From b2b12ae0cb6c7e6dd71092aafa14e06b57a290ec Mon Sep 17 00:00:00 2001 From: Scott McMurray Date: Sun, 19 Jan 2025 16:15:00 -0800 Subject: [PATCH] Add an example of using `carrying_mul_add` to write wider multiplication Just the basic quadratic version that you wouldn't actually want for a true bigint, but it's nice and short so is useful as an example :) --- library/core/src/num/uint_macros.rs | 33 +++++++++++++++++++++++++++-- 1 file changed, 31 insertions(+), 2 deletions(-) diff --git a/library/core/src/num/uint_macros.rs b/library/core/src/num/uint_macros.rs index 404e4bcffd379..c8433b3bb168a 100644 --- a/library/core/src/num/uint_macros.rs +++ b/library/core/src/num/uint_macros.rs @@ -2663,8 +2663,8 @@ macro_rules! uint_impl { /// /// Basic usage: /// - /// Please note that this example is shared between integer types. - /// Which explains why `u32` is used here. + /// Please note that this example is shared between integer types, + /// which explains why `u32` is used here. /// /// ``` /// #![feature(bigint_helper_methods)] @@ -2677,6 +2677,35 @@ macro_rules! uint_impl { "(", stringify!($SelfT), "::MAX, ", stringify!($SelfT), "::MAX));" )] /// ``` + /// + /// This is the core per-digit operation for "grade school" O(n²) multiplication. + /// + /// Please note that this example is shared between integer types, + /// using `u8` for simplicity of the demonstration. + /// + /// ``` + /// #![feature(bigint_helper_methods)] + /// + /// fn quadratic_mul(a: [u8; N], b: [u8; N]) -> [u8; N] { + /// let mut out = [0; N]; + /// for j in 0..N { + /// let mut carry = 0; + /// for i in 0..(N - j) { + /// (out[j + i], carry) = u8::carrying_mul_add(a[i], b[j], out[j + i], carry); + /// } + /// } + /// out + /// } + /// + /// // -1 * -1 == 1 + /// assert_eq!(quadratic_mul([0xFF; 3], [0xFF; 3]), [1, 0, 0]); + /// + /// assert_eq!(u32::wrapping_mul(0x9e3779b9, 0x7f4a7c15), 0xCFFC982D); + /// assert_eq!( + /// quadratic_mul(u32::to_le_bytes(0x9e3779b9), u32::to_le_bytes(0x7f4a7c15)), + /// u32::to_le_bytes(0xCFFC982D) + /// ); + /// ``` #[unstable(feature = "bigint_helper_methods", issue = "85532")] #[rustc_const_unstable(feature = "bigint_helper_methods", issue = "85532")] #[must_use = "this returns the result of the operation, \