//! Defines methods for spawning commands into futures.

use ExitStatus;
use future::{Async, EnvFuture, Poll};
use future_ext::{EnvFutureExt, FlattenedEnvFuture};
use futures::Future;

mod and_or;
mod case;
mod for_cmd;
mod func_exec;
mod if_cmd;
mod local_redirections;
mod loop_cmd;
mod pipeline;
mod rc;
mod sequence;
mod simple;
mod subshell;
mod substitution;
mod swallow_non_fatal;
mod vec_sequence;

pub mod builtin;
#[cfg(feature = "conch-parser")]
pub mod ast_impl;

// Private definitions
use self::vec_sequence::{VecSequence, VecSequenceWithLast};

// Pub reexports
pub use self::and_or::{AndOr, AndOrList, and_or_list};
pub use self::case::{Case, case, PatternBodyPair};
pub use self::for_cmd::{For, ForArgs, for_args, for_loop, for_with_args};
pub use self::func_exec::{Function, function, function_body};
pub use self::if_cmd::{If, if_cmd};
pub use self::local_redirections::{LocalRedirections, spawn_with_local_redirections};
pub use self::loop_cmd::{Loop, loop_cmd};
pub use self::pipeline::{Pipeline, pipeline, SpawnedPipeline};
pub use self::sequence::{Sequence, sequence};
pub use self::simple::{SimpleCommand, simple_command, SpawnedSimpleCommand};
pub use self::subshell::{Subshell, subshell};
pub use self::substitution::{Substitution, SubstitutionEnvFuture, substitution};
pub use self::swallow_non_fatal::{SwallowNonFatal, swallow_non_fatal_errors};

/// A trait for spawning commands into an `EnvFuture` which can be
/// polled to completion.
///
/// Spawning a command is separated into two distinct parts: a future
/// that requires a mutable environment to make progress, and a future
/// which no longer needs any context and can make progress on its own.
///
/// This distinction allows a caller to drop an environment as soon as
/// it is no longer needed, which will free up resources, and especially
/// important in preventing deadlocks between pipelines (since the parent
/// process will contain extra reader/writer ends of a pipe and may prevent
/// processes from exiting).
pub trait Spawn<E: ?Sized> {
    /// The future that represents spawning the command.
    ///
    /// It represents all computations that may need an environment to
    /// progress further.
    type EnvFuture: EnvFuture<E, Item = Self::Future, Error = Self::Error>;
    /// The future that represents the exit status of a fully bootstrapped
    /// command, which no longer requires an environment to be driven to completion.
    type Future: Future<Item = ExitStatus, Error = Self::Error>;
    /// The type of error that a future will resolve with if it fails in a
    /// normal fashion.
    type Error;

    /// Spawn the command as a future.
    ///
    /// Although the implementation is free to make any optimizations or
    /// pre-computations, there should be no observable side-effects until the
    /// very first call to `poll` on the future. That way a constructed future
    /// that was never `poll`ed could be dropped without the risk of unintended
    /// side effects.
    ///
    /// **Note**: There are no guarantees that the environment will not change
    /// between the `spawn` invocation and the first call to `poll()` on the
    /// future. Thus any optimizations the implementation may decide to make
    /// based on the environment should be done with care.
    fn spawn(self, env: &E) -> Self::EnvFuture;
}

impl<'a, 'b: 'a, T, E: ?Sized> Spawn<E> for &'a &'b T
    where &'b T: Spawn<E>
{
    type EnvFuture = <&'b T as Spawn<E>>::EnvFuture;
    type Future = <&'b T as Spawn<E>>::Future;
    type Error = <&'b T as Spawn<E>>::Error;

    fn spawn(self, env: &E) -> Self::EnvFuture {
        (*self).spawn(env)
    }
}

#[cfg_attr(feature = "clippy", allow(boxed_local))]
impl<E: ?Sized, T: Spawn<E>> Spawn<E> for Box<T> {
    type EnvFuture = T::EnvFuture;
    type Future = T::Future;
    type Error = T::Error;

    fn spawn(self, env: &E) -> Self::EnvFuture {
        (*self).spawn(env)
    }
}

#[cfg_attr(feature = "clippy", allow(boxed_local))]
impl<'a, E: ?Sized, T: 'a> Spawn<E> for &'a Box<T>
    where &'a T: Spawn<E>,
{
    type EnvFuture = <&'a T as Spawn<E>>::EnvFuture;
    type Future = <&'a T as Spawn<E>>::Future;
    type Error = <&'a T as Spawn<E>>::Error;

    fn spawn(self, env: &E) -> Self::EnvFuture {
        Spawn::spawn(&**self, env)
    }
}

/// A marker trait for denoting that the receiver of a `Spawn` implementation
/// can also be `spawn`ed by reference without moving. Automatically derived
/// for any `&'a T: Spawn`.
///
/// Until Associated Type Constructors (ATCs) land, we cannot define a version
/// of `Spawn` which takes the receiver by reference since there is no way we
/// reference the receiver's lifetime within the associated futures.
///
/// We can, however, get around this by defining `Spawn` to move its receiver,
/// and then implementing the trait directly on a reference (this moves the
/// reference, but references are `Copy` which is effectively a no-op). That way
/// we can tie the associated futures to the lifetime of the reference since
/// neither can outlive the actual struct.
///
/// This effectively gives rise to two `Spawn` implementations we can add on each
/// type: one that moves the caller and any inner types by value, and one that
/// operates on the outer and inner types by reference only. As long as we don't
/// mix the two kinds, we're golden!
///
/// Except there are situations where we may want to own a type directly, but
/// want to spawn it by reference (imagine we're running a loop on an "owned"
/// implementation chain and need to spawn something repeatedly, but we don't
/// want to clone deeply nested types)... Unfortunately, doing so confuses the
/// compiler which causes it to get stuck in a recursive loop when evaluating
/// bounds (error is `E0275` with a message like "required by the impl for
/// `&T<_>` because of the requirements on the impl for `&T<T<_>>`,
/// `&T<T<T<_>>>`, ...").
///
/// We can apparently point the compiler in the right direction by adding a
/// marker trait only when `Spawn` is implemented directly on a reference,
/// allowing it to avoid the first "owned" implementation on the same type.
pub trait SpawnRef<E: ?Sized> {
    /// The future that represents spawning the command.
    ///
    /// It represents all computations that may need an environment to
    /// progress further.
    type EnvFuture: EnvFuture<E, Item = Self::Future, Error = Self::Error>;
    /// The future that represents the exit status of a fully bootstrapped
    /// command, which no longer requires an environment to be driven to completion.
    type Future: Future<Item = ExitStatus, Error = Self::Error>;
    /// The type of error that a future will resolve with if it fails in a
    /// normal fashion.
    type Error;

    /// Identical to `Spawn::spawn` but does not move `self`.
    fn spawn_ref(&self, env: &E) -> Self::EnvFuture;
}

/// A marker trait for any reference.
pub trait Ref: Copy {}
impl<'a, T> Ref for &'a T {}

impl<S, E: ?Sized> SpawnRef<E> for S
    where S: Spawn<E> + Ref,
{
    type EnvFuture = S::EnvFuture;
    type Future = S::Future;
    type Error = S::Error;

    fn spawn_ref(&self, env: &E) -> Self::EnvFuture {
        (*self).spawn(env)
    }
}

/// Type alias for boxed futures that represent spawning a command.
pub type BoxSpawnEnvFuture<'a, E, ERR> = Box<'a + EnvFuture<
    E,
    Item = BoxStatusFuture<'a, ERR>,
    Error = ERR
>>;

/// Type alias for a boxed future which will resolve to an `ExitStatus`.
pub type BoxStatusFuture<'a, ERR> = Box<'a + Future<Item = ExitStatus, Error = ERR>>;

/// A trait for spawning commands (without moving ownership) into boxed futures.
/// Largely useful for having spawnable trait objects.
pub trait SpawnBoxed<E: ?Sized> {
    /// The type of error that a future will resolve with if it fails in a
    /// normal fashion.
    type Error;

    /// Identical to `Spawn::spawn` but does not move `self` and returns boxed futures.
    fn spawn_boxed<'a>(&'a self, env: &E) -> BoxSpawnEnvFuture<'a, E, Self::Error> where E: 'a;
}

impl<S, ERR, E: ?Sized> SpawnBoxed<E> for S
    where for<'a> &'a S: Spawn<E, Error = ERR>,
{
    type Error = ERR;

    fn spawn_boxed<'a>(&'a self, env: &E) -> BoxSpawnEnvFuture<'a, E, Self::Error> where E: 'a {
        Box::from(self.spawn(env).boxed_result())
    }
}

/// Represents either a ready `ExitStatus` or a future that will resolve to one.
#[must_use = "futures do nothing unless polled"]
#[derive(Debug)]
pub enum ExitResult<F> {
    /// An unresolved future.
    Pending(F),
    /// A ready `ExitStatus` value.
    Ready(ExitStatus),
}

impl<F> From<ExitStatus> for ExitResult<F> {
    fn from(status: ExitStatus) -> Self {
        ExitResult::Ready(status)
    }
}

impl<F> Future for ExitResult<F>
    where F: Future<Item = ExitStatus>
{
    type Item = F::Item;
    type Error = F::Error;

    fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
        match *self {
            ExitResult::Pending(ref mut f) => f.poll(),
            ExitResult::Ready(exit) => Ok(Async::Ready(exit)),
        }
    }
}

/// A grouping of guard and body commands.
#[derive(Debug, PartialEq, Eq, Clone)]
pub struct GuardBodyPair<T> {
    /// The guard commands, which if successful, should lead to the
    /// execution of the body commands.
    pub guard: T,
    /// The body commands to execute if the guard is successful.
    pub body: T,
}