Split EntityMap into entity::PrimaryMap and entity::EntityMap.

The new PrimaryMap replaces the primary EntityMap and the PrimaryEntityData
marker trait which was causing some confusion. We now have a clear
division between the two types of maps:

- PrimaryMap is used to assign entity numbers to the primary data for an
  entity.
- EntityMap is a secondary mapping adding additional info.

The split also means that the secondary EntityMap can now behave as if
all keys have a default value. This means that we can get rid of the
annoying ensure() and get_or_default() methods ther were used everywhere
instead of indexing. Just use normal indexing now; non-existent keys
will return the default value.

lib/cretonne/src/entity/map.rs

@@ -0,0 +1,135 @@
+//! Densely numbered entity references as mapping keys.
+//!
+//! The `EntityMap` data structure uses the dense index space to implement a map with a vector.
+//! Unlike `PrimaryMap`, and `EntityMap` can't be used to allocate entity references. It is used to
+//! associate secondary information with entities.
+
+use entity::{EntityRef, Keys};
+use std::marker::PhantomData;
+use std::ops::{Index, IndexMut};
+
+/// A mapping `K -> V` for densely indexed entity references.
+#[derive(Debug, Clone)]
+pub struct EntityMap<K, V>
+    where K: EntityRef,
+          V: Clone
+{
+    elems: Vec<V>,
+    default: V,
+    unused: PhantomData<K>,
+}
+
+/// Shared `EntityMap` implementation for all value types.
+impl<K, V> EntityMap<K, V>
+    where K: EntityRef,
+          V: Clone
+{
+    /// Create a new empty map.
+    pub fn new() -> Self
+        where V: Default
+    {
+        EntityMap {
+            elems: Vec::new(),
+            default: Default::default(),
+            unused: PhantomData,
+        }
+    }
+
+    /// Get the element at `k` if it exists.
+    pub fn get(&self, k: K) -> Option<&V> {
+        self.elems.get(k.index())
+    }
+
+    /// Is this map completely empty?
+    pub fn is_empty(&self) -> bool {
+        self.elems.is_empty()
+    }
+
+    /// Remove all entries from this map.
+    pub fn clear(&mut self) {
+        self.elems.clear()
+    }
+
+    /// Iterate over all the keys in this map.
+    pub fn keys(&self) -> Keys<K> {
+        Keys::new(self.elems.len())
+    }
+
+    /// Resize the map to have `n` entries by adding default entries as needed.
+    pub fn resize(&mut self, n: usize) {
+        self.elems.resize(n, self.default.clone());
+    }
+}
+
+/// Immutable indexing into an `EntityMap`.
+///
+/// All keys are permitted. Untouched entries have the default value.
+impl<K, V> Index<K> for EntityMap<K, V>
+    where K: EntityRef,
+          V: Clone
+{
+    type Output = V;
+
+    fn index(&self, k: K) -> &V {
+        self.get(k).unwrap_or(&self.default)
+    }
+}
+
+/// Mutable indexing into an `EntityMap`.
+///
+/// The map grows as needed to accommodate new keys.
+impl<K, V> IndexMut<K> for EntityMap<K, V>
+    where K: EntityRef,
+          V: Clone
+{
+    fn index_mut(&mut self, k: K) -> &mut V {
+        let i = k.index();
+        if i >= self.elems.len() {
+            self.resize(i + 1);
+        }
+        &mut self.elems[i]
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    // `EntityRef` impl for testing.
+    #[derive(Clone, Copy, Debug, PartialEq, Eq)]
+    struct E(u32);
+
+    impl EntityRef for E {
+        fn new(i: usize) -> Self {
+            E(i as u32)
+        }
+        fn index(self) -> usize {
+            self.0 as usize
+        }
+    }
+
+    #[test]
+    fn basic() {
+        let r0 = E(0);
+        let r1 = E(1);
+        let r2 = E(2);
+        let mut m = EntityMap::new();
+
+        let v: Vec<E> = m.keys().collect();
+        assert_eq!(v, []);
+
+        m[r2] = 3;
+        m[r1] = 5;
+
+        assert_eq!(m[r1], 5);
+        assert_eq!(m[r2], 3);
+
+        let v: Vec<E> = m.keys().collect();
+        assert_eq!(v, [r0, r1, r2]);
+
+        let shared = &m;
+        assert_eq!(shared[r0], 0);
+        assert_eq!(shared[r1], 5);
+        assert_eq!(shared[r2], 3);
+    }
+}