scroll/
pread.rs

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
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
        })
    }
}