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Implement midpoint for all signed and unsigned integers
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6 files changed

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6 files changed

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‎library/core/src/lib.rs

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@@ -132,6 +132,7 @@
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#![feature(const_maybe_uninit_assume_init)]
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#![feature(const_maybe_uninit_uninit_array)]
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#![feature(const_nonnull_new)]
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#![feature(const_num_midpoint)]
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#![feature(const_option)]
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#![feature(const_option_ext)]
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#![feature(const_pin)]

‎library/core/src/num/int_macros.rs

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@@ -2332,6 +2332,44 @@ macro_rules! int_impl {
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}
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}
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/// Calculates the middle point of `self` and `rhs`.
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///
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/// `midpoint(a, b)` is `(a + b) >> 1` as if it were performed in a
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/// sufficiently-large signed integral type. This implies that the result is
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/// always rounded towards negative infinity and that no overflow will ever occur.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(num_midpoint)]
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#[doc = concat!("assert_eq!(0", stringify!($SelfT), ".midpoint(4), 2);")]
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#[doc = concat!("assert_eq!(0", stringify!($SelfT), ".midpoint(-1), -1);")]
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#[doc = concat!("assert_eq!((-1", stringify!($SelfT), ").midpoint(0), -1);")]
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/// ```
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#[unstable(feature = "num_midpoint", issue = "110840")]
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#[rustc_const_unstable(feature = "const_num_midpoint", issue = "110840")]
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#[rustc_allow_const_fn_unstable(const_num_midpoint)]
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#[must_use = "this returns the result of the operation, \
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without modifying the original"]
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#[inline]
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pub const fn midpoint(self, rhs: Self) -> Self {
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const U: $UnsignedT = <$SelfT>::MIN.unsigned_abs();
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// Map an $SelfT to an $UnsignedT
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// ex: i8 [-128; 127] to [0; 255]
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const fn map(a: $SelfT) -> $UnsignedT {
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(a as $UnsignedT) ^ U
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}
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// Map an $UnsignedT to an $SelfT
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// ex: u8 [0; 255] to [-128; 127]
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const fn demap(a: $UnsignedT) -> $SelfT {
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(a ^ U) as $SelfT
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}
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demap(<$UnsignedT>::midpoint(map(self), map(rhs)))
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}
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/// Returns the logarithm of the number with respect to an arbitrary base,
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/// rounded down.
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///

‎library/core/src/num/mod.rs

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@@ -95,6 +95,57 @@ depending on the target pointer size.
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};
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}
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macro_rules! midpoint_impl {
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($SelfT:ty, unsigned) => {
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/// Calculates the middle point of `self` and `rhs`.
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///
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/// `midpoint(a, b)` is `(a + b) >> 1` as if it were performed in a
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/// sufficiently-large signed integral type. This implies that the result is
104+
/// always rounded towards negative infinity and that no overflow will ever occur.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(num_midpoint)]
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#[doc = concat!("assert_eq!(0", stringify!($SelfT), ".midpoint(4), 2);")]
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#[doc = concat!("assert_eq!(1", stringify!($SelfT), ".midpoint(4), 2);")]
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/// ```
113+
#[unstable(feature = "num_midpoint", issue = "110840")]
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#[rustc_const_unstable(feature = "const_num_midpoint", issue = "110840")]
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#[must_use = "this returns the result of the operation, \
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without modifying the original"]
117+
#[inline]
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pub const fn midpoint(self, rhs: $SelfT) -> $SelfT {
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// Use the well known branchless algorthim from Hacker's Delight to compute
120+
// `(a + b) / 2` without overflowing: `((a ^ b) >> 1) + (a & b)`.
121+
((self ^ rhs) >> 1) + (self & rhs)
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}
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};
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($SelfT:ty, $WideT:ty, unsigned) => {
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/// Calculates the middle point of `self` and `rhs`.
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///
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/// `midpoint(a, b)` is `(a + b) >> 1` as if it were performed in a
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/// sufficiently-large signed integral type. This implies that the result is
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/// always rounded towards negative infinity and that no overflow will ever occur.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(num_midpoint)]
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#[doc = concat!("assert_eq!(0", stringify!($SelfT), ".midpoint(4), 2);")]
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#[doc = concat!("assert_eq!(1", stringify!($SelfT), ".midpoint(4), 2);")]
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/// ```
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#[unstable(feature = "num_midpoint", issue = "110840")]
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#[rustc_const_unstable(feature = "const_num_midpoint", issue = "110840")]
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#[must_use = "this returns the result of the operation, \
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without modifying the original"]
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#[inline]
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pub const fn midpoint(self, rhs: $SelfT) -> $SelfT {
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((self as $WideT + rhs as $WideT) / 2) as $SelfT
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}
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};
147+
}
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98149
macro_rules! widening_impl {
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($SelfT:ty, $WideT:ty, $BITS:literal, unsigned) => {
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/// Calculates the complete product `self * rhs` without the possibility to overflow.
@@ -455,6 +506,7 @@ impl u8 {
455506
bound_condition = "",
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}
457508
widening_impl! { u8, u16, 8, unsigned }
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midpoint_impl! { u8, u16, unsigned }
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/// Checks if the value is within the ASCII range.
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///
@@ -1057,6 +1109,7 @@ impl u16 {
10571109
bound_condition = "",
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}
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widening_impl! { u16, u32, 16, unsigned }
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midpoint_impl! { u16, u32, unsigned }
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/// Checks if the value is a Unicode surrogate code point, which are disallowed values for [`char`].
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///
@@ -1105,6 +1158,7 @@ impl u32 {
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bound_condition = "",
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}
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widening_impl! { u32, u64, 32, unsigned }
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midpoint_impl! { u32, u64, unsigned }
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}
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impl u64 {
@@ -1128,6 +1182,7 @@ impl u64 {
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bound_condition = "",
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}
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widening_impl! { u64, u128, 64, unsigned }
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midpoint_impl! { u64, u128, unsigned }
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}
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11331188
impl u128 {
@@ -1152,6 +1207,7 @@ impl u128 {
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from_xe_bytes_doc = "",
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bound_condition = "",
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}
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midpoint_impl! { u128, unsigned }
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}
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#[cfg(target_pointer_width = "16")]
@@ -1176,6 +1232,7 @@ impl usize {
11761232
bound_condition = " on 16-bit targets",
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}
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widening_impl! { usize, u32, 16, unsigned }
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midpoint_impl! { usize, u32, unsigned }
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}
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#[cfg(target_pointer_width = "32")]
@@ -1200,6 +1257,7 @@ impl usize {
12001257
bound_condition = " on 32-bit targets",
12011258
}
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widening_impl! { usize, u64, 32, unsigned }
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midpoint_impl! { usize, u64, unsigned }
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}
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12051263
#[cfg(target_pointer_width = "64")]
@@ -1224,6 +1282,7 @@ impl usize {
12241282
bound_condition = " on 64-bit targets",
12251283
}
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widening_impl! { usize, u128, 64, unsigned }
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midpoint_impl! { usize, u128, unsigned }
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}
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12291288
impl usize {

‎library/core/tests/lib.rs

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@@ -54,6 +54,7 @@
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#![feature(maybe_uninit_uninit_array_transpose)]
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#![feature(min_specialization)]
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#![feature(numfmt)]
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#![feature(num_midpoint)]
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#![feature(step_trait)]
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#![feature(str_internals)]
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#![feature(std_internals)]

‎library/core/tests/num/int_macros.rs

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@@ -364,6 +364,32 @@ macro_rules! int_module {
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assert_eq!((0 as $T).borrowing_sub($T::MIN, false), ($T::MIN, true));
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assert_eq!((0 as $T).borrowing_sub($T::MIN, true), ($T::MAX, false));
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}
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368+
#[test]
369+
fn test_midpoint() {
370+
assert_eq!(<$T>::midpoint(1, 3), 2);
371+
assert_eq!(<$T>::midpoint(3, 1), 2);
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373+
assert_eq!(<$T>::midpoint(0, 0), 0);
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assert_eq!(<$T>::midpoint(0, 2), 1);
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assert_eq!(<$T>::midpoint(2, 0), 1);
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assert_eq!(<$T>::midpoint(2, 2), 2);
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assert_eq!(<$T>::midpoint(1, 4), 2);
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assert_eq!(<$T>::midpoint(4, 1), 2);
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assert_eq!(<$T>::midpoint(3, 4), 3);
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assert_eq!(<$T>::midpoint(4, 3), 3);
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assert_eq!(<$T>::midpoint(<$T>::MIN, <$T>::MAX), -1);
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assert_eq!(<$T>::midpoint(<$T>::MAX, <$T>::MIN), -1);
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assert_eq!(<$T>::midpoint(<$T>::MIN, <$T>::MIN), <$T>::MIN);
386+
assert_eq!(<$T>::midpoint(<$T>::MAX, <$T>::MAX), <$T>::MAX);
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388+
assert_eq!(<$T>::midpoint(<$T>::MIN, 6), <$T>::MIN / 2 + 3);
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assert_eq!(<$T>::midpoint(6, <$T>::MIN), <$T>::MIN / 2 + 3);
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assert_eq!(<$T>::midpoint(<$T>::MAX, 6), <$T>::MAX / 2 + 3);
391+
assert_eq!(<$T>::midpoint(6, <$T>::MAX), <$T>::MAX / 2 + 3);
392+
}
367393
}
368394
};
369395
}

‎library/core/tests/num/uint_macros.rs

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@@ -252,6 +252,32 @@ macro_rules! uint_module {
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assert_eq!($T::MAX.borrowing_sub(0, true), ($T::MAX - 1, false));
253253
assert_eq!($T::MAX.borrowing_sub($T::MAX, true), ($T::MAX, true));
254254
}
255+
256+
#[test]
257+
fn test_midpoint() {
258+
assert_eq!(<$T>::midpoint(1, 3), 2);
259+
assert_eq!(<$T>::midpoint(3, 1), 2);
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261+
assert_eq!(<$T>::midpoint(0, 0), 0);
262+
assert_eq!(<$T>::midpoint(0, 2), 1);
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assert_eq!(<$T>::midpoint(2, 0), 1);
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assert_eq!(<$T>::midpoint(2, 2), 2);
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266+
assert_eq!(<$T>::midpoint(1, 4), 2);
267+
assert_eq!(<$T>::midpoint(4, 1), 2);
268+
assert_eq!(<$T>::midpoint(3, 4), 3);
269+
assert_eq!(<$T>::midpoint(4, 3), 3);
270+
271+
assert_eq!(<$T>::midpoint(<$T>::MIN, <$T>::MAX), (<$T>::MAX - <$T>::MIN) / 2);
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assert_eq!(<$T>::midpoint(<$T>::MAX, <$T>::MIN), (<$T>::MAX - <$T>::MIN) / 2);
273+
assert_eq!(<$T>::midpoint(<$T>::MIN, <$T>::MIN), <$T>::MIN);
274+
assert_eq!(<$T>::midpoint(<$T>::MAX, <$T>::MAX), <$T>::MAX);
275+
276+
assert_eq!(<$T>::midpoint(<$T>::MIN, 6), <$T>::MIN / 2 + 3);
277+
assert_eq!(<$T>::midpoint(6, <$T>::MIN), <$T>::MIN / 2 + 3);
278+
assert_eq!(<$T>::midpoint(<$T>::MAX, 6), (<$T>::MAX - <$T>::MIN) / 2 + 3);
279+
assert_eq!(<$T>::midpoint(6, <$T>::MAX), (<$T>::MAX - <$T>::MIN) / 2 + 3);
280+
}
255281
}
256282
};
257283
}

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