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Add radix51 impl of reduce

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Henry de Valence 2017-03-12 23:18:36 -07:00
parent 08ac5a9068
commit 7591c74018
1 changed files with 34 additions and 0 deletions
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34
src/field.rs
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@ -455,6 +455,40 @@ impl FieldElement {
output[9] = h[9] as i32;
output
}
#[cfg(feature="radix_51")]
fn reduce(input: &[u128; 5]) -> FieldElement {
// After multiplication,
// 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.
// The carry (c[i] >> 51) fits into a u64 iff 2b+6.27 < 64+51 iff b <= 54.
// After the first carry pass, all c[i] fit into u64.
let low_51_bit_mask = (1u64 << 51) - 1;
c1 += (c0 >> 51) as u128;
let mut c0: u64 = (c0 as u64) & low_51_bit_mask;
c2 += (c1 >> 51) as u128;
let mut c1: u64 = (c1 as u64) & low_51_bit_mask;
c3 += (c2 >> 51) as u128;
let mut c2: u64 = (c2 as u64) & low_51_bit_mask;
c4 += (c3 >> 51) as u128;
let mut c3: u64 = (c3 as u64) & low_51_bit_mask;
c0 += ((c4 >> 51) as u64) * 19;
let mut c4: u64 = (c4 as u64) & low_51_bit_mask;
c1 += c0 >> 51;
c0 = c0 & low_51_bit_mask;
c2 += c1 >> 51;
c1 = c1 & low_51_bit_mask;
c3 += c2 >> 51;
c2 = c2 & low_51_bit_mask;
c4 += c3 >> 51;
c3 = c3 & low_51_bit_mask;
c0 += (c4 >> 51) * 19;
c4 = c4 & low_51_bit_mask;
FieldElement([c0,c1,c2,c3,c4])
}
/// Create a FieldElement by demarshalling an array of 32 bytes.
///