// ---------------------------------------------------------------------------
// VFS — "everything is a file".
//
// A tiny hierarchical filesystem namespace: directories, in-memory regular
// files (ramfs, writable via the shell), and character devices under /dev.
// Device reads re-generate their content on every access, so
// `cat /dev/rtc` shows the *current* time and `cat /dev/mem` hexdumps live
// physical memory.  Writes to /dev/console print to the screen, writes to
// /dev/null disappear.
//
// Nodes live in a flat table; handles are stable indices (removal leaves a
// tombstone).  Single-threaded, interrupt-safe because no ISR touches the
// table.  A real disk filesystem can mount under this same namespace later.
// ---------------------------------------------------------------------------

extern crate alloc;

use crate::gop;
use crate::memory;
use crate::pci;
use crate::rtc;
use crate::serial;
use alloc::boxed::Box;
use alloc::string::String;
use alloc::vec::Vec;

/// /dev/mem covers the low physical-memory window (identity-mapped RAM).
pub const MEM_WINDOW: u64 = 64 * 1024;

#[derive(Clone, Copy, PartialEq)]
pub enum FileType {
    Regular,
    Char,
    Dir,
}

#[derive(Clone, Copy)]
enum CharDevice {
    Mem,
    Rtc,
    Pci,
    Version,
    Meminfo,
    Klog,
    Null,
    Zero,
    Console,
    Serial,
    Mouse,
}

enum NodeData {
    Regular(Vec<u8>),
    Dir(Vec<usize>),
    Char(CharDevice),
    Disk(DiskInfo),
}

/// A node backed by the FAT32 volume (`/disk` mount).  `parent` is the
/// directory cluster holding the entry (used to re-locate it on every
/// op, so it stays correct as the volume changes), `cluster` the entry's
/// own first cluster.
struct DiskInfo {
    vol: usize, // index into fat::VOLS (which FAT32 volume this lives on)
    parent: u32,
    name: String,
    is_dir: bool,
    size: u64,
    cluster: u32,
}

struct Node {
    name: String,
    kind: FileType,
    parent: Option<usize>,
    data: NodeData,
    removed: bool,
}

/// Node table; root is always handle 0.  Boxed so the static stays a raw
/// pointer (matches the rest of the crate's style).
static mut NODES: *mut Vec<Node> = core::ptr::null_mut();

fn nodes() -> &'static mut Vec<Node> {
    unsafe { &mut *NODES }
}

fn add(name: &str, parent: Option<usize>, kind: FileType, data: NodeData) -> usize {
    let h = nodes().len();
    nodes().push(Node {
        name: name.into(),
        kind,
        parent,
        data,
        removed: false,
    });
    if let Some(p) = parent
        && let NodeData::Dir(v) = &mut nodes()[p].data
    {
        v.push(h);
    }
    h
}

/// Build the tree and report (dirs, files).
pub fn init() -> (usize, usize) {
    unsafe {
        NODES = Box::into_raw(Box::new(Vec::new()));
    }
    let root = add("/", None, FileType::Dir, NodeData::Dir(Vec::new()));
    let dev = add("dev", Some(root), FileType::Dir, NodeData::Dir(Vec::new()));
    let _tmp = add("tmp", Some(root), FileType::Dir, NodeData::Dir(Vec::new()));
    let etc = add("etc", Some(root), FileType::Dir, NodeData::Dir(Vec::new()));

    add(
        "mem",
        Some(dev),
        FileType::Char,
        NodeData::Char(CharDevice::Mem),
    );
    add(
        "rtc",
        Some(dev),
        FileType::Char,
        NodeData::Char(CharDevice::Rtc),
    );
    add(
        "pci",
        Some(dev),
        FileType::Char,
        NodeData::Char(CharDevice::Pci),
    );
    add(
        "version",
        Some(dev),
        FileType::Char,
        NodeData::Char(CharDevice::Version),
    );
    add(
        "meminfo",
        Some(dev),
        FileType::Char,
        NodeData::Char(CharDevice::Meminfo),
    );
    add(
        "klog",
        Some(dev),
        FileType::Char,
        NodeData::Char(CharDevice::Klog),
    );
    add(
        "null",
        Some(dev),
        FileType::Char,
        NodeData::Char(CharDevice::Null),
    );
    add(
        "zero",
        Some(dev),
        FileType::Char,
        NodeData::Char(CharDevice::Zero),
    );
    add(
        "console",
        Some(dev),
        FileType::Char,
        NodeData::Char(CharDevice::Console),
    );
    add(
        "serial",
        Some(dev),
        FileType::Char,
        NodeData::Char(CharDevice::Serial),
    );
    add(
        "mouse",
        Some(dev),
        FileType::Char,
        NodeData::Char(CharDevice::Mouse),
    );

    add(
        "motd",
        Some(etc),
        FileType::Regular,
        NodeData::Regular(
            b"Welcome to rs-dos!\nEverything is a file. Try: cat /dev/rtc, cat /dev/pci, cat /dev/mem\n"
                .to_vec(),
        ),
    );

    // Mount every FAT32 volume under /disk/<UID>/ (UID = label-serial).
    // `rescan_disks()` also keeps this table current after hotplug events.
    let _ = rescan_disks();

    let dirs = nodes().iter().filter(|n| n.kind == FileType::Dir).count();
    let files = nodes().len() - dirs;
    (dirs, files)
}

/// (Re)sync the /disk mount table with the FAT volumes: create the /disk
/// dir lazily, ensure a mount node for every mounted volume (refreshing
/// its backing info), and tombstone nodes whose volume was unmounted.
/// Returns (added, removed) node ops.
pub fn rescan_disks() -> (usize, usize) {
    let mut added = 0usize;
    let mut removed = 0usize;
    let disk_dir = match lookup(0, "disk") {
        Some(h) => h,
        None => add("disk", Some(0), FileType::Dir, NodeData::Dir(Vec::new())),
    };
    for vol in 0..crate::fat::volume_count() {
        if !crate::fat::is_mounted(vol) {
            continue;
        }
        let uid = String::from(crate::fat::uid(vol));
        let root_cl = crate::fat::root_cluster(vol);
        match lookup(disk_dir, &uid) {
            Some(h) => {
                // Refresh the backing info in case the volume's slot moved.
                if let NodeData::Disk(i) = &mut nodes()[h].data {
                    i.vol = vol;
                    i.parent = root_cl;
                    i.cluster = root_cl;
                }
                nodes()[h].removed = false;
            }
            None => {
                add(
                    &uid,
                    Some(disk_dir),
                    FileType::Dir,
                    NodeData::Disk(DiskInfo {
                        vol,
                        parent: root_cl,
                        name: String::new(),
                        is_dir: true,
                        size: 0,
                        cluster: root_cl,
                    }),
                );
                added += 1;
            }
        }
    }
    if let NodeData::Dir(v) = &nodes()[disk_dir].data {
        for &c in v.clone().iter() {
            if nodes()[c].removed {
                continue;
            }
            if let NodeData::Disk(i) = &nodes()[c].data
                && !crate::fat::is_mounted(i.vol)
            {
                nodes()[c].removed = true;
                removed += 1;
            }
        }
    }
    (added, removed)
}

fn lookup(dir: usize, name: &str) -> Option<usize> {
    let children = match &nodes()[dir].data {
        NodeData::Dir(v) => v.clone(),
        _ => return None,
    };
    for c in children {
        let n = &nodes()[c];
        if !n.removed && n.name == name {
            return Some(c);
        }
    }
    None
}

/// Resolve an absolute path (supports `.` and `..`).  Paths under
/// `/disk` walk the mounted FAT32 volume through ephemeral Disk nodes.
pub fn resolve(path: &str) -> Option<usize> {
    if nodes().is_empty() {
        return None;
    }
    let mut cur = 0usize; // root
    // (vol, cluster, parent, name) of directories descended into under /disk.
    let mut dstack: Vec<(usize, u32, u32, String)> = Vec::new();
    for seg in path.split('/') {
        match seg {
            "" | "." => {}
            ".." => {
                if let Some((vol, cl, p, n)) = dstack.pop() {
                    cur = materialize_disk(DiskInfo {
                        vol,
                        parent: p,
                        name: n,
                        is_dir: true,
                        size: 0,
                        cluster: cl,
                    });
                } else if let Some(p) = nodes()[cur].parent {
                    cur = p;
                }
            }
            name => {
                let disk_ctx: Option<(usize, u32)> = match &nodes()[cur].data {
                    NodeData::Disk(info) => Some((info.vol, info.cluster)),
                    _ => None,
                };
                if let Some((vol, dir_cluster)) = disk_ctx {
                    let e = crate::fat::find(vol, dir_cluster, name).ok()?;
                    let d = DiskInfo {
                        vol,
                        parent: dir_cluster,
                        name: e.name.clone(),
                        is_dir: e.is_dir,
                        size: e.size,
                        cluster: e.cluster,
                    };
                    if e.is_dir {
                        dstack.push((vol, e.cluster, dir_cluster, e.name));
                    }
                    cur = materialize_disk(d);
                } else {
                    cur = lookup(cur, name)?;
                }
            }
        }
    }
    Some(cur)
}

pub fn kind(h: usize) -> FileType {
    nodes()[h].kind
}

pub fn size(h: usize) -> u64 {
    match &nodes()[h].data {
        NodeData::Regular(v) => v.len() as u64,
        NodeData::Disk(i) => i.size,
        _ => 0,
    }
}

/// Entries of a directory as (name, kind, size) — tombstoned nodes
/// skipped; disk directories are listed fresh from the FAT volume.
pub fn children(h: usize) -> Vec<(String, FileType, u64)> {
    if let NodeData::Disk(i) = &nodes()[h].data {
        let cl = i.cluster;
        return crate::fat::list_dir(i.vol, cl)
            .map(|es| {
                es.into_iter()
                    .map(|e| {
                        (
                            e.name,
                            if e.is_dir {
                                FileType::Dir
                            } else {
                                FileType::Regular
                            },
                            e.size,
                        )
                    })
                    .collect()
            })
            .unwrap_or_default();
    }
    let kids = match &nodes()[h].data {
        NodeData::Dir(v) => v.clone(),
        _ => return Vec::new(),
    };
    let mut out = Vec::new();
    for c in kids {
        let n = &nodes()[c];
        if n.removed {
            continue;
        }
        let sz = match &n.data {
            NodeData::Regular(v) => v.len() as u64,
            _ => 0,
        };
        out.push((n.name.clone(), n.kind, sz));
    }
    out
}

pub fn read(h: usize, buf: &mut [u8], pos: u64) -> Result<usize, ()> {
    if buf.is_empty() {
        return Ok(0);
    }
    match &mut nodes()[h].data {
        NodeData::Regular(v) => {
            let p = pos as usize;
            if p >= v.len() {
                return Ok(0);
            }
            let n = buf.len().min(v.len() - p);
            buf[..n].copy_from_slice(&v[p..p + n]);
            Ok(n)
        }
        NodeData::Char(CharDevice::Mem) => {
            // /dev/mem is a raw byte window: file offset = physical address.
            let p = pos as usize;
            if p >= MEM_WINDOW as usize {
                return Ok(0);
            }
            let n = buf.len().min(MEM_WINDOW as usize - p);
            for i in 0..n {
                buf[i] = unsafe { core::ptr::read_volatile((p + i) as *const u8) };
            }
            Ok(n)
        }
        NodeData::Char(CharDevice::Zero) => {
            buf.fill(0);
            Ok(buf.len())
        }
        NodeData::Char(dev) => {
            let text = dev_text(*dev, pos);
            let bytes = text.as_bytes();
            let p = pos as usize;
            if p >= bytes.len() {
                return Ok(0);
            }
            let n = buf.len().min(bytes.len() - p);
            buf[..n].copy_from_slice(&bytes[p..p + n]);
            Ok(n)
        }
        NodeData::Disk(i) => crate::fat::read_file(i.vol, i.parent, &i.name, buf, pos),
        NodeData::Dir(_) => Err(()),
    }
}

pub fn write(h: usize, buf: &[u8], pos: u64) -> Result<usize, ()> {
    match &mut nodes()[h].data {
        NodeData::Regular(v) => {
            let p = pos as usize;
            if p + buf.len() > v.len() {
                v.resize(p + buf.len(), 0);
            }
            v[p..p + buf.len()].copy_from_slice(buf);
            Ok(buf.len())
        }
        NodeData::Char(dev) => match dev {
            CharDevice::Null | CharDevice::Zero => Ok(buf.len()),
            CharDevice::Console => {
                for &b in buf {
                    if b == b'\n' {
                        gop::put_char('\n');
                        serial::put_byte(b'\n');
                    } else if b.is_ascii() {
                        gop::put_char(b as char);
                        serial::put_byte(b);
                    }
                }
                Ok(buf.len())
            }
            CharDevice::Serial => {
                for &b in buf {
                    serial::put_byte(b);
                }
                Ok(buf.len())
            }
            _ => Err(()), // read-only devices
        },
        NodeData::Disk(i) => crate::fat::write_file(i.vol, i.parent, &i.name, buf, pos),
        NodeData::Dir(_) => Err(()),
    }
}

/// Handle of `crash.log` on the crash-log volume, if it exists.  The
/// crash-log volume is the one labeled CRASHLOG (a dedicated stick that
/// survives reboot writes), else the first mounted volume.  Resolves the
/// FAT path (`/disk/<uid>/crash.log`) because Disk nodes hold no cached
/// children.
pub fn read_crash_log() -> Option<usize> {
    let disk_dir = lookup(0, "disk")?;
    let children = match &nodes()[disk_dir].data {
        NodeData::Dir(v) => v.clone(),
        _ => return None,
    };
    // Prefer a disk node whose name starts with "CRASHLOG-".
    let mut first = None;
    for &c in &children {
        if nodes()[c].removed || !matches!(nodes()[c].data, NodeData::Disk(_)) {
            continue;
        }
        let name = &nodes()[c].name;
        let label = name.split_once('-').map(|(l, _)| l).unwrap_or(name);
        if label.eq_ignore_ascii_case("CRASHLOG") {
            return resolve(&alloc::format!("/disk/{}/crash.log", name));
        }
        if first.is_none() {
            first = Some(c);
        }
    }
    first.and_then(|disk| {
        let name = nodes()[disk].name.clone();
        resolve(&alloc::format!("/disk/{}/crash.log", name))
    })
}

/// Persist a crash report to the mounted FAT volume so it survives a reboot
/// (real hardware has no serial console).  Best-effort: any failure is
/// ignored — the caller (fault handler) must never fault here; the
/// in-dump ISR guard catches that case and halts.
///
/// Goes straight at the FAT layer (the crash-log volume + its root
/// cluster) rather than through the fs node table: the fault may have
/// interrupted the fs mid-operation, and a stale DiskInfo/ACTIVE_VOL
/// would make node-based writes fail with "no volume at slot N".  The
/// crash-log volume is the one labeled CRASHLOG (a dedicated stick that
/// survives reboot writes), else slot 0 (first mounted) as before.
pub fn write_crash_log(lines: &[&str]) {
    // Assemble the report on the stack (heap-free — a corrupted heap is
    // exactly the situation that produces crash reports).
    let mut text = [0u8; 640];
    let mut n = 0usize;
    for line in lines {
        for &b in line.as_bytes() {
            if n < text.len() {
                text[n] = b;
                n += 1;
            }
        }
        if n < text.len() {
            text[n] = b'\n';
            n += 1;
        }
    }
    let Some(vol) = crate::fat::crash_vol() else {
        crate::serial::write_raw("crashlog: no volume\r\n");
        return;
    };
    // Trace the target so a test can verify the right stick got the log.
    let label = crate::fat::volume_label(vol).unwrap_or("?");
    let mut tb = [0u8; 48];
    let tn = {
        let mut w = crate::strbuf::FmtBuf::new(&mut tb);
        let _ = core::fmt::write(&mut w, core::format_args!("crashlog: vol {} ({})\r\n", vol, label));
        w.len()
    };
    crate::serial::write_raw(core::str::from_utf8(&tb[..tn]).unwrap_or("?"));
    let root = crate::fat::root_cluster(vol);
    if crate::fat::create(vol, root, "crash.log", false).is_err()
        && crate::fat::find(vol, root, "crash.log").is_err()
    {
        crate::serial::write_raw("crashlog: create failed\r\n");
        return;
    }
    if crate::fat::set_len(vol, root, "crash.log", 0).is_err() {
        crate::serial::write_raw("crashlog: set_len failed\r\n");
        return;
    }
    match crate::fat::write_file(vol, root, "crash.log", &text[..n], 0) {
        Ok(_) => crate::serial::write_raw("crashlog: wrote\r\n"),
        Err(_) => crate::serial::write_raw("crashlog: write failed\r\n"),
    }
}

/// Truncate/resize a regular file.
pub fn set_len(h: usize, len: u64) -> Result<(), ()> {    match &mut nodes()[h].data {
        NodeData::Regular(v) => {
            v.resize(len as usize, 0);
            Ok(())
        }
        NodeData::Disk(i) => crate::fat::set_len(i.vol, i.parent, &i.name, len),
        _ => Err(()),
    }
}

/// Create a Regular or Dir child.  Fails if the name exists or is invalid.
pub fn create(parent: usize, name: &str, kind: FileType) -> Result<usize, ()> {
    if name.is_empty() || name.contains('/') || kind == FileType::Char {
        return Err(());
    }
    // Disk directory: create through the FAT volume.
    let disk_parent: Option<(usize, u32)> = match &nodes()[parent].data {
        NodeData::Disk(i) => {
            if !i.is_dir {
                return Err(());
            }
            Some((i.vol, i.cluster))
        }
        _ => None,
    };
    if let Some((vol, p)) = disk_parent {
        if crate::fat::find(vol, p, name).is_ok() {
            return Err(());
        }
        crate::fat::create(vol, p, name, kind == FileType::Dir)?;
        let e = crate::fat::find(vol, p, name)?;
        return Ok(materialize_disk(DiskInfo {
            vol,
            parent: p,
            name: e.name,
            is_dir: e.is_dir,
            size: e.size,
            cluster: e.cluster,
        }));
    }
    if lookup(parent, name).is_some() {
        return Err(());
    }
    let data = match kind {
        FileType::Regular => NodeData::Regular(Vec::new()),
        FileType::Dir => NodeData::Dir(Vec::new()),
        FileType::Char => return Err(()),
    };
    Ok(add(name, Some(parent), kind, data))
}

/// Remove a regular file or an empty directory (tombstone).
pub fn remove(h: usize) -> Result<(), ()> {
    // Disk nodes: delete through the FAT volume (dirs must be empty).
    if let NodeData::Disk(i) = &nodes()[h].data {
        if i.name.is_empty() {
            return Err(()); // the /disk mount itself
        }
        let vol = i.vol;
        let p = i.parent;
        let n = i.name.clone();
        return crate::fat::remove(vol, p, &n);
    }
    let parent = match nodes()[h].parent {
        Some(p) => p,
        None => return Err(()),
    };
    let empty = match &nodes()[h].data {
        NodeData::Dir(v) => v.is_empty(),
        _ => true,
    };
    if !empty {
        return Err(());
    }
    if let NodeData::Dir(v) = &mut nodes()[parent].data {
        v.retain(|&c| c != h);
    }
    let n = &mut nodes()[h];
    n.removed = true;
    n.parent = None;
    Ok(())
}

// --- path helpers ---------------------------------------------------------

/// Split `/a/b/c` into (handle of `/a/b`, "c").
fn split_parent_name(path: &str) -> Result<(usize, String), ()> {
    let path = path.trim_end_matches('/');
    if path.is_empty() {
        return Err(());
    }
    let (parent_path, name) = match path.rfind('/') {
        Some(i) => (&path[..i], &path[i + 1..]),
        None => ("/", path),
    };
    if name.is_empty() {
        return Err(());
    }
    Some((resolve(parent_path).ok_or(())?, name.into())).ok_or(())
}

pub fn mkdir(path: &str) -> Result<(), ()> {
    let (p, n) = split_parent_name(path)?;
    create(p, &n, FileType::Dir).map(|_| ())
}

pub fn touch(path: &str) -> Result<(), ()> {
    if resolve(path).is_some() {
        return Ok(());
    }
    let (p, n) = split_parent_name(path)?;
    create(p, &n, FileType::Regular).map(|_| ())
}

/// Resolve a path, creating a regular file if it doesn't exist yet.
pub fn ensure_file(path: &str) -> Result<usize, ()> {
    if let Some(h) = resolve(path) {
        if kind(h) == FileType::Dir {
            Err(())
        } else {
            Ok(h)
        }
    } else {
        touch(path)?;
        resolve(path).ok_or(())
    }
}

pub fn remove_path(path: &str) -> Result<(), ()> {
    let h = resolve(path).ok_or(())?;
    remove(h)
}

// --- disk mount ------------------------------------------------------------

/// Round-robin pool of ephemeral disk nodes so a long-lived shell doesn't
/// grow the node table forever.
const DISK_SLOTS: usize = 24;
static mut DISK_POOL: *mut Vec<usize> = core::ptr::null_mut();
static mut DISK_ROBIN: usize = 0;

fn materialize_disk(info: DiskInfo) -> usize {
    unsafe {
        if DISK_POOL.is_null() {
            DISK_POOL = Box::into_raw(Box::new(Vec::new()));
        }
        let pool = &mut *DISK_POOL;
        let idx = DISK_ROBIN % DISK_SLOTS;
        DISK_ROBIN = DISK_ROBIN.wrapping_add(1);
        let kind = if info.is_dir {
            FileType::Dir
        } else {
            FileType::Regular
        };
        if idx < pool.len() {
            let h = pool[idx];
            let n = &mut nodes()[h];
            n.name = info.name.clone();
            n.kind = kind;
            n.parent = None;
            n.data = NodeData::Disk(info);
            n.removed = false;
            return h;
        }
        let name = info.name.clone();
        let h = add(&name, None, kind, NodeData::Disk(info));
        pool.push(h);
        h
    }
}

// --- character devices ----------------------------------------------------

fn version_text() -> String {
    alloc::format!(
        "rsnix v{} — Official RS-DOS kernel\nUEFI x86_64, identity paging, free-list heap, bitmap pages\n",
        env!("CARGO_PKG_VERSION")
    )
}

fn meminfo_text() -> String {
    let h = memory::heap_stats();
    let p = memory::page_stats();
    let (map, key) = memory::get_map();
    let mut usable_kb = 0u64;
    for d in map {
        if d.kind == 7 {
            usable_kb += d.number_of_pages * 4;
        }
    }
    alloc::format!(
        "heap: {} KB capacity, {} KB free in {} block(s)\npages: {} free / {} used, {} MB span\nmem: {} entries (key 0x{:x}), {} MB usable\n",
        h.heap_size >> 10,
        h.free_bytes >> 10,
        h.free_blocks,
        p.free_pages,
        p.used_pages,
        p.span_bytes >> 20,
        map.len(),
        key,
        usable_kb >> 10,
    )
}

fn pci_listing() -> String {
    let mut out = String::new();
    let devices = pci::probe();
    out += &alloc::format!("{} device(s)\n", devices.len());
    for d in &devices {
        out += &alloc::format!(
            "{:02x}:{:02x}.{}  {:<16} {:04x}:{:04x}\n",
            d.bus,
            d.device,
            d.func,
            pci::class_name(d.class, d.subclass),
            d.vendor,
            d.device_id
        );
    }
    out
}

fn dev_text(dev: CharDevice, _pos: u64) -> String {
    match dev {
        CharDevice::Mem => String::new(), // raw bytes, special-cased in read()
        CharDevice::Rtc => {
            let t = rtc::now();
            alloc::format!(
                "{}-{:02}-{:02} {:02}:{:02}:{:02}\n",
                t.year,
                t.month,
                t.day,
                t.hour,
                t.minute,
                t.second
            )
        }
        CharDevice::Pci => pci_listing(),
        CharDevice::Version => version_text(),
        CharDevice::Meminfo => meminfo_text(),
        CharDevice::Klog => crate::klog::dump(),
        CharDevice::Mouse => mouse_text(),
        CharDevice::Null | CharDevice::Console | CharDevice::Serial => String::new(),
        CharDevice::Zero => String::new(),
    }
}

/// Render `/dev/mouse`: the current cursor state (position + buttons).
/// The cursor task owns the PS/2 ring, so this is a position device —
/// the GUI-correct semantic — rather than an event drain.
fn mouse_text() -> String {
    if !crate::ps2::mouse_present() {
        return String::from("mouse: no PS/2 mouse\n");
    }
    let (x, y) = crate::gop::cursor_pos();
    let b = crate::gop::cursor_buttons();
    alloc::format!(
        "x={} y={} buttons={}{}{}\n",
        x,
        y,
        if b & 1 != 0 { 'L' } else { '-' },
        if b & 2 != 0 { 'R' } else { '-' },
        if b & 4 != 0 { 'M' } else { '-' }
    )
}

// --- self-test ------------------------------------------------------------

pub fn self_test() -> bool {
    let mut buf = [0u8; 8];
    let h = match ensure_file("/tmp/selftest") {
        Ok(h) => h,
        Err(_) => return false,
    };
    if write(h, b"hello", 0) != Ok(5) {
        return false;
    }
    if read(h, &mut buf, 0) != Ok(5) || &buf[..5] != b"hello" {
        return false;
    }
    if remove(h).is_err() || resolve("/tmp/selftest").is_some() {
        return false;
    }
    // device write goes nowhere: echo into /dev/null
    if let Ok(nul) = ensure_file("/dev/null") {
        let _ = write(nul, b"discard me", 0);
    }
    true
}