在流模式下使用时，nom会尝试解析一个切片，然后告诉你它消耗了多少。因此，直到解析器返回之前，你都不知道实际使用了多少数据。Buffer公开了一个`data()`方法，它提供一个不可变的当前可读数据的切片，以及一个`consume()`方法来移动流中的位置。`space()`和`fill()`方法是这些方法的写入对应方法。

extern crate oval;

use oval::Buffer;
use std::io::Write;

fn main() {

  // allocate a new Buffer
  let mut b = Buffer::with_capacity(10);
  assert_eq!(b.available_data(), 0);
  assert_eq!(b.available_space(), 10);

  let res = b.write(&b"abcd"[..]);
  assert_eq!(res.ok(), Some(4));
  assert_eq!(b.available_data(), 4);
  assert_eq!(b.available_space(), 6);

  //the 4 bytes we wrote are immediately available and usable for parsing
  assert_eq!(b.data(), &b"abcd"[..]);

  // this will advance the position from 0 to 2. it does not modify the underlying Vec
  b.consume(2);
  assert_eq!(b.available_data(), 2);
  assert_eq!(b.available_space(), 6);
  assert_eq!(b.data(), &b"cd"[..]);

  // shift moves the available data at the beginning of the buffer.
  // the position is now 0
  b.shift();
  assert_eq!(b.available_data(), 2);
  assert_eq!(b.available_space(), 8);
  assert_eq!(b.data(), &b"cd"[..]);
}