO docs and test
+    /// Returns the expected length of the return telegram, include start byte but excluding ACK byte
+    /// 
+    /// Currently only implemented for Read and Write requests, as the rest is not needed and/or
+    /// unknown behaviour
+    /// 
+    /// # Errors:
+    /// - `MessageReturnLengthError` - Describes the error which occured when converting
+    pub const fn get_expected_return_telegram_length(&self) -> Result<usize, MessageReturnLengthError> {
+        match (self.message_identifier, self.function_code) {
+            (MessageIdentifier::Request, FunctionCode::VirtualREAD) => Ok(8 + self.data_length as usize),
+            (MessageIdentifier::Request, FunctionCode::VirtualWRITE) => Ok(8),
+            (MessageIdentifier::Response, _) => Err(MessageReturnLengthError::NonsensicalRequest),
+            _ => Err(MessageReturnLengthError::Unimplemented())
+        }
+    }
 }
 
 /// Errors that can occur while creating a message
@@ -151,6 +168,18 @@ pub enum MessageCreationError {
     /// The data length is longer than u8
     #[error("Data too long")]
     DataTooLong(#[from] std::num::TryFromIntError),
+
+}
+
+/// Errors that can occur while getting the expected return length of a message
+#[derive(Error, Debug, PartialEq, Eq)]
+pub enum MessageReturnLengthError {
+    /// The data length is longer than u8
+    #[error("Unimplemented")]
+    Unimplemented(),
+    /// It makes no sense to get the expected length for a response
+    #[error("Nonsensical request")]
+    NonsensicalRequest
 }
 
 /// Errors that can occur while decoding a message from bytes
@@ -343,5 +372,33 @@ mod tests {
         assert_eq!(bytes_expected, bytes_generated);
     }
 
+    #[test]
+    fn test_expected_return_length_read() {
+        let expected_message = Protocol300Message{
+            telegram_length: 5,
+            message_identifier: MessageIdentifier::Request,
+            function_code: FunctionCode::VirtualREAD,
+            data_address: 0x0101,
+            data_length: 2,
+            data: vec![],
+            checksum: 0x0A
+        };
+        assert_eq!(10_usize, expected_message.get_expected_return_telegram_length().unwrap());
+    }
+
+    #[test]
+    fn test_expected_return_length_write() {
+        let expected_message = Protocol300Message{
+            telegram_length: 9,
+            message_identifier: MessageIdentifier::Request,
+            function_code: FunctionCode::VirtualWRITE,
+            data_address: 0x0101,
+            data_length: 4,
+            data: vec![0x01, 0x02, 0x03, 0x04],
+            checksum: 0x1B
+        };
+        assert_eq!(8_usize, expected_message.get_expected_return_telegram_length().unwrap());
+    }
+
 
 }