//! /proc (stage 18): a window into the running process, made up on the fly.
//!
//! Linux's /proc has a directory for every process and lots of files. We
//! have just the corner that programs (and C libraries) lean on most:
//!
//!   /proc/self/exe     a symlink to the program that's running
//!   /proc/self/fd/     one symlink per open file descriptor:
//!   /proc/self/fd/N      to the file's path ("/tmp/notes.txt"), or a
//!                        name like "pipe:[4123]" or "socket:[1007]" for
//!                        things that have no path, or "/dev/pts/0"
//!
//! Nothing here is stored anywhere. Every lookup and every listing asks
//! the process that is *asking* (`current()`) for its fd table, so two
//! programs listing /proc/self/fd at once each see their own files. That
//! is the whole trick of "self".
//!
//! Following one of the fd links (open, stat) is special-cased in
//! syscall.rs, as on Linux: opening /proc/self/fd/N opens the file that's
//! behind fd N again (a pipe or terminal: the same open file, like dup),
//! even when the link's text isn't a real path ("pipe:[...]").
//!
//! Differences from Linux: /proc/self is a directory, not a symlink to
//! /proc/<pid>, and there are no /proc/<pid> directories yet.

use super::file::File;
use super::{DirEntry, FileSystem, Inode, InodeKind, StatFs};
use crate::errno::Errno;
use alloc::{format, string::String, sync::Arc, vec::Vec};

/// f_type of a mounted /proc (PROC_SUPER_MAGIC in <linux/magic.h>).
pub const PROC_MAGIC: u64 = 0x9fa0;

pub struct ProcFs;

impl FileSystem for ProcFs {
    fn name(&self) -> &'static str {
        "proc"
    }
    fn root(&self) -> Arc<dyn Inode> {
        Arc::new(Node::Root)
    }
    fn statfs(&self) -> StatFs {
        StatFs { magic: PROC_MAGIC, block_size: 4096, name_max: 255, ..Default::default() }
    }
}

/// The handful of things in our /proc.
#[derive(Clone, Copy, PartialEq, Eq, Debug)]
enum Node {
    Root,
    SelfDir,
    FdDir,
    Exe,
    /// /proc/self/fd/N
    Fd(u64),
}

/// The open files of the process asking, by fd number.
fn my_fds() -> Vec<(u64, Arc<dyn File>)> {
    crate::syscall::current().map(|p| p.open_files()).unwrap_or_default()
}

/// What /proc/self/fd/N says for this open file (readlink's answer).
pub fn fd_link(file: &Arc<dyn File>) -> String {
    if let Some(name) = file.proc_link() {
        return name; // devices and terminals know their /dev name
    }
    if let Some(path) = file.path() {
        return path;
    }
    let ino = super::file::anon_ino(file);
    if file.as_socket().is_some() || file.as_unix().is_some() {
        format!("socket:[{}]", ino)
    } else if file.as_epoll().is_some() {
        String::from("anon_inode:[eventpoll]")
    } else if file.stat().is_ok_and(|s| s.kind == super::file::KIND_PIPE) {
        format!("pipe:[{}]", ino)
    } else {
        String::from("anon_inode:[unknown]")
    }
}

/// Is `path` (absolute, normalized) /proc/self/fd/N? Then N.
pub fn fd_number(path: &str) -> Option<u64> {
    let n = path.strip_prefix("/proc/self/fd/")?;
    if n.is_empty() || !n.bytes().all(|b| b.is_ascii_digit()) || (n.len() > 1 && n.starts_with('0')) {
        return None;
    }
    n.parse().ok()
}

impl Inode for Node {
    fn kind(&self) -> InodeKind {
        match self {
            Node::Root | Node::SelfDir | Node::FdDir => InodeKind::Directory,
            Node::Exe | Node::Fd(_) => InodeKind::Symlink,
        }
    }
    fn size(&self) -> usize {
        match self {
            Node::Root => 1,
            Node::SelfDir => 2,
            Node::FdDir => my_fds().len(),
            _ => self.readlink().map(|t| t.len()).unwrap_or(0),
        }
    }
    fn mode(&self) -> u32 {
        match self {
            Node::Root | Node::SelfDir => 0o555,
            Node::FdDir => 0o500,
            Node::Exe => 0o777,
            Node::Fd(_) => 0o700, // lrwx------, like Linux
        }
    }
    fn ino(&self) -> Option<u64> {
        Some(match *self {
            Node::Root => 1,
            Node::SelfDir => 2,
            Node::FdDir => 3,
            Node::Exe => 4,
            Node::Fd(n) => 1000 + n,
        })
    }
    fn owner(&self) -> (u32, u32) {
        // Everything under /proc/self belongs to whoever is looking.
        match (self, crate::syscall::current()) {
            (Node::Root, _) | (_, Err(_)) => (0, 0),
            (_, Ok(p)) => {
                let c = *p.creds.lock();
                (c.euid, c.egid)
            }
        }
    }
    fn readlink(&self) -> Result<String, Errno> {
        match *self {
            Node::Exe => Ok(crate::syscall::current()?.exe.lock().clone()),
            Node::Fd(n) => {
                let file = crate::syscall::current()?.file(n).map_err(|_| Errno::ENOENT)?;
                Ok(fd_link(&file))
            }
            _ => Err(Errno::EINVAL),
        }
    }
    fn lookup(&self, name: &str) -> Result<Arc<dyn Inode>, Errno> {
        let node = match (self, name) {
            (Node::Root, "self") => Node::SelfDir,
            (Node::SelfDir, "fd") => Node::FdDir,
            (Node::SelfDir, "exe") => Node::Exe,
            (Node::FdDir, n) => {
                let fd = fd_number(&format!("/proc/self/fd/{}", n)).ok_or(Errno::ENOENT)?;
                crate::syscall::current()?.file(fd).map_err(|_| Errno::ENOENT)?;
                Node::Fd(fd)
            }
            (Node::Root | Node::SelfDir, _) => return Err(Errno::ENOENT),
            _ => return Err(Errno::ENOTDIR),
        };
        Ok(Arc::new(node))
    }
    fn list(&self) -> Result<Vec<DirEntry>, Errno> {
        let entry = |name: String, kind, ino| DirEntry { name, kind, ino };
        Ok(match self {
            Node::Root => alloc::vec![entry(String::from("self"), InodeKind::Directory, 2)],
            Node::SelfDir => alloc::vec![
                entry(String::from("exe"), InodeKind::Symlink, 4),
                entry(String::from("fd"), InodeKind::Directory, 3),
            ],
            Node::FdDir => my_fds().into_iter().map(|(fd, _)| entry(format!("{}", fd), InodeKind::Symlink, 1000 + fd)).collect(),
            _ => return Err(Errno::ENOTDIR),
        })
    }
    fn create(&self, _name: &str, _kind: InodeKind) -> Result<Arc<dyn Inode>, Errno> {
        Err(if self.kind() == InodeKind::Directory { Errno::EACCES } else { Errno::ENOTDIR })
    }
    fn remove(&self, _name: &str) -> Result<(), Errno> {
        Err(if self.kind() == InodeKind::Directory { Errno::EACCES } else { Errno::ENOTDIR })
    }
}