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
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
// SPDX-License-Identifier: Apache-2.0

//! Simple, Low-level I/O traits
//!
//! This crate provides two simple traits: `Read` and `Write`. These traits
//! mimic their counterparts in `std::io`, but are trimmed for simplicity
//! and can be used in `no_std` and `no_alloc` environments. Since this
//! crate contains only traits, inline functions and unit structs, it should
//! be a zero-cost abstraction.
//!
//! If the `std` feature is enabled, we provide blanket implementations for
//! all `std::io` types. If the `alloc` feature is enabled, we provide
//! implementations for `Vec<u8>`. In all cases, you get implementations
//! for byte slices. You can, of course, implement the traits for your own
//! types.

#![cfg_attr(not(feature = "std"), no_std)]
#![deny(missing_docs)]
#![deny(clippy::all)]
#![deny(clippy::cargo)]

#[cfg(feature = "alloc")]
extern crate alloc;

/// A trait indicating a type that can read bytes
///
/// Note that this is similar to `std::io::Read`, but simplified for use in a
/// `no_std` context.
pub trait Read {
    /// The error type
    type Error;

    /// Reads exactly `data.len()` bytes or fails
    fn read_exact(&mut self, data: &mut [u8]) -> Result<(), Self::Error>;
}

/// A trait indicating a type that can write bytes
///
/// Note that this is similar to `std::io::Write`, but simplified for use in a
/// `no_std` context.
pub trait Write {
    /// The error type
    type Error;

    /// Writes all bytes from `data` or fails
    fn write_all(&mut self, data: &[u8]) -> Result<(), Self::Error>;

    /// Flushes all output
    fn flush(&mut self) -> Result<(), Self::Error>;
}

#[cfg(feature = "std")]
impl<T: std::io::Read> Read for T {
    type Error = std::io::Error;

    #[inline]
    fn read_exact(&mut self, data: &mut [u8]) -> Result<(), Self::Error> {
        self.read_exact(data)
    }
}

#[cfg(feature = "std")]
impl<T: std::io::Write> Write for T {
    type Error = std::io::Error;

    #[inline]
    fn write_all(&mut self, data: &[u8]) -> Result<(), Self::Error> {
        self.write_all(data)
    }

    #[inline]
    fn flush(&mut self) -> Result<(), Self::Error> {
        self.flush()
    }
}

#[cfg(not(feature = "std"))]
impl<R: Read + ?Sized> Read for &mut R {
    type Error = R::Error;

    #[inline]
    fn read_exact(&mut self, data: &mut [u8]) -> Result<(), Self::Error> {
        (**self).read_exact(data)
    }
}

#[cfg(not(feature = "std"))]
impl<W: Write + ?Sized> Write for &mut W {
    type Error = W::Error;

    #[inline]
    fn write_all(&mut self, data: &[u8]) -> Result<(), Self::Error> {
        (**self).write_all(data)
    }

    #[inline]
    fn flush(&mut self) -> Result<(), Self::Error> {
        (**self).flush()
    }
}

/// An error indicating there are no more bytes to read
#[cfg(not(feature = "std"))]
#[derive(Debug)]
pub struct EndOfFile(());

#[cfg(not(feature = "std"))]
impl Read for &[u8] {
    type Error = EndOfFile;

    #[inline]
    fn read_exact(&mut self, data: &mut [u8]) -> Result<(), Self::Error> {
        if data.len() > self.len() {
            return Err(EndOfFile(()));
        }

        let (prefix, suffix) = self.split_at(data.len());
        data.copy_from_slice(prefix);
        *self = suffix;
        Ok(())
    }
}

/// An error indicating that the output cannot accept more bytes
#[cfg(not(feature = "std"))]
#[derive(Debug)]
pub struct OutOfSpace(());

#[cfg(not(feature = "std"))]
impl Write for &mut [u8] {
    type Error = OutOfSpace;

    #[inline]
    fn write_all(&mut self, data: &[u8]) -> Result<(), Self::Error> {
        if data.len() > self.len() {
            return Err(OutOfSpace(()));
        }

        let (prefix, suffix) = core::mem::take(self).split_at_mut(data.len());
        prefix.copy_from_slice(data);
        *self = suffix;
        Ok(())
    }

    #[inline]
    fn flush(&mut self) -> Result<(), Self::Error> {
        Ok(())
    }
}

#[cfg(all(not(feature = "std"), feature = "alloc"))]
impl Write for alloc::vec::Vec<u8> {
    type Error = core::convert::Infallible;

    #[inline]
    fn write_all(&mut self, data: &[u8]) -> Result<(), Self::Error> {
        self.extend_from_slice(data);
        Ok(())
    }

    #[inline]
    fn flush(&mut self) -> Result<(), Self::Error> {
        Ok(())
    }
}

#[cfg(test)]
mod test {
    use super::*;

    #[test]
    fn read_eof() {
        let mut reader = &[1u8; 0][..];
        let mut buffer = [0u8; 1];

        reader.read_exact(&mut buffer[..]).unwrap_err();
    }

    #[test]
    fn read_one() {
        let mut reader = &[1u8; 1][..];
        let mut buffer = [0u8; 1];

        reader.read_exact(&mut buffer[..]).unwrap();
        assert_eq!(buffer[0], 1);

        reader.read_exact(&mut buffer[..]).unwrap_err();
    }

    #[test]
    fn read_two() {
        let mut reader = &[1u8; 2][..];
        let mut buffer = [0u8; 1];

        reader.read_exact(&mut buffer[..]).unwrap();
        assert_eq!(buffer[0], 1);

        reader.read_exact(&mut buffer[..]).unwrap();
        assert_eq!(buffer[0], 1);

        reader.read_exact(&mut buffer[..]).unwrap_err();
    }

    #[test]
    #[cfg(feature = "std")]
    fn read_std() {
        let mut reader = std::io::repeat(1);
        let mut buffer = [0u8; 2];

        reader.read_exact(&mut buffer[..]).unwrap();
        assert_eq!(buffer[0], 1);
        assert_eq!(buffer[1], 1);
    }

    #[test]
    fn write_oos() {
        let mut writer = &mut [0u8; 0][..];

        writer.write_all(&[1u8; 1][..]).unwrap_err();
    }

    #[test]
    fn write_one() {
        let mut buffer = [0u8; 1];
        let mut writer = &mut buffer[..];

        writer.write_all(&[1u8; 1][..]).unwrap();
        writer.write_all(&[1u8; 1][..]).unwrap_err();
        assert_eq!(buffer[0], 1);
    }

    #[test]
    fn write_two() {
        let mut buffer = [0u8; 2];
        let mut writer = &mut buffer[..];

        writer.write_all(&[1u8; 1][..]).unwrap();
        writer.write_all(&[1u8; 1][..]).unwrap();
        writer.write_all(&[1u8; 1][..]).unwrap_err();
        assert_eq!(buffer[0], 1);
        assert_eq!(buffer[1], 1);
    }

    #[test]
    #[cfg(feature = "alloc")]
    fn write_vec() {
        let mut buffer = alloc::vec::Vec::new();

        buffer.write_all(&[1u8; 1][..]).unwrap();
        buffer.write_all(&[1u8; 1][..]).unwrap();

        assert_eq!(buffer.len(), 2);
        assert_eq!(buffer[0], 1);
        assert_eq!(buffer[1], 1);
    }

    #[test]
    #[cfg(feature = "std")]
    fn write_std() {
        let mut writer = std::io::sink();

        writer.write_all(&[1u8; 1][..]).unwrap();
        writer.write_all(&[1u8; 1][..]).unwrap();
    }
}