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⚡ Optimize KeySwitchingKey generation to avoid repeated allocations #368

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⚡ Optimize KeySwitchingKey generation to avoid repeated allocations #368

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21 changes: 13 additions & 8 deletions crates/fhe/src/bfv/keys/key_switching_key.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -10,7 +10,7 @@ use fhe_math::{
rq::{Ntt, NttShoup, Poly, PowerBasis},
};
use fhe_traits::{DeserializeWithContext, Serialize};
use itertools::{Itertools, izip};
use itertools::izip;
use num_bigint::BigUint;
use rand::{CryptoRng, Rng, RngCore, SeedableRng};
use rand_chacha::ChaCha8Rng;
Expand Down Expand Up @@ -305,25 +305,29 @@ impl KeySwitchingKey {
.ilog2() as usize;

let mut coefficients = p.coefficients().to_slice().unwrap().to_vec();
let mut c2i = vec![];
let mask = (1u64 << self.log_base) - 1;
(0..log_modulus.div_ceil(self.log_base)).for_each(|_| {
c2i.push(coefficients.iter().map(|c| c & mask).collect_vec());
coefficients.iter_mut().for_each(|c| *c >>= self.log_base);
});

let mut c0 = Poly::<Ntt>::zero(&self.ctx_ksk);
let mut c1 = Poly::<Ntt>::zero(&self.ctx_ksk);
for (c2_i_coefficients, c0_i, c1_i) in izip!(c2i.iter(), self.c0.iter(), self.c1.iter()) {

let decomposition_count = log_modulus.div_ceil(self.log_base);
let mut c2_i_buffer = vec![0u64; coefficients.len()];

for (c0_i, c1_i) in izip!(self.c0.iter(), self.c1.iter()).take(decomposition_count) {
for (dest, src) in izip!(c2_i_buffer.iter_mut(), coefficients.iter()) {
*dest = src & mask;
}
let mut c2_i = unsafe {
Poly::<Ntt>::create_constant_ntt_polynomial_with_lazy_coefficients_and_variable_time(
c2_i_coefficients.as_slice(),
&c2_i_buffer,
&self.ctx_ksk,
)
};
c0 += &(&c2_i * c0_i);
c2_i *= c1_i;
c1 += &c2_i;

coefficients.iter_mut().for_each(|c| *c >>= self.log_base);
}
Ok((c0, c1))
}
Expand Down Expand Up @@ -576,4 +580,5 @@ mod tests {
}
Ok(())
}

}
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