Add LayerSet::get_or_create_layer

src/layer.rs

@@ -144,14 +144,23 @@ impl LayerSet {
             Err(NamingError::Duplicate(name.to_string()))
         } else {
             let name = Name::new(name).map_err(|_| NamingError::Invalid(name.into()))?;
-            let path = crate::util::default_file_name_for_layer_name(&name, &self.path_set);
+            let path = util::default_file_name_for_layer_name(&name, &self.path_set);
             let layer = Layer::new(name, path);
             self.path_set.insert(layer.path.to_string_lossy().to_lowercase());
             self.layers.push(layer);
             Ok(self.layers.last_mut().unwrap())
         }
     }
 
+    /// Returns a mutable reference to a layer, by name, creating the layer if it doesn't exist
+    pub fn get_or_create_layer(&mut self, name: &str) -> Result<&mut Layer, NamingError> {
+        let index = self.layers.iter().position(|l| l.name.as_str() == name);
+        match index {
+            Some(its_here) => Ok(&mut self.layers[its_here]),
+            None => self.new_layer(name),
+        }
+    }
+
     /// Remove a layer.
     ///
     /// The default layer cannot be removed.
@@ -729,6 +738,36 @@ mod tests {
         );
     }
 
+    #[test]
+    fn layer_get_or_create() {
+        let mut ufo = crate::Font::load("testdata/MutatorSansLightWide.ufo").unwrap();
+        assert_eq!(
+            ufo.layers.len(),
+            2,
+            "number of layers in test data MutatorSansLightWide changed"
+        );
+
+        let result = ufo.layers.get_or_create_layer("background");
+        assert!(result.is_ok());
+        assert_eq!(ufo.layers.len(), 2);
+
+        let result = ufo.layers.get_or_create_layer("middleground");
+        assert!(result.is_ok());
+        assert_eq!(ufo.layers.len(), 3, "new layer wasn't created");
+        assert!(
+            ufo.layers.layers.iter().any(|layer| layer.name().as_str() == "foreground"),
+            "new layer was created with the wrong name"
+        );
+
+        let result = ufo.layers.get_or_create_layer("\t");
+        assert!(matches!(result, Err(NamingError::Invalid(_))));
+        assert_eq!(ufo.layers.len(), 3);
+
+        let result = ufo.layers.get_or_create_layer(DEFAULT_LAYER_NAME);
+        assert!(matches!(result, Err(NamingError::ReservedName)));
+        assert_eq!(ufo.layers.len(), 3);
+    }
+
     #[test]
     fn rename_layer() {
         let mut layer_set = LayerSet::default();