//! A tiny "libc" for user programs: syscall wrappers, print!/println!,
//! the `_start` entry point and a panic handler.
//!
//! Each user program includes it with
//!     #[macro_use] #[path = "ulib.rs"] mod ulib;
//! and defines `fn main() -> i64` (the exit code).
//!
//! User programs are built for the `x86_64-unknown-none` target, which has
//! SSE/floating point disabled. That matters: the kernel doesn't save SSE
//! registers on a context switch.

#![allow(dead_code)]

use core::arch::asm;
use core::fmt;

pub const SYS_READ: u64 = 0;
pub const SYS_WRITE: u64 = 1;
pub const SYS_OPEN: u64 = 2;
pub const SYS_CLOSE: u64 = 3;
pub const SYS_SCHED_YIELD: u64 = 24;
pub const SYS_GETPID: u64 = 39;
pub const SYS_EXIT: u64 = 60;

pub const O_CREAT: u64 = 0o100;
pub const ENOENT: i64 = 2;
pub const EBADF: i64 = 9;
pub const EFAULT: i64 = 14;

/// Raw system call: number in rax, args in rdi/rsi/rdx, result in rax.
/// The `syscall` instruction overwrites rcx and r11.
pub fn syscall3(nr: u64, a1: u64, a2: u64, a3: u64) -> i64 {
    let ret: i64;
    unsafe {
        asm!(
            "syscall",
            inlateout("rax") nr as i64 => ret,
            in("rdi") a1,
            in("rsi") a2,
            in("rdx") a3,
            lateout("rcx") _,
            lateout("r11") _,
            options(nostack),
        );
    }
    ret
}

pub fn read(fd: i64, buf: &mut [u8]) -> i64 {
    syscall3(SYS_READ, fd as u64, buf.as_mut_ptr() as u64, buf.len() as u64)
}
pub fn write(fd: i64, buf: &[u8]) -> i64 {
    syscall3(SYS_WRITE, fd as u64, buf.as_ptr() as u64, buf.len() as u64)
}
pub fn open(path: &str, flags: u64) -> i64 {
    syscall3(SYS_OPEN, path.as_ptr() as u64, path.len() as u64, flags)
}
pub fn close(fd: i64) -> i64 {
    syscall3(SYS_CLOSE, fd as u64, 0, 0)
}
pub fn sched_yield() {
    syscall3(SYS_SCHED_YIELD, 0, 0, 0);
}
pub fn getpid() -> i64 {
    syscall3(SYS_GETPID, 0, 0, 0)
}
pub fn exit(code: i64) -> ! {
    syscall3(SYS_EXIT, code as u64, 0, 0);
    loop {} // not reached
}

pub struct Stdout;

impl fmt::Write for Stdout {
    fn write_str(&mut self, s: &str) -> fmt::Result {
        write(1, s.as_bytes());
        Ok(())
    }
}

macro_rules! print {
    ($($arg:tt)*) => {{
        use core::fmt::Write;
        let _ = write!($crate::ulib::Stdout, $($arg)*);
    }};
}

macro_rules! println {
    ($($arg:tt)*) => {{
        use core::fmt::Write;
        let _ = writeln!($crate::ulib::Stdout, $($arg)*);
    }};
}

/// The kernel jumps here (see process.rs). Run main, exit with its result.
#[unsafe(no_mangle)]
pub extern "C" fn _start() -> ! {
    exit(crate::main())
}

#[panic_handler]
fn panic(info: &core::panic::PanicInfo) -> ! {
    println!("[user panic] {}", info);
    exit(101)
}