//! A wrapper around the system's curses/ncurses library, exposing some lower-level functionality
//! that's not directly exposed in any of the popular ncurses crates.
//!
//! In addition to exposing the C library ffi calls, we also shim around some functionality that's
//! only made available via the the ncurses headers to C code via macro magic, such as polyfilling
//! missing capability strings to shoe-in missing support for certain terminal sequences.
//!
//! This is intentionally very bare bones and only implements the subset of curses functionality
//! used by fish

use self::sys::*;
use std::ffi::{CStr, CString};
use std::sync::Arc;
use std::sync::Mutex;

/// The [`Term`] singleton, providing a façade around the system curses library. Initialized via a
/// successful call to [`setup()`] and surfaced to the outside world via [`term()`].
///
/// It isn't guaranteed that fish will ever be able to successfully call `setup()`, so this must
/// remain an `Option` instead of returning `Term` by default and just panicking if [`term()`] was
/// called before `setup()`.
///
/// We can't just use an AtomicPtr<Arc<Term>> here because there's a race condition when the old Arc
/// gets dropped - we would obtain the current (non-null) value of `TERM` in [`term()`] but there's
/// no guarantee that a simultaneous call to [`setup()`] won't result in this refcount being
/// decremented to zero and the memory being reclaimed before we can clone it, since we can only
/// atomically *read* the value of the pointer, not clone the `Arc` it points to.
pub static TERM: Mutex<Option<Arc<Term>>> = Mutex::new(None);

/// Returns a reference to the global [`Term`] singleton or `None` if not preceded by a successful
/// call to [`curses::setup()`].
pub fn term() -> Option<Arc<Term>> {
    TERM.lock()
        .expect("Mutex poisoned!")
        .as_ref()
        .map(Arc::clone)
}

/// Private module exposing system curses ffi.
mod sys {
    pub const OK: i32 = 0;
    pub const ERR: i32 = -1;

    extern "C" {
        /// The ncurses `cur_term` TERMINAL pointer.
        pub static mut cur_term: *const core::ffi::c_void;

        /// setupterm(3) is a low-level call to begin doing any sort of `term.h`/`curses.h` work.
        /// It's called internally by ncurses's `initscr()` and `newterm()`, but the C++ code called
        /// it directly from [`initialize_curses_using_fallbacks()`].
        pub fn setupterm(
            term: *const libc::c_char,
            filedes: libc::c_int,
            errret: *mut libc::c_int,
        ) -> libc::c_int;

        /// Frees the `cur_term` TERMINAL  pointer.
        pub fn del_curterm(term: *const core::ffi::c_void) -> libc::c_int;

        /// Checks for the presence of a termcap flag identified by the first two characters of
        /// `id`.
        pub fn tgetflag(id: *const libc::c_char) -> libc::c_int;

        /// Checks for the presence and value of a number capability in the termcap/termconf
        /// database. A return value of `-1` indicates not found.
        pub fn tgetnum(id: *const libc::c_char) -> libc::c_int;

        pub fn tgetstr(
            id: *const libc::c_char,
            area: *mut *mut libc::c_char,
        ) -> *const libc::c_char;
    }
}

/// The safe wrapper around curses functionality, initialized by a successful call to [`setup()`]
/// and obtained thereafter by calls to [`term()`].
///
/// An extant `Term` instance means the curses `TERMINAL *cur_term` pointer is non-null. Any
/// functionality that is normally performed using `cur_term` should be done via `Term` instead.
pub struct Term {
    // String capabilities
    pub enter_italics_mode: Option<CString>,
    pub exit_italics_mode: Option<CString>,
    pub enter_dim_mode: Option<CString>,

    // Number capabilities
    pub max_colors: Option<i32>,

    // Flag/boolean capabilities
    pub eat_newline_glitch: bool,
}

impl Term {
    /// Initialize a new `Term` instance, prepopulating the values of all the curses string
    /// capabilities we care about in the process.
    fn new() -> Self {
        Term {
            // String capabilities
            enter_italics_mode: StringCap::new("ZH").lookup(),
            exit_italics_mode: StringCap::new("ZR").lookup(),
            enter_dim_mode: StringCap::new("mh").lookup(),

            // Number capabilities
            max_colors: NumberCap::new("Co").lookup(),

            // Flag/boolean capabilities
            eat_newline_glitch: FlagCap::new("xn").lookup(),
        }
    }
}

trait Capability {
    type Result: Sized;
    fn lookup(&self) -> Self::Result;
}

impl Capability for StringCap {
    type Result = Option<CString>;

    fn lookup(&self) -> Self::Result {
        unsafe {
            const NULL: *const i8 = core::ptr::null();
            match sys::tgetstr(self.code.as_ptr(), core::ptr::null_mut()) {
                NULL => None,
                // termcap spec says nul is not allowed in terminal sequences and must be encoded;
                // so the terminating NUL is the end of the string.
                result => Some(CStr::from_ptr(result).to_owned()),
            }
        }
    }
}

impl Capability for NumberCap {
    type Result = Option<i32>;

    fn lookup(&self) -> Self::Result {
        unsafe {
            match tgetnum(self.0.as_ptr()) {
                -1 => None,
                n => Some(n),
            }
        }
    }
}

impl Capability for FlagCap {
    type Result = bool;

    fn lookup(&self) -> Self::Result {
        unsafe { tgetflag(self.0.as_ptr()) != 0 }
    }
}

/// Calls the curses `setupterm()` function with the provided `$TERM` value `term` (or a null
/// pointer in case `term` is null) for the file descriptor `fd`. Returns a reference to the newly
/// initialized [`Term`] singleton on success or `None` if this failed.
///
/// The `configure` parameter may be set to a callback that takes an `&mut Term` reference to
/// override any capabilities before the `Term` is permanently made immutable.
///
/// Note that the `errret` parameter is provided to the function, meaning curses will not write
/// error output to stderr in case of failure.
///
/// Any existing references from `curses::term()` will be invalidated by this call!
pub fn setup<F>(term: Option<&CStr>, fd: i32, configure: F) -> Option<Arc<Term>>
where
    F: Fn(&mut Term),
{
    // For now, use the same TERM lock when using `cur_term` to prevent any race conditions in
    // curses itself. We might split this to another lock in the future.
    let mut global_term = TERM.lock().expect("Mutex poisoned!");

    let result = unsafe {
        // If cur_term is already initialized for a different $TERM value, calling setupterm() again
        // will leak memory. Call del_curterm() first to free previously allocated resources.
        let _ = sys::del_curterm(cur_term);

        let mut err = 0;
        if let Some(term) = term {
            sys::setupterm(term.as_ptr(), fd, &mut err)
        } else {
            sys::setupterm(core::ptr::null(), fd, &mut err)
        }
    };

    // Safely store the new Term instance or replace the old one. We have the lock so it's safe to
    // drop the old TERM value and have its refcount decremented - no one will be cloning it.
    if result == sys::OK {
        // Create a new `Term` instance, prepopulate the capabilities we care about, and allow the
        // caller to override any as needed.
        let mut term = Term::new();
        (configure)(&mut term);

        let term = Arc::new(term);
        *global_term = Some(term.clone());
        Some(term)
    } else {
        *global_term = None;
        None
    }
}

/// Resets the curses `cur_term` TERMINAL pointer. Subsequent calls to [`curses::term()`](term())
/// will return `None`.
pub fn reset() {
    let mut term = TERM.lock().expect("Mutex poisoned!");
    if term.is_some() {
        unsafe {
            // Ignore the result of del_curterm() as the only documented error is that
            // `cur_term` was already null.
            let _ = sys::del_curterm(cur_term);
            sys::cur_term = core::ptr::null();
        }
        *term = None;
    }
}

#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
struct Code {
    /// The two-char termcap code for the capability, followed by a nul.
    code: [u8; 3],
}

impl Code {
    /// `code` is the two-digit termcap code. See termcap(5) for a reference.
    ///
    /// Panics if anything other than a two-ascii-character `code` is passed into the function. It
    /// would take a hard-coded `[u8; 2]` parameter but that is less ergonomic. Since all our
    /// termcap `Code`s are compile-time constants, the panic is a compile-time error, meaning
    /// there's no harm to going this more ergonomic route.
    const fn new(code: &str) -> Code {
        let code = code.as_bytes();
        if code.len() != 2 {
            panic!("Invalid termcap code provided!");
        }
        Code {
            code: [code[0], code[1], b'\0'],
        }
    }

    /// The nul-terminated termcap id of the capability.
    pub const fn as_ptr(&self) -> *const libc::c_char {
        self.code.as_ptr().cast()
    }
}

#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
struct StringCap {
    code: Code,
}
impl StringCap {
    const fn new(code: &str) -> Self {
        StringCap {
            code: Code::new(code),
        }
    }
}

#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
struct NumberCap(Code);
impl NumberCap {
    const fn new(code: &str) -> Self {
        NumberCap(Code::new(code))
    }
}

#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
struct FlagCap(Code);
impl FlagCap {
    const fn new(code: &str) -> Self {
        FlagCap(Code::new(code))
    }
}