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
172
173
174
175
176
177
178
179
180
181
182
183
184

// SPDX-License-Identifier: CC0-1.0

//! Provides a wrapper for types that can technically implement `PartialOrd`/`Ord`
//! but for semantic reasons it is nonsensical.
//!
//! For examples see the docs on [`Ordered`] or the code in `examples/point.rs`.

#![no_std]
// Experimental features we need.
#![cfg_attr(docsrs, feature(doc_auto_cfg))]
// Coding conventions.
#![warn(missing_docs)]

use core::borrow::{Borrow, BorrowMut};
use core::cmp::Ordering;
use core::fmt;
use core::ops::{Deref, DerefMut};

/// Trait for types that perform an arbitrary ordering.
///
/// More specifically, this trait is for types that perform either a partial or
/// total order but semantically it is nonsensical.
pub trait ArbitraryOrd: Eq + PartialEq {
    /// Implements a meaningless, arbitrary ordering.
    fn arbitrary_cmp(&self, other: &Self) -> Ordering;
}

/// A wrapper type that implements `PartialOrd` and `Ord`.
///
/// # Examples
///
/// ```
/// use core::{cmp::Ordering, fmt};
/// use ordered::{ArbitraryOrd, Ordered};
///
/// /// A point in 2D space.
/// ///
/// /// We do not want users to be able to write `a < b` because it is not well defined.
/// #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
/// struct Point {
///     x: u32,
///     y: u32,
/// }
///
/// impl ArbitraryOrd for Point {
///     fn arbitrary_cmp(&self, other: &Self) -> Ordering {
///         // Just use whatever order tuple cmp gives us.
///         (self.x, self.y).cmp(&(other.x, other.y))
///     }
/// }
///
/// impl fmt::Display for Point {
///     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
///         write!(f, "({}, {})", self.x, self.y)
///     }
/// }
///
/// let point = Point { x: 0, y: 1 };
/// let ordered = Ordered(point);
///
/// println!("We can explicitly deref (*ordered): {}", *ordered);
/// println!("Or use deref coercion (ordered): {}", ordered);
/// println!("Or we can use borrow (&ordered): {}", &ordered);
/// ```
#[derive(Debug, Clone, PartialEq, Eq, Hash)]
#[repr(transparent)]
pub struct Ordered<T>(pub T);

impl<T: Copy> Copy for Ordered<T> {}

impl<T> Ordered<T> {
    /// Creates a new wrapped ordered type.
    ///
    /// The inner type is public so this function is never explicitly needed.
    pub const fn new(inner: T) -> Self { Self(inner) }

    /// Returns a reference to the inner object.
    ///
    /// We also implement [`core::borrow::Borrow`] so this function is never explicitly needed.
    pub const fn as_inner(&self) -> &T { &self.0 }

    /// Returns the inner object.
    ///
    /// We also implement [`core::ops::Deref`] so this function is never explicitly needed.
    pub fn into_inner(self) -> T { self.0 }

    /// Creates an `Ordered<T>` from a reference.
    ///
    /// This allows: `let found = map.get(Ordered::from_ref(&a));`
    pub fn from_ref(value: &T) -> &Self {
        unsafe { &*(value as *const _ as *const Self) }
    }
}

impl<T: ArbitraryOrd> ArbitraryOrd for &T {
    fn arbitrary_cmp(&self, other: &Self) -> Ordering {
        (*self).arbitrary_cmp(other)
    }
}

impl<T: ArbitraryOrd> PartialOrd for Ordered<T> {
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> { Some(self.cmp(other)) }
}

impl<T: ArbitraryOrd> Ord for Ordered<T> {
    fn cmp(&self, other: &Self) -> Ordering { self.0.arbitrary_cmp(&other.0) }
}

impl<T: fmt::Display> fmt::Display for Ordered<T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { fmt::Display::fmt(&self.0, f) }
}

impl<T> From<T> for Ordered<T> {
    fn from(inner: T) -> Self { Self(inner) }
}

impl<T> AsRef<T> for Ordered<T> {
    fn as_ref(&self) -> &T { &self.0 }
}

impl<T> AsMut<T> for Ordered<T> {
    fn as_mut(&mut self) -> &mut T { &mut self.0 }
}

impl<T> Borrow<T> for Ordered<T> {
    fn borrow(&self) -> &T { &self.0 }
}

impl<T> BorrowMut<T> for Ordered<T> {
    fn borrow_mut(&mut self) -> &mut T { &mut self.0 }
}

impl<T> Deref for Ordered<T> {
    type Target = T;

    fn deref(&self) -> &Self::Target { &self.0 }
}

impl<T> DerefMut for Ordered<T> {
    fn deref_mut(&mut self) -> &mut T { &mut self.0 }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
    struct Point {
        x: u32,
        y: u32,
    }

    impl Point {
        fn new(x: u32, y: u32) -> Self { Point { x, y } }
    }

    impl ArbitraryOrd for Point {
        fn arbitrary_cmp(&self, other: &Self) -> Ordering { (self.x, self.y).cmp(&(other.x, other.y)) }
    }

    #[test]
    fn can_compare() {
        let a = Point::new(2, 3);
        let b = Point::new(5, 7);

        assert!(Ordered(a) < Ordered(b));
    }

    #[test]
    fn can_compare_with_from_ref() {
        let a = Point::new(2, 3);
        let b = Point::new(5, 7);

        assert!(Ordered::from_ref(&a) < Ordered::from_ref(&b));
    }

    #[test]
    fn can_compare_with_reference() {
        let a = Point::new(2, 3);
        let b = Point::new(5, 7);

        assert!(Ordered(&a) < Ordered(&b));
    }
}