//! system/services/vfs — the user-space VFS server. Shipped in the initial_ramdisk, spawned as a
//! ring-3 process, and reached by every other process through IPC (the `runtime`
//! file API marshals open/read/write/stat/close into calls to this server's
//! endpoint, published under the well-known `vfs` service id).
//!
//! For now the namespace is a small in-memory ramfs (opening a name creates it):
//! enough to prove the whole path — client file API -> IPC -> server dispatch ->
//! reply. Device nodes backed by user-space drivers (/device) layer on top in M10,
//! where `open` on a /device name forwards to the owning driver's endpoint.

const std = @import("std");
const runtime = @import("runtime");
const protocol = runtime.vfs_protocol;

const Node = struct {
    used: bool = false,
    name: [24]u8 = undefined,
    name_len: usize = 0,
    data: [512]u8 = undefined,
    size: usize = 0,
};

const OpenFile = struct {
    used: bool = false,
    node: usize = 0,
    // The client (task id — an IPC badge is one) that opened this handle. What
    // release-on-death sweeps by: a service must never depend on its clients
    // cleaning up after themselves (docs/process-lifecycle.md).
    owner: u32 = 0,
};

var nodes = [_]Node{.{}} ** 8;
var opens = [_]OpenFile{.{}} ** 16;

fn findNode(name: []const u8) ?usize {
    for (&nodes, 0..) |*n, i| {
        if (n.used and std.mem.eql(u8, n.name[0..n.name_len], name)) return i;
    }
    return null;
}

fn createNode(name: []const u8) ?usize {
    for (&nodes, 0..) |*n, i| {
        if (!n.used) {
            const l = @min(name.len, n.name.len);
            @memcpy(n.name[0..l], name[0..l]);
            n.* = .{ .used = true, .name = n.name, .name_len = l, .size = 0 };
            return i;
        }
    }
    return null;
}

fn openAt(id: u64) ?*OpenFile {
    if (id >= opens.len) return null;
    const o = &opens[@intCast(id)];
    return if (o.used) o else null;
}

/// Serialise a reply header + payload into `out`; returns the total length.
fn writeReply(out: []u8, reply: protocol.Reply, payload: []const u8) usize {
    @memcpy(out[0..protocol.reply_size], std.mem.asBytes(&reply));
    const n = @min(payload.len, out.len - protocol.reply_size);
    @memcpy(out[protocol.reply_size..][0..n], payload[0..n]);
    return protocol.reply_size + n;
}

fn fail(out: []u8) usize {
    return writeReply(out, .{ .status = -1 }, &.{});
}

/// Format one whole log line and emit it in a single `debug_write`, so lines from
/// concurrent processes can never land in the middle of it.
fn writeLine(comptime fmt: []const u8, arguments: anytype) void {
    var line: [96]u8 = undefined;
    _ = runtime.system.write(std.fmt.bufPrint(&line, fmt, arguments) catch return);
}

/// Release every open handle `client` held — called on that client's published
/// exit event. The nodes (the files) stay: ramfs contents outlive their writers,
/// only the dead client's handles go.
fn releaseClientHandles(client: u32) void {
    var released: u32 = 0;
    for (&opens) |*o| {
        if (o.used and o.owner == client) {
            o.used = false;
            released += 1;
        }
    }
    if (released != 0) writeLine("system/services/vfs: released {d} handle(s) for dead client {d}\n", .{ released, client });
}

/// Handle one request from `sender`; write the reply into `out`, return its length.
fn handle(message: []const u8, out: []u8, sender: u32, capability: ?runtime.ipc.Handle) usize {
    _ = capability;
    if (message.len < protocol.request_size) return fail(out);
    const request = std.mem.bytesToValue(protocol.Request, message[0..protocol.request_size]);
    const payload = message[protocol.request_size..];

    switch (request.operation) {
        .open => {
            const name = payload[0..@min(payload.len, request.len)];
            const ni = findNode(name) orelse createNode(name) orelse return fail(out);
            for (&opens, 0..) |*o, i| {
                if (!o.used) {
                    o.* = .{ .used = true, .node = ni, .owner = sender };
                    return writeReply(out, .{ .status = 0, .node = i }, &.{});
                }
            }
            return fail(out);
        },
        .read => {
            const of = openAt(request.node) orelse return fail(out);
            const nd = &nodes[of.node];
            const off: usize = @intCast(request.offset);
            if (off >= nd.size) return writeReply(out, .{ .status = 0, .len = 0 }, &.{}); // EOF
            const n = @min(@min(nd.size - off, request.len), protocol.maximum_payload);
            return writeReply(out, .{ .status = 0, .len = @intCast(n) }, nd.data[off .. off + n]);
        },
        .write => {
            const of = openAt(request.node) orelse return fail(out);
            const nd = &nodes[of.node];
            const off: usize = @intCast(request.offset);
            if (off > nd.data.len) return fail(out);
            const n = @min(@min(payload.len, request.len), nd.data.len - off);
            @memcpy(nd.data[off .. off + n], payload[0..n]);
            if (off + n > nd.size) nd.size = off + n;
            return writeReply(out, .{ .status = 0, .len = @intCast(n) }, &.{});
        },
        .status => {
            const of = openAt(request.node) orelse return fail(out);
            const st = protocol.FileStatus{ .size = nodes[of.node].size, .kind = 0 };
            return writeReply(out, .{ .status = 0, .len = @sizeOf(protocol.FileStatus) }, std.mem.asBytes(&st));
        },
        .close => {
            if (request.node < opens.len) opens[@intCast(request.node)].used = false;
            return writeReply(out, .{ .status = 0 }, &.{});
        },
    }
}

/// Startup, under the harness: subscribe to the published exit events — when a
/// client dies holding open handles, the exit notification is how the VFS learns
/// to release them (docs/process-lifecycle.md).
fn initialise(endpoint: runtime.ipc.Handle) bool {
    if (!runtime.process.subscribeExits(endpoint)) {
        _ = runtime.system.write("system/services/vfs: exit subscription failed\n");
    }
    _ = runtime.system.write("system/services/vfs: ready\n");
    return true;
}

/// A non-signal notification: the only kind the VFS subscribes to is exit events.
fn onNotification(badge: u64) void {
    if (badge & runtime.ipc.notify_exit_bit != 0) {
        releaseClientHandles(@intCast(badge & ~(runtime.ipc.notify_badge_bit | runtime.ipc.notify_exit_bit)));
    }
}

pub fn main() void {
    // The harness owns the loop: requests dispatch to handle(), exit events to
    // onNotification(), ping and terminate are answered for free — this service
    // gained the whole lifecycle contract by deleting its hand-rolled loop.
    runtime.service.run(protocol.message_maximum, .{
        .service = .vfs,
        .init = initialise,
        .on_message = handle,
        .on_notification = onNotification,
    });
}

pub const panic = runtime.panic;
comptime {
    _ = &runtime.start._start;
}