prepare for media query merging

src/atrule/media.rs

@@ -1,3 +1,4 @@
+#![allow(dead_code)]
 use std::fmt;
 
 use crate::{parse::Stmt, selector::Selector};
@@ -9,7 +10,7 @@ pub(crate) struct MediaRule {
     pub body: Vec<Stmt>,
 }
 
-#[derive(Debug, Eq, PartialEq, Clone)]
+#[derive(Clone, Debug, Eq, PartialEq, Hash)]
 pub(crate) struct MediaQuery {
     /// The modifier, probably either "not" or "only".
     ///
@@ -25,7 +26,6 @@ pub(crate) struct MediaQuery {
     pub features: Vec<String>,
 }
 
-#[allow(dead_code)]
 impl MediaQuery {
     pub fn is_condition(&self) -> bool {
         self.modifier.is_none() && self.media_type.is_none()
@@ -47,9 +47,159 @@ impl MediaQuery {
         }
     }
 
-    #[allow(dead_code, unused_variables)]
-    pub fn merge(other: &Self) -> Self {
-        todo!()
+    fn merge(&self, other: &Self) -> MediaQueryMergeResult {
+        let this_modifier = self.modifier.as_ref().map(|m| m.to_ascii_lowercase());
+        let this_type = self.media_type.as_ref().map(|m| m.to_ascii_lowercase());
+        let other_modifier = other.modifier.as_ref().map(|m| m.to_ascii_lowercase());
+        let other_type = other.media_type.as_ref().map(|m| m.to_ascii_lowercase());
+
+        if this_type.is_none() && other_type.is_none() {
+            return MediaQueryMergeResult::Success(Self::condition(
+                self.features
+                    .iter()
+                    .chain(&other.features)
+                    .cloned()
+                    .collect(),
+            ));
+        }
+
+        let modifier;
+        let media_type;
+        let features;
+
+        if (this_modifier.as_deref() == Some("not")) != (other_modifier.as_deref() == Some("not")) {
+            if this_modifier == other_modifier {
+                let negative_features = if this_modifier.as_deref() == Some("not") {
+                    &self.features
+                } else {
+                    &other.features
+                };
+
+                let positive_features = if this_modifier.as_deref() == Some("not") {
+                    &other.features
+                } else {
+                    &self.features
+                };
+
+                // If the negative features are a subset of the positive features, the
+                // query is empty. For example, `not screen and (color)` has no
+                // intersection with `screen and (color) and (grid)`.
+                //
+                // However, `not screen and (color)` *does* intersect with `screen and
+                // (grid)`, because it means `not (screen and (color))` and so it allows
+                // a screen with no color but with a grid.
+
+                if negative_features
+                    .iter()
+                    .all(|feat| positive_features.contains(&feat))
+                {
+                    return MediaQueryMergeResult::Empty;
+                } else {
+                    return MediaQueryMergeResult::Unrepresentable;
+                }
+            } else if self.matches_all_types() || other.matches_all_types() {
+                return MediaQueryMergeResult::Unrepresentable;
+            }
+
+            if this_modifier.as_deref() == Some("not") {
+                modifier = &other_modifier;
+                media_type = &other_type;
+                features = other.features.clone();
+            } else {
+                modifier = &this_modifier;
+                media_type = &this_type;
+                features = self.features.clone();
+            }
+        } else if this_modifier.as_deref() == Some("not") {
+            debug_assert_eq!(other_modifier.as_deref(), Some("not"));
+
+            // CSS has no way of representing "neither screen nor print".
+            if this_type != other_type {
+                return MediaQueryMergeResult::Unrepresentable;
+            }
+
+            let more_features = if self.features.len() > other.features.len() {
+                &self.features
+            } else {
+                &other.features
+            };
+
+            let fewer_features = if self.features.len() > other.features.len() {
+                &other.features
+            } else {
+                &self.features
+            };
+
+            // If one set of features is a superset of the other, use those features
+            // because they're strictly narrower.
+            if fewer_features
+                .iter()
+                .all(|feat| more_features.contains(feat))
+            {
+                modifier = &this_modifier; // "not"
+                media_type = &this_type;
+                features = more_features.clone();
+            } else {
+                // Otherwise, there's no way to represent the intersection.
+                return MediaQueryMergeResult::Unrepresentable;
+            }
+        } else if self.matches_all_types() {
+            modifier = &other_modifier;
+
+            // Omit the type if either input query did, since that indicates that they
+            // aren't targeting a browser that requires "all and".
+            media_type = if other.matches_all_types() && this_type.is_none() {
+                &None
+            } else {
+                &other_type
+            };
+
+            features = self
+                .features
+                .iter()
+                .chain(&other.features)
+                .cloned()
+                .collect();
+        } else if other.matches_all_types() {
+            modifier = &this_modifier;
+            media_type = &this_type;
+            features = self
+                .features
+                .iter()
+                .chain(&other.features)
+                .cloned()
+                .collect();
+        } else if this_type != other_type {
+            return MediaQueryMergeResult::Empty;
+        } else {
+            if this_modifier.is_some() {
+                modifier = &this_modifier
+            } else {
+                modifier = &other_modifier;
+            }
+
+            media_type = &this_type;
+            features = self
+                .features
+                .iter()
+                .chain(&other.features)
+                .cloned()
+                .collect();
+        }
+
+        MediaQueryMergeResult::Success(MediaQuery {
+            media_type: if media_type == &this_type {
+                self.media_type.clone()
+            } else {
+                other.media_type.clone()
+            },
+            modifier: if modifier == &this_modifier {
+                self.modifier.clone()
+            } else {
+                other.modifier.clone()
+            },
+            features,
+        })
     }
 }
 
@@ -67,3 +217,10 @@ impl fmt::Display for MediaQuery {
         f.write_str(&self.features.join(" and "))
     }
 }
+
+#[derive(Clone, Debug, Eq, PartialEq, Hash)]
+enum MediaQueryMergeResult {
+    Empty,
+    Unrepresentable,
+    Success(MediaQuery),
+}