1 files changed, 60 insertions, 0 deletions
diff --git a/src/compress.rs b/src/compress.rs
new file mode 100644
index 0000000..77e634d
--- /dev/null
+++ b/src/compress.rs
@@ -0,0 +1,60 @@
+//! Deflate for what goes over the wire: large sync messages (`swarm::sync`). Automerge
+//! sends changes as they are stored, uncompressed; text and JSON in them shrink to about
+//! half.
+
+use std::io::{Read, Write};
+
+use anyhow::{Result, bail};
+use flate2::Compression;
+use flate2::read::DeflateDecoder;
+use flate2::write::DeflateEncoder;
+
+/// Below this, deflating saves too little to be worth it.
+pub(crate) const WORTH_FROM: usize = 1024;
+
+/// `bytes` deflated, if that makes them at least a tenth smaller.
+pub(crate) fn deflate(bytes: &[u8]) -> Option<Vec<u8>> {
+    if bytes.len() < WORTH_FROM {
+        return None;
+    }
+    let mut encoder = DeflateEncoder::new(Vec::with_capacity(bytes.len() / 2), Compression::fast());
+    encoder.write_all(bytes).ok()?;
+    let deflated = encoder.finish().ok()?;
+    (deflated.len() < bytes.len() - bytes.len() / 10).then_some(deflated)
+}
+
+/// `bytes` inflated, if they come to at most `limit` bytes: a small message can't make
+/// us hold a large one.
+pub(crate) fn inflate(bytes: &[u8], limit: usize) -> Result<Vec<u8>> {
+    let mut inflated = Vec::with_capacity((bytes.len() * 3).min(limit));
+    DeflateDecoder::new(bytes)
+        .take(limit as u64 + 1)
+        .read_to_end(&mut inflated)?;
+    if inflated.len() > limit {
+        bail!("inflates to more than {limit} bytes");
+    }
+    Ok(inflated)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// What deflates inflates back; little, or what doesn't shrink, isn't deflated; more
+    /// than the limit is refused.
+    #[test]
+    fn deflates_and_inflates_within_limits() {
+        let text = "a key, a value; ".repeat(1000);
+        let deflated = deflate(text.as_bytes()).expect("text shrinks");
+        assert!(deflated.len() < text.len() / 4);
+        assert_eq!(inflate(&deflated, text.len()).unwrap(), text.as_bytes());
+        assert!(
+            inflate(&deflated, text.len() - 1).is_err(),
+            "over the limit"
+        );
+        assert!(deflate(b"short").is_none());
+        let mut rng = crate::testrand::Rng::new(1);
+        let noise: Vec<u8> = (0..4096).map(|_| rng.next_u64() as u8).collect();
+        assert!(deflate(&noise).is_none(), "noise doesn't shrink");
+    }
+}