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+//! Tools for converting file format structures to and from bytes.
+//!
+//! This module should be replaced once rust provides safe transmutes.
+
+// This module provides functions for both read and write features.
+#![cfg_attr(
+ not(all(feature = "read_core", feature = "write_core")),
+ allow(dead_code)
+)]
+
+use core::{mem, result, slice};
+
+type Result<T> = result::Result<T, ()>;
+
+/// A trait for types that can safely be converted from and to byte slices.
+///
+/// # Safety
+/// A type that is `Pod` must:
+/// - be `#[repr(C)]` or `#[repr(transparent)]`
+/// - have no invalid byte values
+/// - have no padding
+pub unsafe trait Pod: Copy + 'static {}
+
+/// Cast a byte slice to a `Pod` type.
+///
+/// Returns the type and the tail of the slice.
+#[inline]
+pub fn from_bytes<T: Pod>(data: &[u8]) -> Result<(&T, &[u8])> {
+ let size = mem::size_of::<T>();
+ let tail = data.get(size..).ok_or(())?;
+ let ptr = data.as_ptr();
+ if (ptr as usize) % mem::align_of::<T>() != 0 {
+ return Err(());
+ }
+ // Safety:
+ // The alignment and size are checked by this function.
+ // The Pod trait ensures the type is valid to cast from bytes.
+ let val = unsafe { &*ptr.cast() };
+ Ok((val, tail))
+}
+
+/// Cast a mutable byte slice to a `Pod` type.
+///
+/// Returns the type and the tail of the slice.
+#[inline]
+pub fn from_bytes_mut<T: Pod>(data: &mut [u8]) -> Result<(&mut T, &mut [u8])> {
+ let size = mem::size_of::<T>();
+ if size > data.len() {
+ return Err(());
+ }
+ let (data, tail) = data.split_at_mut(size);
+ let ptr = data.as_mut_ptr();
+ if (ptr as usize) % mem::align_of::<T>() != 0 {
+ return Err(());
+ }
+ // Safety:
+ // The alignment and size are checked by this function.
+ // The Pod trait ensures the type is valid to cast from bytes.
+ let val = unsafe { &mut *ptr.cast() };
+ Ok((val, tail))
+}
+
+/// Cast a byte slice to a slice of a `Pod` type.
+///
+/// Returns the type slice and the tail of the byte slice.
+#[inline]
+pub fn slice_from_bytes<T: Pod>(data: &[u8], count: usize) -> Result<(&[T], &[u8])> {
+ let size = count.checked_mul(mem::size_of::<T>()).ok_or(())?;
+ let tail = data.get(size..).ok_or(())?;
+ let ptr = data.as_ptr();
+ if (ptr as usize) % mem::align_of::<T>() != 0 {
+ return Err(());
+ }
+ // Safety:
+ // The alignment and size are checked by this function.
+ // The Pod trait ensures the type is valid to cast from bytes.
+ let slice = unsafe { slice::from_raw_parts(ptr.cast(), count) };
+ Ok((slice, tail))
+}
+
+/// Cast a mutable byte slice to a slice of a `Pod` type.
+///
+/// Returns the type slice and the tail of the byte slice.
+#[inline]
+pub fn slice_from_bytes_mut<T: Pod>(
+ data: &mut [u8],
+ count: usize,
+) -> Result<(&mut [T], &mut [u8])> {
+ let size = count.checked_mul(mem::size_of::<T>()).ok_or(())?;
+ if size > data.len() {
+ return Err(());
+ }
+ let (data, tail) = data.split_at_mut(size);
+ let ptr = data.as_mut_ptr();
+ if (ptr as usize) % mem::align_of::<T>() != 0 {
+ return Err(());
+ }
+ // Safety:
+ // The alignment and size are checked by this function.
+ // The Pod trait ensures the type is valid to cast from bytes.
+ let slice = unsafe { slice::from_raw_parts_mut(ptr.cast(), count) };
+ Ok((slice, tail))
+}
+
+/// Cast a `Pod` type to a byte slice.
+#[inline]
+pub fn bytes_of<T: Pod>(val: &T) -> &[u8] {
+ let size = mem::size_of::<T>();
+ // Safety:
+ // Any alignment is allowed.
+ // The size is determined in this function.
+ // The Pod trait ensures the type is valid to cast to bytes.
+ unsafe { slice::from_raw_parts(slice::from_ref(val).as_ptr().cast(), size) }
+}
+
+/// Cast a `Pod` type to a mutable byte slice.
+#[inline]
+pub fn bytes_of_mut<T: Pod>(val: &mut T) -> &mut [u8] {
+ let size = mem::size_of::<T>();
+ // Safety:
+ // Any alignment is allowed.
+ // The size is determined in this function.
+ // The Pod trait ensures the type is valid to cast to bytes.
+ unsafe { slice::from_raw_parts_mut(slice::from_mut(val).as_mut_ptr().cast(), size) }
+}
+
+/// Cast a slice of a `Pod` type to a byte slice.
+#[inline]
+pub fn bytes_of_slice<T: Pod>(val: &[T]) -> &[u8] {
+ let size = val.len().wrapping_mul(mem::size_of::<T>());
+ // Safety:
+ // Any alignment is allowed.
+ // The size is determined in this function.
+ // The Pod trait ensures the type is valid to cast to bytes.
+ unsafe { slice::from_raw_parts(val.as_ptr().cast(), size) }
+}
+
+/// Cast a slice of a `Pod` type to a mutable byte slice.
+#[inline]
+pub fn bytes_of_slice_mut<T: Pod>(val: &mut [T]) -> &mut [u8] {
+ let size = val.len().wrapping_mul(mem::size_of::<T>());
+ // Safety:
+ // Any alignment is allowed.
+ // The size is determined in this function.
+ // The Pod trait ensures the type is valid to cast to bytes.
+ unsafe { slice::from_raw_parts_mut(val.as_mut_ptr().cast(), size) }
+}
+
+macro_rules! unsafe_impl_pod {
+ ($($struct_name:ident),+ $(,)?) => {
+ $(
+ unsafe impl Pod for $struct_name { }
+ )+
+ }
+}
+
+unsafe_impl_pod!(u8, u16, u32, u64);
+
+#[cfg(test)]
+mod tests {
+ use super::*;
+
+ #[test]
+ fn single() {
+ let x = u32::to_be(0x0123_4567);
+ let mut x_mut = x;
+ let bytes = bytes_of(&x);
+ let bytes_mut = bytes_of_mut(&mut x_mut);
+ assert_eq!(bytes, [0x01, 0x23, 0x45, 0x67]);
+ assert_eq!(bytes, bytes_mut);
+
+ let x16 = [u16::to_be(0x0123), u16::to_be(0x4567)];
+
+ let (y, tail) = from_bytes::<u32>(bytes).unwrap();
+ let (y_mut, tail_mut) = from_bytes_mut::<u32>(bytes_mut).unwrap();
+ assert_eq!(*y, x);
+ assert_eq!(y, y_mut);
+ assert_eq!(tail, &[]);
+ assert_eq!(tail, tail_mut);
+
+ let (y, tail) = from_bytes::<u16>(bytes).unwrap();
+ let (y_mut, tail_mut) = from_bytes_mut::<u16>(bytes_mut).unwrap();
+ assert_eq!(*y, x16[0]);
+ assert_eq!(y, y_mut);
+ assert_eq!(tail, &bytes[2..]);
+ assert_eq!(tail, tail_mut);
+
+ let (y, tail) = from_bytes::<u16>(&bytes[2..]).unwrap();
+ let (y_mut, tail_mut) = from_bytes_mut::<u16>(&mut bytes_mut[2..]).unwrap();
+ assert_eq!(*y, x16[1]);
+ assert_eq!(y, y_mut);
+ assert_eq!(tail, &[]);
+ assert_eq!(tail, tail_mut);
+
+ assert_eq!(from_bytes::<u16>(&bytes[1..]), Err(()));
+ assert_eq!(from_bytes::<u16>(&bytes[3..]), Err(()));
+ assert_eq!(from_bytes::<u16>(&bytes[4..]), Err(()));
+ assert_eq!(from_bytes_mut::<u16>(&mut bytes_mut[1..]), Err(()));
+ assert_eq!(from_bytes_mut::<u16>(&mut bytes_mut[3..]), Err(()));
+ assert_eq!(from_bytes_mut::<u16>(&mut bytes_mut[4..]), Err(()));
+ }
+
+ #[test]
+ fn slice() {
+ let x = [
+ u16::to_be(0x0123),
+ u16::to_be(0x4567),
+ u16::to_be(0x89ab),
+ u16::to_be(0xcdef),
+ ];
+ let mut x_mut = x;
+
+ let bytes = bytes_of_slice(&x);
+ let bytes_mut = bytes_of_slice_mut(&mut x_mut);
+ assert_eq!(bytes, [0x01, 0x23, 0x45, 0x67, 0x89, 0xab, 0xcd, 0xef]);
+ assert_eq!(bytes, bytes_mut);
+
+ let (y, tail) = slice_from_bytes::<u16>(bytes, 4).unwrap();
+ let (y_mut, tail_mut) = slice_from_bytes_mut::<u16>(bytes_mut, 4).unwrap();
+ assert_eq!(y, x);
+ assert_eq!(y, y_mut);
+ assert_eq!(tail, &[]);
+ assert_eq!(tail, tail_mut);
+
+ let (y, tail) = slice_from_bytes::<u16>(&bytes[2..], 2).unwrap();
+ let (y_mut, tail_mut) = slice_from_bytes::<u16>(&mut bytes_mut[2..], 2).unwrap();
+ assert_eq!(y, &x[1..3]);
+ assert_eq!(y, y_mut);
+ assert_eq!(tail, &bytes[6..]);
+ assert_eq!(tail, tail_mut);
+
+ assert_eq!(slice_from_bytes::<u16>(bytes, 5), Err(()));
+ assert_eq!(slice_from_bytes::<u16>(&bytes[2..], 4), Err(()));
+ assert_eq!(slice_from_bytes::<u16>(&bytes[1..], 2), Err(()));
+ assert_eq!(slice_from_bytes_mut::<u16>(bytes_mut, 5), Err(()));
+ assert_eq!(slice_from_bytes_mut::<u16>(&mut bytes_mut[2..], 4), Err(()));
+ assert_eq!(slice_from_bytes_mut::<u16>(&mut bytes_mut[1..], 2), Err(()));
+ }
+}
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