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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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18
src/ringct/bulletproofs.cc
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@ -202,13 +202,27 @@ 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 */ /* 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) if (n == 0)
return rct::zero(); return rct::zero();
rct::key res = rct::identity(); rct::key res = rct::identity();
if (n == 1) if (n == 1)
return res; return res;
const bool is_power_of_2 = (n & (n - 1)) == 0;
if (is_power_of_2)
{
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; rct::key prev = x;
for (size_t i = 1; i < n; ++i) for (size_t i = 1; i < n; ++i)
{ {
@ -216,6 +230,8 @@ static rct::key vector_power_sum(const rct::key &x, size_t n)
sc_mul(prev.bytes, prev.bytes, x.bytes); sc_mul(prev.bytes, prev.bytes, x.bytes);
sc_add(res.bytes, res.bytes, prev.bytes); sc_add(res.bytes, res.bytes, prev.bytes);
} }
}
return res; return res;
} }