//! 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");
    }
}