compress level 2

1 files changed, 20 insertions, 1 deletions

diff --git a/src/compress.rs b/src/compress.rs
index 77e634d..d175cda 100644
--- a/src/compress.rs
+++ b/src/compress.rs
@@ -12,12 +12,19 @@ use flate2::write::DeflateEncoder;
 /// Below this, deflating saves too little to be worth it.
 pub(crate) const WORTH_FROM: usize = 1024;
 
+/// Not 1 (`Compression::fast`): `flate2` runs on zlib-rs here (another dependency asks
+/// for it), whose level 1 is zlib-ng's quick deflate, with fixed Huffman codes only.
+/// Those can't use a skewed alphabet: 64 KiB of random hex shrinks to 87% at level 1, to
+/// 54% at level 2 (as at level 6), still quick.
+const LEVEL: u32 = 2;
+
 /// `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());
+    let mut encoder =
+        DeflateEncoder::new(Vec::with_capacity(bytes.len() / 2), Compression::new(LEVEL));
     encoder.write_all(bytes).ok()?;
     let deflated = encoder.finish().ok()?;
     (deflated.len() < bytes.len() - bytes.len() / 10).then_some(deflated)
@@ -53,6 +60,18 @@ mod tests {
             "over the limit"
         );
         assert!(deflate(b"short").is_none());
+        // Hex has 4 bits in 8: about half, as Huffman codes alone get it.
+        let mut rng = crate::testrand::Rng::new(2);
+        let hex: String = (0..4096)
+            .map(|_| format!("{:016x}", rng.next_u64()))
+            .collect();
+        let deflated = deflate(hex.as_bytes()).expect("hex shrinks");
+        assert!(
+            deflated.len() < hex.len() * 6 / 10,
+            "{} of {}",
+            deflated.len(),
+            hex.len()
+        );
         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");