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

use std::io::Error;
use std::io::ErrorKind;
use std::io::Read;
use std::io::Result;
use std::io::Seek;
use std::io::SeekFrom;
use std::io::Write;


pub struct IoSlice<T> where T: Seek {
    // note: the value of `begin`, `length`, `remaining`, and `begin + length` will never be greater than the value of `std::i64::max`
    // note: `length` and `remaining` will never be greater than `std::usize::max`

    underlying: T,
    begin:      u64,
    length:     u64,
    remaining:  u64,
}

impl<T> IoSlice<T> where T: Seek {
    pub fn new(mut source: T, begin: u64, length: u64) -> Result<IoSlice<T>> {
        // :: check invariants
        let u64_max   = std::i64::MAX as u64;
        let usize_max = std::usize::MAX as u64;

        if begin > u64_max || length > usize_max || begin + length > u64_max {
            return Err(Error::from(ErrorKind::InvalidInput));
        }


        // :: attempt to seek to the requested position.
        let seek = SeekFrom::Start(begin);

        match source.seek(seek) {
            Ok(position) => {
                if position == begin {
                    Ok(())
                } else {
                    // if the source does not seek us to the `begin` position, then this stream is "invalid".
                    Err(Error::from(ErrorKind::InvalidInput))
                }
            },
            Err(error) => {
                Err(error)
            },
        }?;


        // :: seek ok, now return the struct
        let slice = IoSlice {
            underlying: source,
            begin:      begin,
            length:     length,
            remaining:  length,
        };

        Ok(slice)
    }

    pub fn len(&self) -> u64 {
        self.length
    }

    pub fn position(&self) -> u64 {
        self.length - self.remaining
    }
}

impl<T> Seek for IoSlice<T> where T: Seek {
    fn seek(&mut self, position: SeekFrom) -> Result<u64> {
        // :: make sure that position(i64) is not more `std::i64::max`.
        if match position { SeekFrom::Start(x) => x as u64, SeekFrom::Current(x) => x as u64, SeekFrom::End(x) => x as u64 } > std::i64::MAX as u64 {
            return Err(Error::from(ErrorKind::InvalidInput));
        }


        // :: then calculate the new stream offset.
        let absolute = match position {
            SeekFrom::Start(value)   => self.begin + value as u64,
            SeekFrom::Current(value) => self.begin + self.length - self.remaining + value as u64,
            SeekFrom::End(value)     => self.begin + self.length + value as u64,
        };

        // :: seek.
        //
        // if the new requested position is in bounds. seek to it, and make sure that the new position is the one we
        // requested.
        if absolute >= self.begin && absolute <= self.begin + self.length {
            if self.underlying.seek(SeekFrom::Start(absolute))? == absolute {
                let new = absolute - self.begin;

                self.remaining = self.length - new;
                return Ok(new);
            }

            return Err(Error::from(ErrorKind::Other));
        }

        // the new requested position is out of bounds, return eof. we don't allow seeking out of bounds.
        Err(Error::from(ErrorKind::UnexpectedEof))
    }
}

impl<T> Read for IoSlice<T> where T: Read + Seek {
    fn read(&mut self, buffer: &mut [u8]) -> Result<usize> {
        // `std::io::read::read()` can only read `usize::max` bytes at once.
        let remaining   = std::cmp::min(self.remaining, std::usize::MAX as u64) as usize;
        let request     = std::cmp::min(remaining, buffer.len());
        let actual      = self.underlying.read(&mut buffer[..request])?;
        self.remaining -= actual as u64;

        Ok(actual)
    }

    fn read_to_end(&mut self, buffer: &mut Vec<u8>) -> Result<usize> {
        let length    = buffer.len();
        let remaining = self.remaining as usize;

        buffer.reserve(remaining);

        unsafe {
            let ptr   = buffer.as_mut_ptr().offset(length as isize);
            let slice = std::slice::from_raw_parts_mut(ptr, remaining);

            self.underlying.read_exact(slice)?;

            buffer.set_len(length + remaining);
            self.remaining = 0;
        }

        Ok(remaining)
    }
}

impl<T> Write for IoSlice<T> where T: Write + Seek {
    fn write(&mut self, buffer: &[u8]) -> Result<usize> {
        if buffer.len() as u64 > self.remaining {
            return Err(Error::from(ErrorKind::UnexpectedEof));
        }

        let actual = self.underlying.write(buffer)?;
        self.remaining -= actual as u64;

        Ok(actual)
    }

    fn flush(&mut self) -> Result<()> {
        self.underlying.flush()
    }
}