add method for getting accept type preference (#1793)

actix-http/CHANGES.md

@@ -3,6 +3,7 @@
 ## Unreleased - 2020-xx-xx
 ### Added
 * HttpResponse builders for 1xx status codes. [#1768]
+* `Accept::mime_precedence` and `Accept::mime_preference`. [#1793]
 
 ### Fixed
 * Started dropping `transfer-encoding: chunked` and `Content-Length` for 1XX and 204 responses. [#1767]
@@ -12,6 +13,7 @@
 
 [#1767]: https://github.com/actix/actix-web/pull/1767
 [#1768]: https://github.com/actix/actix-web/pull/1768
+[#1793]: https://github.com/actix/actix-web/pull/1793
 
 
 ## 2.1.0 - 2020-10-30

actix-http/src/header/common/accept.rs

@@ -1,3 +1,5 @@
+use std::cmp::Ordering;
+
 use mime::Mime;
 
 use crate::header::{qitem, QualityItem};
@@ -7,7 +9,7 @@ header! {
     /// `Accept` header, defined in [RFC7231](http://tools.ietf.org/html/rfc7231#section-5.3.2)
     ///
     /// The `Accept` header field can be used by user agents to specify
-    /// response media types that are acceptable.  Accept header fields can
+    /// response media types that are acceptable. Accept header fields can
     /// be used to indicate that the request is specifically limited to a
     /// small set of desired types, as in the case of a request for an
     /// in-line image
@@ -97,14 +99,14 @@ header! {
          test_header!(
             test1,
             vec![b"audio/*; q=0.2, audio/basic"],
-            Some(HeaderField(vec![
+            Some(Accept(vec![
                 QualityItem::new("audio/*".parse().unwrap(), q(200)),
                 qitem("audio/basic".parse().unwrap()),
                 ])));
         test_header!(
             test2,
             vec![b"text/plain; q=0.5, text/html, text/x-dvi; q=0.8, text/x-c"],
-            Some(HeaderField(vec![
+            Some(Accept(vec![
                 QualityItem::new(mime::TEXT_PLAIN, q(500)),
                 qitem(mime::TEXT_HTML),
                 QualityItem::new(
@@ -138,23 +140,148 @@ header! {
 }
 
 impl Accept {
-    /// A constructor to easily create `Accept: */*`.
+    /// Construct `Accept: */*`.
     pub fn star() -> Accept {
         Accept(vec![qitem(mime::STAR_STAR)])
     }
 
-    /// A constructor to easily create `Accept: application/json`.
+    /// Construct `Accept: application/json`.
     pub fn json() -> Accept {
         Accept(vec![qitem(mime::APPLICATION_JSON)])
     }
 
-    /// A constructor to easily create `Accept: text/*`.
+    /// Construct `Accept: text/*`.
     pub fn text() -> Accept {
         Accept(vec![qitem(mime::TEXT_STAR)])
     }
 
-    /// A constructor to easily create `Accept: image/*`.
+    /// Construct `Accept: image/*`.
     pub fn image() -> Accept {
         Accept(vec![qitem(mime::IMAGE_STAR)])
     }
+
+    /// Construct `Accept: text/html`.
+    pub fn html() -> Accept {
+        Accept(vec![qitem(mime::TEXT_HTML)])
+    }
+
+    /// Returns a sorted list of mime types from highest to lowest preference, accounting for
+    /// [q-factor weighting] and specificity.
+    ///
+    /// [q-factor weighting]: https://tools.ietf.org/html/rfc7231#section-5.3.2
+    pub fn mime_precedence(&self) -> Vec<Mime> {
+        let mut types = self.0.clone();
+
+        // use stable sort so items with equal q-factor and specificity retain listed order
+        types.sort_by(|a, b| {
+            // sort by q-factor descending
+            b.quality.cmp(&a.quality).then_with(|| {
+                // use specificity rules on mime types with
+                // same q-factor (eg. text/html > text/* > */*)
+
+                // subtypes are not comparable if main type is star, so return
+                match (a.item.type_(), b.item.type_()) {
+                    (mime::STAR, mime::STAR) => return Ordering::Equal,
+
+                    // a is sorted after b
+                    (mime::STAR, _) => return Ordering::Greater,
+
+                    // a is sorted before b
+                    (_, mime::STAR) => return Ordering::Less,
+
+                    _ => {}
+                }
+
+                // in both these match expressions, the returned ordering appears
+                // inverted because sort is high-to-low ("descending") precedence
+                match (a.item.subtype(), b.item.subtype()) {
+                    (mime::STAR, mime::STAR) => Ordering::Equal,
+
+                    // a is sorted after b
+                    (mime::STAR, _) => Ordering::Greater,
+
+                    // a is sorted before b
+                    (_, mime::STAR) => Ordering::Less,
+
+                    _ => Ordering::Equal,
+                }
+            })
+        });
+
+        types.into_iter().map(|qitem| qitem.item).collect()
+    }
+
+    /// Extracts the most preferable mime type, accounting for [q-factor weighting].
+    ///
+    /// If no q-factors are provided, the first mime type is chosen. Note that items without
+    /// q-factors are given the maximum preference value.
+    ///
+    /// Returns `None` if contained list is empty.
+    ///
+    /// [q-factor weighting]: https://tools.ietf.org/html/rfc7231#section-5.3.2
+    pub fn mime_preference(&self) -> Option<Mime> {
+        let types = self.mime_precedence();
+        types.first().cloned()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::header::q;
+
+    #[test]
+    fn test_mime_precedence() {
+        let test = Accept(vec![]);
+        assert!(test.mime_precedence().is_empty());
+
+        let test = Accept(vec![qitem(mime::APPLICATION_JSON)]);
+        assert_eq!(test.mime_precedence(), vec!(mime::APPLICATION_JSON));
+
+        let test = Accept(vec![
+            qitem(mime::TEXT_HTML),
+            "application/xhtml+xml".parse().unwrap(),
+            QualityItem::new("application/xml".parse().unwrap(), q(0.9)),
+            QualityItem::new(mime::STAR_STAR, q(0.8)),
+        ]);
+        assert_eq!(
+            test.mime_precedence(),
+            vec![
+                mime::TEXT_HTML,
+                "application/xhtml+xml".parse().unwrap(),
+                "application/xml".parse().unwrap(),
+                mime::STAR_STAR,
+            ]
+        );
+
+        let test = Accept(vec![
+            qitem(mime::STAR_STAR),
+            qitem(mime::IMAGE_STAR),
+            qitem(mime::IMAGE_PNG),
+        ]);
+        assert_eq!(
+            test.mime_precedence(),
+            vec![mime::IMAGE_PNG, mime::IMAGE_STAR, mime::STAR_STAR]
+        );
+    }
+
+    #[test]
+    fn test_mime_preference() {
+        let test = Accept(vec![
+            qitem(mime::TEXT_HTML),
+            "application/xhtml+xml".parse().unwrap(),
+            QualityItem::new("application/xml".parse().unwrap(), q(0.9)),
+            QualityItem::new(mime::STAR_STAR, q(0.8)),
+        ]);
+        assert_eq!(test.mime_preference(), Some(mime::TEXT_HTML));
+
+        let test = Accept(vec![
+            QualityItem::new("video/*".parse().unwrap(), q(0.8)),
+            qitem(mime::IMAGE_PNG),
+            QualityItem::new(mime::STAR_STAR, q(0.5)),
+            qitem(mime::IMAGE_SVG),
+            QualityItem::new(mime::IMAGE_STAR, q(0.8)),
+        ]);
+        assert_eq!(test.mime_preference(), Some(mime::IMAGE_PNG));
+    }
 }

actix-http/src/header/shared/quality_item.rs

@@ -18,7 +18,7 @@ use self::internal::IntoQuality;
 ///
 /// [RFC7231 Section 5.3.1](https://tools.ietf.org/html/rfc7231#section-5.3.1)
 /// gives more information on quality values in HTTP header fields.
-#[derive(Copy, Clone, Debug, Eq, Ord, PartialEq, PartialOrd)]
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
 pub struct Quality(u16);
 
 impl Default for Quality {
@@ -121,7 +121,7 @@ fn from_f32(f: f32) -> Quality {
 /// Convenience function to wrap a value in a `QualityItem`
 /// Sets `q` to the default 1.0
 pub fn qitem<T>(item: T) -> QualityItem<T> {
-    QualityItem::new(item, Default::default())
+    QualityItem::new(item, Quality::default())
 }
 
 /// Convenience function to create a `Quality` from a float or integer.