From ba357ee99b48a5bcde8a5e061043058e4af344fb Mon Sep 17 00:00:00 2001
From: Quaternions <krakow20@gmail.com>
Date: Sat, 21 Sep 2024 15:42:29 -0700
Subject: [PATCH] efficient fixed mul

---
 fixed_wide/src/fixed.rs | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/fixed_wide/src/fixed.rs b/fixed_wide/src/fixed.rs
index a566151..05e8b26 100644
--- a/fixed_wide/src/fixed.rs
+++ b/fixed_wide/src/fixed.rs
@@ -295,10 +295,14 @@ macro_rules! impl_multiply_operator_not_const_generic {
 		impl<const F:usize> $struct<$width,F>{
 			paste::item!{
 			#[inline]
-			pub fn [<fixed_ $method>](self, other: Self) -> Self {
-				let lhs=self.bits.as_::<BInt::<{$width*2}>>();
-				let rhs=other.bits.as_::<BInt::<{$width*2}>>();
-				Self::from_bits(lhs.mul(rhs).shr(F as u32).as_())
+			pub fn [<fixed_ $method>](self, rhs: Self) -> Self {
+				let (low,high)=self.bits.unsigned_abs().widening_mul(rhs.bits.unsigned_abs());
+				let out:BInt::<{$width*2}>=unsafe{core::mem::transmute([low,high])};
+				if self.is_negative()==rhs.is_negative(){
+					Self::from_bits(out.shr(F as u32).as_())
+				}else{
+					-Self::from_bits(out.shr(F as u32).as_())
+				}
 			}
 			}
 		}