//! Defines tools for interacting with the environment
//! of the shell.

use super::{ Error, Result, exec, };

use std::{
    io::Write,
    collections::HashMap,
    iter::FromIterator
};

/// An environment used to [evaluate expressions](mod@super::eval) or 
/// [execute programs](mod@super::exec).
pub trait Env {
    
    /// Get the arguments passed to the environment.
    fn args (&self) -> Args;

    /// Bind a value to a name.
    fn bind <N, V> (&mut self, name: N, value: V)
    where
        N: AsRef<str>,
        V: AsRef<str>;

    /// Get a bound value from the env.
    fn get <N> (&self, name: N) -> Option<String>
    where
        N: AsRef<str>;

    /// Get the current working directory.
    fn working_dir (&self) -> Result<std::path::PathBuf>;

    /// Change the working directory.
    fn set_working_dir <P> (&mut self, path: P) -> Result<()>
    where
        P: AsRef<std::path::Path>;

    /// Write to stdout.
    fn stdout <R> (&mut self, data: R) -> Result<()>
    where
        R: ToString;

    /// Search the `PATH` variable for the `query`. The default implementation
    /// is derived from [`Env::get`].
    fn search <S> (&self, query: S) -> Option<exec::Program> 
    where
        S: AsRef<str>
    {
        self.get("PATH")?
            .split(':')
            .filter_map(|path| {
                std::fs::read_dir(path).ok()
            })
            .flat_map(|dir| {
                dir.filter_map(|entry| entry.ok())
            })
            .map(|file| file.path())
            .find_map(|path| {
                if path.file_name()? == query.as_ref() { 
                    Some (path)
                } else {
                    None
                }
            })
            .map(exec::Program::new)
    }

    /// Create a scoped environment to isolate variable
    /// bindings.
    fn scope (&mut self) -> Scope<'_, Self> 
    where
        Self: Sized
    {
        Scope {
            bindings: HashMap::new(),
            parent: self,
        }
    }

    /// Create a new environment that inherits this environments'
    /// behavior, except the args of this env are overwritten with
    /// `args`.
    fn set_args <A> (self, args: A) -> SetArgs<Self> 
    where
        Self: Sized,
        A: IntoIterator<Item = String>,
    {
        SetArgs {
            parent: self, 
            args: Args::from_iter(args),
        }
    }

}

/// Overwrites the arguments in the wrapped [`Env`].
pub struct SetArgs<E: Env> {
    parent: E,
    args: Args,
}

impl<E: Env> Env for SetArgs<E> {
    
    fn working_dir (&self) -> Result<std::path::PathBuf> {
        self.parent.working_dir()
    }

    fn set_working_dir <P> (&mut self, path: P) -> Result<()>
    where
        P: AsRef<std::path::Path>
    {
        self.parent.set_working_dir(path)
    }

    fn search <S> (&self, query: S) -> Option<exec::Program> 
    where
        S: AsRef<str>
    {
        self.parent.search(query)
    }

    fn stdout <R> (&mut self, data: R) -> Result<()>
    where
        R: ToString
    {
        self.parent.stdout(data)
    }

    fn args (&self) -> Args {
        self.args.clone()
    }

    fn bind <N, V> (&mut self, name: N, value: V)
    where
        N: AsRef<str>,
        V: AsRef<str>,
    {
        self.parent.bind(name, value);
    }

    fn get <N> (&self, name: N) -> Option<String>
    where
        N: AsRef<str>
    {
        self.parent.get(name)
    }

}

/// A one-way barrier for variable bindings. Things executed in
/// a `Scope` can read the parent's bindings and shadow them, but
/// not mutate them. 
pub struct Scope <'parent, E: Env> {
    bindings: HashMap <String, String>,
    parent: &'parent mut E,
}

impl<E: Env> Env for Scope<'_, E> {

    fn working_dir (&self) -> Result<std::path::PathBuf> {
        self.parent.working_dir()
    }

    fn set_working_dir <P> (&mut self, path: P) -> Result<()>
    where
        P: AsRef<std::path::Path>
    {
        self.parent.set_working_dir(path)
    }

    fn search <S> (&self, query: S) -> Option<exec::Program> 
    where
        S: AsRef<str>
    {
        self.parent.search(query)
    }

    fn stdout <R> (&mut self, data: R) -> Result<()>
    where
        R: ToString
    {
        self.parent.stdout(data)
    }

    fn args (&self) -> Args {
        self.parent.args()
    }

    fn bind <N, V> (&mut self, name: N, value: V)
    where
        N: AsRef<str>,
        V: AsRef<str>,
    {
        self.bindings.insert(
            name.as_ref().to_string(),
            value.as_ref().to_string()
        );
    }

    fn get <N> (&self, name: N) -> Option<String>
    where
        N: AsRef<str>
    {
        self.bindings
            .get(name.as_ref())
            .cloned()
            .or_else(|| {
                self.parent.get(name)
            })
    }
}

/// An iterator of arguments.
#[derive(Default, Clone)]
pub struct Args {
    args: Vec<String>,
    cur: usize
}

impl<S: ToString> FromIterator<S> for Args {
    fn from_iter <T: IntoIterator<Item = S>> (iter: T) -> Args {
        Args {
            args: iter
                .into_iter()
                .map(|s| s.to_string())
                .collect(),
            cur: 0
        }
    }
}

impl Iterator for Args {
    type Item = String;

    fn next (&mut self) -> Option<Self::Item> {
        let Args { args, cur } = self;

        let val = args.get(*cur)?;
        *cur += 1;
        
        Some (val.clone())
    }
}

/// A completely empty environment.
pub struct Pure {
    bindings: HashMap <String, String>,
    cwd: std::path::PathBuf,
    buf: String,
}

impl Pure {
    pub fn init (cwd: impl AsRef<std::path::Path>) -> Pure {
        Pure { 
            cwd: cwd.as_ref().to_owned(), 
            bindings: HashMap::new(),
            buf: String::new(),
        }
    }
}

impl Env for Pure {
    fn args (&self) -> Args {
        std::iter::empty::<String>().collect()
    }

    fn bind <N, V> (&mut self, name: N, value: V)
    where
        N: AsRef<str>,
        V: AsRef<str>,
    {
        self.bindings.insert(
            name.as_ref().to_string(),
            value.as_ref().to_string()
        );
    }

    fn get <N> (&self, name: N) -> Option<String>
    where
        N: AsRef<str>
    {
        self.bindings.get(name.as_ref()).cloned()
    }

    fn stdout <R> (&mut self, data: R) -> Result<()>
    where
        R: ToString
    {
        self.buf += &data.to_string();
        Ok (())
    }

    fn working_dir (&self) -> Result<std::path::PathBuf> {
        if self.cwd.exists() {
            Ok (self.cwd.clone())
        } else {
            use std::io::ErrorKind;
            Err (Error::Io (ErrorKind::NotFound.into()))
        }
    }

    fn set_working_dir <P> (&mut self, path: P) -> Result<()>
    where
        P: AsRef<std::path::Path>
    {
        let path = path.as_ref();

        if path.is_dir() && path.exists() {
            Ok (self.cwd = path.to_owned())
        } else {
            use std::io::ErrorKind;
            Err (Error::Io (ErrorKind::InvalidInput.into()))
        }
    }
}

/// The default global environment, which inherits from the environment of the
/// shell process.
pub struct Inherit {}

impl Env for Inherit {
    fn args (&self) -> Args {
        std::env::args().skip(1).collect()
    }

    fn bind <N, V> (&mut self, name: N, value: V)
    where
        N: AsRef<str>,
        V: AsRef<str>,
    {
        std::env::set_var(name.as_ref(), value.as_ref());
    }

    fn get <N> (&self, name: N) -> Option<String>
    where
        N: AsRef<str>
    {
        std::env::var(name.as_ref()).ok()
    }

    fn working_dir (&self) -> Result<std::path::PathBuf> {
        std::env::current_dir()
            .map_err(Error::Io)
    }

    fn set_working_dir <P> (&mut self, path: P) -> Result<()>
    where
        P: AsRef<std::path::Path>
    {
        let path = path.as_ref();

        std::env::set_current_dir(path)
            .map_err(Error::Io)
    }

    fn stdout <R> (&mut self, data: R) -> Result<()>
    where
        R: ToString
    {
        std::io::stdout().write(
            data.to_string()
                .as_bytes()
        )?;
        
        Ok (())
    }
}

/// Initialize the default environment by inheriting from the environment of
/// the shell process.
pub fn inherit () -> impl Env {
    Inherit {}
}

pub mod history;
pub mod job;
pub mod io;