Move entity into a separate crate (#297)

lib/entity/src/lib.rs

@@ -0,0 +1,122 @@
+//! Array-based data structures using densely numbered entity references as mapping keys.
+//!
+//! This crate defines a number of data structures based on arrays. The arrays are not indexed by
+//! `usize` as usual, but by *entity references* which are integers wrapped in new-types. This has
+//! a couple advantages:
+//!
+//! - Improved type safety. The various map and set types accept a specific key type, so there is
+//!   no confusion about the meaning of an array index, as there is with plain arrays.
+//! - Smaller indexes. The normal `usize` index is often 64 bits which is way too large for most
+//!   purposes. The entity reference types can be smaller, allowing for more compact data
+//!   structures.
+//!
+//! The `EntityRef` trait should be implemented by types to be used as indexed. The `entity_impl!`
+//! macro provides convenient defaults for types wrapping `u32` which is common.
+//!
+//! - [`PrimaryMap`](struct.PrimaryMap.html) is used to keep track of a vector of entities,
+//!   assigning a unique entity reference to each.
+//! - [`EntityMap`](struct.EntityMap.html) is used to associate secondary information to an entity.
+//!   The map is implemented as a simple vector, so it does not keep track of which entities have
+//!   been inserted. Instead, any unknown entities map to the default value.
+//! - [`SparseMap`](struct.SparseMap.html) is used to associate secondary information to a small
+//!   number of entities. It tracks accurately which entities have been inserted. This is a
+//!   specialized data structure which can use a lot of memory, so read the documentation before
+//!   using it.
+//! - [`EntitySet`](struct.EntitySet.html) is used to represent a secondary set of entities.
+//!   The set is implemented as a simple vector, so it does not keep track of which entities have
+//!   been inserted into the primary map. Instead, any unknown entities are not in the set.
+//! - [`EntityList`](struct.EntityList.html) is a compact representation of lists of entity
+//!   references allocated from an associated memory pool. It has a much smaller footprint than
+//!   `Vec`.
+
+#![deny(missing_docs, trivial_numeric_casts, unused_extern_crates)]
+#![cfg_attr(feature = "clippy", plugin(clippy(conf_file = "../../clippy.toml")))]
+#![cfg_attr(feature = "cargo-clippy",
+            allow(new_without_default, new_without_default_derive, redundant_field_names))]
+
+// Turns on no_std and alloc features if std is not available.
+#![cfg_attr(not(feature = "std"), no_std)]
+#![cfg_attr(not(feature = "std"), feature(alloc))]
+
+/// This replaces `std` in builds with `core`.
+#[cfg(not(feature = "std"))]
+mod std {
+    extern crate alloc;
+    pub use self::alloc::{boxed, string, vec};
+    pub use core::*;
+}
+
+// Re-export core so that the macros works with both std and no_std crates
+#[doc(hidden)]
+pub extern crate core as __core;
+
+/// A type wrapping a small integer index should implement `EntityRef` so it can be used as the key
+/// of an `EntityMap` or `SparseMap`.
+pub trait EntityRef: Copy + Eq {
+    /// Create a new entity reference from a small integer.
+    /// This should crash if the requested index is not representable.
+    fn new(usize) -> Self;
+
+    /// Get the index that was used to create this entity reference.
+    fn index(self) -> usize;
+}
+
+/// Macro which provides the common implementation of a 32-bit entity reference.
+#[macro_export]
+macro_rules! entity_impl {
+    // Basic traits.
+    ($entity:ident) => {
+        impl $crate::EntityRef for $entity {
+            fn new(index: usize) -> Self {
+                debug_assert!(index < ($crate::__core::u32::MAX as usize));
+                $entity(index as u32)
+            }
+
+            fn index(self) -> usize {
+                self.0 as usize
+            }
+        }
+
+        impl $crate::packed_option::ReservedValue for $entity {
+            fn reserved_value() -> $entity {
+                $entity($crate::__core::u32::MAX)
+            }
+        }
+    };
+
+    // Include basic `Display` impl using the given display prefix.
+    // Display an `Ebb` reference as "ebb12".
+    ($entity:ident, $display_prefix:expr) => {
+        entity_impl!($entity);
+
+        impl $crate::__core::fmt::Display for $entity {
+            fn fmt(&self, f: &mut $crate::__core::fmt::Formatter) -> $crate::__core::fmt::Result {
+                write!(f, "{}{}", $display_prefix, self.0)
+            }
+        }
+
+        impl $crate::__core::fmt::Debug for $entity {
+            fn fmt(&self, f: &mut $crate::__core::fmt::Formatter) -> $crate::__core::fmt::Result {
+                (self as &$crate::__core::fmt::Display).fmt(f)
+            }
+        }
+    };
+}
+
+pub mod packed_option;
+
+mod iter;
+mod keys;
+mod list;
+mod map;
+mod primary;
+mod set;
+mod sparse;
+
+pub use self::iter::{Iter, IterMut};
+pub use self::keys::Keys;
+pub use self::list::{EntityList, ListPool};
+pub use self::map::EntityMap;
+pub use self::primary::PrimaryMap;
+pub use self::set::EntitySet;
+pub use self::sparse::{SparseMap, SparseMapValue, SparseSet};