perf: cache blurred wallpaper to disk to avoid re-blur on startup

First launch with blur blurs and saves to ~/.cache/moonset/. Subsequent
starts load the cached PNG directly (~9x faster). Cache invalidates
when wallpaper path, size, mtime, or sigma changes.

CHANGELOG.md

@@ -8,6 +8,7 @@ Format based on [Keep a Changelog](https://keepachangelog.com/).
 ### Added
 
 - Optional background blur via `background_blur` config option (Gaussian blur, `image` crate)
+- Disk cache for blurred wallpaper (`~/.cache/moonset/`) — avoids re-blurring on subsequent starts
 
 ## [0.1.1] - 2026-03-28
 

Cargo.lock

@@ -805,7 +805,7 @@ dependencies = [
 
 [[package]]
 name = "moonset"
-version = "0.1.1"
+version = "0.4.0"
 dependencies = [
  "dirs",
  "env_logger",

src/panel.rs

@@ -8,8 +8,12 @@ use gtk4::prelude::*;
 use gtk4::{self as gtk, gio};
 use image::imageops;
 use std::cell::RefCell;
-use std::path::Path;
+use std::fs;
+use std::io::Write;
+use std::os::unix::fs::OpenOptionsExt;
+use std::path::{Path, PathBuf};
 use std::rc::Rc;
+use std::time::SystemTime;
 
 use crate::i18n::{load_strings, Strings};
 use crate::power::{self, PowerError};
@@ -96,11 +100,118 @@ pub fn load_background_texture(bg_path: &Path, blur_radius: Option<f32>) -> gdk:
     };
 
     match blur_radius {
-        Some(sigma) if sigma > 0.0 => apply_blur(&texture, sigma),
+        Some(sigma) if sigma > 0.0 => load_blurred_with_cache(bg_path, &texture, sigma),
         _ => texture,
     }
 }
 
+// -- Blur cache ----------------------------------------------------------------
+
+const CACHE_PNG: &str = "blur-cache.png";
+const CACHE_META: &str = "blur-cache.meta";
+
+fn blur_cache_dir() -> Option<PathBuf> {
+    dirs::cache_dir().map(|d| d.join("moonset"))
+}
+
+/// Build the cache key string for the current wallpaper + sigma.
+fn build_cache_meta(bg_path: &Path, sigma: f32) -> Option<String> {
+    if bg_path.starts_with("/dev/moonarch/") {
+        let binary = std::env::current_exe().ok()?;
+        let binary_mtime = fs::metadata(&binary)
+            .ok()?
+            .modified()
+            .ok()?
+            .duration_since(SystemTime::UNIX_EPOCH)
+            .ok()?
+            .as_secs();
+        Some(format!(
+            "path={}\nbinary_mtime={}\nsigma={}\n",
+            bg_path.display(), binary_mtime, sigma,
+        ))
+    } else {
+        let meta = fs::metadata(bg_path).ok()?;
+        let mtime = meta
+            .modified()
+            .ok()?
+            .duration_since(SystemTime::UNIX_EPOCH)
+            .ok()?
+            .as_secs();
+        Some(format!(
+            "path={}\nsize={}\nmtime={}\nsigma={}\n",
+            bg_path.display(), meta.len(), mtime, sigma,
+        ))
+    }
+}
+
+/// Try to load a cached blurred texture if the cache key matches.
+fn load_cached_blur(cache_dir: &Path, expected_meta: &str) -> Option<gdk::Texture> {
+    let stored_meta = fs::read_to_string(cache_dir.join(CACHE_META)).ok()?;
+    if stored_meta != expected_meta {
+        log::debug!("Blur cache meta mismatch, will re-blur");
+        return None;
+    }
+    let file = gio::File::for_path(cache_dir.join(CACHE_PNG));
+    match gdk::Texture::from_file(&file) {
+        Ok(texture) => {
+            log::debug!("Loaded blurred wallpaper from cache");
+            Some(texture)
+        }
+        Err(e) => {
+            log::debug!("Failed to load cached blur PNG: {e}");
+            None
+        }
+    }
+}
+
+/// Save a blurred texture to the cache directory.
+fn save_blur_cache(cache_dir: &Path, texture: &gdk::Texture, meta: &str) {
+    if let Err(e) = save_blur_cache_inner(cache_dir, texture, meta) {
+        log::debug!("Failed to save blur cache: {e}");
+    }
+}
+
+fn save_blur_cache_inner(
+    cache_dir: &Path,
+    texture: &gdk::Texture,
+    meta: &str,
+) -> Result<(), Box<dyn std::error::Error>> {
+    fs::create_dir_all(cache_dir)?;
+
+    let png_bytes = texture.save_to_png_bytes();
+
+    let mut f = fs::OpenOptions::new()
+        .create(true).write(true).truncate(true).mode(0o600)
+        .open(cache_dir.join(CACHE_PNG))?;
+    f.write_all(&png_bytes)?;
+
+    // Meta last — incomplete cache is treated as a miss on next start
+    let mut f = fs::OpenOptions::new()
+        .create(true).write(true).truncate(true).mode(0o600)
+        .open(cache_dir.join(CACHE_META))?;
+    f.write_all(meta.as_bytes())?;
+
+    log::debug!("Saved blur cache to {}", cache_dir.display());
+    Ok(())
+}
+
+/// Load blurred texture, using disk cache when available.
+fn load_blurred_with_cache(bg_path: &Path, texture: &gdk::Texture, sigma: f32) -> gdk::Texture {
+    if let Some(cache_dir) = blur_cache_dir() {
+        if let Some(meta) = build_cache_meta(bg_path, sigma) {
+            if let Some(cached) = load_cached_blur(&cache_dir, &meta) {
+                return cached;
+            }
+            let blurred = apply_blur(texture, sigma);
+            save_blur_cache(&cache_dir, &blurred, &meta);
+            return blurred;
+        }
+    }
+    apply_blur(texture, sigma)
+}
+
+// -- Blur implementation -------------------------------------------------------
+
 /// Apply Gaussian blur to a texture and return a blurred texture.
 fn apply_blur(texture: &gdk::Texture, sigma: f32) -> gdk::Texture {
     let width = texture.width() as u32;
@@ -627,4 +738,61 @@ mod tests {
         let confirm_fn = defs[1].confirm_attr.unwrap();
         assert_eq!(confirm_fn(strings), "Wirklich abmelden?");
     }
+
+    // -- Blur cache tests --
+
+    #[test]
+    fn build_cache_meta_for_file() {
+        let dir = tempfile::tempdir().unwrap();
+        let file = dir.path().join("wallpaper.jpg");
+        fs::write(&file, b"fake image").unwrap();
+        let meta = build_cache_meta(&file, 20.0);
+        assert!(meta.is_some());
+        let meta = meta.unwrap();
+        assert!(meta.contains("path="));
+        assert!(meta.contains("size=10"));
+        assert!(meta.contains("sigma=20"));
+        assert!(meta.contains("mtime="));
+    }
+
+    #[test]
+    fn build_cache_meta_for_gresource() {
+        let path = Path::new("/dev/moonarch/moonset/wallpaper.jpg");
+        let meta = build_cache_meta(path, 15.0);
+        assert!(meta.is_some());
+        let meta = meta.unwrap();
+        assert!(meta.contains("binary_mtime="));
+        assert!(meta.contains("sigma=15"));
+        assert!(!meta.contains("size="));
+    }
+
+    #[test]
+    fn build_cache_meta_missing_file() {
+        let meta = build_cache_meta(Path::new("/nonexistent/wallpaper.jpg"), 20.0);
+        assert!(meta.is_none());
+    }
+
+    #[test]
+    fn cache_meta_mismatch_returns_none() {
+        let dir = tempfile::tempdir().unwrap();
+        fs::write(
+            dir.path().join(CACHE_META),
+            "path=/old.jpg\nsize=100\nmtime=1\nsigma=20\n",
+        ).unwrap();
+        let result = load_cached_blur(
+            dir.path(),
+            "path=/new.jpg\nsize=200\nmtime=2\nsigma=20\n",
+        );
+        assert!(result.is_none());
+    }
+
+    #[test]
+    fn cache_missing_meta_returns_none() {
+        let dir = tempfile::tempdir().unwrap();
+        let result = load_cached_blur(
+            dir.path(),
+            "path=/any.jpg\nsize=1\nmtime=1\nsigma=20\n",
+        );
+        assert!(result.is_none());
+    }
 }