doc: rewrite doc for signed int::{carrying_add,borrowing_sub}

Author: tspiteri

library/core/src/num/int_macros.rs

@@ -1506,37 +1506,50 @@ 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")]
@@ -1600,25 +1613,51 @@ 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")]