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bulletproofs: speed up vector_power_sum 

found by sarang
This commit is contained in:
moneromooo-monero 2019-01-18 19:51:29 +00:00
parent 4e72384318
commit 1bc5f9fa4b
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GPG Key ID: 686F07454D6CEFC3
1 changed files with 22 additions and 6 deletions
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28
src/ringct/bulletproofs.cc
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@ -202,20 +202,36 @@ static rct::keyV vector_powers(const rct::key &x, size_t n)
}
/* Given a scalar, return the sum of its powers from 0 to n-1 */
static rct::key vector_power_sum(const rct::key &x, size_t n)
static rct::key vector_power_sum(rct::key x, size_t n)
{
if (n == 0)
return rct::zero();
rct::key res = rct::identity();
if (n == 1)
return res;
rct::key prev = x;
for (size_t i = 1; i < n; ++i)
const bool is_power_of_2 = (n & (n - 1)) == 0;
if (is_power_of_2)
{
if (i > 1)
sc_mul(prev.bytes, prev.bytes, x.bytes);
sc_add(res.bytes, res.bytes, prev.bytes);
sc_add(res.bytes, res.bytes, x.bytes);
while (n > 2)
{
sc_mul(x.bytes, x.bytes, x.bytes);
sc_muladd(res.bytes, x.bytes, res.bytes, res.bytes);
n /= 2;
}
}
else
{
rct::key prev = x;
for (size_t i = 1; i < n; ++i)
{
if (i > 1)
sc_mul(prev.bytes, prev.bytes, x.bytes);
sc_add(res.bytes, res.bytes, prev.bytes);
}
}
return res;
}