feat(headers): Add If-None-Match header field

Add the HTTP/1.1 `If-None-Match` header field makes the request method conditional
on a recipient cache or origin server either not having any current
representation of the target resource, when the field-value is "*",
or having a selected representation with an entity-tag that does not
match any of those listed in the field-value.

Closes #238

src/header/common/if_none_match.rs

@@ -0,0 +1,84 @@
+use header::{Header, HeaderFormat, EntityTag};
+use header::parsing::{from_comma_delimited, fmt_comma_delimited, from_one_raw_str};
+use std::fmt::{self};
+
+/// The `If-None-Match` header defined by HTTP/1.1.
+///
+/// The "If-None-Match" header field makes the request method conditional
+/// on a recipient cache or origin server either not having any current
+/// representation of the target resource, when the field-value is "*",
+/// or having a selected representation with an entity-tag that does not
+/// match any of those listed in the field-value.
+///
+/// A recipient MUST use the weak comparison function when comparing
+/// entity-tags for If-None-Match (Section 2.3.2), since weak entity-tags
+/// can be used for cache validation even if there have been changes to
+/// the representation data.
+///
+/// Spec: https://tools.ietf.org/html/rfc7232#section-3.2
+
+/// The `If-None-Match` header field.
+#[derive(Clone, PartialEq, Debug)]
+pub enum IfNoneMatch {
+    /// This corresponds to '*'.
+    Any,
+    /// The header field names which will influence the response representation.
+    EntityTags(Vec<EntityTag>)
+}
+
+impl Header for IfNoneMatch {
+    fn header_name() -> &'static str {
+        "If-None-Match"
+    }
+
+    fn parse_header(raw: &[Vec<u8>]) -> Option<IfNoneMatch> {
+        from_one_raw_str(raw).and_then(|s: String| {
+            let slice = &s[];
+            match slice {
+                "" => None,
+                "*" => Some(IfNoneMatch::Any),
+                _ => from_comma_delimited(raw).map(|vec| IfNoneMatch::EntityTags(vec)),
+            }
+        })
+    }
+}
+
+impl HeaderFormat for IfNoneMatch {
+    fn fmt_header(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        match *self {
+            IfNoneMatch::Any => { write!(fmt, "*") }
+            IfNoneMatch::EntityTags(ref fields) => { fmt_comma_delimited(fmt, &fields[]) }
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::IfNoneMatch;
+    use header::Header;
+    use header::EntityTag;
+
+    #[test]
+    fn test_if_none_match() {
+        let mut if_none_match: Option<IfNoneMatch>;
+
+        if_none_match = Header::parse_header([b"*".to_vec()].as_slice());
+        assert_eq!(if_none_match, Some(IfNoneMatch::Any));
+
+        if_none_match = Header::parse_header([b"\"foobar\", W/\"weak-etag\"".to_vec()].as_slice());
+        let mut entities: Vec<EntityTag> = Vec::new();
+        let foobar_etag = EntityTag {
+            weak: false,
+            tag: "foobar".to_string()
+        };
+        let weak_etag = EntityTag {
+            weak: true,
+            tag: "weak-etag".to_string()
+        };
+        entities.push(foobar_etag);
+        entities.push(weak_etag);
+        assert_eq!(if_none_match, Some(IfNoneMatch::EntityTags(entities)));
+    }
+}
+
+bench_header!(bench, IfNoneMatch, { vec![b"W/\"nonemptytag\"".to_vec()] });

src/header/common/mod.rs

@@ -21,6 +21,7 @@ pub use self::etag::Etag;
 pub use self::expires::Expires;
 pub use self::host::Host;
 pub use self::if_modified_since::IfModifiedSince;
+pub use self::if_none_match::IfNoneMatch;
 pub use self::if_unmodified_since::IfUnmodifiedSince;
 pub use self::last_modified::LastModified;
 pub use self::location::Location;
@@ -158,6 +159,7 @@ mod expires;
 mod host;
 mod last_modified;
 mod if_modified_since;
+mod if_none_match;
 mod if_unmodified_since;
 mod location;
 mod pragma;