1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131

use super::*;

/// 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,
}

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)
  }
}