look_at_that/src/main.rs

use anyhow::{Context as _, Result, anyhow};
use camino::{Utf8Path, Utf8PathBuf};
use std::{
    borrow::Cow,
    fs,
    io::{Read as _, Write as _},
    rc::Rc,
};

fn main() -> Result<()> {
    let mut app = App::try_new()?;
    app.run()
}

struct App {
    is_terminal: bool,
    lk: Option<Lk>,
    paths: Vec<Cow<'static, Utf8Path>>,
    path_index: usize,
}

impl App {
    fn try_new() -> Result<Self> {
        use std::io::IsTerminal as _;
        let is_terminal = std::io::stdout().is_terminal();

        let mut args = std::env::args();
        let _exe_name = args.next().context("How do we not have an exe name?")?;
        let mut paths = vec![];
        for arg in args {
            paths.push(Cow::Owned(Utf8PathBuf::from(arg)));
        }
        let paths = if paths.is_empty() {
            vec![Cow::Borrowed(Utf8Path::new("."))]
        } else {
            paths
        };

        Ok(App {
            is_terminal,
            lk: None,
            paths,
            path_index: 0,
        })
    }

    fn poll(&mut self) -> Result<Option<Rc<[u8]>>> {
        loop {
            if self.path_index >= self.paths.len() {
                return Ok(None);
            }
            let path = &self.paths[self.path_index];
            match self.lk.as_mut() {
                Some(lk) => {
                    if let Some(buf) = lk.poll()? {
                        return Ok(Some(buf));
                    } else {
                        self.lk = None;
                        self.path_index += 1;
                    }
                }
                None => {
                    self.lk = Some(Lk::try_new(path)?);
                }
            };
        }
    }

    fn run(&mut self) -> Result<()> {
        while let Some(buf) = self.poll()? {
            std::io::stdout().write_all(&buf)?;
        }
        Ok(())
    }
}

enum Lk {
    Cat(Cat),
    Ls(Ls),
}

impl Lk {
    fn try_new(path: &Utf8Path) -> Result<Self> {
        let metadata = fs::metadata(path)?;
        if metadata.is_dir() {
            Self::try_new_ls(path)
        } else {
            Self::try_new_cat(path)
        }
    }

    fn try_new_cat(path: &Utf8Path) -> Result<Self> {
        Ok(Self::Cat(Cat::try_new(path)?))
    }

    fn try_new_ls(path: &Utf8Path) -> Result<Self> {
        Ok(Self::Ls(Ls::try_new(path)?))
    }

    // I don't understand why, but using `Rc` here instead of `Cow` satisfies the borrow checker.
    fn poll(&mut self) -> Result<Option<Rc<[u8]>>> {
        match self {
            Lk::Cat(x) => x.poll(),
            Lk::Ls(x) => x.poll(),
        }
    }
}

struct Cat {
    f: fs::File,
    buf: Vec<u8>,
}

impl Cat {
    fn try_new(path: &Utf8Path) -> Result<Self> {
        let f = fs::File::open(path)?;
        let buf = vec![0u8; 4_096];
        Ok(Self { f, buf })
    }

    fn poll(&mut self) -> Result<Option<Rc<[u8]>>> {
        let bytes_read = self.f.read(&mut self.buf)?;
        let buf = &self.buf[0..bytes_read];
        if bytes_read == 0 {
            return Ok(None);
        }
        Ok(Some(buf.into()))
    }
}

struct Ls {
    names: Vec<String>,
    name_index: usize,
}

impl Ls {
    fn try_new(path: &Utf8Path) -> Result<Self> {
        let d = fs::read_dir(path)?;
        let mut names = vec![];
        for entry in d {
            let entry = entry?;
            names.push(
                entry
                    .file_name()
                    .into_string()
                    .map_err(|_| anyhow!("name is not UTF-8"))?,
            );
        }
        names.sort();
        Ok(Self {
            names,
            name_index: 0,
        })
    }

    fn poll(&mut self) -> Result<Option<Rc<[u8]>>> {
        if self.name_index >= self.names.len() {
            return Ok(None);
        }
        let name = &self.names[self.name_index];
        self.name_index += 1;
        Ok(Some(format!("{name}\n").into_bytes().into()))
    }
}

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

    #[test]
    fn test() -> Result<()> {
        let dir = tempfile::tempdir()?;
        let dir = dir.as_ref();
        fs::write(dir.join("alfa.txt"), "alfa\n")?;
        fs::write(dir.join("bravo.txt"), "bravo\n")?;

        for (input, expected) in [
            (vec![dir], b"alfa.txt\nbravo.txt\n".to_vec()),
            (
                vec![&dir.join("alfa.txt"), &dir.join("bravo.txt")],
                b"alfa\nbravo\n".to_vec(),
            ),
        ] {
            assert_eq!(lk(input)?, expected);
        }

        Ok(())
    }

    fn lk(paths: Vec<&Path>) -> Result<Vec<u8>> {
        let paths = paths
            .iter()
            .map(|p| Cow::Owned(Utf8PathBuf::from_path_buf(p.to_path_buf()).unwrap()))
            .collect();
        let mut app = App {
            is_terminal: false,
            lk: None,
            paths,
            path_index: 0,
        };
        let mut out = vec![];
        while let Some(buf) = app.poll()? {
            out.write_all(&buf)?;
        }
        Ok(out)
    }
}