xan 0.49.2

The CSV magician
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use std::io::{self, Read, Seek, SeekFrom};

pub struct ReverseRead<R> {
    input: R,
    offset: u64,
    ptr: u64,
}

impl<R: Seek + Read> Read for ReverseRead<R> {
    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
        let buff_size = buf.len() as u64;

        if self.ptr == self.offset {
            return Ok(0);
        }

        if self.offset + buff_size > self.ptr {
            let e = (self.ptr - self.offset) as usize;

            self.input.seek(SeekFrom::Start(self.offset))?;
            self.input.read_exact(&mut buf[0..e])?;

            buf[0..e].reverse();

            self.ptr = self.offset;

            Ok(e)
        } else {
            let new_position = self.ptr - buff_size;

            self.input.seek(SeekFrom::Start(new_position))?;
            self.input.read_exact(buf)?;
            buf.reverse();

            self.ptr -= buff_size;

            Ok(buff_size as usize)
        }
    }
}

impl<R: Seek + Read> ReverseRead<R> {
    pub fn new(input: R, filesize: u64, offset: u64) -> Self {
        Self {
            input,
            offset,
            ptr: filesize,
        }
    }
}