use std::collections::BTreeMap;use crate::ExecServerError;const MAX_REORDER_DISTANCE: u32 = 64;const MAX_PENDING_BYTES: usize = 1024 * 1024;/// Reorders relay records before they reach Noise's implicit receive nonce./// The window is bounded, and each sequence number is released at most once.#[derive(Default)]pub(crate) struct OrderedCiphertextFrames {    next_seq: u32,    pending: BTreeMap<u32, Vec<u8>>,    pending_bytes: usize,}impl OrderedCiphertextFrames {    /// Accept one relay record and return the newly contiguous ciphertext run.    ///    /// Returns nothing for duplicates or while a gap remains. Closing a gap also    /// releases any buffered records that now follow it contiguously.    pub(crate) fn push(        &mut self,        seq: u32,        payload: Vec<u8>,    ) -> Result<Vec<Vec<u8>>, ExecServerError> {        // Keep the first ciphertext for a sequence. Later copies are duplicates.        if seq < self.next_seq || self.pending.contains_key(&seq) {            return Ok(Vec::new());        }        if seq > self.next_seq {            // Bound both the sequence gap and buffered bytes.            if seq - self.next_seq > MAX_REORDER_DISTANCE {                return Err(ExecServerError::Protocol(                    "Noise relay ciphertext exceeds reorder window".to_string(),                ));            }            let pending_bytes = self.pending_bytes + payload.len();            if pending_bytes > MAX_PENDING_BYTES {                return Err(ExecServerError::Protocol(                    "Noise relay pending ciphertext buffer is full".to_string(),                ));            }            self.pending.insert(seq, payload);            self.pending_bytes = pending_bytes;            return Ok(Vec::new());        }        // Release the expected record and anything now contiguous behind it.        let mut ready = vec![payload];        self.advance()?;        while let Some(payload) = self.pending.remove(&self.next_seq) {            self.pending_bytes -= payload.len();            ready.push(payload);            self.advance()?;        }        Ok(ready)    }    fn advance(&mut self) -> Result<(), ExecServerError> {        self.next_seq = self.next_seq.checked_add(1).ok_or_else(|| {            ExecServerError::Protocol("Noise relay sequence number exhausted".to_string())        })?;        Ok(())    }}#[cfg(test)]#[path = "ordered_ciphertext_tests.rs"]mod tests;