Support merging --style arguments

The `overrides_with` clap builder option was removed
because it interfered with the matcher's ability to
retain all occurrences of `--style`.

The behavior it covered is expressed within the new
`forced_style_components` function.

src/style.rs

@@ -138,3 +138,197 @@ impl StyleComponents {
         self.0.clear();
     }
 }
+
+#[derive(Debug, PartialEq)]
+enum ComponentAction {
+    Override,
+    Add,
+    Remove,
+}
+
+impl ComponentAction {
+    fn extract_from_str(string: &str) -> (ComponentAction, &str) {
+        match string.chars().next() {
+            Some('-') => (ComponentAction::Remove, string.strip_prefix('-').unwrap()),
+            Some('+') => (ComponentAction::Add, string.strip_prefix('+').unwrap()),
+            _ => (ComponentAction::Override, string),
+        }
+    }
+}
+
+/// A list of [StyleComponent] that can be parsed from a string.
+pub struct StyleComponentList(Vec<(ComponentAction, StyleComponent)>);
+
+impl StyleComponentList {
+    fn expand_into(&self, components: &mut HashSet<StyleComponent>, interactive_terminal: bool) {
+        for (action, component) in self.0.iter() {
+            let subcomponents = component.components(interactive_terminal);
+
+            use ComponentAction::*;
+            match action {
+                Override | Add => components.extend(subcomponents),
+                Remove => components.retain(|c| !subcomponents.contains(c)),
+            }
+        }
+    }
+
+    /// Returns `true` if any component in the list was not prefixed with `+` or `-`.
+    fn contains_override(&self) -> bool {
+        self.0.iter().any(|(a, _)| *a == ComponentAction::Override)
+    }
+
+    /// Combines multiple [StyleComponentList]s into a single [StyleComponents] set.
+    ///
+    /// ## Precedence
+    /// The most recent list will take precedence and override all previous lists
+    /// unless it only contains components prefixed with `-` or `+`. When this
+    /// happens, the list's components will be merged into the previous list.
+    ///
+    /// ## Example
+    /// ```text
+    /// [numbers,grid] + [header,changes]  -> [header,changes]
+    /// [numbers,grid] + [+header,-grid]   -> [numbers,header]
+    /// ```
+    pub fn to_components(
+        lists: impl IntoIterator<Item = StyleComponentList>,
+        interactive_terminal: bool,
+    ) -> StyleComponents {
+        StyleComponents(
+            lists
+                .into_iter()
+                .fold(HashSet::new(), |mut components, list| {
+                    if list.contains_override() {
+                        components.clear();
+                    }
+
+                    list.expand_into(&mut components, interactive_terminal);
+                    components
+                }),
+        )
+    }
+}
+
+impl Default for StyleComponentList {
+    fn default() -> Self {
+        StyleComponentList(vec![(ComponentAction::Override, StyleComponent::Default)])
+    }
+}
+
+impl FromStr for StyleComponentList {
+    type Err = Error;
+
+    fn from_str(s: &str) -> Result<Self> {
+        Ok(StyleComponentList(
+            s.split(",")
+                .map(|s| ComponentAction::extract_from_str(s)) // If the component starts with "-", it's meant to be removed
+                .map(|(a, s)| Ok((a, StyleComponent::from_str(s)?)))
+                .collect::<Result<Vec<(ComponentAction, StyleComponent)>>>()?,
+        ))
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use std::collections::HashSet;
+    use std::str::FromStr;
+
+    use super::ComponentAction::*;
+    use super::StyleComponent::*;
+    use super::StyleComponentList;
+
+    #[test]
+    pub fn style_component_list_parse() {
+        assert_eq!(
+            StyleComponentList::from_str("grid,+numbers,snip,-snip,header")
+                .expect("no error")
+                .0,
+            vec![
+                (Override, Grid),
+                (Add, LineNumbers),
+                (Override, Snip),
+                (Remove, Snip),
+                (Override, Header),
+            ]
+        );
+
+        assert!(StyleComponentList::from_str("not-a-component").is_err());
+        assert!(StyleComponentList::from_str("grid,not-a-component").is_err());
+        assert!(StyleComponentList::from_str("numbers,-not-a-component").is_err());
+    }
+
+    #[test]
+    pub fn style_component_list_to_components() {
+        assert_eq!(
+            StyleComponentList::to_components(
+                vec![StyleComponentList::from_str("grid,numbers").expect("no error")],
+                false
+            )
+            .0,
+            HashSet::from([Grid, LineNumbers])
+        );
+    }
+
+    #[test]
+    pub fn style_component_list_to_components_removes_negated() {
+        assert_eq!(
+            StyleComponentList::to_components(
+                vec![StyleComponentList::from_str("grid,numbers,-grid").expect("no error")],
+                false
+            )
+            .0,
+            HashSet::from([LineNumbers])
+        );
+    }
+
+    #[test]
+    pub fn style_component_list_to_components_expands_subcomponents() {
+        assert_eq!(
+            StyleComponentList::to_components(
+                vec![StyleComponentList::from_str("full").expect("no error")],
+                false
+            )
+            .0,
+            HashSet::from_iter(Full.components(true).to_owned())
+        );
+    }
+
+    #[test]
+    pub fn style_component_list_expand_negates_subcomponents() {
+        assert!(!StyleComponentList::to_components(
+            vec![StyleComponentList::from_str("full,-numbers").expect("no error")],
+            true
+        )
+        .numbers());
+    }
+
+    #[test]
+    pub fn style_component_list_to_components_precedence_overrides_previous_lists() {
+        assert_eq!(
+            StyleComponentList::to_components(
+                vec![
+                    StyleComponentList::from_str("grid").expect("no error"),
+                    StyleComponentList::from_str("numbers").expect("no error"),
+                ],
+                false
+            )
+            .0,
+            HashSet::from([LineNumbers])
+        );
+    }
+
+    #[test]
+    pub fn style_component_list_to_components_precedence_merges_previous_lists() {
+        assert_eq!(
+            StyleComponentList::to_components(
+                vec![
+                    StyleComponentList::from_str("grid,header").expect("no error"),
+                    StyleComponentList::from_str("-grid").expect("no error"),
+                    StyleComponentList::from_str("+numbers").expect("no error"),
+                ],
+                false
+            )
+            .0,
+            HashSet::from([HeaderFilename, LineNumbers])
+        );
+    }
+}