cranelift_entity/
packed_option.rs

1//! Compact representation of `Option<T>` for types with a reserved value.
2//!
3//! Small Cranelift types like the 32-bit entity references are often used in tables and linked
4//! lists where an `Option<T>` is needed. Unfortunately, that would double the size of the tables
5//! because `Option<T>` is twice as big as `T`.
6//!
7//! This module provides a `PackedOption<T>` for types that have a reserved value that can be used
8//! to represent `None`.
9
10use core::fmt;
11use core::mem;
12
13#[cfg(feature = "enable-serde")]
14use serde_derive::{Deserialize, Serialize};
15
16/// Types that have a reserved value which can't be created any other way.
17pub trait ReservedValue {
18    /// Create an instance of the reserved value.
19    fn reserved_value() -> Self;
20    /// Checks whether value is the reserved one.
21    fn is_reserved_value(&self) -> bool;
22}
23
24/// Packed representation of `Option<T>`.
25#[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Hash)]
26#[cfg_attr(feature = "enable-serde", derive(Serialize, Deserialize))]
27pub struct PackedOption<T: ReservedValue>(T);
28
29impl<T: ReservedValue> PackedOption<T> {
30    /// Returns `true` if the packed option is a `None` value.
31    pub fn is_none(&self) -> bool {
32        self.0.is_reserved_value()
33    }
34
35    /// Returns `true` if the packed option is a `Some` value.
36    pub fn is_some(&self) -> bool {
37        !self.0.is_reserved_value()
38    }
39
40    /// Expand the packed option into a normal `Option`.
41    pub fn expand(self) -> Option<T> {
42        if self.is_none() {
43            None
44        } else {
45            Some(self.0)
46        }
47    }
48
49    /// Maps a `PackedOption<T>` to `Option<U>` by applying a function to a contained value.
50    pub fn map<U, F>(self, f: F) -> Option<U>
51    where
52        F: FnOnce(T) -> U,
53    {
54        self.expand().map(f)
55    }
56
57    /// Unwrap a packed `Some` value or panic.
58    pub fn unwrap(self) -> T {
59        self.expand().unwrap()
60    }
61
62    /// Unwrap a packed `Some` value or panic.
63    pub fn expect(self, msg: &str) -> T {
64        self.expand().expect(msg)
65    }
66
67    /// Takes the value out of the packed option, leaving a `None` in its place.
68    pub fn take(&mut self) -> Option<T> {
69        mem::replace(self, None.into()).expand()
70    }
71}
72
73impl<T: ReservedValue> Default for PackedOption<T> {
74    /// Create a default packed option representing `None`.
75    fn default() -> Self {
76        Self(T::reserved_value())
77    }
78}
79
80impl<T: ReservedValue> From<T> for PackedOption<T> {
81    /// Convert `t` into a packed `Some(x)`.
82    fn from(t: T) -> Self {
83        debug_assert!(
84            !t.is_reserved_value(),
85            "Can't make a PackedOption from the reserved value."
86        );
87        Self(t)
88    }
89}
90
91impl<T: ReservedValue> From<Option<T>> for PackedOption<T> {
92    /// Convert an option into its packed equivalent.
93    fn from(opt: Option<T>) -> Self {
94        match opt {
95            None => Self::default(),
96            Some(t) => t.into(),
97        }
98    }
99}
100
101impl<T: ReservedValue> Into<Option<T>> for PackedOption<T> {
102    fn into(self) -> Option<T> {
103        self.expand()
104    }
105}
106
107impl<T> fmt::Debug for PackedOption<T>
108where
109    T: ReservedValue + fmt::Debug,
110{
111    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
112        if self.is_none() {
113            write!(f, "None")
114        } else {
115            write!(f, "Some({:?})", self.0)
116        }
117    }
118}
119
120#[cfg(test)]
121mod tests {
122    use super::*;
123
124    // Dummy entity class, with no Copy or Clone.
125    #[derive(Debug, PartialEq, Eq)]
126    struct NoC(u32);
127
128    impl ReservedValue for NoC {
129        fn reserved_value() -> Self {
130            NoC(13)
131        }
132
133        fn is_reserved_value(&self) -> bool {
134            self.0 == 13
135        }
136    }
137
138    #[test]
139    fn moves() {
140        let x = NoC(3);
141        let somex: PackedOption<NoC> = x.into();
142        assert!(!somex.is_none());
143        assert_eq!(somex.expand(), Some(NoC(3)));
144
145        let none: PackedOption<NoC> = None.into();
146        assert!(none.is_none());
147        assert_eq!(none.expand(), None);
148    }
149
150    // Dummy entity class, with Copy.
151    #[derive(Clone, Copy, Debug, PartialEq, Eq)]
152    struct Ent(u32);
153
154    impl ReservedValue for Ent {
155        fn reserved_value() -> Self {
156            Ent(13)
157        }
158
159        fn is_reserved_value(&self) -> bool {
160            self.0 == 13
161        }
162    }
163
164    #[test]
165    fn copies() {
166        let x = Ent(2);
167        let some: PackedOption<Ent> = x.into();
168        assert_eq!(some.expand(), x.into());
169        assert_eq!(some, x.into());
170    }
171}
cranelift_entity/packed_option.rs

cranelift_entity/
packed_option.rs
