//! Does all the magic to have you potentially long output piped through the
//! external pager. Similar to what git does for its output.
//!
//! # Quick Start
//!
//! ```rust
//! extern crate pager;
//! use pager::Pager;
//! fn main() {
//!     Pager::new().setup();
//!     // The rest of your program goes here
//! }
//! ```
//!
//! Under the hood this forks the current process, connects child' stdout
//! to parent's stdin, and then replaces the parent with the pager of choice
//! (environment variable PAGER). The child just continues as normal. If PAGER
//! environment variable is not present `Pager` probes current PATH for `more`.
//! If found it is used as a default pager.
//!
//! You can control pager to a limited degree. For example you can change the
//! environment variable used for finding pager executable.
//!
//! ```rust
//! extern crate pager;
//! use pager::Pager;
//! fn main() {
//!     Pager::with_env("MY_PAGER").setup();
//!     // The rest of your program goes here
//! }
//! ```
//!
//! Also you can set alternative default (fallback) pager to be used instead of
//! `more`. PAGER environment variable (if set) will still have precedence.
//!
//! ```rust
//! extern crate pager;
//! use pager::Pager;
//! fn main() {
//!     Pager::with_default_pager("pager").setup();
//!     // The rest of your program goes here
//! }
//! ```
//! Alternatively you can specify directly the desired pager command, exactly
//! as it would appear in PAGER environment variable. This is useful if you
//! need some specific pager and/or flags (like "less -r") and would like to
//! avoid forcing your consumers into modifying their existing PAGER
//! configuration just for your application.
//!
//! ```rust
//! extern crate pager;
//! use pager::Pager;
//! fn main() {
//!     Pager::with_pager("pager -r").setup();
//!     // The rest of your program goes here
//! }
//! ```
//!
//! If no suitable pager found `setup()` does nothing and your executable keeps
//! running as usual. `Pager` cleans after itself and doesn't leak resources in
//! case of setup failure.
//!
//! Sometimes you may want to bypass pager if the output of you executable is not a `tty`.
//! If this case you may use `.skip_on_notty()` to get the desirable effect.
//!
//! ```rust
//! extern crate pager;
//! use pager::Pager;
//! fn main() {
//!     Pager::new().skip_on_notty().setup();
//!     // The rest of your program goes here
//! }
//! ```
//!
//! If you need to disable pager altogether set environment variable `NOPAGER` and `Pager::setup()`
//! will skip initialization. The host application will continue as normal. `Pager::is_on()` will
//! reflect the fact that no Pager is active.

#![doc(html_root_url = "https://docs.rs/pager/0.14.0")]
#![cfg_attr(all(feature = "cargo-clippy", feature = "pedantic"), warn(clippy_pedantic))]

extern crate errno;
extern crate libc;

mod utils;

use std::env;
use std::ffi::{OsStr, OsString};

/// Default pager environment variable
const DEFAULT_PAGER_ENV: &str = "PAGER";

/// Environment variable to disable pager altogether
const NOPAGER_ENV: &str = "NOPAGER";

/// Last resort pager. Should work everywhere.
const DEFAULT_PAGER: &str = "more";

/// Keeps track of the current pager state
#[derive(Debug)]
pub struct Pager {
    default_pager: Option<OsString>,
    pager: Option<OsString>,
    on: bool,
    skip_on_notty: bool,
}

impl Default for Pager {
    fn default() -> Self {
        Self {
            default_pager: None,
            pager: env::var_os(DEFAULT_PAGER_ENV),
            on: true,
            skip_on_notty: true,
        }
    }
}

impl Pager {
    /// Creates new instance of `Pager` with default settings
    pub fn new() -> Self {
        Default::default()
    }

    /// Creates new instance of pager using `env` environment variable instead of PAGER
    pub fn with_env(env: &str) -> Self {
        Self {
            pager: env::var_os(env),
            ..Default::default()
        }
    }

    #[deprecated(since = "0.12.0", note = "use with_env() instead")]
    pub fn env(env: &str) -> Self {
        Self::with_env(env)
    }

    /// Creates a new `Pager` instance with the specified default fallback
    pub fn with_default_pager<S>(pager: S) -> Self
    where
        S: Into<OsString>,
    {
        let default_pager = Some(pager.into());
        Self {
            default_pager,
            ..Default::default()
        }
    }

    /// Creates a new `Pager` instance directly specifying the desired pager
    pub fn with_pager(pager: &str) -> Self {
        Self {
            pager: Some(pager.into()),
            ..Default::default()
        }
    }

    /// Instructs `Pager` to bypass invoking pager if output is not a `tty`
    #[deprecated(since = "0.14.0", note = "'skip_on_notty' is default now")]
    pub fn skip_on_notty(self) -> Self {
        Self {
            skip_on_notty: true,
            ..self
        }
    }

    /// Gives quick assessment of successful `Pager` setup
    pub fn is_on(&self) -> bool {
        self.on
    }

    fn pager(&self) -> Option<OsString> {
        let fallback_pager = || Some(OsStr::new(DEFAULT_PAGER).into());

        if env::var_os(NOPAGER_ENV).is_some() {
            None
        } else {
            self.pager
                .clone()
                .or_else(|| self.default_pager.clone())
                .or_else(fallback_pager)
        }
    }

    /// Initiates Pager framework and sets up all the necessary environment for sending standard
    /// output to the activated pager.
    pub fn setup(&mut self) {
        if self.skip_on_notty && !utils::isatty(libc::STDOUT_FILENO) {
            self.on = false;
            return;
        }
        if let Some(ref pager) = self.pager() {
            let (pager_stdin, main_stdout) = utils::pipe();
            let pid = utils::fork();
            match pid {
                -1 => {
                    // Fork failed
                    utils::close(pager_stdin);
                    utils::close(main_stdout);
                    self.on = false
                }
                0 => {
                    // I am child
                    utils::dup2(main_stdout, libc::STDOUT_FILENO);
                    utils::close(pager_stdin);
                }
                _ => {
                    // I am parent
                    utils::dup2(pager_stdin, libc::STDIN_FILENO);
                    utils::close(main_stdout);
                    utils::execvp(pager);
                }
            }
        } else {
            self.on = false;
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::ops::Drop;

    enum PagerEnv {
        Reinstate(OsString, OsString),
        Remove(OsString)
    }

    impl PagerEnv {
        fn new<S: AsRef<OsStr>>(env: S) -> Self {
            let env = env.as_ref().into();
            if let Some(value) = env::var_os(&env) {
                PagerEnv::Reinstate(env, value)
            } else {
                PagerEnv::Remove(env)
            }
        }

        fn set<S: AsRef<OsStr>>(&self, value: S) {
            match self {
                | PagerEnv::Reinstate(env, _)
                | PagerEnv::Remove(env) => env::set_var(env, value)
            }
        }

        fn remove(&self) {
            match self {
                | PagerEnv::Reinstate(env, _)
                | PagerEnv::Remove(env) => env::remove_var(env)
            }
        }
    }

    impl Drop for PagerEnv {
        fn drop(&mut self) {
            match self {
                PagerEnv::Reinstate(env, value) => env::set_var(env, value),
                PagerEnv::Remove(env) => env::remove_var(env),
            }
        }
    }

    fn assert_pager(pager: &Pager, result: &str) {
        assert_eq!(pager.pager(), Some(OsStr::new(result).into()));
    }

    #[test]
    fn nopager() {
        let nopager = PagerEnv::new(NOPAGER_ENV);
        nopager.set("");

        let pager = Pager::new();
        assert!(pager.pager().is_none());
    }

    #[test]
    fn fallback_uses_more() {
        let pager = Pager::new();
        assert_pager(&pager, DEFAULT_PAGER);
    }

    #[test]
    fn with_default_pager_without_env() {
        let pagerenv = PagerEnv::new(DEFAULT_PAGER_ENV);
        pagerenv.remove();

        let pager = Pager::with_default_pager("more_or_less");
        assert_pager(&pager, "more_or_less");
    }

    #[test]
    fn with_default_pager_with_env() {
        let pagerenv = PagerEnv::new(DEFAULT_PAGER_ENV);
        pagerenv.set("something_else");

        let pager = Pager::with_default_pager("more_or_less");
        assert_pager(&pager, "something_else");
    }

    #[test]
    fn with_default_pager() {
        let pager = Pager::with_default_pager("more_or_less");
        assert_pager(&pager, "more_or_less");
    }

    #[test]
    fn with_pager() {
        let pager = Pager::with_pager("now_or_never");
        assert_pager(&pager, "now_or_never");
    }
}