fix(config_edit): byte-clean writes matching Loxone's format

src/config_edit/write.rs

@@ -13,7 +13,11 @@ impl ConfigEditor {
         let config = xmltree::EmitterConfig::new()
             .perform_indent(true)
             .indent_string("\t")
-            .write_document_declaration(false);
+            .write_document_declaration(false)
+            // Loxone writes self-closing tags without a leading space (`<C/>`, not `<C />`).
+            // xml-rs pads by default, which makes every self-closing element differ on
+            // round-trip and drowns real changes in formatting noise.
+            .pad_self_closing(false);
         self.root
             .write_with_config(&mut buf, config)
             .context("Failed to write XML")?;
@@ -29,6 +33,23 @@ impl ConfigEditor {
             buf = s.into_bytes();
         }
 
+        // Post-process: Loxone writes attribute-less empty elements expanded
+        // (`<IoData></IoData>`, never `<IoData/>`). xml-rs always self-closes, so expand
+        // the attribute-less self-closing tags to match and keep round-trips byte-clean.
+        {
+            let s = String::from_utf8(buf).context("XML is not valid UTF-8")?;
+            let mut s = Self::expand_attrless_empty_tags(&s);
+            // Loxone keeps literal newlines inside attribute values (e.g. multi-line PicoC
+            // code, notification texts); xml-rs escapes them to `&#xA;`. Un-escape to match.
+            // Loxone never emits `&#xA;`, so this only reverses xml-rs's own escaping.
+            s = s.replace("&#xA;", "\n");
+            // Loxone terminates the file with a trailing newline; xml-rs does not.
+            if !s.ends_with('\n') {
+                s.push('\n');
+            }
+            buf = s.into_bytes();
+        }
+
         // Post-process: restore BOM
         if self.had_bom {
             let mut result = Vec::with_capacity(3 + buf.len());
@@ -45,4 +66,76 @@ impl ConfigEditor {
 
         Ok(buf)
     }
+
+    /// Expand attribute-less self-closing tags (`<IoData/>` → `<IoData></IoData>`).
+    ///
+    /// Loxone writes empty elements that have no attributes in expanded form. xml-rs always
+    /// self-closes; only tags of the shape `<Name/>` (name immediately followed by `/>`, i.e.
+    /// no attributes) are rewritten — attributed empty tags like `<Co K="I" U="…"/>` are left
+    /// self-closed, matching Loxone.
+    fn expand_attrless_empty_tags(s: &str) -> String {
+        let bytes = s.as_bytes();
+        let n = bytes.len();
+        let mut out = String::with_capacity(n);
+        let mut last = 0;
+        let mut i = 0;
+        while i < n {
+            if bytes[i] == b'<'
+                && i + 1 < n
+                && (bytes[i + 1].is_ascii_alphabetic() || bytes[i + 1] == b'_')
+            {
+                let name_start = i + 1;
+                let mut j = name_start;
+                while j < n
+                    && (bytes[j].is_ascii_alphanumeric() || matches!(bytes[j], b'_' | b':' | b'-'))
+                {
+                    j += 1;
+                }
+                if j + 1 < n && bytes[j] == b'/' && bytes[j + 1] == b'>' {
+                    out.push_str(&s[last..i]);
+                    let name = &s[name_start..j];
+                    out.push('<');
+                    out.push_str(name);
+                    out.push_str("></");
+                    out.push_str(name);
+                    out.push('>');
+                    i = j + 2;
+                    last = i;
+                    continue;
+                }
+                i = j; // skip past the element name
+                continue;
+            }
+            i += 1;
+        }
+        out.push_str(&s[last..]);
+        out
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::ConfigEditor;
+
+    #[test]
+    fn test_write_matches_loxone_formatting() {
+        // Loxone's on-disk conventions the emitter must reproduce for byte-clean round-trips:
+        // no space before '/>', attribute-less empties expanded, attributed empties
+        // self-closed, literal newlines in attribute values, trailing newline.
+        let xml = "<?xml version=\"1.0\" encoding=\"utf-8\"?>\n\
+<ControlList Version=\"1\">\n\
+\t<C U=\"a\" NTXT=\"line1\nline2\">\n\
+\t\t<IoData></IoData>\n\
+\t\t<Co K=\"Q\" U=\"b\"></Co>\n\
+\t</C>\n\
+</ControlList>\n";
+        let editor = ConfigEditor::load(xml.as_bytes()).unwrap();
+        let out = String::from_utf8(editor.to_bytes().unwrap()).unwrap();
+        assert!(!out.contains(" />"), "no padded self-close");
+        assert!(out.contains("<IoData></IoData>"), "attr-less empty stays expanded");
+        assert!(out.contains(r#"<Co K="Q" U="b"/>"#), "attributed empty self-closes");
+        assert!(out.contains("line1\nline2"), "literal newline in attr value");
+        assert!(!out.contains("&#xA;"), "no escaped newline");
+        assert!(out.ends_with('\n'), "trailing newline");
+    }
 }