range-reader 0.2.0

use std::io::{Read, Result, Seek, SeekFrom};

type FnAPI = Box<dyn Fn(usize, &mut [u8]) -> Result<()>>;

/// Implements `Read + Seek` for a (blocking) function that reads ranges of bytes.
/// # Implementation
/// This struct has an internal `Vec<u8>` that buffers calls.
pub struct RangedReader {
    pos: u64,        // position of the seek
    length: u64,     // total size
    buffer: Vec<u8>, // a ring
    offset: usize,   // offset in the ring: buffer[:offset] have been read
    range_fn: FnAPI,
}

impl RangedReader {
    /// Creates a new [`RangedReader`] with internal buffer `buffer`
    pub fn new(length: usize, range_fn: FnAPI, mut buffer: Vec<u8>) -> Self {
        let length = length as u64;
        buffer.clear();
        Self {
            pos: 0,
            range_fn,
            length,
            buffer,
            offset: 0,
        }
    }

    fn read_more(&mut self, to_consume: usize) -> Result<()> {
        let remaining = self.buffer.len() - self.offset;

        if to_consume < remaining {
            return Ok(());
        }
        let to_read = std::cmp::max(
            std::cmp::max(self.offset, to_consume),
            self.buffer.capacity(),
        ) - remaining;

        self.buffer.rotate_left(self.offset);
        self.buffer.resize(remaining + to_read, 0);

        (self.range_fn)(self.pos as usize, &mut self.buffer[remaining..])?;
        self.pos += to_read as u64;
        self.offset = 0;
        Ok(())
    }
}

impl Read for RangedReader {
    fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
        let to_consume = buf.len();
        self.read_more(to_consume)?;

        // copy from the internal buffer.
        buf[..to_consume].copy_from_slice(&self.buffer[self.offset..self.offset + to_consume]);
        // and offset
        self.offset += to_consume;
        Ok(to_consume)
    }
}

impl Seek for RangedReader {
    fn seek(&mut self, pos: SeekFrom) -> Result<u64> {
        match pos {
            SeekFrom::Start(pos) => self.pos = pos,
            SeekFrom::End(pos) => self.pos = (self.length as i64 + pos) as u64,
            SeekFrom::Current(pos) => self.pos = (self.pos as i64 + pos) as u64,
        };
        // todo: optimize: do not clear buffer and instead check whether we can just move the offset.
        self.offset = 0;
        self.buffer.clear();
        Ok(self.pos)
    }
}