// Copyright 2020 Shin Yoshida
//
// "LGPL-3.0-or-later OR Apache-2.0 OR BSD-2-Clause OR MIT"
//
// This is part of cpu-endian
//
//  cpu-endian is free software: you can redistribute it and/or modify
//  it under the terms of the GNU Lesser General Public License as published by
//  the Free Software Foundation, either version 3 of the License, or
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// Licensed under the Apache License, Version 2.0 (the "License");
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#![deny(missing_docs)]

//! # cpu-endian
//!
//! `cpu-endian` is a portable crate to detect CPU byte order.
//!
//! It detects how CPU native scalar type is ordered; little-endian or big-endian, or something else (like PDP-endian, mixed-endian, middle-endian, and so on.)
//!
//! ## Examples
//!
//! ```
//! use cpu_endian::{Endian, working};
//!
//! // Takes first octet of 0x00ff: u16.
//! let v: u16 = 0x00ff;
//! let first_octet: u8 = unsafe {
//!     let ptr = &v as *const u16;
//!     let ptr = ptr as *const u8;
//!     *ptr
//! };
//!
//! // If the byte-order is little-endian, the first octet should be 0xff, or if big-endian,
//! // it should be 0x00.
//! match working() {
//!     Endian::Little => assert_eq!(0xff, first_octet),
//!     Endian::Big => assert_eq!(0x00, first_octet),
//!     _ => {},
//! }
//! ```

use core::cell::Cell;

/// Byte order of scalar types.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Endian {
    /// little-endian
    Little,
    /// big-endian
    Big,
    /// Neither little-endian nor big-endian. For example, PDP-endian, mixed-endian, middle-endian, and so on.
    /// (Such endian is very rare today.)
    Minor,
}

thread_local!(
    /// Cache of native_().
    ///
    /// Some CPUs change the byte order, however I don't think none of them changes it after process started.
    static CACHE: Cell<u8> = Cell::new(0)
);

/// Returns the CPU byte order.
///
/// ```
/// use cpu_endian::*;
///
/// // Takes first octet of 0x00ff: u16.
/// let v: u16 = 0x00ff;
/// let first_octet: u8 = unsafe {
///     let ptr = &v as *const u16;
///     let ptr = ptr as *const u8;
///     *ptr
/// };
///
/// // If the byte-order is little-endian, the first octet should be 0xff, or if big-endian,
/// // it should be 0x00.
/// match working() {
///     Endian::Little => assert_eq!(0xff, first_octet),
///     Endian::Big => assert_eq!(0x00, first_octet),
///     _ => {},
/// }
/// ```
#[inline]
pub fn working() -> Endian {
    inner_working()
}

/// Implementation for `working` .
///
/// This function is only for `x86` and `x86_64` architecture, and const function to return
/// `Endian::Little` . ( `x86` and `x86_64` are typical little-endian CPU.)
#[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
const fn inner_working() -> Endian {
    Endian::Little
}

/// Implementation for `working` .
///
/// This function is for CPUs but `x86` nor `x86_64` and calls C function to detect the endian.
/// (Some CPUs can switch the endian, so it is difficult to hard cord.)
#[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
#[inline]
fn inner_working() -> Endian {
    // No cache is hit.
    // Because native()_ always returns the same value, Ordering::Relaxed will do.
    if CACHE.with(|c| c.get()) == 0 {
        let order = unsafe { native_() };
        debug_assert_ne!(0, order);

        CACHE.with(|c| c.set(order as u8));
    }

    match CACHE.with(|c| c.get()) {
        1 => Endian::Little,
        2 => Endian::Big,
        _ => Endian::Minor,
    }
}

#[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
use std::os::raw::c_int;

#[link(name = "native_endian_")]
#[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
extern "C" {

    /// Returns the cpu native endian.
    ///
    /// - Little endian: 1
    /// - Big endian: 2
    /// - Other: 3
    fn native_() -> c_int;
}