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
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
use std::ops::{Index, IndexMut};

/// An AST node ID for lookups in the arena
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
pub struct NodeId(pub usize);

/// Arenas are efficient ways of storing trees: All nodes are actually on a
/// flat surface, but have IDs to other nodes.
#[derive(Debug)]
pub enum Arena<'a, T: 'a> {
    Owned(Vec<Option<T>>),
    Borrowed(&'a mut Vec<Option<T>>)
}
impl<'a, T> Default for Arena<'a, T> {
    fn default() -> Self {
        Arena::Owned(Vec::new())
    }
}
impl<'a, T> Arena<'a, T> {
    /// Create a new arena instance
    pub fn new() -> Self {
        Self::default()
    }
    /// Create a new arena instance that reuses an existing vector
    pub fn with_arena(arena: &'a mut Vec<Option<T>>) -> Self {
        Arena::Borrowed(arena)
    }
    /// Create a new arena reference that shares the inner vector. This
    /// reference will make the existing instance invalid for use while the
    /// reference is active.
    pub fn reference<'b>(&'b mut self) -> Arena<'b, T> {
        Arena::with_arena(self.get_mut())
    }

    /// Return the arena instance
    ///
    /// # Panics
    /// Panics if this arena instance was created with Borrowed
    pub fn into_inner(self) -> Vec<Option<T>> {
        match self {
            Arena::Owned(inner) => inner,
            Arena::Borrowed(_) => panic!("can't move out of borrowed arena")
        }
    }
    /// Return a slice pointing to the inner arena representation
    pub fn get_ref(&self) -> &[Option<T>] {
        match self {
            Arena::Owned(inner) => inner,
            Arena::Borrowed(inner) => inner
        }
    }
    fn get_mut(&mut self) -> &mut Vec<Option<T>> {
        match self {
            Arena::Owned(inner) => inner,
            Arena::Borrowed(inner) => inner
        }
    }
    /// Return an iterator over the nodes in this arena
    pub fn iter(&self) -> ArenaIter<T> {
        self.into_iter()
    }
    /// Place an element into the arena
    pub fn insert(&mut self, elem: T) -> NodeId {
        let inner = self.get_mut();
        inner.push(Some(elem));
        NodeId(inner.len() - 1)
    }
    /// Move out of an element in the arena. This will make following indexing
    /// calls on that index panic. This will not deallocate any space. This is
    /// due to the fact that all indexes need to stay the same. We could use a
    /// different data type instead of a tree to prevent this, but that instead
    /// means lookup will be slightly slower.
    pub fn take(&mut self, index: NodeId) -> T {
        self.get_mut()[index.0].take().expect("this index has been taken")
    }

    /// Get an iterator over all existing NodeIds in the arena.
    pub fn ids(&'a self) -> impl Iterator<Item = NodeId> + 'a {
        let inner = self.get_ref();

        inner
            .iter()
            .enumerate()
            .filter(|(_, val)| val.is_some())
            .map(|(id, _)| NodeId(id))
    }
}
impl<'a, T> Index<NodeId> for Arena<'a, T> {
    type Output = T;

    fn index(&self, index: NodeId) -> &Self::Output {
        &self.get_ref()[index.0].as_ref().expect("this index has been taken")
    }
}
impl<'a, T> IndexMut<NodeId> for Arena<'a, T> {
    fn index_mut(&mut self, index: NodeId) -> &mut Self::Output {
        self.get_mut()[index.0].as_mut().expect("this index has been taken")
    }
}
impl<'r, 'a, T> IntoIterator for &'r Arena<'a, T> {
    type Item = &'r T;
    type IntoIter = ArenaIter<'r, T>;

    fn into_iter(self) -> Self::IntoIter {
        ArenaIter {
            slice: self.get_ref(),
            index: 0
        }
    }
}

impl<'a, T> Clone for Arena<'a, T> where T: Clone {
    fn clone(&self) -> Self {
        match self {
            Arena::Owned(inner) => Arena::Owned(inner.to_vec()),
            Arena::Borrowed(inner) => Arena::Owned(inner.to_vec())
        }
    }
}

/// An iterator over an arena, created with `iter`
pub struct ArenaIter<'a, T: 'a> {
    slice: &'a [Option<T>],
    index: usize
}
impl<'a, T> Iterator for ArenaIter<'a, T> {
    type Item = &'a T;

    fn next(&mut self) -> Option<Self::Item> {
        while let Some(None) = self.slice.get(self.index) {
            self.index += 1;
        }
        let item = self.slice.get(self.index).map(|item| item.as_ref().unwrap());
        self.index += 1;
        item
    }
}

#[cfg(test)]
mod tests {
    use super::{Arena, NodeId};

    #[test]
    fn it_should_return_node_ids() {
        let mut arena: Arena<'static, u32> = Arena::new();

        arena.insert(100);
        arena.insert(101);

        let ids: Vec<NodeId> = arena.ids().collect();
        assert_eq!(
            ids,
            vec![NodeId(0), NodeId(1)]
        )
    }

    #[test]
    fn it_should_not_return_node_ids_of_removed_elements() {
        let mut arena: Arena<'static, u32> = Arena::new();

        arena.insert(100);
        arena.insert(101);
        arena.take(NodeId(0));

        let ids: Vec<NodeId> = arena.ids().collect();
        assert_eq!(
            ids,
            vec![NodeId(1)]
        )
    }
}