Add XOR diff + zstd frame encoding in wayray-protocol

Encoder: xor_diff + encode_region (per damage rectangle).
Decoder: apply_region (decompress + XOR apply to framebuffer).
Both live in wayray-protocol::encoding for shared access.
14 tests including 3 end-to-end round-trip integration tests.

Cargo.lock

@@ -2192,6 +2192,7 @@ dependencies = [
  "postcard",
  "serde",
  "thiserror 2.0.18",
+ "zstd",
 ]
 
 [[package]]
@@ -2597,3 +2598,31 @@ name = "zmij"
 version = "1.0.21"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b8848ee67ecc8aedbaf3e4122217aff892639231befc6a1b58d29fff4c2cabaa"
+
+[[package]]
+name = "zstd"
+version = "0.13.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e91ee311a569c327171651566e07972200e76fcfe2242a4fa446149a3881c08a"
+dependencies = [
+ "zstd-safe",
+]
+
+[[package]]
+name = "zstd-safe"
+version = "7.2.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8f49c4d5f0abb602a93fb8736af2a4f4dd9512e36f7f570d66e65ff867ed3b9d"
+dependencies = [
+ "zstd-sys",
+]
+
+[[package]]
+name = "zstd-sys"
+version = "2.0.16+zstd.1.5.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "91e19ebc2adc8f83e43039e79776e3fda8ca919132d68a1fed6a5faca2683748"
+dependencies = [
+ "cc",
+ "pkg-config",
+]

crates/wayray-protocol/Cargo.toml

@@ -8,3 +8,4 @@ license.workspace = true
 serde.workspace = true
 postcard.workspace = true
 thiserror.workspace = true
+zstd.workspace = true

crates/wayray-protocol/src/encoding.rs

@@ -0,0 +1,134 @@
+//! Frame encoding and decoding: XOR diff + zstd compression.
+//!
+//! The encoder (server-side) produces `DamageRegion` messages from
+//! framebuffer changes. The decoder (client-side) applies them to
+//! reconstruct the framebuffer.
+
+use crate::messages::DamageRegion;
+
+// ── Encoder (server side) ───────────────────────────────────────────
+
+/// Compute XOR diff between two ARGB8888 framebuffers.
+///
+/// Returns a new buffer where unchanged pixels are zero.
+/// Both buffers must have the same length.
+pub fn xor_diff(current: &[u8], previous: &[u8]) -> Vec<u8> {
+    assert_eq!(
+        current.len(),
+        previous.len(),
+        "framebuffer size mismatch: {} vs {}",
+        current.len(),
+        previous.len()
+    );
+    current
+        .iter()
+        .zip(previous.iter())
+        .map(|(a, b)| a ^ b)
+        .collect()
+}
+
+/// Extract a rectangular region from an XOR diff buffer and compress it.
+///
+/// `stride` is the number of bytes per row in the full framebuffer.
+/// The region is compressed with zstd at level 1 (fast).
+pub fn encode_region(
+    diff: &[u8],
+    stride: usize,
+    x: u32,
+    y: u32,
+    width: u32,
+    height: u32,
+) -> DamageRegion {
+    let bpp = 4; // ARGB8888
+    let region_stride = width as usize * bpp;
+    let mut region_data = Vec::with_capacity(region_stride * height as usize);
+
+    for row in 0..height as usize {
+        let src_offset = (y as usize + row) * stride + x as usize * bpp;
+        region_data.extend_from_slice(&diff[src_offset..src_offset + region_stride]);
+    }
+
+    let compressed = zstd::encode_all(region_data.as_slice(), 1).expect("zstd encode failed");
+
+    DamageRegion {
+        x,
+        y,
+        width,
+        height,
+        data: compressed,
+    }
+}
+
+// ── Decoder (client side) ───────────────────────────────────────────
+
+/// Apply a compressed XOR diff region onto a framebuffer.
+///
+/// Decompresses the region data with zstd, then XOR-applies it at the
+/// specified position. Modifies `framebuffer` in place.
+///
+/// `stride` is the number of bytes per row in the full framebuffer.
+pub fn apply_region(framebuffer: &mut [u8], stride: usize, region: &DamageRegion) {
+    let bpp = 4;
+    let decompressed = zstd::decode_all(region.data.as_slice()).expect("zstd decode failed");
+    let region_stride = region.width as usize * bpp;
+
+    for row in 0..region.height as usize {
+        let dst_offset = (region.y as usize + row) * stride + region.x as usize * bpp;
+        let src_offset = row * region_stride;
+        let dst_row = &mut framebuffer[dst_offset..dst_offset + region_stride];
+        let src_row = &decompressed[src_offset..src_offset + region_stride];
+        for (d, s) in dst_row.iter_mut().zip(src_row.iter()) {
+            *d ^= s;
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn xor_diff_identical_frames() {
+        let frame = vec![100u8; 400];
+        let diff = xor_diff(&frame, &frame);
+        assert!(diff.iter().all(|&b| b == 0));
+    }
+
+    #[test]
+    fn xor_diff_changed_pixel() {
+        let prev = vec![0u8; 16];
+        let mut curr = vec![0u8; 16];
+        curr[0..4].copy_from_slice(&[255, 128, 64, 255]);
+        let diff = xor_diff(&curr, &prev);
+        assert_eq!(&diff[0..4], &[255, 128, 64, 255]);
+        assert!(diff[4..].iter().all(|&b| b == 0));
+    }
+
+    #[test]
+    fn encode_region_compresses_zeros() {
+        let width = 100usize;
+        let height = 100usize;
+        let stride = width * 4;
+        let diff = vec![0u8; stride * height];
+        let region = encode_region(&diff, stride, 0, 0, width as u32, height as u32);
+        assert!(region.data.len() < 1000);
+    }
+
+    #[test]
+    fn apply_region_zeros_is_noop() {
+        let stride = 40; // 10 pixels * 4 bpp
+        let mut framebuffer = vec![42u8; stride * 10];
+        let original = framebuffer.clone();
+        let zero_data = vec![0u8; stride * 10];
+        let compressed = zstd::encode_all(zero_data.as_slice(), 1).unwrap();
+        let region = DamageRegion {
+            x: 0,
+            y: 0,
+            width: 10,
+            height: 10,
+            data: compressed,
+        };
+        apply_region(&mut framebuffer, stride, &region);
+        assert_eq!(framebuffer, original);
+    }
+}

crates/wayray-protocol/src/lib.rs

@@ -5,6 +5,7 @@
 //! length prefix for transmission over QUIC streams.
 
 pub mod codec;
+pub mod encoding;
 pub mod messages;
 
 /// Current protocol version. Incremented on breaking changes.

crates/wayray-protocol/tests/encoding_roundtrip.rs

@@ -0,0 +1,82 @@
+//! Integration test: encode → compress → decompress → apply.
+//!
+//! Verifies the full XOR diff → zstd → XOR apply pipeline
+//! reconstructs the original framebuffer correctly.
+
+use wayray_protocol::encoding::{apply_region, encode_region, xor_diff};
+
+#[test]
+fn roundtrip_with_subregion() {
+    let width = 100u32;
+    let height = 100u32;
+    let stride = width as usize * 4;
+    let prev = vec![0u8; stride * height as usize];
+    let mut curr = prev.clone();
+
+    // Paint a 10x10 red rectangle at (5, 5)
+    for y in 5..15u32 {
+        for x in 5..15u32 {
+            let offset = y as usize * stride + x as usize * 4;
+            curr[offset..offset + 4].copy_from_slice(&[255, 0, 0, 255]);
+        }
+    }
+
+    let diff = xor_diff(&curr, &prev);
+    let region = encode_region(&diff, stride, 5, 5, 10, 10);
+
+    let mut reconstructed = prev;
+    apply_region(&mut reconstructed, stride, &region);
+    assert_eq!(reconstructed, curr);
+}
+
+#[test]
+fn roundtrip_unchanged_frame() {
+    let width = 50u32;
+    let height = 50u32;
+    let stride = width as usize * 4;
+    let frame = vec![128u8; stride * height as usize];
+
+    let diff = xor_diff(&frame, &frame);
+    let region = encode_region(&diff, stride, 0, 0, width, height);
+
+    assert!(
+        region.data.len() < 200,
+        "all-zero diff should compress well"
+    );
+
+    let mut reconstructed = frame.clone();
+    apply_region(&mut reconstructed, stride, &region);
+    assert_eq!(reconstructed, frame);
+}
+
+#[test]
+fn roundtrip_multiple_regions() {
+    let width = 200u32;
+    let height = 200u32;
+    let stride = width as usize * 4;
+    let prev = vec![0u8; stride * height as usize];
+    let mut curr = prev.clone();
+
+    for y in 0..10u32 {
+        for x in 0..10u32 {
+            let offset = y as usize * stride + x as usize * 4;
+            curr[offset..offset + 4].copy_from_slice(&[255, 0, 0, 255]);
+        }
+    }
+
+    for y in 100..120u32 {
+        for x in 150..180u32 {
+            let offset = y as usize * stride + x as usize * 4;
+            curr[offset..offset + 4].copy_from_slice(&[0, 255, 0, 255]);
+        }
+    }
+
+    let diff = xor_diff(&curr, &prev);
+    let region1 = encode_region(&diff, stride, 0, 0, 10, 10);
+    let region2 = encode_region(&diff, stride, 150, 100, 30, 20);
+
+    let mut reconstructed = prev;
+    apply_region(&mut reconstructed, stride, &region1);
+    apply_region(&mut reconstructed, stride, &region2);
+    assert_eq!(reconstructed, curr);
+}

crates/wrsrvd/src/handlers/compositor.rs

@@ -1,9 +1,6 @@
 use smithay::{
     delegate_compositor, delegate_shm,
-    reexports::wayland_server::{
-        Client,
-        protocol::wl_surface::WlSurface,
-    },
+    reexports::wayland_server::{Client, protocol::wl_surface::WlSurface},
     wayland::{
         buffer::BufferHandler,
         compositor::{CompositorClientState, CompositorHandler, CompositorState},

crates/wrsrvd/src/handlers/output.rs

@@ -1,8 +1,4 @@
-use smithay::{
-    delegate_output,
-    output::Output,
-    wayland::output::OutputHandler,
-};
+use smithay::{delegate_output, output::Output, wayland::output::OutputHandler};
 
 use crate::state::WayRay;
 

crates/wrsrvd/src/handlers/xdg_shell.rs

@@ -2,8 +2,7 @@ use smithay::{
     delegate_xdg_decoration, delegate_xdg_shell,
     desktop::Window,
     reexports::{
-        wayland_protocols::xdg::decoration::zv1::server::zxdg_toplevel_decoration_v1::Mode
-            as DecorationMode,
+        wayland_protocols::xdg::decoration::zv1::server::zxdg_toplevel_decoration_v1::Mode as DecorationMode,
         wayland_server::protocol::wl_seat,
     },
     utils::Serial,

crates/wrsrvd/src/main.rs

@@ -41,8 +41,8 @@ fn main() -> Result<()> {
     info!("wrsrvd starting");
 
     // Create the Wayland display.
-    let mut display = Display::<WayRay>::new()
-        .map_err(|e| errors::WayRayError::DisplayInit(Box::new(e)))?;
+    let mut display =
+        Display::<WayRay>::new().map_err(|e| errors::WayRayError::DisplayInit(Box::new(e)))?;
 
     // Initialize the Winit backend (opens a window, creates a GlesRenderer).
     let (backend, winit_event_loop) = winit::init::<GlesRenderer>().map_err(|e| {