//! Open files: what a file descriptor refers to.
//!
//! `File` is the interface the read/write/lseek/fstat/getdents syscalls use.
//! It's separate from `Inode` because the same inode can be open several
//! times with different offsets, and because some files aren't in any file
//! system at all (the console, pipes). Linux calls this `struct file` +
//! `file_operations`.

use super::{Inode, InodeKind};
use crate::errno::Errno;
use alloc::{string::String, sync::Arc};
use spin::Mutex;

/// What a `File` or inode tells the kernel about itself. Our own small
/// layout; the syscalls turn it into Linux's `struct stat` (below), which
/// is what user programs get.
#[repr(C)]
#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
pub struct Stat {
    pub kind: u64, // one of the KIND_* constants
    pub size: u64, // bytes (files), entries (directories)
    pub mode: u64, // permission bits like 0o644 (0 = the file system has none)
}

pub const KIND_FILE: u64 = 1;
pub const KIND_DIR: u64 = 2;
pub const KIND_CONSOLE: u64 = 3;
pub const KIND_PIPE: u64 = 4;
pub const KIND_SYMLINK: u64 = 5;
pub const KIND_SOCKET: u64 = 6;
pub const KIND_DEVICE: u64 = 7; // /dev/null, /dev/fb0, ...

impl Stat {
    pub fn of(inode: &dyn Inode) -> Stat {
        let kind = match inode.kind() {
            InodeKind::File => KIND_FILE,
            InodeKind::Directory => KIND_DIR,
            InodeKind::Symlink => KIND_SYMLINK,
            InodeKind::Device => KIND_DEVICE,
        };
        Stat { kind, size: inode.size() as u64, mode: inode.mode() as u64 }
    }
}

/// Linux's `struct stat` on x86_64 (144 bytes), what stat/fstat/lstat/
/// newfstatat write to user memory since stage 15.1: a C library reads
/// these exact offsets. We fill in what we know and make up the rest the
/// way a simple Linux file system would.
#[repr(C)]
#[derive(Debug, Clone, Copy, Default)]
pub struct LinuxStat {
    pub st_dev: u64,     // which file system (we say 1 for all)
    pub st_ino: u64,     // file number: we hash the path (see below)
    pub st_nlink: u64,   // hard links: 1 (directories 2)
    pub st_mode: u32,    // S_IF* type bits | permission bits
    pub st_uid: u32,     // owner: everything belongs to root (0)
    pub st_gid: u32,
    _pad0: u32,
    pub st_rdev: u64,    // device number for device files (0 here)
    pub st_size: i64,    // bytes
    pub st_blksize: i64, // good I/O size: 4096
    pub st_blocks: i64,  // 512-byte blocks used
    pub st_atime: [i64; 2], // last access (seconds, nanoseconds): we don't keep times yet
    pub st_mtime: [i64; 2], // last change of the contents
    pub st_ctime: [i64; 2], // last change of the inode
    _unused: [i64; 3],
}

/// File type bits of `st_mode` (octal, as in <sys/stat.h>).
pub const S_IFIFO: u32 = 0o010000;
pub const S_IFCHR: u32 = 0o020000;
pub const S_IFDIR: u32 = 0o040000;
pub const S_IFREG: u32 = 0o100000;
pub const S_IFLNK: u32 = 0o120000;
pub const S_IFSOCK: u32 = 0o140000;

impl LinuxStat {
    /// `ino` should tell files apart: callers pass a hash of the path (or of
    /// the open file's address for pipes and sockets). Programs use
    /// (st_dev, st_ino) to spot "same file" (`cp a a`), nothing more.
    pub fn new(stat: &Stat, ino: u64) -> LinuxStat {
        let (ty, default_perm) = match stat.kind {
            KIND_DIR => (S_IFDIR, 0o755),
            KIND_SYMLINK => (S_IFLNK, 0o777),
            KIND_CONSOLE | KIND_DEVICE => (S_IFCHR, 0o666),
            KIND_PIPE => (S_IFIFO, 0o600),
            KIND_SOCKET => (S_IFSOCK, 0o777),
            _ => (S_IFREG, 0o644),
        };
        let perm = if stat.mode == 0 { default_perm } else { stat.mode as u32 & 0o7777 };
        LinuxStat {
            st_dev: 1,
            st_ino: ino.max(1), // 0 means "no inode" to some programs
            st_nlink: if stat.kind == KIND_DIR { 2 } else { 1 },
            st_mode: ty | perm,
            st_size: stat.size as i64,
            st_blksize: 4096,
            st_blocks: stat.size.div_ceil(512) as i64,
            ..Default::default()
        }
    }
}

/// A 64-bit FNV-1a hash: our stand-in for inode numbers (from the path).
pub fn ino_hash(bytes: &[u8]) -> u64 {
    bytes.iter().fold(0xcbf2_9ce4_8422_2325, |h, &b| (h ^ b as u64).wrapping_mul(0x100_0000_01b3))
}

/// `whence` values for `seek`, as in Linux's lseek.
pub const SEEK_SET: u64 = 0;
pub const SEEK_CUR: u64 = 1;
pub const SEEK_END: u64 = 2;

pub trait File: Send + Sync {
    /// Read up to `buf.len()` bytes; 0 means end of file.
    fn read(&self, buf: &mut [u8]) -> Result<usize, Errno>;
    /// Write bytes; returns how many were written.
    fn write(&self, buf: &[u8]) -> Result<usize, Errno>;
    /// Move the offset; returns the new offset. Consoles and pipes can't.
    fn seek(&self, _offset: i64, _whence: u64) -> Result<u64, Errno> {
        Err(Errno::ESPIPE)
    }
    fn stat(&self) -> Result<Stat, Errno>;
    /// The absolute path this was opened by, if it was (files and
    /// directories): for openat & co. relative to a directory fd, fchdir,
    /// and fstat's inode number.
    fn path(&self) -> Option<String> {
        None
    }
    /// Fill `buf` with Linux `dirent64` records for the next directory
    /// entries; returns bytes used, 0 at the end.
    /// Socket syscalls (connect, accept, ...) need the socket behind an fd;
    /// everything that isn't a socket says "no" (ENOTSOCK).
    fn as_socket(&self) -> Option<&crate::net::socket::Socket> {
        None
    }
    fn getdents(&self, _buf: &mut [u8]) -> Result<usize, Errno> {
        Err(Errno::ENOTDIR)
    }
    /// The framebuffer ioctls need /dev/fb0 behind the fd.
    fn as_framebuffer(&self) -> Option<&crate::dev::Framebuffer> {
        None
    }
    /// mmap of a file needs the regular file's inode behind the fd.
    fn inode(&self) -> Option<Arc<dyn Inode>> {
        None
    }

    // --- stage 15: what event loops and a C library need -----------------

    /// Readiness right now, as poll(2) bits (crate::poll::POLLIN...): can a
    /// read / a write go ahead without blocking? Files that never block
    /// (regular files, /dev/null) are always ready.
    fn poll(&self) -> u16 {
        crate::poll::POLLIN | crate::poll::POLLOUT
    }
    /// O_NONBLOCK (fcntl F_SETFL, ioctl FIONBIO): instead of blocking,
    /// fail with EAGAIN. It belongs to the open file, so dup'ed and
    /// inherited fds share it. Only files that can block keep it.
    fn set_nonblocking(&self, _on: bool) {}
    fn nonblocking(&self) -> bool {
        false
    }
    /// Device-specific ioctls (ttys: termios, window size...).
    fn ioctl(&self, _cmd: u64, _arg: u64) -> Result<u64, Errno> {
        Err(Errno::ENOTTY)
    }
    /// pread/pwrite: at an explicit offset, leaving the file offset alone.
    fn read_at(&self, _buf: &mut [u8], _offset: u64) -> Result<usize, Errno> {
        Err(Errno::ESPIPE)
    }
    fn write_at(&self, _buf: &[u8], _offset: u64) -> Result<usize, Errno> {
        Err(Errno::ESPIPE)
    }
    /// epoll_ctl / epoll_wait need the epoll instance behind the fd.
    fn as_epoll(&self) -> Option<&crate::poll::Epoll> {
        None
    }
    /// The terminal behind the fd, if it is one (tty.rs).
    fn as_tty(&self) -> Option<&crate::tty::TtyFile> {
        None
    }
}

/// A file or directory opened through the VFS: inode + current position.
/// For directories the "position" is the index of the next entry to list.
pub struct InodeFile {
    inode: Arc<dyn Inode>,
    offset: Mutex<usize>,
    /// O_APPEND: every write goes to the current end of the file (`>>`).
    append: bool,
    /// The absolute path it was opened by (see `File::path`).
    path: String,
}

impl InodeFile {
    pub fn open(inode: Arc<dyn Inode>, path: String) -> Result<Self, Errno> {
        Ok(InodeFile { inode, offset: Mutex::new(0), append: false, path })
    }

    pub fn open_append(inode: Arc<dyn Inode>, path: String) -> Result<Self, Errno> {
        Ok(InodeFile { inode, offset: Mutex::new(0), append: true, path })
    }
}

impl File for InodeFile {
    fn read(&self, buf: &mut [u8]) -> Result<usize, Errno> {
        let mut offset = self.offset.lock();
        let n = self.inode.read_at(*offset, buf)?;
        *offset += n;
        Ok(n)
    }

    fn write(&self, buf: &[u8]) -> Result<usize, Errno> {
        let mut offset = self.offset.lock();
        if self.append {
            *offset = self.inode.size();
        }
        let n = self.inode.write_at(*offset, buf)?;
        *offset += n;
        Ok(n)
    }

    fn seek(&self, offset: i64, whence: u64) -> Result<u64, Errno> {
        if self.inode.kind() == InodeKind::Directory {
            return if offset == 0 && whence == SEEK_SET { *self.offset.lock() = 0; Ok(0) } else { Err(Errno::EINVAL) };
        }
        let mut current = self.offset.lock();
        let base = match whence {
            SEEK_SET => 0,
            SEEK_CUR => *current as i64,
            SEEK_END => self.inode.size() as i64,
            _ => return Err(Errno::EINVAL),
        };
        let new = base.checked_add(offset).filter(|&n| n >= 0).ok_or(Errno::EINVAL)?;
        *current = new as usize;
        Ok(new as u64)
    }

    fn stat(&self) -> Result<Stat, Errno> {
        Ok(Stat::of(&*self.inode))
    }

    fn path(&self) -> Option<String> {
        Some(self.path.clone())
    }

    fn read_at(&self, buf: &mut [u8], offset: u64) -> Result<usize, Errno> {
        match self.inode.kind() {
            InodeKind::Directory => Err(Errno::EISDIR),
            _ => self.inode.read_at(offset as usize, buf),
        }
    }

    fn write_at(&self, buf: &[u8], offset: u64) -> Result<usize, Errno> {
        match self.inode.kind() {
            InodeKind::Directory => Err(Errno::EISDIR),
            _ => self.inode.write_at(offset as usize, buf),
        }
    }

    fn inode(&self) -> Option<Arc<dyn Inode>> {
        (self.inode.kind() == InodeKind::File).then(|| self.inode.clone())
    }

    fn getdents(&self, buf: &mut [u8]) -> Result<usize, Errno> {
        // Re-list the directory each call and skip what we've returned
        // already (simple; a real fs would keep a cursor into the directory).
        let entries = self.inode.list()?;
        let mut index = self.offset.lock();
        let mut used = 0;
        for entry in entries.iter().skip(*index) {
            // struct linux_dirent64 { u64 ino; i64 off; u16 reclen; u8 type; char name[]; }
            // padded to a multiple of 8 bytes.
            let reclen = (19 + entry.name.len() + 1).next_multiple_of(8);
            if used + reclen > buf.len() {
                if used == 0 {
                    return Err(Errno::EINVAL); // buffer too small for even one entry
                }
                break;
            }
            let rec = &mut buf[used..used + reclen];
            rec.fill(0);
            rec[0..8].copy_from_slice(&(*index as u64 + 1).to_le_bytes()); // fake inode number
            rec[8..16].copy_from_slice(&(*index as i64 + 1).to_le_bytes()); // offset of next
            rec[16..18].copy_from_slice(&(reclen as u16).to_le_bytes());
            rec[18] = match entry.kind {
                InodeKind::Directory => 4, // DT_DIR
                InodeKind::File => 8,      // DT_REG
                InodeKind::Symlink => 10,  // DT_LNK
                InodeKind::Device => 2,    // DT_CHR (a "character device")
            };
            rec[19..19 + entry.name.len()].copy_from_slice(entry.name.as_bytes());
            used += reclen;
            *index += 1;
        }
        Ok(used)
    }
}