//! Execution of commands, builtins and code.

use super::{
    Result,
    env::Env,
};

/// Execute the code in the given environment.
pub trait Exec {
    
    /// Run the executable.
    fn exec <E> (&self, env: &mut E) -> Result<()>
    where
        E: Env;

}

/// A shell command.
pub enum Command {
    Program (Program),
    Builtin (Builtin),
}

impl Exec for Command {
    fn exec<E>(&self, env: &mut E) -> Result<()>
    where
        E: Env
    {
        match self {
            Command::Builtin (builtin) => builtin.exec(env),
            Command::Program (program) => program.exec(env),
        }
    }
}

/// A path to an executable file representing an external program.
pub struct Program (std::ffi::OsString);

impl Program {

    /// Create a new [`Program`] struct.
    pub (super) fn new (str: impl AsRef<std::ffi::OsStr>) -> Program {
        Program (str.as_ref().to_owned())
    }

    /// Get the command name as an [`OsStr`](std::ffi::OsStr)
    pub fn as_os_str (&self) -> &std::ffi::OsStr {
        &self.0
    }

    /// Get the [`Path`](std::path::Path) of the file the name references.
    pub fn as_path (&self) -> &std::path::Path {
        self.as_ref()
    }
}

impl AsRef<std::path::Path> for Program {
    fn as_ref (&self) -> &std::path::Path {
        std::path::Path::new(&self.0)
    }
}

impl Exec for Program {
    fn exec <E> (&self, env: &mut E) -> Result<()>
    where
        E: Env
    {
        use ptyprocess::PtyProcess;
        use std::io::{ BufRead, Read };

        let mut cmd = std::process::Command::new(self.as_os_str());

        cmd.current_dir(env.working_dir()?);
        cmd.args(env.args());

        let proc = PtyProcess::spawn(cmd)
            .unwrap();

        let mut pty = proc
            .get_pty_handle()
            .unwrap();

        while let Ok (true) = proc.is_alive() {
            let mut buf = String::new();
            pty.read_to_string(&mut buf);
            env.stdout(&buf);
        }

        Ok (())
    }
}


/// Represents the case that a string fails to parse to a [`Builtin`] 
/// because the associated builtin does not exist. Valid builtin names
/// are listed in the [`builtin`] module.
#[derive(Debug)]
pub struct NotFound;

/// A shell builtin. See also the [`builtin`] module.
///
/// # Examples
/// 
/// The only way to get a `Builtin` is through [`FromStr`].
///
/// ```
/// # use rush::exec::Builtin;
/// "type".parse::<Builtin>().unwrap();
/// ```
///
/// Parsing an unknown builtin name yields an error.
///
/// ```should_panic
/// # use rush::exec::Builtin;
/// "sjdkgfhgfkfg"
///     .parse::<Builtin>()
///     .unwrap();
/// ```
///
/// [`FromStr`]: std::str::FromStr
pub struct Builtin (BuiltinName);

enum BuiltinName {
    Disown,
    Type,
    Exit,
    Pwd,
    Fg,
    Bg,
    Cd,
}

impl std::str::FromStr for Builtin {
    type Err = NotFound;

    fn from_str (s: &str) -> Result<Self, Self::Err> {
        let ret = Builtin (match s {
            "disown" => BuiltinName::Disown,
            "type"   => BuiltinName::Type,
            "exit"   => BuiltinName::Exit,
            "pwd"    => BuiltinName::Pwd,
            "fg"     => BuiltinName::Fg,
            "bg"     => BuiltinName::Bg,
            "cd"     => BuiltinName::Cd,
            _ => return Err (NotFound)
        });

        Ok (ret)
    }
}

impl Exec for Builtin {
    fn exec <E> (&self, env: &mut E) -> Result<()>
    where
        E: Env
    {
        use BuiltinName::*;

        let f = match self.0 {
            Disown => builtin::disown,
            Type   => builtin::r#type,
            Exit   => builtin::exit,
            Pwd    => builtin::pwd,
            Fg     => builtin::fg,
            Bg     => builtin::bg,
            Cd     => builtin::cd,
        };

        f (env)
    }
}

pub mod builtin {
    
    //! Shell builtins.

    use super::{ Env, Result, };

    /// Change the current working directory.
    pub fn cd (env: &mut impl Env) -> Result<()> {
        let arg = env.args().next().unwrap();
        env.set_working_dir(arg)
    }

    /// Print the current working directory.
    pub fn pwd (env: &mut impl Env) -> Result<()> {
        let cwd = env.working_dir()?;
        env.stdout(format!("{}", cwd.display()).as_str());
        Ok (())
    }

    /// Stop the program. Optionally takes an exit code.
    pub fn exit (env: &mut impl Env) -> Result<()> {
        if let Some (code) = env.args().next().and_then(|s| s.parse::<i32>().ok()) {
            std::process::exit(code)
        } else {
            std::process::exit(0)
        }
    }

    /// Display information about the type of a command.
    pub fn r#type (env: &mut impl Env) -> Result<()> {
        use crate::exec::Builtin;

        for arg in env.args() {
            let output = if let Ok (_) = arg.parse::<Builtin>() {
                format!("{} is a shell builtin", arg)
            } else if let Some (path) = env.search(&arg) {
                format!("{} is {}", arg, path.as_path().display())
            } else {
                format!("type: {}: not found", arg)
            };

            env.stdout((output + "\n").as_str())?;
        }

        Ok (())
    }

    /// Send a job to the background. See also [`Job::bg`].
    ///
    /// [`Job::bg`]: crate::job::Job::bg
    pub fn bg (_: &mut impl Env) -> Result<()> {
        todo!()
    }

    /// Bring a job to the foreground. See also [`Job::fg`].
    ///
    /// [`Job::fg`]: crate::job::Job::fg
    pub fn fg (_: &mut impl Env) -> Result<()> {
        todo!()
    }

    /// Disown all background jobs.
    pub fn disown (_: &mut impl Env) -> Result<()> {
        todo!()
    }

}