//  * This file is part of the uutils coreutils package.
//  *
//  * (c) Ben Eggers <ben.eggers36@gmail.com>
//  * (c) Akira Hayakawa <ruby.wktk@gmail.com>
//  *
//  * For the full copyright and license information, please view the LICENSE
//  * file that was distributed with this source code.
use clap::{crate_version, Arg, Command};
use std::collections::{HashMap, HashSet};
use std::fs::File;
use std::io::{stdin, BufRead, BufReader, Read};
use std::path::Path;
use uucore::display::Quotable;
use uucore::error::{FromIo, UResult, USimpleError};
use uucore::format_usage;

static ABOUT: &str = "Topological sort the strings in FILE.
Strings are defined as any sequence of tokens separated by whitespace (tab, space, or newline).
If FILE is not passed in, stdin is used instead.";
static USAGE: &str = "{} [OPTIONS] FILE";

mod options {
    pub const FILE: &str = "file";
}

#[uucore::main]
pub fn uumain(args: impl uucore::Args) -> UResult<()> {
    let args = args.collect_lossy();

    let matches = uu_app().try_get_matches_from(args)?;

    let input = matches
        .get_one::<String>(options::FILE)
        .expect("Value is required by clap");

    let mut stdin_buf;
    let mut file_buf;
    let mut reader = BufReader::new(if input == "-" {
        stdin_buf = stdin();
        &mut stdin_buf as &mut dyn Read
    } else {
        file_buf = File::open(Path::new(&input)).map_err_context(|| input.to_string())?;
        &mut file_buf as &mut dyn Read
    });

    let mut g = Graph::new();
    loop {
        let mut line = String::new();
        match reader.read_line(&mut line) {
            Ok(_) => {
                let tokens: Vec<String> = line.split_whitespace().map(|s| s.to_owned()).collect();
                if tokens.is_empty() {
                    break;
                }
                for ab in tokens.chunks(2) {
                    match ab.len() {
                        2 => g.add_edge(&ab[0], &ab[1]),
                        _ => {
                            return Err(USimpleError::new(
                                1,
                                format!(
                                    "{}: input contains an odd number of tokens",
                                    input.maybe_quote()
                                ),
                            ))
                        }
                    }
                }
            }
            _ => break,
        }
    }

    g.run_tsort();

    if !g.is_acyclic() {
        return Err(USimpleError::new(
            1,
            format!("{}, input contains a loop:", input),
        ));
    }

    for x in &g.result {
        println!("{}", x);
    }

    Ok(())
}

pub fn uu_app() -> Command {
    Command::new(uucore::util_name())
        .version(crate_version!())
        .override_usage(format_usage(USAGE))
        .about(ABOUT)
        .infer_long_args(true)
        .arg(
            Arg::new(options::FILE)
                .default_value("-")
                .hide(true)
                .value_hint(clap::ValueHint::FilePath),
        )
}

// We use String as a representation of node here
// but using integer may improve performance.
#[derive(Default)]
struct Graph {
    in_edges: HashMap<String, HashSet<String>>,
    out_edges: HashMap<String, Vec<String>>,
    result: Vec<String>,
}

impl Graph {
    fn new() -> Self {
        Self::default()
    }

    fn has_node(&self, n: &str) -> bool {
        self.in_edges.contains_key(n)
    }

    fn has_edge(&self, from: &str, to: &str) -> bool {
        self.in_edges[to].contains(from)
    }

    fn init_node(&mut self, n: &str) {
        self.in_edges.insert(n.to_string(), HashSet::new());
        self.out_edges.insert(n.to_string(), vec![]);
    }

    fn add_edge(&mut self, from: &str, to: &str) {
        if !self.has_node(to) {
            self.init_node(to);
        }

        if !self.has_node(from) {
            self.init_node(from);
        }

        if from != to && !self.has_edge(from, to) {
            self.in_edges.get_mut(to).unwrap().insert(from.to_string());
            self.out_edges.get_mut(from).unwrap().push(to.to_string());
        }
    }

    // Kahn's algorithm
    // O(|V|+|E|)
    fn run_tsort(&mut self) {
        let mut start_nodes = vec![];
        for (n, edges) in &self.in_edges {
            if edges.is_empty() {
                start_nodes.push(n.clone());
            }
        }

        while !start_nodes.is_empty() {
            let n = start_nodes.remove(0);

            self.result.push(n.clone());

            let n_out_edges = self.out_edges.get_mut(&n).unwrap();
            for m in n_out_edges.iter() {
                let m_in_edges = self.in_edges.get_mut(m).unwrap();
                m_in_edges.remove(&n);

                // If m doesn't have other in-coming edges add it to start_nodes
                if m_in_edges.is_empty() {
                    start_nodes.push(m.clone());
                }
            }
            n_out_edges.clear();
        }
    }

    fn is_acyclic(&self) -> bool {
        for edges in self.out_edges.values() {
            if !edges.is_empty() {
                return false;
            }
        }
        true
    }
}
