use std::iter;

#[derive(Debug)]
struct Book {
  pub title: String,
}

#[derive(Debug)]
struct Library {
  pub books: Vec<Book>,
}

#[derive(Debug)]
struct LibraryIterator<'b> {
  /// Keeps track which index we're currently at.
  pub cursor: usize,
  /// Borrow of the Bookshelf we're going to iterate over.
  pub inner: &'b Library,
}

#[derive(Debug)]
struct LibraryIter {
  /// Borrow of the Bookshelf we're going to iterate over.
  pub inner: Library,
}

impl Library {
  /// Create an iterator that iterates over all books in the library.
  pub fn iter(&self) -> LibraryIterator {
    LibraryIterator {
      inner: self,
      cursor: 0,
    }
  }
}

impl<'b> iter::Iterator for LibraryIterator<'b> {
  type Item = &'b Book;

  fn next(&mut self) -> Option<Self::Item> {
    let cursor = self.cursor;
    self.cursor += 1;

    if cursor >= self.inner.books.len() {
      None
    } else {
      Some(&self.inner.books[cursor])
    }
  }
}

impl iter::Iterator for LibraryIter {
  type Item = Book;

  fn next(&mut self) -> Option<Self::Item> {
      if self.inner.books.len() > 0 {
          Some(self.inner.books.remove(0))
      } else {
          None
      }
  }
}

impl<'b> iter::IntoIterator for &'b Library {
  type Item = &'b Book;
  type IntoIter = LibraryIterator<'b>;

  fn into_iter(self) -> Self::IntoIter {
    Self::IntoIter {
      cursor: 0,
      inner: self,
    }
  }
}

impl<'b> iter::IntoIterator for Library {
  type Item = Book;
  type IntoIter = LibraryIter;

  fn into_iter(self) -> Self::IntoIter {
    Self::IntoIter {
      inner: self,
    }
  }
}

fn main() {
  let library = Library {
    books: vec![
      Book { title: "Das Kapital I".into() },
      Book { title: "Das Kapital II".into() },
      Book { title: "Das Kapital III".into() },
    ],
  };

  // Borrow library and iterate.
  for book in library.iter() {
    println!("book {}", book.title);
  }

  // Consume library and iterate. `library` can no longer be used after this
  // point.
  for book in &library {
    println!("book {}", book.title);
  }

  for book in library {
    println!("book {}", book.title);
  }
}