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bigint::monty: simplify redc masks

This commit is contained in:
Josh Stone 2017-10-22 15:34:46 -07:00
parent 5a0de140c9
commit 5708db0f67
1 changed files with 3 additions and 4 deletions
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7
bigint/src/monty.rs
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@ -67,17 +67,16 @@ fn monty_redc(a: BigUint, mr: &MontyReducer) -> BigUint {
// β is the size of a word, in this case 32 bits. So "a mod β" is // β is the size of a word, in this case 32 bits. So "a mod β" is
// equivalent to masking a to 32 bits. // equivalent to masking a to 32 bits.
let beta_mask = u32::max_value() as u64;
// mu <- -N^(-1) mod β // mu <- -N^(-1) mod β
let mu = (beta_mask-mr.n0inv as u64)+1; let mu = 0u32.wrapping_sub(mr.n0inv);
// 1: for i = 0 to (n-1) // 1: for i = 0 to (n-1)
for i in 0..n_size { for i in 0..n_size {
// 2: q_i <- mu*c_i mod β // 2: q_i <- mu*c_i mod β
let q_i = ((c[i] as u64) * mu) & beta_mask; let q_i = c[i].wrapping_mul(mu);
// 3: C <- C + q_i * N * β^i // 3: C <- C + q_i * N * β^i
super::algorithms::mac_digit(&mut c[i..], n, q_i as u32); super::algorithms::mac_digit(&mut c[i..], n, q_i);
} }
// 4: R <- C * β^(-n) // 4: R <- C * β^(-n)