//! 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::sync::Arc;
use spin::Mutex;

/// What `stat`/`fstat` return. Our own small layout (Linux's `struct stat`
/// has ~15 fields we have no use for yet). `#[repr(C)]` because it's copied
/// to user space as is; user/ulib.rs has the same struct.
#[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;

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,
        };
        Stat { kind, size: inode.size() as u64, mode: inode.mode() as u64 }
    }
}

/// `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>;
    /// Fill `buf` with Linux `dirent64` records for the next directory
    /// entries; returns bytes used, 0 at the end.
    fn getdents(&self, _buf: &mut [u8]) -> Result<usize, Errno> {
        Err(Errno::ENOTDIR)
    }
}

/// 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,
}

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

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

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 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
            };
            rec[19..19 + entry.name.len()].copy_from_slice(entry.name.as_bytes());
            used += reclen;
            *index += 1;
        }
        Ok(used)
    }
}