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#![deny(missing_docs)]
#![feature(external_doc)]
#![doc(include = "../README.md")]

use std::ops::{Deref, DerefMut};

/// Memory pager instance. Manages [`Page`] instances.
///
/// [`Page`]: struct.Page.html
#[derive(Debug)]
pub struct Pager {
  /// The size of each page held in memory.
  pub page_size: usize,
  /// A vector of pages that are held in memory.
  pub pages: Vec<Option<Page>>,
  length: usize,
}

/// Memory pages returned by [`Pager`].
///
/// [`Pager`]: struct.Pager.html
#[derive(Debug, Clone)]
pub struct Page {
  /// Byte offset for the start of the `Page` relative to all other `Page`
  /// instances.
  offset: usize,
  /// Buffer with capacity of size [`page_size`].
  ///
  /// [`page_size`]: struct.Pager.html#structfield.page_size
  buffer: Vec<u8>,
}

impl Page {
  fn new(i: usize, buf: Vec<u8>) -> Self {
    Page {
      offset: i * buf.capacity(),
      buffer: buf,
    }
  }

  /// Byte offset for the start of the `Page` relative to all other `Page`
  /// instances.
  pub fn offset(&self) -> usize {
    self.offset
  }
}

impl Deref for Page {
  type Target = [u8];
  fn deref(&self) -> &Self::Target {
    &self.buffer
  }
}

impl DerefMut for Page {
  fn deref_mut(&mut self) -> &mut Self::Target {
    &mut self.buffer
  }
}

impl Pager {
  /// Create a new [`Pager`] instance with a [`page_size`].
  ///
  /// [`Pager`]: struct.Pager.html
  /// [`page_size`]: struct.Pager.html#structfield.page_size
  pub fn new(page_size: usize) -> Self {
    Pager {
      page_size,
      length: 0,
      pages: vec![None; 16],
    }
  }

  /// Create a new [`Pager`] instance with a [`page_size`] and [`pages`]. Useful
  /// to restore a [`Pager`] instance from disk.
  ///
  /// [`Pager`]: struct.Pager.html
  /// [`page_size`]: struct.Pager.html#structfield.page_size
  /// [`pages`]: struct.Pager.html#structfield.pages
  pub fn with_pages(page_size: usize, pages: Vec<Option<Page>>) -> Self {
    for page in &pages {
      if let Some(ref page) = *page {
        assert_eq!(page.len(), page_size);
      }
    }

    Pager {
      page_size,
      length: pages.len(),
      pages,
    }
  }

  /// Get a [`Page`] mutably. The page will be allocated on first access.
  ///
  /// [`Page`]: struct.Page.html
  pub fn get_mut_or_alloc(&mut self, page_num: usize) -> &mut Page {
    if page_num >= self.pages.capacity() {
      self.grow_pages(page_num);
    }

    // This should never be out of bounds.
    if self.pages[page_num].is_none() {
      let buf = vec![0; self.page_size];
      let page = Page::new(page_num, buf);
      self.pages.insert(page_num, Some(page));
    }

    self.pages[page_num].as_mut().unwrap()
  }

  /// Get a [`Page`] wrapped in an `Option` enum. Does not allocate on access.
  ///
  /// [`Page`]: struct.Page.html
  pub fn get(&mut self, page_num: usize) -> Option<&Page> {
    match self.pages.get(page_num) {
      None => None,
      Some(page) => match page.as_ref() {
        None => None,
        Some(page) => Some(page),
      },
    }
  }

  /// Get a mutable [`Page`] wrapped in an `Option` enum. Does not allocate on
  /// access.
  ///
  /// [`Page`]: struct.Page.html
  pub fn get_mut(&mut self, page_num: usize) -> Option<&mut Page> {
    match self.pages.get_mut(page_num) {
      None => None,
      Some(page) => match page.as_mut() {
        None => None,
        Some(page) => Some(page),
      },
    }
  }

  /// Grow the page buffer capacity to accomodate more elements.
  fn grow_pages(&mut self, index: usize) {
    let start_len = self.pages.capacity();
    let mut new_len = start_len * 2;

    // Guard against a page size of 0.
    if new_len == 0 {
      new_len += 1
    }

    while new_len <= index {
      new_len *= 2;
    }

    self.pages.resize(new_len, None);
  }

  /// Return the highest index number for the Pages held. Not exactly the
  /// "length" in the classical sence, but the same for all intents and
  /// purposes.
  pub fn len(&self) -> usize {
    self.length
  }

  /// check whether the `length` is zero.
  pub fn is_empty(&self) -> bool {
    self.length == 0
  }
}

/// Create a new [`Pager`] instance with a [`page_size`] of `1024`.
///
/// [`Pager`]: struct.Pager.html
/// [`page_size`]: struct.Pager.html#structfield.page_size
impl Default for Pager {
  fn default() -> Self {
    Pager::new(1024)
  }
}