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use core::result;
use crate::ctx::TryFromCtx;
use crate::error;
/// A very generic, contextual pread interface in Rust.
///
/// Like [Pwrite](trait.Pwrite.html) — but for reading!
///
/// Implementing `Pread` on a data store allows you to then read almost arbitrarily complex types
/// efficiently.
///
/// To this end the Pread trait works in conjuction with the [TryFromCtx](ctx/trait.TryFromCtx.html);
/// The `TryFromCtx` trait implemented on a type defines how to convert data to an object of that
/// type, the Pread trait implemented on a data store defines how to extract said data from that
/// store.
///
/// It should be noted though that in this context, data does not necessarily mean `&[u8]` —
/// `Pread` and `TryFromCtx` are generic over what 'data' means and could be implemented instead
/// over chunks of memory or any other indexable type — but scroll does come with a set of powerful
/// blanket implementations for data being a continous block of byte-addressable memory.
///
/// Note that in the particular case of the implementation of `Pread` for `[u8]`,
/// reading it at the length boundary of that slice will cause to read from an empty slice.
/// i.e. we make use of the fact that `&bytes[bytes.len()..]` will return an empty slice, rather
/// than returning an error. In the past, scroll returned an offset error.
///
/// Pread provides two main groups of functions: pread and gread.
///
/// `pread` is the basic function that simply extracts a given type from a given data store - either
/// using a provided Context in the case of [pread_with](trait.Pread.html#method.pread_with) or
/// with the default context for the given type in the case of [pread](trait.Pread.html#method.pread)
///
/// `gread` does in addition to that update the offset it's currently at, allowing for a cursored
/// read — `gread_inout` expands on that and reads a number of continous types from the data store.
/// gread again comes with `_with` variants to allow using a specific context.
///
/// Since pread and friends are very generic functions their types are rather complex, but very
/// much understandable; `TryFromCtx` is generic over `Ctx` ([described
/// here](ctx/index.html#context)), `Output` and `Error`. The Error type is hopefully
/// self-explanatory, however the `Output` type is rather important; it defines what Pread extracts
/// from the data store and has to match up with what `TryFromCtx` expects as input to convert into
/// the resulting type. scroll defaults to `&[u8]` here.
///
/// Unless you need to implement your own data store — that is either can't convert to `&[u8]` or
/// have a data that does not expose a `&[u8]` — you will probably want to implement
/// [TryFromCtx](ctx/trait.TryFromCtx.html) on your Rust types to be extracted.
///
pub trait Pread<Ctx: Copy, E> {
#[inline]
/// Reads a value from `self` at `offset` with a default `Ctx`. For the primitive numeric values, this will read at the machine's endianness.
/// # Example
/// ```rust
/// use scroll::Pread;
/// let bytes = [0x7fu8; 0x01];
/// let byte = bytes.pread::<u8>(0).unwrap();
fn pread<'a, N: TryFromCtx<'a, Ctx, Self, Error = E>>(
&'a self,
offset: usize,
) -> result::Result<N, E>
where
Ctx: Default,
{
self.pread_with(offset, Ctx::default())
}
#[inline]
/// Reads a value from `self` at `offset` with the given `ctx`
/// # Example
/// ```rust
/// use scroll::Pread;
/// let bytes: [u8; 2] = [0xde, 0xad];
/// let dead: u16 = bytes.pread_with(0, scroll::BE).unwrap();
/// assert_eq!(dead, 0xdeadu16);
fn pread_with<'a, N: TryFromCtx<'a, Ctx, Self, Error = E>>(
&'a self,
offset: usize,
ctx: Ctx,
) -> result::Result<N, E> {
let mut ignored = offset;
self.gread_with(&mut ignored, ctx)
}
#[inline]
/// Reads a value from `self` at `offset` with a default `Ctx`. For the primitive numeric values, this will read at the machine's endianness. Updates the offset
/// # Example
/// ```rust
/// use scroll::Pread;
/// let offset = &mut 0;
/// let bytes = [0x7fu8; 0x01];
/// let byte = bytes.gread::<u8>(offset).unwrap();
/// assert_eq!(*offset, 1);
fn gread<'a, N: TryFromCtx<'a, Ctx, Self, Error = E>>(
&'a self,
offset: &mut usize,
) -> result::Result<N, E>
where
Ctx: Default,
{
let ctx = Ctx::default();
self.gread_with(offset, ctx)
}
/// Reads a value from `self` at `offset` with the given `ctx`, and updates the offset.
/// # Example
/// ```rust
/// use scroll::Pread;
/// let offset = &mut 0;
/// let bytes: [u8; 2] = [0xde, 0xad];
/// let dead: u16 = bytes.gread_with(offset, scroll::BE).unwrap();
/// assert_eq!(dead, 0xdeadu16);
/// assert_eq!(*offset, 2);
fn gread_with<'a, N: TryFromCtx<'a, Ctx, Self, Error = E>>(
&'a self,
offset: &mut usize,
ctx: Ctx,
) -> result::Result<N, E>;
/// Tries to write `inout.len()` `N`s into `inout` from `Self` starting at `offset`, using the default context for `N`, and updates the offset.
/// # Example
/// ```rust
/// use scroll::Pread;
/// let mut bytes: Vec<u8> = vec![0, 0];
/// let offset = &mut 0;
/// let bytes_from: [u8; 2] = [0x48, 0x49];
/// bytes_from.gread_inout(offset, &mut bytes).unwrap();
/// assert_eq!(&bytes, &bytes_from);
/// assert_eq!(*offset, 2);
#[inline]
fn gread_inout<'a, N: TryFromCtx<'a, Ctx, Self, Error = E>>(
&'a self,
offset: &mut usize,
inout: &mut [N],
) -> result::Result<(), E>
where
Ctx: Default,
{
for i in inout.iter_mut() {
*i = self.gread(offset)?;
}
Ok(())
}
/// Tries to write `inout.len()` `N`s into `inout` from `Self` starting at `offset`, using the context `ctx`
/// # Example
/// ```rust
/// use scroll::{ctx, LE, Pread};
/// let mut bytes: Vec<u8> = vec![0, 0];
/// let offset = &mut 0;
/// let bytes_from: [u8; 2] = [0x48, 0x49];
/// bytes_from.gread_inout_with(offset, &mut bytes, LE).unwrap();
/// assert_eq!(&bytes, &bytes_from);
/// assert_eq!(*offset, 2);
#[inline]
fn gread_inout_with<'a, N: TryFromCtx<'a, Ctx, Self, Error = E>>(
&'a self,
offset: &mut usize,
inout: &mut [N],
ctx: Ctx,
) -> result::Result<(), E> {
for i in inout.iter_mut() {
*i = self.gread_with(offset, ctx)?;
}
Ok(())
}
}
impl<Ctx: Copy, E: From<error::Error>> Pread<Ctx, E> for [u8] {
fn gread_with<'a, N: TryFromCtx<'a, Ctx, Self, Error = E>>(
&'a self,
offset: &mut usize,
ctx: Ctx,
) -> result::Result<N, E> {
let start = *offset;
if start > self.len() {
return Err(error::Error::BadOffset(start).into());
}
N::try_from_ctx(&self[start..], ctx).map(|(n, size)| {
*offset += size;
n
})
}
}