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    use header::{HeaderValue, IntoHeaderValue, InvalidHeaderValueBytes, Writer};
    use std::fmt::{self, Display, Write};
    use std::str::FromStr;

    /// check that each char in the slice is either:
    /// 1. `%x21`, or
    /// 2. in the range `%x23` to `%x7E`, or
    /// 3. above `%x80`
    fn check_slice_validity(slice: &str) -> bool {
        slice
            .bytes()
            .all(|c| c == b'\x21' || (c >= b'\x23' && c <= b'\x7e') | (c >= b'\x80'))
    }

    /// An entity tag, defined in [RFC7232](https://tools.ietf.org/html/rfc7232#section-2.3)
    ///
    /// An entity tag consists of a string enclosed by two literal double quotes.
    /// Preceding the first double quote is an optional weakness indicator,
    /// which always looks like `W/`. Examples for valid tags are `"xyzzy"` and
    /// `W/"xyzzy"`.
    ///
    /// # ABNF
    ///
    /// ```text
    /// entity-tag = [ weak ] opaque-tag
    /// weak       = %x57.2F ; "W/", case-sensitive
    /// opaque-tag = DQUOTE *etagc DQUOTE
    /// etagc      = %x21 / %x23-7E / obs-text
    ///            ; VCHAR except double quotes, plus obs-text
    /// ```
    ///
    /// # Comparison
    /// To check if two entity tags are equivalent in an application always use the
    /// `strong_eq` or `weak_eq` methods based on the context of the Tag. Only use
    /// `==` to check if two tags are identical.
    ///
    /// The example below shows the results for a set of entity-tag pairs and
    /// both the weak and strong comparison function results:
    ///
    /// | `ETag 1`| `ETag 2`| Strong Comparison | Weak Comparison |
    /// |---------|---------|-------------------|-----------------|
    /// | `W/"1"` | `W/"1"` | no match          | match           |
    /// | `W/"1"` | `W/"2"` | no match          | no match        |
    /// | `W/"1"` | `"1"`   | no match          | match           |
    /// | `"1"`   | `"1"`   | match             | match           |
    #[derive(Clone, Debug, Eq, PartialEq)]
    pub struct EntityTag {
        /// Weakness indicator for the tag
        pub weak: bool,
        /// The opaque string in between the DQUOTEs
        tag: String,
    }

    impl EntityTag {
        /// Constructs a new EntityTag.
        /// # Panics
        /// If the tag contains invalid characters.
        pub fn new(weak: bool, tag: String) -> EntityTag {
            assert!(check_slice_validity(&tag), "Invalid tag: {:?}", tag);
            EntityTag { weak, tag }
        }

        /// Constructs a new weak EntityTag.
        /// # Panics
        /// If the tag contains invalid characters.
        pub fn weak(tag: String) -> EntityTag {
            EntityTag::new(true, tag)
        }

        /// Constructs a new strong EntityTag.
        /// # Panics
        /// If the tag contains invalid characters.
        pub fn strong(tag: String) -> EntityTag {
            EntityTag::new(false, tag)
        }

        /// Get the tag.
        pub fn tag(&self) -> &str {
            self.tag.as_ref()
        }

        /// Set the tag.
        /// # Panics
        /// If the tag contains invalid characters.
        pub fn set_tag(&mut self, tag: String) {
            assert!(check_slice_validity(&tag), "Invalid tag: {:?}", tag);
            self.tag = tag
        }

        /// For strong comparison two entity-tags are equivalent if both are not
        /// weak and their opaque-tags match character-by-character.
        pub fn strong_eq(&self, other: &EntityTag) -> bool {
            !self.weak && !other.weak && self.tag == other.tag
        }

        /// For weak comparison two entity-tags are equivalent if their
        /// opaque-tags match character-by-character, regardless of either or
        /// both being tagged as "weak".
        pub fn weak_eq(&self, other: &EntityTag) -> bool {
            self.tag == other.tag
        }

        /// The inverse of `EntityTag.strong_eq()`.
        pub fn strong_ne(&self, other: &EntityTag) -> bool {
            !self.strong_eq(other)
        }

        /// The inverse of `EntityTag.weak_eq()`.
        pub fn weak_ne(&self, other: &EntityTag) -> bool {
            !self.weak_eq(other)
        }
    }

    impl Display for EntityTag {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            if self.weak {
                write!(f, "W/\"{}\"", self.tag)
            } else {
                write!(f, "\"{}\"", self.tag)
            }
        }
    }

    impl FromStr for EntityTag {
        type Err = ::error::ParseError;

        fn from_str(s: &str) -> Result<EntityTag, ::error::ParseError> {
            let length: usize = s.len();
            let slice = &s[..];
            // Early exits if it doesn't terminate in a DQUOTE.
            if !slice.ends_with('"') || slice.len() < 2 {
                return Err(::error::ParseError::Header);
            }
            // The etag is weak if its first char is not a DQUOTE.
            if slice.len() >= 2
                && slice.starts_with('"')
                && check_slice_validity(&slice[1..length - 1])
            {
                // No need to check if the last char is a DQUOTE,
                // we already did that above.
                return Ok(EntityTag {
                    weak: false,
                    tag: slice[1..length - 1].to_owned(),
                });
            } else if slice.len() >= 4
                && slice.starts_with("W/\"")
                && check_slice_validity(&slice[3..length - 1])
            {
                return Ok(EntityTag {
                    weak: true,
                    tag: slice[3..length - 1].to_owned(),
                });
            }
            Err(::error::ParseError::Header)
        }
    }

    impl IntoHeaderValue for EntityTag {
        type Error = InvalidHeaderValueBytes;

        fn try_into(self) -> Result<HeaderValue, Self::Error> {
            let mut wrt = Writer::new();
            write!(wrt, "{}", self).unwrap();
            HeaderValue::from_shared(wrt.take())
        }
    }

    #[cfg(test)]
    mod tests {
        use super::EntityTag;

        #[test]
        fn test_etag_parse_success() {
            // Expected success
            assert_eq!(
                "\"foobar\"".parse::<EntityTag>().unwrap(),
                EntityTag::strong("foobar".to_owned())
            );
            assert_eq!(
                "\"\"".parse::<EntityTag>().unwrap(),
                EntityTag::strong("".to_owned())
            );
            assert_eq!(
                "W/\"weaktag\"".parse::<EntityTag>().unwrap(),
                EntityTag::weak("weaktag".to_owned())
            );
            assert_eq!(
                "W/\"\x65\x62\"".parse::<EntityTag>().unwrap(),
                EntityTag::weak("\x65\x62".to_owned())
            );
            assert_eq!(
                "W/\"\"".parse::<EntityTag>().unwrap(),
                EntityTag::weak("".to_owned())
            );
        }

        #[test]
        fn test_etag_parse_failures() {
            // Expected failures
            assert!("no-dquotes".parse::<EntityTag>().is_err());
            assert!(
                "w/\"the-first-w-is-case-sensitive\""
                    .parse::<EntityTag>()
                    .is_err()
            );
            assert!("".parse::<EntityTag>().is_err());
            assert!("\"unmatched-dquotes1".parse::<EntityTag>().is_err());
            assert!("unmatched-dquotes2\"".parse::<EntityTag>().is_err());
            assert!("matched-\"dquotes\"".parse::<EntityTag>().is_err());
        }

        #[test]
        fn test_etag_fmt() {
            assert_eq!(
                format!("{}", EntityTag::strong("foobar".to_owned())),
                "\"foobar\""
            );
            assert_eq!(format!("{}", EntityTag::strong("".to_owned())), "\"\"");
            assert_eq!(
                format!("{}", EntityTag::weak("weak-etag".to_owned())),
                "W/\"weak-etag\""
            );
            assert_eq!(
                format!("{}", EntityTag::weak("\u{0065}".to_owned())),
                "W/\"\x65\""
            );
            assert_eq!(format!("{}", EntityTag::weak("".to_owned())), "W/\"\"");
        }

        #[test]
        fn test_cmp() {
            // | ETag 1  | ETag 2  | Strong Comparison | Weak Comparison |
            // |---------|---------|-------------------|-----------------|
            // | `W/"1"` | `W/"1"` | no match          | match           |
            // | `W/"1"` | `W/"2"` | no match          | no match        |
            // | `W/"1"` | `"1"`   | no match          | match           |
            // | `"1"`   | `"1"`   | match             | match           |
            let mut etag1 = EntityTag::weak("1".to_owned());
            let mut etag2 = EntityTag::weak("1".to_owned());
            assert!(!etag1.strong_eq(&etag2));
            assert!(etag1.weak_eq(&etag2));
            assert!(etag1.strong_ne(&etag2));
            assert!(!etag1.weak_ne(&etag2));

            etag1 = EntityTag::weak("1".to_owned());
            etag2 = EntityTag::weak("2".to_owned());
            assert!(!etag1.strong_eq(&etag2));
            assert!(!etag1.weak_eq(&etag2));
            assert!(etag1.strong_ne(&etag2));
            assert!(etag1.weak_ne(&etag2));

            etag1 = EntityTag::weak("1".to_owned());
            etag2 = EntityTag::strong("1".to_owned());
            assert!(!etag1.strong_eq(&etag2));
            assert!(etag1.weak_eq(&etag2));
            assert!(etag1.strong_ne(&etag2));
            assert!(!etag1.weak_ne(&etag2));

            etag1 = EntityTag::strong("1".to_owned());
            etag2 = EntityTag::strong("1".to_owned());
            assert!(etag1.strong_eq(&etag2));
            assert!(etag1.weak_eq(&etag2));
            assert!(!etag1.strong_ne(&etag2));
            assert!(!etag1.weak_ne(&etag2));
        }
    }

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