You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: mod.rs
    path of file: ./repos/webrtc/media/src/io/ogg_reader
    
    the code of the file until where you have to start completion: #[cfg(test)]
mod ogg_reader_test;

use std::io::{Cursor, Read};

use byteorder::{LittleEndian, ReadBytesExt};
use bytes::BytesMut;

use crate::error::{Error, Result};
use crate::io::ResetFn;

pub const PAGE_HEADER_TYPE_CONTINUATION_OF_STREAM: u8 = 0x00;
pub const PAGE_HEADER_TYPE_BEGINNING_OF_STREAM: u8 = 0x02;
pub const PAGE_HEADER_TYPE_END_OF_STREAM: u8 = 0x04;
pub const DEFAULT_PRE_SKIP: u16 = 3840; // 3840 recommended in the RFC
pub const PAGE_HEADER_SIGNATURE: &[u8] = b"OggS";
pub const ID_PAGE_SIGNATURE: &[u8] = b"OpusHead";
pub const COMMENT_PAGE_SIGNATURE: &[u8] = b"OpusTags";
pub const PAGE_HEADER_SIZE: usize = 27;
pub const ID_PAGE_PAYLOAD_SIZE: usize = 19;

/// OggReader is used to read Ogg files and return page payloads
pub struct OggReader<R: Read> {
    reader: R,
    bytes_read: usize,
    checksum_table: [u32; 256],
    do_checksum: bool,
}

/// OggHeader is the metadata from the first two pages
/// in the file (ID and Comment)
/// https://tools.ietf.org/html/rfc7845.html#section-3
pub struct OggHeader {
    pub channel_map: u8,
    pub channels: u8,
    pub output_gain: u16,
    pub pre_skip: u16,
    pub sample_rate: u32,
    pub version: u8,
}

/// OggPageHeader is the metadata for a Page
/// Pages are the fundamental unit of multiplexing in an Ogg stream
/// https://tools.ietf.org/html/rfc7845.html#section-1
pub struct OggPageHeader {
    pub granule_position: u64,

    sig: [u8; 4],
    version: u8,
    header_type: u8,
    serial: u32,
    index: u32,
    segments_count: u8,
}

impl<R: Read> OggReader<R> {
    /// new returns a new Ogg reader and Ogg header
    /// with an io.Reader input
    pub fn new(reader: R, do_checksum: bool) -> Result<(OggReader<R>, OggHeader)> {
        let mut r = OggR
    
    pub fn parse_next_page(&mut self) -> Result<(BytesMut, OggPageHeader)> {
        let mut h = [0u8; PAGE_HEADER_SIZE];
        self.reader.read_exact(&mut h)?;

        let mut head_reader = Cursor::new(h);
        let mut sig = [0u8; 4]; //0-3
        head_reader.read_exact(&mut sig)?;
        let version = head_reader.read_u8()?; //4
        let header_type = head_reader.read_u8()?; //5
        let granule_position = head_reader.read_u64::<LittleEndian>()?; //6-13
        let serial = head_reader.read_u32::<LittleEndian>()?; //14-17
        let index = head_reader.read_u32::<LittleEndian>()?; //18-21
        let checksum = head_reader.read_u32::<LittleEndian>()?; //22-25
        let segments_count = head_reader.read_u8()?; //26

        let mut size_buffer = vec![0u8; segments_count as usize];
        self.reader.read_exact(&mut size_buffer)?;

        let mut payload_size = 0usize;
        for s in &size_buffer {
            payload_size += *s as usize;
        }

        let mut payload = BytesMut::with_capacity(payload_size);
        payload.resize(payload_size, 0);
        self.reader.read_exact(&mut payload)?;

        if self.do_checksum {
            let mut sum = 0;

            for (index, v) in h.iter().enumerate() {
                // Don't include expected checksum in our generation
                if index > 21 && index < 26 {
                    sum = self.update_checksum(0, sum);
                    continue;
                }
                sum = self.update_checksum(*v, sum);
            }

            for v in &size_buffer {
                sum = self.update_checksum(*v, sum);
            }
            for v in &payload[..] {
                sum = self.update_checksum(*v, sum);
            }

            if sum != checksum {
                return Err(Error::ErrChecksumMismatch);
            }
        }

        let page_header = OggPageHeader {
            granule_position,
            sig,
            version,
            header_type,
            serial,
            index,
            segments_count,
        };

        Ok((payload, page_header))
    }