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    use std::error;
    use std::fmt;
    use std::str::{self, FromStr};

    use serde::{de, ser};

    /// A parsed TOML datetime value
    ///
    /// This structure is intended to represent the datetime primitive type that can
    /// be encoded into TOML documents. This type is a parsed version that contains
    /// all metadata internally.
    ///
    /// Currently this type is intentionally conservative and only supports
    /// `to_string` as an accessor. Over time though it's intended that it'll grow
    /// more support!
    ///
    /// Note that if you're using `Deserialize` to deserialize a TOML document, you
    /// can use this as a placeholder for where you're expecting a datetime to be
    /// specified.
    ///
    /// Also note though that while this type implements `Serialize` and
    /// `Deserialize` it's only recommended to use this type with the TOML format,
    /// otherwise encoded in other formats it may look a little odd.
    ///
    /// Depending on how the option values are used, this struct will correspond
    /// with one of the following four datetimes from the [TOML v1.0.0 spec]:
    ///
    /// | `date`    | `time`    | `offset`  | TOML type          |
    /// | --------- | --------- | --------- | ------------------ |
    /// | `Some(_)` | `Some(_)` | `Some(_)` | [Offset Date-Time] |
    /// | `Some(_)` | `Some(_)` | `None`    | [Local Date-Time]  |
    /// | `Some(_)` | `None`    | `None`    | [Local Date]       |
    /// | `None`    | `Some(_)` | `None`    | [Local Time]       |
    ///
    /// **1. Offset Date-Time**: If all the optional values are used, `Datetime`
    /// corresponds to an [Offset Date-Time]. From the TOML v1.0.0 spec:
    ///
    /// > To unambiguously represent a specific instant in time, you may use an
    /// > RFC 3339 formatted date-time with offset.
    /// >
    /// > ```toml
    /// > odt1 = 1979-05-27T07:32:00Z
    /// > odt2 = 1979-05-27T00:32:00-07:00
    /// > odt3 = 1979-05-27T00:32:00.999999-07:00
    /// > ```
    /// >
    /// > For the sake of readability, you may replace the T delimiter between date
    /// > and time with a space character (as permitted by RFC 3339 section 5.6).
    /// >
    /// > ```toml
    /// > odt4 = 1979-05-27 07:32:00Z
    /// > ```
    ///
    /// **2. Local Date-Time**: If `date` and `time` are given but `offset` is
    /// `None`, `Datetime` corresponds to a [Local Date-Time]. From the spec:
    ///
    /// > If you omit the offset from an RFC 3339 formatted date-time, it will
    /// > represent the given date-time without any relation to an offset or
    /// > timezone. It cannot be converted to an instant in time without additional
    /// > information. Conversion to an instant, if required, is implementation-
    /// > specific.
    /// >
    /// > ```toml
    /// > ldt1 = 1979-05-27T07:32:00
    /// > ldt2 = 1979-05-27T00:32:00.999999
    /// > ```
    ///
    /// **3. Local Date**: If only `date` is given, `Datetime` corresponds to a
    /// [Local Date]; see the docs for [`Date`].
    ///
    /// **4. Local Time**: If only `time` is given, `Datetime` corresponds to a
    /// [Local Time]; see the docs for [`Time`].
    ///
    /// [TOML v1.0.0 spec]: https://toml.io/en/v1.0.0
    /// [Offset Date-Time]: https://toml.io/en/v1.0.0#offset-date-time
    /// [Local Date-Time]: https://toml.io/en/v1.0.0#local-date-time
    /// [Local Date]: https://toml.io/en/v1.0.0#local-date
    /// [Local Time]: https://toml.io/en/v1.0.0#local-time
    #[derive(PartialEq, Eq, Clone)]
    pub struct Datetime {
        /// Optional date.
        /// Required for: *Offset Date-Time*, *Local Date-Time*, *Local Date*.
        pub date: Option<Date>,

        /// Optional time.
        /// Required for: *Offset Date-Time*, *Local Date-Time*, *Local Time*.
        pub time: Option<Time>,

        /// Optional offset.
        /// Required for: *Offset Date-Time*.
        pub offset: Option<Offset>,
    }

    /// Error returned from parsing a `Datetime` in the `FromStr` implementation.
    #[derive(Debug, Clone)]
    pub struct DatetimeParseError {
        _private: (),
    }

    // Currently serde itself doesn't have a datetime type, so we map our `Datetime`
    // to a special valid in the serde data model. Namely one with these special
    // fields/struct names.
    //
    // In general the TOML encoder/decoder will catch this and not literally emit
    // these strings but rather emit datetimes as they're intended.
    pub const FIELD: &str = "$__toml_private_datetime";
    pub const NAME: &str = "$__toml_private_Datetime";

    /// A parsed TOML date value
    ///
    /// May be part of a [`Datetime`]. Alone, `Date` corresponds to a [Local Date].
    /// From the TOML v1.0.0 spec:
    ///
    /// > If you include only the date portion of an RFC 3339 formatted date-time,
    /// > it will represent that entire day without any relation to an offset or
    /// > timezone.
    /// >
    /// > ```toml
    /// > ld1 = 1979-05-27
    /// > ```
    ///
    /// [Local Date]: https://toml.io/en/v1.0.0#local-date
    #[derive(PartialEq, Eq, Clone)]
    pub struct Date {
        /// Year: four digits
        pub year: u16,
        /// Month: 1 to 12
        pub month: u8,
        /// Day: 1 to {28, 29, 30, 31} (based on month/year)
        pub day: u8,
    }

    /// A parsed TOML time value
    ///
    /// May be part of a [`Datetime`]. Alone, `Time` corresponds to a [Local Time].
    /// From the TOML v1.0.0 spec:
    ///
    /// > If you include only the time portion of an RFC 3339 formatted date-time,
    /// > it will represent that time of day without any relation to a specific
    /// > day or any offset or timezone.
    /// >
    /// > ```toml
    /// > lt1 = 07:32:00
    /// > lt2 = 00:32:00.999999
    /// > ```
    /// >
    /// > Millisecond precision is required. Further precision of fractional
    /// > seconds is implementation-specific. If the value contains greater
    /// > precision than the implementation can support, the additional precision
    /// > must be truncated, not rounded.
    ///
    /// [Local Time]: https://toml.io/en/v1.0.0#local-time
    #[derive(PartialEq, Eq, Clone)]
    pub struct Time {
        /// Hour: 0 to 23
        pub hour: u8,
        /// Minute: 0 to 59
        pub minute: u8,
        /// Second: 0 to {58, 59, 60} (based on leap second rules)
        pub second: u8,
        /// Nanosecond: 0 to 999_999_999
        pub nanosecond: u32,
    }

    /// A parsed TOML time offset
    ///
    #[derive(PartialEq, Eq, Clone)]
    pub enum Offset {
        /// > A suffix which, when applied to a time, denotes a UTC offset of 00:00;
        /// > often spoken "Zulu" from the ICAO phonetic alphabet representation of
        /// > the letter "Z". --- [RFC 3339 section 2]
        ///
        /// [RFC 3339 section 2]: https://datatracker.ietf.org/doc/html/rfc3339#section-2
        Z,

        /// Offset between local time and UTC
        Custom {
            /// Hours: -12 to +12
            hours: i8,

            /// Minutes: 0 to 59
            minutes: u8,
        },
    }

    impl fmt::Debug for Datetime {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt::Display::fmt(self, f)
        }
    }

    impl fmt::Display for Datetime {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            if let Some(ref date) = self.date {
                write!(f, "{}", date)?;
            }
            if let Some(ref time) = self.time {
                if self.date.is_some() {
                    write!(f, "T")?;
                }
                write!(f, "{}", time)?;
            }
            if let Some(ref offset) = self.offset {
                write!(f, "{}", offset)?;
            }
            Ok(())
        }
    }

    impl fmt::Display for Date {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            write!(f, "{:04}-{:02}-{:02}", self.year, self.month, self.day)
        }
    }

    impl fmt::Display for Time {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            write!(f, "{:02}:{:02}:{:02}", self.hour, self.minute, self.second)?;
            if self.nanosecond != 0 {
                let s = format!("{:09}", self.nanosecond);
                write!(f, ".{}", s.trim_end_matches('0'))?;
            }
            Ok(())
        }
    }

    impl fmt::Display for Offset {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            match *self {
                Offset::Z => write!(f, "Z"),
                Offset::Custom { hours, minutes } => write!(f, "{:+03}:{:02}", hours, minutes),
            }
        }
    }

    impl FromStr for Datetime {
        type Err = DatetimeParseError;

        fn from_str(date: &str) -> Result<Datetime, DatetimeParseError> {
            // Accepted formats:
            //
            // 0000-00-00T00:00:00.00Z
            // 0000-00-00T00:00:00.00
            // 0000-00-00
            // 00:00:00.00
            if date.len() < 3 {
                return Err(DatetimeParseError { _private: () });
            }
            let mut offset_allowed = true;
            let mut chars = date.chars();

            // First up, parse the full date if we can
            let full_date = if chars.clone().nth(2) == Some(':') {
                offset_allowed = false;
                None
            } else {
                let y1 = u16::from(digit(&mut chars)?);
                let y2 = u16::from(digit(&mut chars)?);
                let y3 = u16::from(digit(&mut chars)?);
                let y4 = u16::from(digit(&mut chars)?);

                match chars.next() {
                    Some('-') => {}
                    _ => return Err(DatetimeParseError { _private: () }),
                }

                let m1 = digit(&mut chars)?;
                let m2 = digit(&mut chars)?;

                match chars.next() {
                    Some('-') => {}
                    _ => return Err(DatetimeParseError { _private: () }),
                }

                let d1 = digit(&mut chars)?;
                let d2 = digit(&mut chars)?;

                let date = Date {
                    year: y1 * 1000 + y2 * 100 + y3 * 10 + y4,
                    month: m1 * 10 + m2,
                    day: d1 * 10 + d2,
                };

                if date.month < 1 || date.month > 12 {
                    return Err(DatetimeParseError { _private: () });
                }
                if date.day < 1 || date.day > 31 {
                    return Err(DatetimeParseError { _private: () });
                }

                Some(date)
            };

            // Next parse the "partial-time" if available
            let next = chars.clone().next();
            let partial_time = if full_date.is_some()
                && (next == Some('T') || next == Some('t') || next == Some(' '))
            {
                chars.next();
                true
            } else {
                full_date.is_none()
            };

            let time = if partial_time {
                let h1 = digit(&mut chars)?;
                let h2 = digit(&mut chars)?;
                match chars.next() {
                    Some(':') => {}
                    _ => return Err(DatetimeParseError { _private: () }),
                }
                let m1 = digit(&mut chars)?;
                let m2 = digit(&mut chars)?;
                match chars.next() {
                    Some(':') => {}
                    _ => return Err(DatetimeParseError { _private: () }),
                }
                let s1 = digit(&mut chars)?;
                let s2 = digit(&mut chars)?;

                let mut nanosecond = 0;
                if chars.clone().next() == Some('.') {
                    chars.next();
                    let whole = chars.as_str();

                    let mut end = whole.len();
                    for (i, byte) in whole.bytes().enumerate() {
                        match byte {
                            b'0'..=b'9' => {
                                if i < 9 {
                                    let p = 10_u32.pow(8 - i as u32);
                                    nanosecond += p * u32::from(byte - b'0');
                                }
                            }
                            _ => {
                                end = i;
                                break;
                            }
                        }
                    }
                    if end == 0 {
                        return Err(DatetimeParseError { _private: () });
                    }
                    chars = whole[end..].chars();
                }

                let time = Time {
                    hour: h1 * 10 + h2,
                    minute: m1 * 10 + m2,
                    second: s1 * 10 + s2,
                    nanosecond,
                };

                if time.hour > 24 {
                    return Err(DatetimeParseError { _private: () });
                }
                if time.minute > 59 {
                    return Err(DatetimeParseError { _private: () });
                }
                if time.second > 59 {
                    return Err(DatetimeParseError { _private: () });
                }
                if time.nanosecond > 999_999_999 {
                    return Err(DatetimeParseError { _private: () });
                }

                Some(time)
            } else {
                offset_allowed = false;
                None
            };

            // And finally, parse the offset
            let offset = if offset_allowed {
                let next = chars.clone().next();
                if next == Some('Z') || next == Some('z') {
                    chars.next();
                    Some(Offset::Z)
                } else if next.is_none() {
                    None
                } else {
                    let sign = match next {
                        Some('+') => 1,
                        Some('-') => -1,
                        _ => return Err(DatetimeParseError { _private: () }),
                    };
                    chars.next();
                    let h1 = digit(&mut chars)? as i8;
                    let h2 = digit(&mut chars)? as i8;
                    match chars.next() {
                        Some(':') => {}
                        _ => return Err(DatetimeParseError { _private: () }),
                    }
                    let m1 = digit(&mut chars)?;
                    let m2 = digit(&mut chars)?;

                    Some(Offset::Custom {
                        hours: sign * (h1 * 10 + h2),
                        minutes: m1 * 10 + m2,
                    })
                }
            } else {
                None
            };

            // Return an error if we didn't hit eof, otherwise return our parsed
            // date
            if chars.next().is_some() {
                return Err(DatetimeParseError { _private: () });
            }

            Ok(Datetime {
                date: full_date,
                time,
                offset,
            })
        }
    }

    fn digit(chars: &mut str::Chars<'_>) -> Result<u8, DatetimeParseError> {
        match chars.next() {
            Some(c) if ('0'..='9').contains(&c) => Ok(c as u8 - b'0'),
            _ => Err(DatetimeParseError { _private: () }),
        }
    }

    impl ser::Serialize for Datetime {
        fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
        where
            S: ser::Serializer,
        {
            use serde::ser::SerializeStruct;

            let mut s = serializer.serialize_struct(NAME, 1)?;
            s.serialize_field(FIELD, &self.to_string())?;
            s.end()
        }
    }

    impl<'de> de::Deserialize<'de> for Datetime {
        fn deserialize<D>(deserializer: D) -> Result<Datetime, D::Error>
        where
            D: de::Deserializer<'de>,
        {
            struct DatetimeVisitor;

            impl<'de> de::Visitor<'de> for DatetimeVisitor {
                type Value = Datetime;

                fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
                    formatter.write_str("a TOML datetime")
                }

                fn visit_map<V>(self, mut visitor: V) -> Result<Datetime, V::Error>
                where
                    V: de::MapAccess<'de>,
                {
                    let value = visitor.next_key::<DatetimeKey>()?;
                    if value.is_none() {
                        return Err(de::Error::custom("datetime key not found"));
                    }
                    let v: DatetimeFromString = visitor.next_value()?;
                    Ok(v.value)
                }
            }

            static FIELDS: [&str; 1] = [FIELD];
            deserializer.deserialize_struct(NAME, &FIELDS, DatetimeVisitor)
        }
    }

    struct DatetimeKey;

    impl<'de> de::Deserialize<'de> for DatetimeKey {
        fn deserialize<D>(deserializer: D) -> Result<DatetimeKey, D::Error>
        where
            D: de::Deserializer<'de>,
        {
            struct FieldVisitor;

            impl<'de> de::Visitor<'de> for FieldVisitor {
                type Value = ();

                fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
                    formatter.write_str("a valid datetime field")
                }

                fn visit_str<E>(self, s: &str) -> Result<(), E>
                where
                    E: de::Error,
                {
                    if s == FIELD {
                        Ok(())
                    } else {
                        Err(de::Error::custom("expected field with custom name"))
                    }
                }
            }

            deserializer.deserialize_identifier(FieldVisitor)?;
            Ok(DatetimeKey)
        }
    }

    pub struct DatetimeFromString {
        pub value: Datetime,
    }

    impl<'de> de::Deserialize<'de> for DatetimeFromString {
        fn deserialize<D>(deserializer: D) -> Result<DatetimeFromString, D::Error>
        where
            D: de::Deserializer<'de>,
        {
            struct Visitor;

            impl<'de> de::Visitor<'de> for Visitor {
                type Value = DatetimeFromString;

                fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
                    formatter.write_str("string containing a datetime")
                }

                fn visit_str<E>(self, s: &str) -> Result<DatetimeFromString, E>
                where
                    E: de::Error,
                {
                    match s.parse() {
                        Ok(date) => Ok(DatetimeFromString { value: date }),
                        Err(e) => Err(de::Error::custom(e)),
                    }
                }
            }

            deserializer.deserialize_str(Visitor)
        }
    }

    impl fmt::Display for DatetimeParseError {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            "failed to parse datetime".fmt(f)
        }
    }

    impl error::Error for DatetimeParseError {}