//! 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).

use super::{now_timespec, DirEntry, FileSystem, Inode, InodeKind, MODE_SET};
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()
    }
}

pub enum Node {
    File(Mutex<Vec<u8>>),
    Dir(Mutex<BTreeMap<String, Arc<RamInode>>>),
}

/// 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],
}

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] }) }
    }

    /// 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,
        }
    }

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

    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) };
        if name.is_empty() || name.contains('/') || name.len() > 255 {
            return Err(Errno::EINVAL);
        }
        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::Symlink | InodeKind::Device => return Err(Errno::EOPNOTSUPP), // ramfs has neither
        }));
        entries.insert(String::from(name), node.clone());
        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() }).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);
            }
        }
        entries.remove(name);
        Ok(())
    }
}