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Fix an issue in Barrett reduction.

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This commit is contained in:
Adam Wick
2018-10-04 20:00:46 -07:00
parent 78750598a5
commit fe43949684
2 changed files with 46 additions and 43 deletions
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4
generate.hs
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@@ -50,8 +50,10 @@ needs = [ Need ModExp (\ size -> [Req size ModMul
,Req (size + 64) Mul
,Req (size * 2) (Convert ((size * 2) + 64))
,Req ((size * 2) + 64) Shifts
,Req ((size * 2) + 128) Shifts
,Req ((size * 2) + 64) Div
,Req (size + 64) (Convert (size * 2))
,Req (size + 64) (Convert ((size * 2) + 128))
,Req ((size * 2) + 64)
(Convert ((size * 2) + 128))
])
@@ -254,7 +256,7 @@ generateAllTheTests =
let (db3, gen3) = emptyDatabase gen2
generateTests Barretts "barrett_reduce" db3 $ \ size memory0 ->
let (m, memory1) = generateNum memory0 "m" size
(x, memory2) = generateNum memory1 "x" (min size (2 * k * 64))
(x, memory2) = generateNum memory1 "x" (min (2 * size) (2 * k * 64))
k = computeK m
u = barrett m
r = x `mod` m

5
src/unsigned/barrett.rs
View File

@@ -29,10 +29,9 @@ macro_rules! barrett_impl {
}
pub fn reduce(&self, x: &$dbl) -> $name {
let m2: $dbl64 = $dbl64::from(&self.m);
// 1. q1←⌊x/bk−1⌋, q2←q1 · μ, q3←⌊q2/bk+1⌋.
let q1: $name64 = $name64::from(x >> ((self.k - 1) * 64));
let q2: $dbl64 = $dbl64::from(q1 * &self.mu);
let q2 = q1 * &self.mu;
let q3: $name64 = $name64::from(q2 >> ((self.k + 1) * 64));
// 2. r1←x mod bk+1, r2←q3 · m mod bk+1, r←r1 − r2.
let mut r: $dbl64 = $dbl64::from(x);
@@ -45,9 +44,11 @@ macro_rules! barrett_impl {
if went_negative {
let mut bk1 = $dbl64::zero();
bk1.value[self.k+1] = 1;
// this may overflow, and we should probably be OK with it.
r += &bk1;
}
// 4. While r≥m do: r←r−m.
let m2 = $dbl64::from(&self.m);
while &r > &m2 {
r -= &m2;
}