src/endian.rs

// Copyright 2017 The Chromium OS Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE-BSD-3-Clause file.
//
// SPDX-License-Identifier: Apache-2.0 OR BSD-3-Clause

//! Explicit endian types useful for embedding in structs or reinterpreting data.
//!
//! Each endian type is guaarnteed to have the same size and alignment as a regular unsigned
//! primitive of the equal size.
//!
//! # Examples
//!
//! ```
//! # use vm_memory::{Be32, Le32};
//! #
//! let b: Be32 = From::from(3);
//! let l: Le32 = From::from(3);
//!
//! assert_eq!(b.to_native(), 3);
//! assert_eq!(l.to_native(), 3);
//! assert!(b == 3);
//! assert!(l == 3);
//!
//! let b_trans: u32 = unsafe { std::mem::transmute(b) };
//! let l_trans: u32 = unsafe { std::mem::transmute(l) };
//!
//! #[cfg(target_endian = "little")]
//! assert_eq!(l_trans, 3);
//! #[cfg(target_endian = "big")]
//! assert_eq!(b_trans, 3);
//!
//! assert_ne!(b_trans, l_trans);
//! ```

use std::mem::{align_of, size_of};

use crate::bytes::ByteValued;

macro_rules! const_assert {
    ($condition:expr) => {
        let _ = [(); 0 - !$condition as usize];
    };
}

macro_rules! endian_type {
    ($old_type:ident, $new_type:ident, $to_new:ident, $from_new:ident) => {
        /// An unsigned integer type of with an explicit endianness.
        ///
        /// See module level documentation for examples.
        #[derive(Copy, Clone, Eq, PartialEq, Debug, Default)]
        pub struct $new_type($old_type);

        impl $new_type {
            fn _assert() {
                const_assert!(align_of::<$new_type>() == align_of::<$old_type>());
                const_assert!(size_of::<$new_type>() == size_of::<$old_type>());
            }

            /// Converts `self` to the native endianness.
            pub fn to_native(self) -> $old_type {
                $old_type::$from_new(self.0)
            }
        }

        // SAFETY: Safe because we are using this for implementing ByteValued for endian types
        // which are POD.
        unsafe impl ByteValued for $new_type {}

        impl PartialEq<$old_type> for $new_type {
            fn eq(&self, other: &$old_type) -> bool {
                self.0 == $old_type::$to_new(*other)
            }
        }

        impl PartialEq<$new_type> for $old_type {
            fn eq(&self, other: &$new_type) -> bool {
                $old_type::$to_new(other.0) == *self
            }
        }

        impl From<$new_type> for $old_type {
            fn from(v: $new_type) -> $old_type {
                v.to_native()
            }
        }

        impl From<$old_type> for $new_type {
            fn from(v: $old_type) -> $new_type {
                $new_type($old_type::$to_new(v))
            }
        }
    };
}

endian_type!(u16, Le16, to_le, from_le);
endian_type!(u32, Le32, to_le, from_le);
endian_type!(u64, Le64, to_le, from_le);
endian_type!(usize, LeSize, to_le, from_le);
endian_type!(u16, Be16, to_be, from_be);
endian_type!(u32, Be32, to_be, from_be);
endian_type!(u64, Be64, to_be, from_be);
endian_type!(usize, BeSize, to_be, from_be);

#[cfg(test)]
mod tests {
    #![allow(clippy::undocumented_unsafe_blocks)]
    use super::*;

    use std::convert::From;
    use std::mem::transmute;

    #[cfg(target_endian = "little")]
    const NATIVE_LITTLE: bool = true;
    #[cfg(target_endian = "big")]
    const NATIVE_LITTLE: bool = false;
    const NATIVE_BIG: bool = !NATIVE_LITTLE;

    macro_rules! endian_test {
        ($old_type:ty, $new_type:ty, $test_name:ident, $native:expr) => {
            mod $test_name {
                use super::*;

                #[allow(overflowing_literals)]
                #[test]
                fn test_endian_type() {
                    <$new_type>::_assert();

                    let v = 0x0123_4567_89AB_CDEF as $old_type;
                    let endian_v: $new_type = From::from(v);
                    let endian_into: $old_type = endian_v.into();
                    let endian_transmute: $old_type = unsafe { transmute(endian_v) };

                    if $native {
                        assert_eq!(endian_v, endian_transmute);
                    } else {
                        assert_eq!(endian_v, endian_transmute.swap_bytes());
                    }

                    assert_eq!(endian_into, v);
                    assert_eq!(endian_v.to_native(), v);

                    assert!(v == endian_v);
                    assert!(endian_v == v);
                }
            }
        };
    }

    endian_test!(u16, Le16, test_le16, NATIVE_LITTLE);
    endian_test!(u32, Le32, test_le32, NATIVE_LITTLE);
    endian_test!(u64, Le64, test_le64, NATIVE_LITTLE);
    endian_test!(usize, LeSize, test_le_size, NATIVE_LITTLE);
    endian_test!(u16, Be16, test_be16, NATIVE_BIG);
    endian_test!(u32, Be32, test_be32, NATIVE_BIG);
    endian_test!(u64, Be64, test_be64, NATIVE_BIG);
    endian_test!(usize, BeSize, test_be_size, NATIVE_BIG);
}