use crate::{convert, ops};

/// 用于告诉操作是应该提前退出还是像往常一样继续操作。
///
/// 在将您希望用户能够选择是否提前退出的事物 (例如图形遍历或访问者) 公开时使用。
/// 有了枚举可以使它更清晰 - 不必再奇怪 "wait, what did `false` mean again?" 了 - 并允许包含一个值。
///
/// # Examples
///
/// 从 [`Iterator::try_for_each`] 提前退出:
///
/// ```
/// use std::ops::ControlFlow;
///
/// let r = (2..100).try_for_each(|x| {
///     if 403 % x == 0 {
///         return ControlFlow::Break(x)
///     }
///
///     ControlFlow::Continue(())
/// });
/// assert_eq!(r, ControlFlow::Break(13));
/// ```
///
/// 基本的树遍历:
///
/// ```no_run
/// use std::ops::ControlFlow;
///
/// pub struct TreeNode<T> {
///     value: T,
///     left: Option<Box<TreeNode<T>>>,
///     right: Option<Box<TreeNode<T>>>,
/// }
///
/// impl<T> TreeNode<T> {
///     pub fn traverse_inorder<B>(&self, mut f: impl FnMut(&T) -> ControlFlow<B>) -> ControlFlow<B> {
///         if let Some(left) = &self.left {
///             left.traverse_inorder(&mut f)?;
///         }
///         f(&self.value)?;
///         if let Some(right) = &self.right {
///             right.traverse_inorder(&mut f)?;
///         }
///         ControlFlow::Continue(())
///     }
/// }
/// ```
#[stable(feature = "control_flow_enum_type", since = "1.55.0")]
#[derive(Debug, Clone, Copy, PartialEq)]
pub enum ControlFlow<B, C = ()> {
    /// 照常进行操作的下一个阶段。
    #[stable(feature = "control_flow_enum_type", since = "1.55.0")]
    #[lang = "Continue"]
    Continue(C),
    /// 退出操作而不运行后续阶段。
    #[stable(feature = "control_flow_enum_type", since = "1.55.0")]
    #[lang = "Break"]
    Break(B),
    // 是的，成员的顺序与类型参数不匹配。
    // 它们是按此顺序排列的，因此 `ControlFlow<A, B>` <-> `Result<B, A>` 是 `Try` 实现中的无操作转换。
    //
}

#[unstable(feature = "try_trait_v2", issue = "84277")]
impl<B, C> ops::TryV2 for ControlFlow<B, C> {
    type Output = C;
    type Residual = ControlFlow<B, convert::Infallible>;

    #[inline]
    fn from_output(output: Self::Output) -> Self {
        ControlFlow::Continue(output)
    }

    #[inline]
    fn branch(self) -> ControlFlow<Self::Residual, Self::Output> {
        match self {
            ControlFlow::Continue(c) => ControlFlow::Continue(c),
            ControlFlow::Break(b) => ControlFlow::Break(ControlFlow::Break(b)),
        }
    }
}

#[unstable(feature = "try_trait_v2", issue = "84277")]
impl<B, C> ops::FromResidual for ControlFlow<B, C> {
    #[inline]
    fn from_residual(residual: ControlFlow<B, convert::Infallible>) -> Self {
        match residual {
            ControlFlow::Break(b) => ControlFlow::Break(b),
        }
    }
}

impl<B, C> ControlFlow<B, C> {
    /// 如果这是 `Break` 成员，则返回 `true`。
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(control_flow_enum)]
    /// use std::ops::ControlFlow;
    ///
    /// assert!(ControlFlow::<i32, String>::Break(3).is_break());
    /// assert!(!ControlFlow::<String, i32>::Continue(3).is_break());
    /// ```
    #[inline]
    #[unstable(feature = "control_flow_enum", reason = "new API", issue = "75744")]
    pub fn is_break(&self) -> bool {
        matches!(*self, ControlFlow::Break(_))
    }

    /// 如果这是 `Continue` 成员，则返回 `true`。
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(control_flow_enum)]
    /// use std::ops::ControlFlow;
    ///
    /// assert!(!ControlFlow::<i32, String>::Break(3).is_continue());
    /// assert!(ControlFlow::<String, i32>::Continue(3).is_continue());
    /// ```
    #[inline]
    #[unstable(feature = "control_flow_enum", reason = "new API", issue = "75744")]
    pub fn is_continue(&self) -> bool {
        matches!(*self, ControlFlow::Continue(_))
    }

    /// 如果 `ControlFlow` 为 `Break`，则将 `ControlFlow` 转换为 `Some`，否则为 `None`。
    ///
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(control_flow_enum)]
    /// use std::ops::ControlFlow;
    ///
    /// assert_eq!(ControlFlow::<i32, String>::Break(3).break_value(), Some(3));
    /// assert_eq!(ControlFlow::<String, i32>::Continue(3).break_value(), None);
    /// ```
    #[inline]
    #[unstable(feature = "control_flow_enum", reason = "new API", issue = "75744")]
    pub fn break_value(self) -> Option<B> {
        match self {
            ControlFlow::Continue(..) => None,
            ControlFlow::Break(x) => Some(x),
        }
    }

    /// Maps `ControlFlow<B, C>` 到 `ControlFlow<T, C>` 通过在中断值 (如果存在) 上应用函数来实现。
    ///
    #[inline]
    #[unstable(feature = "control_flow_enum", reason = "new API", issue = "75744")]
    pub fn map_break<T, F>(self, f: F) -> ControlFlow<T, C>
    where
        F: FnOnce(B) -> T,
    {
        match self {
            ControlFlow::Continue(x) => ControlFlow::Continue(x),
            ControlFlow::Break(x) => ControlFlow::Break(f(x)),
        }
    }
}

/// 这些仅用作实现迭代器适配器的一部分。
/// 它们具有普通的名称和不明显的语义，因此目前还没有走上潜在的稳定之路。
///
impl<R: ops::TryV2> ControlFlow<R, R::Output> {
    /// 从实现 `Try` 的任何类型创建 `ControlFlow`。
    #[inline]
    pub(crate) fn from_try(r: R) -> Self {
        match R::branch(r) {
            ControlFlow::Continue(v) => ControlFlow::Continue(v),
            ControlFlow::Break(v) => ControlFlow::Break(R::from_residual(v)),
        }
    }

    /// 将 `ControlFlow` 转换为实现 `Try` 的任何类型;
    #[inline]
    pub(crate) fn into_try(self) -> R {
        match self {
            ControlFlow::Continue(v) => R::from_output(v),
            ControlFlow::Break(v) => v,
        }
    }
}

impl<B> ControlFlow<B, ()> {
    /// 通常，`Continue` 不需要任何值，因此，如果愿意，这提供了一种避免输入 `(())` 的方法。
    ///
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(control_flow_enum)]
    /// use std::ops::ControlFlow;
    ///
    /// let mut partial_sum = 0;
    /// let last_used = (1..10).chain(20..25).try_for_each(|x| {
    ///     partial_sum += x;
    ///     if partial_sum > 100 { ControlFlow::Break(x) }
    ///     else { ControlFlow::CONTINUE }
    /// });
    /// assert_eq!(last_used.break_value(), Some(22));
    /// ```
    #[unstable(feature = "control_flow_enum", reason = "new API", issue = "75744")]
    pub const CONTINUE: Self = ControlFlow::Continue(());
}

impl<C> ControlFlow<(), C> {
    /// 像 `try_for_each` 这样的 API 不需要 `Break` 的值，因此，如果您愿意的话，这提供了一种避免输入 `(())` 的方法。
    ///
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(control_flow_enum)]
    /// use std::ops::ControlFlow;
    ///
    /// let mut partial_sum = 0;
    /// (1..10).chain(20..25).try_for_each(|x| {
    ///     if partial_sum > 100 { ControlFlow::BREAK }
    ///     else { partial_sum += x; ControlFlow::CONTINUE }
    /// });
    /// assert_eq!(partial_sum, 108);
    /// ```
    #[unstable(feature = "control_flow_enum", reason = "new API", issue = "75744")]
    pub const BREAK: Self = ControlFlow::Break(());
}