//! ramfs: a file system that lives entirely on the heap (like Linux's ramfs /
//! tmpfs without the size limit). The simplest possible `FileSystem`: a tree
//! of nodes; files are a `Vec<u8>`, directories a sorted map name -> node.
//! Since stage 16 every node also remembers its permission bits (chmod)
//! and its three times (utimensat, and writes update the modification time).
//! Stage 17 adds symlinks, hard links (two directory entries holding the
//! same `Arc`), owners (chown) and renames that just move the `Arc`.

use super::{now_timespec, DirEntry, FileSystem, Inode, InodeKind, StatFs, MODE_SET, RAMFS_MAGIC};
use crate::errno::Errno;
use alloc::{collections::BTreeMap, string::String, sync::Arc, vec::Vec};
use spin::Mutex;

pub struct RamFs {
    root: Arc<RamInode>,
}

impl RamFs {
    pub fn new() -> Arc<Self> {
        Arc::new(RamFs { root: Arc::new(RamInode::new(Node::Dir(Mutex::new(BTreeMap::new())))) })
    }
}

impl FileSystem for RamFs {
    fn name(&self) -> &'static str {
        "ramfs"
    }
    fn root(&self) -> Arc<dyn Inode> {
        self.root.clone()
    }
    /// A ramfs can grow until memory runs out, so that's its "size" (Linux's
    /// ramfs says 0 for everything; numbers are friendlier to `df`).
    fn statfs(&self) -> StatFs {
        let (total, free) = crate::memory::frame_totals();
        StatFs {
            magic: RAMFS_MAGIC,
            block_size: 4096,
            blocks: total,
            free_blocks: free,
            avail_blocks: free,
            files: total,
            free_files: free,
            name_max: 255,
            read_only: false,
        }
    }
}

pub enum Node {
    File(Mutex<Vec<u8>>),
    Dir(Mutex<BTreeMap<String, Arc<RamInode>>>),
    /// A symlink's target (stage 17).
    Symlink(String),
    /// A Unix socket's name (stage 18): no contents.
    Socket,
}

/// What `stat` shows besides the contents (stage 16).
struct Meta {
    /// 0 = never chmod'ed (stat shows the default 0o644 / 0o755);
    /// otherwise the bits | MODE_SET.
    mode: u32,
    /// atime, mtime, ctime as [seconds, nanoseconds].
    times: [[i64; 2]; 3],
    /// Owner and group (stage 17: chown; new files get the creator's).
    uid: u32,
    gid: u32,
    /// Directory entries naming this node (hard links; directories count
    /// their subdirectories' ".." on top, see `nlink`).
    links: u32,
}

pub struct RamInode {
    node: Node,
    meta: Mutex<Meta>,
}

impl RamInode {
    fn new(node: Node) -> RamInode {
        let now = now_timespec();
        RamInode { node, meta: Mutex::new(Meta { mode: 0, times: [now; 3], uid: 0, gid: 0, links: 1 }) }
    }

    fn entries(&self) -> Result<&Mutex<BTreeMap<String, Arc<RamInode>>>, Errno> {
        match &self.node {
            Node::Dir(entries) => Ok(entries),
            _ => Err(Errno::ENOTDIR),
        }
    }

    /// A directory entry pointing at `node` went away.
    fn unlinked(node: &RamInode) {
        let mut meta = node.meta.lock();
        meta.links = meta.links.saturating_sub(1);
        meta.times[2] = now_timespec();
    }

    /// The status-change time (ctime) is now.
    fn changed(&self) {
        self.meta.lock().times[2] = now_timespec();
    }

    /// The contents changed: new modification and status-change time.
    fn touch(&self) {
        let now = now_timespec();
        let mut meta = self.meta.lock();
        meta.times[1] = now;
        meta.times[2] = now;
    }
}

impl Inode for RamInode {
    fn kind(&self) -> InodeKind {
        match &self.node {
            Node::File(_) => InodeKind::File,
            Node::Dir(_) => InodeKind::Directory,
            Node::Symlink(_) => InodeKind::Symlink,
            Node::Socket => InodeKind::Socket,
        }
    }

    fn size(&self) -> usize {
        match &self.node {
            Node::File(data) => data.lock().len(),
            Node::Dir(entries) => entries.lock().len(),
            Node::Symlink(target) => target.len(),
            Node::Socket => 0,
        }
    }

    fn readlink(&self) -> Result<String, Errno> {
        match &self.node {
            Node::Symlink(target) => Ok(target.clone()),
            _ => Err(Errno::EINVAL),
        }
    }

    fn read_at(&self, offset: usize, buf: &mut [u8]) -> Result<usize, Errno> {
        let Node::File(data) = &self.node else { return Err(Errno::EISDIR) };
        let data = data.lock();
        if offset >= data.len() {
            return Ok(0); // EOF
        }
        let n = buf.len().min(data.len() - offset);
        buf[..n].copy_from_slice(&data[offset..offset + n]);
        Ok(n)
    }

    fn write_at(&self, offset: usize, buf: &[u8]) -> Result<usize, Errno> {
        let Node::File(data) = &self.node else { return Err(Errno::EISDIR) };
        let mut data = data.lock();
        let end = offset.checked_add(buf.len()).ok_or(Errno::EINVAL)?;
        if data.len() < end {
            data.resize(end, 0); // writing past the end grows the file (gap = zeros)
        }
        data[offset..end].copy_from_slice(buf);
        drop(data);
        self.touch();
        Ok(buf.len())
    }

    fn lookup(&self, name: &str) -> Result<Arc<dyn Inode>, Errno> {
        let Node::Dir(entries) = &self.node else { return Err(Errno::ENOTDIR) };
        let node = entries.lock().get(name).cloned().ok_or(Errno::ENOENT)?;
        Ok(node)
    }

    fn create(&self, name: &str, kind: InodeKind) -> Result<Arc<dyn Inode>, Errno> {
        let Node::Dir(entries) = &self.node else { return Err(Errno::ENOTDIR) };
        check_name(name)?;
        let mut entries = entries.lock();
        if entries.contains_key(name) {
            return Err(Errno::EEXIST);
        }
        let node = Arc::new(RamInode::new(match kind {
            InodeKind::File => Node::File(Mutex::new(Vec::new())),
            InodeKind::Directory => Node::Dir(Mutex::new(BTreeMap::new())),
            InodeKind::Socket => Node::Socket,
            InodeKind::Symlink | InodeKind::Device => return Err(Errno::EOPNOTSUPP), // symlinks: `symlink`
        }));
        entries.insert(String::from(name), node.clone());
        drop(entries);
        self.touch();
        Ok(node)
    }

    fn list(&self) -> Result<Vec<DirEntry>, Errno> {
        let Node::Dir(entries) = &self.node else { return Err(Errno::ENOTDIR) };
        let entries = entries.lock();
        Ok(entries.iter().map(|(name, node)| DirEntry { name: name.clone(), kind: node.kind(), ino: node.ino_number() }).collect())
    }

    fn truncate(&self) -> Result<(), Errno> {
        let Node::File(data) = &self.node else { return Err(Errno::EISDIR) };
        data.lock().clear();
        self.touch();
        Ok(())
    }

    fn set_len(&self, len: usize) -> Result<(), Errno> {
        let Node::File(data) = &self.node else { return Err(Errno::EISDIR) };
        data.lock().resize(len, 0); // shorter: cut; longer: zeros
        self.touch();
        Ok(())
    }

    fn mode(&self) -> u32 {
        self.meta.lock().mode
    }

    fn set_mode(&self, mode: u32) -> Result<(), Errno> {
        let mut meta = self.meta.lock();
        meta.mode = (mode & 0o7777) | MODE_SET;
        meta.times[2] = now_timespec();
        Ok(())
    }

    fn times(&self) -> Option<[[i64; 2]; 3]> {
        Some(self.meta.lock().times)
    }

    fn set_times(&self, atime: Option<[i64; 2]>, mtime: Option<[i64; 2]>) -> Result<(), Errno> {
        let mut meta = self.meta.lock();
        if let Some(t) = atime {
            meta.times[0] = t;
        }
        if let Some(t) = mtime {
            meta.times[1] = t;
        }
        meta.times[2] = now_timespec();
        Ok(())
    }

    fn remove(&self, name: &str) -> Result<(), Errno> {
        let Node::Dir(entries) = &self.node else { return Err(Errno::ENOTDIR) };
        let mut entries = entries.lock();
        let node = entries.get(name).ok_or(Errno::ENOENT)?;
        if let Node::Dir(children) = &node.node {
            if !children.lock().is_empty() {
                return Err(Errno::ENOTEMPTY);
            }
        }
        if let Some(node) = entries.remove(name) {
            Self::unlinked(&node);
        }
        drop(entries);
        self.touch();
        Ok(())
    }

    // --- stage 17 ----------------------------------------------------------

    fn ino(&self) -> Option<u64> {
        Some(self.ino_number())
    }

    fn nlink(&self) -> Option<u64> {
        let links = self.meta.lock().links as u64;
        Some(match &self.node {
            // "." plus each subdirectory's ".." (like a disk file system)
            Node::Dir(entries) => links + 1 + entries.lock().values().filter(|n| matches!(n.node, Node::Dir(_))).count() as u64,
            _ => links,
        })
    }

    fn owner(&self) -> (u32, u32) {
        let meta = self.meta.lock();
        (meta.uid, meta.gid)
    }

    fn set_owner(&self, uid: Option<u32>, gid: Option<u32>) -> Result<(), Errno> {
        let mut meta = self.meta.lock();
        if let Some(uid) = uid {
            meta.uid = uid;
        }
        if let Some(gid) = gid {
            meta.gid = gid;
        }
        meta.times[2] = now_timespec();
        Ok(())
    }

    fn symlink(&self, name: &str, target: &str) -> Result<Arc<dyn Inode>, Errno> {
        check_name(name)?;
        let mut entries = self.entries()?.lock();
        if entries.contains_key(name) {
            return Err(Errno::EEXIST);
        }
        let node = Arc::new(RamInode::new(Node::Symlink(String::from(target))));
        entries.insert(String::from(name), node.clone());
        drop(entries);
        self.touch();
        Ok(node)
    }

    fn link(&self, name: &str, target: &Arc<dyn Inode>) -> Result<(), Errno> {
        check_name(name)?;
        let node = super::downcast::<RamInode>(target).ok_or(Errno::EXDEV)?;
        let mut entries = self.entries()?.lock();
        if entries.contains_key(name) {
            return Err(Errno::EEXIST);
        }
        {
            let mut meta = node.meta.lock();
            meta.links += 1;
            meta.times[2] = now_timespec();
        }
        entries.insert(String::from(name), node);
        drop(entries);
        self.touch();
        Ok(())
    }

    /// Move the `Arc` from one map to the other. With two directories, the
    /// two locks are taken in address order, so two renames going opposite
    /// ways can't each hold one lock and wait for the other.
    fn rename_entry(&self, old: &str, new_dir: &Arc<dyn Inode>, new: &str) -> Result<(), Errno> {
        check_name(new)?;
        let target = super::downcast::<RamInode>(new_dir).ok_or(Errno::EXDEV)?;
        let same = core::ptr::eq(self, &*target);
        let (src, dst) = (self.entries()?, target.entries()?);
        let replaced;
        if same {
            let mut map = src.lock();
            let node = map.remove(old).ok_or(Errno::ENOENT)?;
            replaced = map.insert(String::from(new), node);
        } else {
            let self_first = (self as *const RamInode) < Arc::as_ptr(&target);
            let (mut a, mut b) = if self_first {
                let a = src.lock();
                (a, dst.lock())
            } else {
                let b = dst.lock();
                (src.lock(), b)
            };
            let node = a.remove(old).ok_or(Errno::ENOENT)?;
            node.changed();
            replaced = b.insert(String::from(new), node);
        }
        if let Some(gone) = replaced {
            Self::unlinked(&gone);
        }
        self.touch();
        target.touch();
        Ok(())
    }

    fn as_any(self: Arc<Self>) -> Option<Arc<dyn core::any::Any + Send + Sync>> {
        Some(self)
    }
}

impl RamInode {
    /// Inode number: the node's address (unique while it exists).
    fn ino_number(&self) -> u64 {
        self as *const RamInode as u64
    }
}

fn check_name(name: &str) -> Result<(), Errno> {
    match name {
        "" | "." | ".." => Err(Errno::EINVAL),
        _ if name.contains('/') => Err(Errno::EINVAL),
        _ if name.len() > 255 => Err(Errno::ENAMETOOLONG),
        _ => Ok(()),
    }
}