From 513ce26942cce4171337f35a04ab5452aa6a70ff Mon Sep 17 00:00:00 2001 From: Henry de Valence Date: Thu, 20 Jul 2017 21:17:32 -0700 Subject: [PATCH] avoid 128-bit multiplications --- src/field.rs | 16 +++++++++++----- 1 file changed, 11 insertions(+), 5 deletions(-) diff --git a/src/field.rs b/src/field.rs index bcf8d17..ad1603c 100644 --- a/src/field.rs +++ b/src/field.rs @@ -202,12 +202,18 @@ impl<'a, 'b> Mul<&'b FieldElement> for &'a FieldElement { let a: &[u64; 5] = &self.0; let b: &[u64; 5] = &_rhs.0; + // 64-bit precomputations + let b1_19 = b[1] * 19; + let b2_19 = b[2] * 19; + let b3_19 = b[3] * 19; + let b4_19 = b[4] * 19; + // Multiply to get 128-bit coefficients of output - let c0: u128 = m(a[0],b[0]) + ( m(a[4],b[1]) + m(a[3],b[2]) + m(a[2],b[3]) + m(a[1],b[4]) )*19; - let mut c1: u128 = m(a[1],b[0]) + m(a[0],b[1]) + ( m(a[4],b[2]) + m(a[3],b[3]) + m(a[2],b[4]) )*19; - let mut c2: u128 = m(a[2],b[0]) + m(a[1],b[1]) + m(a[0],b[2]) + ( m(a[4],b[3]) + m(a[3],b[4]) )*19; - let mut c3: u128 = m(a[3],b[0]) + m(a[2],b[1]) + m(a[1],b[2]) + m(a[0],b[3]) + ( m(a[4],b[4]) )*19; - let mut c4: u128 = m(a[4],b[0]) + m(a[3],b[1]) + m(a[2],b[2]) + m(a[1],b[3]) + m(a[0],b[4]); + let c0: u128 = m(a[0],b[0]) + m(a[4],b1_19) + m(a[3],b2_19) + m(a[2],b3_19) + m(a[1],b4_19); + let mut c1: u128 = m(a[1],b[0]) + m(a[0],b[1]) + m(a[4],b2_19) + m(a[3],b3_19) + m(a[2],b4_19); + let mut c2: u128 = m(a[2],b[0]) + m(a[1],b[1]) + m(a[0],b[2]) + m(a[4],b3_19) + m(a[3],b4_19); + let mut c3: u128 = m(a[3],b[0]) + m(a[2],b[1]) + m(a[1],b[2]) + m(a[0],b[3]) + m(a[4],b4_19); + let mut c4: u128 = m(a[4],b[0]) + m(a[3],b[1]) + m(a[2],b[2]) + m(a[1],b[3]) + m(a[0],b[4]); // Now c[i] < 2^2b * (1+i + (4-i)*19) < 2^(2b + lg(1+4*19)) < 2^(2b + 6.27) // where b is the bitlength of the input limbs.