Refactor `MessageWriter`

src/link/writer.rs

@@ -1,79 +1,86 @@
 use std::cmp::{max, min};
 use std::collections::VecDeque;
 use std::convert::TryFrom;
+use std::iter::Extend;
 
-const LENGTH_FIELD_SIZE: usize = 4;
+//  Defines how many bytes is used to encode "LENGTH" field in the chunk sent to ue-server
+const CHUNK_LENGTH_FIELD: usize = 2;
+//  Maximum amount of bytes ue-server is able to receive at once
 const UE_INPUT_BUFFER: usize = 4095;
-
+//  Minimal payload size (in bytes) to send, unless there is not enough data left
-struct PendingMessage {
+const MIN_PAYLOAD_SIZE: usize = 50;
-    contents: Vec<u8>,
-    bytes_sent: usize,
-}
 
 pub struct MessageWriter {
     sent_bytes: u64,
     ue_received_bytes: u64,
-    pending_messages: VecDeque<PendingMessage>,
+    pending_data: VecDeque<u8>,
-}
+}
-
+
-impl PendingMessage {
+///     For converting byte stream that is expected from the ue-server into actual messages.
-    fn bytes_left(&self) -> usize {
+///     Conversion process has two steps:
-        max(0, self.contents.len() - self.bytes_sent)
+///     1.  Every string message is converted into it's utf8 representation and is pre-pended with
-    }
+///         it's own length in format:
-}
+///             [MESSAGE_LENGTH: length of the message in utf8 encoding | 4 bytes: u32 BE]
-
+///             [MESSAGE: utf8-encoded string | `MESSAGE_LENGTH` bytes]
+///         Resulting byte sequences from all the messages then concatenated, in order, into
+///         a single data stream.
+///     2.  Resulting data stream is then separated into "chunks" that can be accepted by
+///         the ue-server (each no longer than `UE_INPUT_BUFFER` in total) and are sent in a format:
+///             [LENGTH: length of the chunk | 2 bytes: u16 BE]
+///             [PAYLOAD: utf8-encoded string | `LENGTH` bytes]
+///     Use `push()` to input string messages and `try_pop()` to retrieve next chunk, if ue-server
+/// can accept it. Call `update_ue_received_bytes()` to update `MessageWriter`'s information about
+/// how many bytes ue-server has received so far.
+///     NOTE: `try_pop()` can return `None` even if not all message data has been transferred,
+/// in case ue-server's buffer does not have enough space. Use `is_empty()` call to check for
+/// whether `MessageWriter` has depleted all it's data.
 impl MessageWriter {
     pub fn new() -> MessageWriter {
         MessageWriter {
             sent_bytes: 0,
             ue_received_bytes: 0,
-            pending_messages: VecDeque::with_capacity(100),
+            //  This value should be more than enough for typical use:
+            //  it will take at least one second to send this much data to a 30 tick rate ue-server.
+            pending_data: VecDeque::with_capacity(30 * UE_INPUT_BUFFER),
         }
     }
 
     pub fn push(&mut self, message: &str) {
         let message_as_utf8 = message.as_bytes();
-        self.pending_messages.push_back(PendingMessage {
+        let message_as_utf8 = [
-            contents: [
             &(message_as_utf8.len() as u32).to_be_bytes(),
             message_as_utf8,
         ]
-            .concat(),
+        .concat();
-            bytes_sent: 0,
+        self.pending_data.extend(message_as_utf8.into_iter());
-        });
     }
 
+    /// This method will always return chunk if all remaining data will fit inside it, otherwise it
+    /// will wait until ue-server's buffer has enough space for at least `MIN_PAYLOAD_SIZE` bytes.
     pub fn try_pop(&mut self) -> Option<Vec<u8>> {
-        if !self.should_send_now() {
+        if self.is_empty() {
             return None;
         }
-        let max_output_bytes = self.available_ue_buffer_capacity();
+        let required_payload_size = min(self.pending_data.len(), MIN_PAYLOAD_SIZE);
-        let next_payload_size = max_output_bytes - 2;
+        let available_payload_space = self
-        let next_message = match self.pending_messages.get_mut(0) {
+            .available_ue_buffer_capacity()
-            Some(message) => message,
+            .checked_sub(CHUNK_LENGTH_FIELD)
-            _ => return None,
+            .unwrap_or_default();
-        };
+        if required_payload_size > available_payload_space {
-        let first_index = next_message.bytes_sent;
+            return None;
-        let last_index = min(
+        }
-            next_message.bytes_sent + next_payload_size - 1,
+        let payload_size = min(available_payload_space, self.pending_data.len());
-            next_message.contents.len() - 1,
+        let mut bytes_to_send = Vec::with_capacity(CHUNK_LENGTH_FIELD + payload_size);
-        );
+        bytes_to_send.extend((payload_size as u16).to_be_bytes().iter());
-        next_message.bytes_sent = last_index + 1;
+        for next_byte in self.pending_data.drain(..payload_size) {
-        let chunk_length = last_index - first_index + 1;
+            bytes_to_send.push(next_byte);
-        let bytes_to_send = [
-            &(chunk_length as u16).to_be_bytes(),
-            &next_message.contents[first_index..last_index + 1],
-        ]
-        .concat();
-        self.sent_bytes += bytes_to_send.len() as u64;
-        if next_message.bytes_sent >= next_message.contents.len() {
-            self.pending_messages.pop_front();
         }
+        self.sent_bytes += bytes_to_send.len() as u64;
         Some(bytes_to_send)
     }
 
     pub fn is_empty(&self) -> bool {
-        self.pending_messages.is_empty()
+        self.pending_data.is_empty()
     }
 
     pub fn update_ue_received_bytes(&mut self, ue_received_bytes: u64) {
@@ -86,39 +93,27 @@ impl MessageWriter {
             _ => 0,
         }
     }
-
-    fn should_send_now(&self) -> bool {
-        if self.pending_messages.is_empty() {
-            return false;
-        }
-        let bytes_left_in_message = match self.pending_messages.get(0) {
-            Some(message) => message.bytes_left(),
-            _ => return false,
-        };
-        let max_output_bytes = self.available_ue_buffer_capacity();
-        if bytes_left_in_message <= max_output_bytes {
-            return true;
-        };
-        let max_bytes_in_payload = UE_INPUT_BUFFER - LENGTH_FIELD_SIZE;
-        let next_payload_size = max_output_bytes.checked_sub(4).unwrap_or_default();
-        let chunks_in_message = (bytes_left_in_message / max_bytes_in_payload) + 1;
-        let chunks_in_message_after_sending =
-            ((bytes_left_in_message - next_payload_size) / max_bytes_in_payload) + 1;
-        chunks_in_message_after_sending < chunks_in_message
-    }
 }
 
 #[test]
-fn new_writer_is_empty() {
+fn writer_contents_after_creation() {
-    let mut writer = MessageWriter::new();
+    let writer = MessageWriter::new();
     assert_eq!(writer.is_empty(), true);
+    assert_eq!(writer.available_ue_buffer_capacity(), UE_INPUT_BUFFER);
 }
 
 #[test]
-fn single_short_message() {
+fn writer_content_after_push() {
     let mut writer = MessageWriter::new();
     writer.push("Hello, world!");
     assert_eq!(writer.is_empty(), false);
+    assert_eq!(writer.available_ue_buffer_capacity(), UE_INPUT_BUFFER);
+}
+
+#[test]
+fn writing_single_short_message() {
+    let mut writer = MessageWriter::new();
+    writer.push("Hello, world!");
     let resulting_bytes = writer.try_pop().unwrap();
     let expected_bytes = [
         0, 17, // Bytes in the chunk = message length (4 bytes) + message (13 bytes)
@@ -126,44 +121,95 @@ fn single_short_message() {
         b'H', b'e', b'l', b'l', b'o', b',', b' ', b'w', b'o', b'r', b'l', b'd', b'!',
     ];
     assert_eq!(writer.is_empty(), true);
+    assert_eq!(
+        writer.available_ue_buffer_capacity(),
+        UE_INPUT_BUFFER - "Hello, world!".len() - 2 - 4
+    );
     assert_eq!(resulting_bytes, expected_bytes);
     assert_eq!(writer.sent_bytes, expected_bytes.len() as u64);
 }
 
 #[test]
-fn single_long_message() {
+fn writing_first_chunk_of_single_long_message() {
     let mut writer = MessageWriter::new();
     //  Because we also have to pass lengths, this will go over the sending limit
     let long_message = "Q".repeat(UE_INPUT_BUFFER);
     writer.push(&long_message);
-    assert_eq!(writer.is_empty(), false);
     let resulting_bytes = writer.try_pop().unwrap();
     assert_eq!(writer.is_empty(), false);
-    assert_eq!(resulting_bytes.len(), 4095);
+    assert_eq!(resulting_bytes.len(), UE_INPUT_BUFFER);
+    assert_eq!(writer.available_ue_buffer_capacity(), 0);
     //  Bytes in the chunk = 4095 - 2 = 4093 = 0x0ffd
     assert_eq!(resulting_bytes[0], 0x0f);
-    assert_eq!(resulting_bytes[1], 0x0fd);
+    assert_eq!(resulting_bytes[1], 0xfd);
     //  Bytes in message = 4095 = 0x0fff
     assert_eq!(resulting_bytes[2], 0);
     assert_eq!(resulting_bytes[3], 0);
     assert_eq!(resulting_bytes[4], 0x0f);
-    assert_eq!(resulting_bytes[5], 0x0ff);
+    assert_eq!(resulting_bytes[5], 0xff);
     for &byte in resulting_bytes[6..].iter() {
         assert_eq!(byte, b'Q');
     }
-
     assert_eq!(writer.try_pop(), None);
     assert_eq!(writer.is_empty(), false);
-    writer.update_ue_received_bytes(UE_INPUT_BUFFER as u64);
+}
+
+#[test]
+fn writing_second_chunk_of_single_long_message() {
+    let mut writer = MessageWriter::new();
+    //  Because we also have to pass lengths, this will go over the sending limit
+    let long_message = "Q".repeat(UE_INPUT_BUFFER);
+    writer.push(&long_message);
+    //  This pops all but 6 bytes of `long_message`, that were required to encode lengths of
+    //  message and first chunk
+    let first_bytes = writer.try_pop().unwrap();
+    writer.update_ue_received_bytes(first_bytes.len() as u64);
     let resulting_bytes = writer.try_pop().unwrap();
+    assert_eq!(
+        writer.available_ue_buffer_capacity(),
+        UE_INPUT_BUFFER - resulting_bytes.len()
+    );
     assert_eq!(writer.is_empty(), true);
-    //  Last time we have popped all but 6 bytes from `long_message`
+    //  Bytes in the chunk = 6
-    //  Bytes in the chunk = 6 = 0x06
     assert_eq!(resulting_bytes[0], 0);
-    assert_eq!(resulting_bytes[1], 0x06);
+    assert_eq!(resulting_bytes[1], 6);
     assert_eq!(resulting_bytes[2..], [b'Q', b'Q', b'Q', b'Q', b'Q', b'Q'])
 }
 
-// TODO: test that `update_ue_received_bytes()` cannot reduce that value
+#[test]
-// TODO: test several messages, message only partially fitting into remaining buffer
+fn will_write_small_chunks_if_no_more_data() {
-// TODO: test giving just barely enough free buffer space
+    let mut writer = MessageWriter::new();
+    //  Because we also have to pass lengths (of chunk `CHUNK_LENGTH_FIELD` amd of message `4`),
+    //  sending this will leave us with exactly 10 free bytes in the buffer
+    let long_message = "Q".repeat(UE_INPUT_BUFFER / 2);
+    writer.push(&long_message);
+    writer.try_pop();
+    let short_message = "Hello, world!";
+    writer.push(&short_message);
+    let expected_bytes = [
+        0, 17, // Bytes in the chunk = message length (4 bytes) + message (13 bytes)
+        0, 0, 0, 13, // Bytes in the message
+        b'H', b'e', b'l', b'l', b'o', b',', b' ', b'w', b'o', b'r', b'l', b'd', b'!',
+    ];
+    //  There should be enough space in the ue-server buffer to send `short_message`
+    let resulting_bytes = writer.try_pop().unwrap();
+    assert_eq!(resulting_bytes, expected_bytes);
+    assert_eq!(writer.try_pop(), None);
+    assert_eq!(writer.is_empty(), true);
+}
+
+#[test]
+fn will_not_write_small_chunks_if_more_data_remains() {
+    let mut writer = MessageWriter::new();
+    //  Because we also have to pass lengths (of chunk `CHUNK_LENGTH_FIELD` amd of message `4`),
+    //  sending this will leave us with exactly 10 free bytes in the buffer
+    let long_message = "Q".repeat(UE_INPUT_BUFFER - CHUNK_LENGTH_FIELD - 4 - 10);
+    writer.push(&long_message);
+    writer.try_pop();
+    let short_message = "Hello, world!";
+    writer.push(&short_message);
+    //  `MessageWriter` can neither send full message, nor a chunk of size 10
+    //  (because it is too short)
+    assert_eq!(writer.try_pop(), None);
+    assert_eq!(writer.is_empty(), false);
+}