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use core::fmt::Debug;

pub trait Partial: Default {
    type Target: HasPartial<Partial = Self>;

    type Error: Debug;

    fn build(self) -> Result<Self::Target, Self::Error>;

    fn source(self, value: impl Source<Self::Target>) -> Result<Self, Self::Error>;

    fn override_with(self, other: Self) -> Self;
}

pub trait HasPartial {
    type Partial: Partial<Target = Self>;
}

pub trait Source<C: HasPartial> {
    type Error: Debug;

    fn to_partial(self) -> Result<C::Partial, Self::Error>;

    fn name(&self) -> String;
}

impl<T, C, E> Source<C> for Option<T>
where
    C: HasPartial,
    T: Source<C, Error = E>,
    E: Debug
{
    type Error = E;

    fn to_partial(self) -> Result<C::Partial, E> {
        self.map_or_else(
            || Ok(C::Partial::default()),
            |v| v.to_partial()
        )
    }

    fn name(&self) -> String {
        self.as_ref().map_or(
            "Unspecified".to_owned(),
            |v| v.name(),
        )
    }
}

#[cfg(feature = "serde")]
pub mod serde {
    use super::{HasPartial, Source};

    #[cfg(feature = "toml")]
    use std::io::Read;

    #[derive(Debug)]
    #[non_exhaustive]
    pub enum FileReadError {
        Open (std::io::Error),

        #[cfg(feature = "toml")]
        Toml (toml::de::Error),

        #[cfg(feature = "json")]
        Json (serde_json::Error),

        NoFile(std::path::PathBuf),

        UnsupportedExtension(String),

        NoExtension,
    }

    impl From<std::io::Error> for FileReadError {
        fn from(value: std::io::Error) -> Self {
            Self::Open(value)
        }
    }

    #[cfg(feature = "toml")]
    pub struct Toml<'a>(pub &'a std::path::Path);

    #[cfg(feature = "json")]
    pub struct Json<'a>(pub &'a std::path::Path);

    #[cfg(feature = "json")]
    impl<'pth, C> Source<C> for Json<'pth> where
            C: HasPartial,
            C::Partial: serde::de::DeserializeOwned,
        {
            type Error = FileReadError;

            fn to_partial(self) -> Result<C::Partial, FileReadError> {
                let Self(path) = self;
                let file = std::fs::OpenOptions::new().read(true).open(path)?;
                let partial: C::Partial = serde_json::from_reader(file).map_err(FileReadError::Json)?;

                Ok(partial)
            }

            fn name(&self) -> String {
                format!("JSON file at {:?}", self.0)
            }
        }


    #[cfg(feature = "toml")]
    impl<'pth, C> Source<C> for Toml<'pth> where
            C: HasPartial,
            C::Partial: serde::de::DeserializeOwned,
        {
            type Error = FileReadError;

            fn to_partial(self) -> Result<C::Partial, FileReadError> {
                let Self(path) = self;
                let mut file = std::fs::OpenOptions::new().read(true).open(path)?;
                let mut buffer: String = String::new();
                file.read_to_string(&mut buffer)?;
                let partial: C::Partial = toml::from_str(&buffer).map_err(FileReadError::Toml)?;

                Ok(partial)
            }

            fn name(&self) -> String {
                format!("TOML file at {:?}", self.0)
            }
        }


    impl<C> Source<C> for std::path::PathBuf
    where
       C: HasPartial,
       C::Partial: serde::de::DeserializeOwned
    {
        type Error = FileReadError;

        fn to_partial(self) -> Result<C::Partial, FileReadError> {
            if !self.exists() {
                Err(FileReadError::NoFile(self))
            } else {
                match self.extension() {
                    Some(os_str) => match os_str.to_str().expect("Failed convrsion from OsStr") {
                        #[cfg(feature = "toml")]
                        "toml" | "tml" => {
                            <Toml<'_> as Source<C>>::to_partial(Toml(&self))
                        }
                        #[cfg(feature = "json")]
                        "json" | "js" => {
                            <Json<'_> as Source<C>>::to_partial(Json(&self))
                        }
                        rest => Err(FileReadError::UnsupportedExtension(rest.to_owned())),
                    },
                    None => Err(FileReadError::NoExtension),
                }
            }
        }

     fn name(&self) -> String {
         format!("Configuration file at `{:?}`", self)
     }
 }
}