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Rollup merge of rust-lang#103236 - tspiteri:redoc-int-adc-sbb, r=m-ou-se
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doc: rewrite doc for signed int::{carrying_add,borrowing_sub}

Reword the documentation for bigint helper methods, signed `int::{carrying_add,borrowing_sub}` (rust-lang#85532).

This change is a follow-up to rust-lang#101889, which was for the unsigned methods.
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@compiler-errors
compiler-errors authored Jan 12, 2023
2 parents d4203ed + 4e38d06 commit e23b0fd
Showing 1 changed file with 75 additions and 36 deletions.
111 changes: 75 additions & 36 deletions library/core/src/num/int_macros.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1514,37 +1514,50 @@ macro_rules! int_impl {
(a as Self, b)
}

/// Calculates `self + rhs + carry` without the ability to overflow.
/// Calculates `self` + `rhs` + `carry` and checks for 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.
/// Performs "ternary addition" of two integer operands and a carry-in
/// bit, and returns a tuple of the sum along with a boolean indicating
/// whether an arithmetic overflow would occur. On overflow, the wrapped
/// value is returned.
///
/// # Examples
/// This allows chaining together multiple additions to create a wider
/// addition, and can be useful for bignum addition. This method should
/// only be used for the most significant word; for the less significant
/// words the unsigned method
#[doc = concat!("[`", stringify!($UnsignedT), "::carrying_add`]")]
/// should be used.
///
/// Basic usage:
/// The output boolean returned by this method is *not* a carry flag,
/// and should *not* be added to a more significant word.
///
/// ```
/// #![feature(bigint_helper_methods)]
#[doc = concat!("assert_eq!(5", stringify!($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 the input carry is false, this method is equivalent to
/// [`overflowing_add`](Self::overflowing_add).
///
/// If `carry` is false, this method is equivalent to [`overflowing_add`](Self::overflowing_add):
/// # Examples
///
/// ```
/// #![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));")]
/// // Only the most significant word is signed.
/// //
#[doc = concat!("// 10 MAX (a = 10 × 2^", stringify!($BITS), " + 2^", stringify!($BITS), " - 1)")]
#[doc = concat!("// + -5 9 (b = -5 × 2^", stringify!($BITS), " + 9)")]
/// // ---------
#[doc = concat!("// 6 8 (sum = 6 × 2^", stringify!($BITS), " + 8)")]
///
#[doc = concat!("let (a1, a0): (", stringify!($SelfT), ", ", stringify!($UnsignedT), ") = (10, ", stringify!($UnsignedT), "::MAX);")]
#[doc = concat!("let (b1, b0): (", stringify!($SelfT), ", ", stringify!($UnsignedT), ") = (-5, 9);")]
/// let carry0 = false;
///
#[doc = concat!("// ", stringify!($UnsignedT), "::carrying_add for the less significant words")]
/// let (sum0, carry1) = a0.carrying_add(b0, carry0);
/// assert_eq!(carry1, true);
///
#[doc = concat!("// ", stringify!($SelfT), "::carrying_add for the most significant word")]
/// let (sum1, overflow) = a1.carrying_add(b1, carry1);
/// assert_eq!(overflow, false);
///
/// assert_eq!((sum1, sum0), (6, 8));
/// ```
#[unstable(feature = "bigint_helper_methods", issue = "85532")]
#[rustc_const_unstable(feature = "const_bigint_helper_methods", issue = "85532")]
Expand Down Expand Up @@ -1608,25 +1621,51 @@ macro_rules! int_impl {
(a as Self, b)
}

/// Calculates `self - rhs - borrow` without the ability to overflow.
/// Calculates `self` − `rhs` − `borrow` and checks for
/// 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.
/// Performs "ternary subtraction" by subtracting both an integer
/// operandand a borrow-in bit from `self`, and returns a tuple of the
/// difference along with a boolean indicating whether an arithmetic
/// overflow would occur. On overflow, the wrapped value is returned.
///
/// # Examples
/// This allows chaining together multiple subtractions to create a
/// wider subtraction, and can be useful for bignum subtraction. This
/// method should only be used for the most significant word; for the
/// less significant words the unsigned method
#[doc = concat!("[`", stringify!($UnsignedT), "::borrowing_sub`]")]
/// should be used.
///
/// Basic usage:
/// The output boolean returned by this method is *not* a borrow flag,
/// and should *not* be subtracted from a more significant word.
///
/// If the input borrow is false, this method is equivalent to
/// [`overflowing_sub`](Self::overflowing_sub).
///
/// # Examples
///
/// ```
/// #![feature(bigint_helper_methods)]
#[doc = concat!("assert_eq!(5", stringify!($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!($SelfT), "::MAX, false));")]
/// // Only the most significant word is signed.
/// //
#[doc = concat!("// 6 8 (a = 6 × 2^", stringify!($BITS), " + 8)")]
#[doc = concat!("// - -5 9 (b = -5 × 2^", stringify!($BITS), " + 9)")]
/// // ---------
#[doc = concat!("// 10 MAX (diff = 10 × 2^", stringify!($BITS), " + 2^", stringify!($BITS), " - 1)")]
///
#[doc = concat!("let (a1, a0): (", stringify!($SelfT), ", ", stringify!($UnsignedT), ") = (6, 8);")]
#[doc = concat!("let (b1, b0): (", stringify!($SelfT), ", ", stringify!($UnsignedT), ") = (-5, 9);")]
/// let borrow0 = false;
///
#[doc = concat!("// ", stringify!($UnsignedT), "::borrowing_sub for the less significant words")]
/// let (diff0, borrow1) = a0.borrowing_sub(b0, borrow0);
/// assert_eq!(borrow1, true);
///
#[doc = concat!("// ", stringify!($SelfT), "::borrowing_sub for the most significant word")]
/// let (diff1, overflow) = a1.borrowing_sub(b1, borrow1);
/// assert_eq!(overflow, false);
///
#[doc = concat!("assert_eq!((diff1, diff0), (10, ", stringify!($UnsignedT), "::MAX));")]
/// ```
#[unstable(feature = "bigint_helper_methods", issue = "85532")]
#[rustc_const_unstable(feature = "const_bigint_helper_methods", issue = "85532")]
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