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Auto merge of #93873 - Stovent:big-ints, r=m-ou-se

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Reimplement `carrying_add` and `borrowing_sub` for signed integers.

As per the discussion in #85532, this PR reimplements `carrying_add` and `borrowing_sub` for signed integers.

It also adds unit tests for both unsigned and signed integers, emphasing on the behaviours of the methods.
This commit is contained in:
bors 2022-09-09 00:59:08 +00:00
commit 7200da0217
4 changed files with 127 additions and 0 deletions
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78
library/core/src/num/int_macros.rs
View File

@ -1518,6 +1518,51 @@ macro_rules! int_impl {
(a as Self, b)
}
/// Calculates `self + rhs + carry` without the ability to overflow.
///
/// Performs "signed ternary addition" which takes in an extra bit to add, and may return an
/// additional bit of overflow. This signed function is used only on the highest-ordered data,
/// for which the signed overflow result indicates whether the big integer overflowed or not.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// #![feature(bigint_helper_methods)]
#[doc = concat!("assert_eq!(5", stringify!($SelfT), ".carrying_add(2, false), (7, false));")]
#[doc = concat!("assert_eq!(5", stringify!($SelfT), ".carrying_add(2, true), (8, false));")]
#[doc = concat!("assert_eq!(", stringify!($SelfT), "::MAX.carrying_add(1, false), (", stringify!($SelfT), "::MIN, true));")]
#[doc = concat!("assert_eq!(", stringify!($SelfT), "::MAX.carrying_add(0, true), (", stringify!($SelfT), "::MIN, true));")]
#[doc = concat!("assert_eq!(", stringify!($SelfT), "::MAX.carrying_add(1, true), (", stringify!($SelfT), "::MIN + 1, true));")]
#[doc = concat!("assert_eq!(",
stringify!($SelfT), "::MAX.carrying_add(", stringify!($SelfT), "::MAX, true), ",
"(-1, true));"
)]
#[doc = concat!("assert_eq!(", stringify!($SelfT), "::MIN.carrying_add(-1, true), (", stringify!($SelfT), "::MIN, false));")]
#[doc = concat!("assert_eq!(0", stringify!($SelfT), ".carrying_add(", stringify!($SelfT), "::MAX, true), (", stringify!($SelfT), "::MIN, true));")]
/// ```
///
/// If `carry` is false, this method is equivalent to [`overflowing_add`](Self::overflowing_add):
///
/// ```
/// #![feature(bigint_helper_methods)]
#[doc = concat!("assert_eq!(5_", stringify!($SelfT), ".carrying_add(2, false), 5_", stringify!($SelfT), ".overflowing_add(2));")]
#[doc = concat!("assert_eq!(", stringify!($SelfT), "::MAX.carrying_add(1, false), ", stringify!($SelfT), "::MAX.overflowing_add(1));")]
/// ```
#[unstable(feature = "bigint_helper_methods", issue = "85532")]
#[rustc_const_unstable(feature = "const_bigint_helper_methods", issue = "85532")]
#[must_use = "this returns the result of the operation, \
without modifying the original"]
#[inline]
pub const fn carrying_add(self, rhs: Self, carry: bool) -> (Self, bool) {
// note: longer-term this should be done via an intrinsic.
// note: no intermediate overflow is required (https://github.com/rust-lang/rust/issues/85532#issuecomment-1032214946).
let (a, b) = self.overflowing_add(rhs);
let (c, d) = a.overflowing_add(carry as $SelfT);
(c, b != d)
}
/// Calculates `self` + `rhs` with an unsigned `rhs`
///
/// Returns a tuple of the addition along with a boolean indicating
@ -1569,6 +1614,39 @@ macro_rules! int_impl {
(a as Self, b)
}
/// Calculates `self - rhs - borrow` without the ability to overflow.
///
/// Performs "signed ternary subtraction" which takes in an extra bit to subtract, and may return an
/// additional bit of overflow. This signed function is used only on the highest-ordered data,
/// for which the signed overflow result indicates whether the big integer overflowed or not.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// #![feature(bigint_helper_methods)]
#[doc = concat!("assert_eq!(5", stringify!($SelfT), ".borrowing_sub(2, false), (3, false));")]
#[doc = concat!("assert_eq!(5", stringify!($SelfT), ".borrowing_sub(2, true), (2, false));")]
#[doc = concat!("assert_eq!(0", stringify!($SelfT), ".borrowing_sub(1, false), (-1, false));")]
#[doc = concat!("assert_eq!(0", stringify!($SelfT), ".borrowing_sub(1, true), (-2, false));")]
#[doc = concat!("assert_eq!(", stringify!($SelfT), "::MIN.borrowing_sub(1, true), (", stringify!($SelfT), "::MAX - 1, true));")]
#[doc = concat!("assert_eq!(", stringify!($SelfT), "::MAX.borrowing_sub(-1, false), (", stringify!($SelfT), "::MIN, true));")]
#[doc = concat!("assert_eq!(", stringify!($SelfT), "::MAX.borrowing_sub(-1, true), (", stringify!($SelfT), "::MAX, false));")]
/// ```
#[unstable(feature = "bigint_helper_methods", issue = "85532")]
#[rustc_const_unstable(feature = "const_bigint_helper_methods", issue = "85532")]
#[must_use = "this returns the result of the operation, \
without modifying the original"]
#[inline]
pub const fn borrowing_sub(self, rhs: Self, borrow: bool) -> (Self, bool) {
// note: longer-term this should be done via an intrinsic.
// note: no intermediate overflow is required (https://github.com/rust-lang/rust/issues/85532#issuecomment-1032214946).
let (a, b) = self.overflowing_sub(rhs);
let (c, d) = a.overflowing_sub(borrow as $SelfT);
(c, b != d)
}
/// Calculates `self` - `rhs` with an unsigned `rhs`
///
/// Returns a tuple of the subtraction along with a boolean indicating

1
library/core/tests/lib.rs
View File

@ -3,6 +3,7 @@
#![feature(array_methods)]
#![feature(array_windows)]
#![feature(bench_black_box)]
#![feature(bigint_helper_methods)]
#![feature(cell_update)]
#![feature(const_assume)]
#![feature(const_black_box)]

26
library/core/tests/num/int_macros.rs
View File

@ -338,6 +338,32 @@ macro_rules! int_module {
assert_eq!(MIN.checked_next_multiple_of(-3), None);
assert_eq!(MIN.checked_next_multiple_of(-1), Some(MIN));
}
#[test]
fn test_carrying_add() {
assert_eq!($T::MAX.carrying_add(1, false), ($T::MIN, true));
assert_eq!($T::MAX.carrying_add(0, true), ($T::MIN, true));
assert_eq!($T::MAX.carrying_add(1, true), ($T::MIN + 1, true));
assert_eq!($T::MAX.carrying_add(-1, false), ($T::MAX - 1, false));
assert_eq!($T::MAX.carrying_add(-1, true), ($T::MAX, false)); // no intermediate overflow
assert_eq!($T::MIN.carrying_add(-1, false), ($T::MAX, true));
assert_eq!($T::MIN.carrying_add(-1, true), ($T::MIN, false)); // no intermediate overflow
assert_eq!((0 as $T).carrying_add($T::MAX, true), ($T::MIN, true));
assert_eq!((0 as $T).carrying_add($T::MIN, true), ($T::MIN + 1, false));
}
#[test]
fn test_borrowing_sub() {
assert_eq!($T::MIN.borrowing_sub(1, false), ($T::MAX, true));
assert_eq!($T::MIN.borrowing_sub(0, true), ($T::MAX, true));
assert_eq!($T::MIN.borrowing_sub(1, true), ($T::MAX - 1, true));
assert_eq!($T::MIN.borrowing_sub(-1, false), ($T::MIN + 1, false));
assert_eq!($T::MIN.borrowing_sub(-1, true), ($T::MIN, false)); // no intermediate overflow
assert_eq!($T::MAX.borrowing_sub(-1, false), ($T::MIN, true));
assert_eq!($T::MAX.borrowing_sub(-1, true), ($T::MAX, false)); // no intermediate overflow
assert_eq!((0 as $T).borrowing_sub($T::MIN, false), ($T::MIN, true));
assert_eq!((0 as $T).borrowing_sub($T::MIN, true), ($T::MAX, false));
}
}
};
}

22
library/core/tests/num/uint_macros.rs
View File

@ -230,6 +230,28 @@ macro_rules! uint_module {
assert_eq!((1 as $T).checked_next_multiple_of(0), None);
assert_eq!(MAX.checked_next_multiple_of(2), None);
}
#[test]
fn test_carrying_add() {
assert_eq!($T::MAX.carrying_add(1, false), (0, true));
assert_eq!($T::MAX.carrying_add(0, true), (0, true));
assert_eq!($T::MAX.carrying_add(1, true), (1, true));
assert_eq!($T::MIN.carrying_add($T::MAX, false), ($T::MAX, false));
assert_eq!($T::MIN.carrying_add(0, true), (1, false));
assert_eq!($T::MIN.carrying_add($T::MAX, true), (0, true));
}
#[test]
fn test_borrowing_sub() {
assert_eq!($T::MIN.borrowing_sub(1, false), ($T::MAX, true));
assert_eq!($T::MIN.borrowing_sub(0, true), ($T::MAX, true));
assert_eq!($T::MIN.borrowing_sub(1, true), ($T::MAX - 1, true));
assert_eq!($T::MAX.borrowing_sub($T::MAX, false), (0, false));
assert_eq!($T::MAX.borrowing_sub(0, true), ($T::MAX - 1, false));
assert_eq!($T::MAX.borrowing_sub($T::MAX, true), ($T::MAX, true));
}
}
};
}