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use crate::{Context, Error};
use std::convert::Infallible;
use std::error::Error as StdError;
use std::fmt::{self, Debug, Display};

#[cfg(backtrace)]
use std::backtrace::Backtrace;

mod ext {
    use super::*;

    pub trait StdError {
        fn ext_context<C>(self, context: C) -> Error
        where
            C: Display + Send + Sync + 'static;
    }

    impl<E> StdError for E
    where
        E: std::error::Error + Send + Sync + 'static,
    {
        fn ext_context<C>(self, context: C) -> Error
        where
            C: Display + Send + Sync + 'static,
        {
            Error::new(ContextError {
                error: self,
                context,
            })
        }
    }

    impl StdError for Error {
        fn ext_context<C>(self, context: C) -> Error
        where
            C: Display + Send + Sync + 'static,
        {
            self.context(context)
        }
    }
}

impl<T, E> Context<T, E> for Result<T, E>
where
    E: ext::StdError + Send + Sync + 'static,
{
    fn context<C>(self, context: C) -> Result<T, Error>
    where
        C: Display + Send + Sync + 'static,
    {
        self.map_err(|error| error.ext_context(context))
    }

    fn with_context<C, F>(self, context: F) -> Result<T, Error>
    where
        C: Display + Send + Sync + 'static,
        F: FnOnce() -> C,
    {
        self.map_err(|error| error.ext_context(context()))
    }
}

/// ```
/// # type T = ();
/// #
/// use anyhow::{Context, Result};
///
/// fn maybe_get() -> Option<T> {
///     # const IGNORE: &str = stringify! {
///     ...
///     # };
///     # unimplemented!()
/// }
///
/// fn demo() -> Result<()> {
///     let t = maybe_get().context("there is no T")?;
///     # const IGNORE: &str = stringify! {
///     ...
///     # };
///     # unimplemented!()
/// }
/// ```
impl<T> Context<T, Infallible> for Option<T> {
    fn context<C>(self, context: C) -> Result<T, Error>
    where
        C: Display + Send + Sync + 'static,
    {
        self.ok_or_else(|| Error::from_display(context, backtrace!()))
    }

    fn with_context<C, F>(self, context: F) -> Result<T, Error>
    where
        C: Display + Send + Sync + 'static,
        F: FnOnce() -> C,
    {
        self.ok_or_else(|| Error::from_display(context(), backtrace!()))
    }
}

pub(crate) struct ContextError<E, C> {
    pub error: E,
    pub context: C,
}

impl<E, C> Debug for ContextError<E, C>
where
    E: Debug,
    C: Display,
{
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{:?}\n\n{}", self.error, self.context)
    }
}

impl<E, C> Display for ContextError<E, C>
where
    C: Display,
{
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        Display::fmt(&self.context, f)
    }
}

impl<E, C> StdError for ContextError<E, C>
where
    E: StdError + 'static,
    C: Display,
{
    #[cfg(backtrace)]
    fn backtrace(&self) -> Option<&Backtrace> {
        self.error.backtrace()
    }

    fn source(&self) -> Option<&(dyn StdError + 'static)> {
        Some(&self.error)
    }
}

impl<C> StdError for ContextError<Error, C>
where
    C: Display,
{
    #[cfg(backtrace)]
    fn backtrace(&self) -> Option<&Backtrace> {
        Some(self.error.backtrace())
    }

    fn source(&self) -> Option<&(dyn StdError + 'static)> {
        Some(&*self.error)
    }
}

pub(crate) mod private {
    use super::*;

    pub trait Sealed {}

    impl<T, E> Sealed for Result<T, E> where E: ext::StdError {}
    impl<T> Sealed for Option<T> {}
}