1use math::poly::PolyRNS;
 2use math::ring::RingRNS;
 3use num_bigint::BigInt;
 4use num_bigint::Sign;
 5use sampling::source::Source;
 6
 7#[test]
 8fn rescaling_rns_u64() {
 9    let n = 1 << 10;
10    let moduli: Vec<u64> = vec![0x1fffffffffc80001u64, 0x1fffffffffe00001u64];
11    let ring_rns: RingRNS<u64> = RingRNS::new(n, moduli);
12
13    test_div_floor_by_last_modulus::<false>(&ring_rns);
14    test_div_floor_by_last_modulus::<true>(&ring_rns);
15}
16
17fn test_div_floor_by_last_modulus<const NTT: bool>(ring_rns: &RingRNS<u64>) {
18    let seed: [u8; 32] = [0; 32];
19    let mut source: Source = Source::new(seed);
20
21    let mut a: PolyRNS<u64> = ring_rns.new_polyrns();
22    let mut b: PolyRNS<u64> = ring_rns.new_polyrns();
23    let mut c: PolyRNS<u64> = ring_rns.at_level(ring_rns.level() - 1).new_polyrns();
24
25    // Allocates a random PolyRNS
26    ring_rns.fill_uniform(&mut source, &mut a);
27
28    // Maps PolyRNS to [BigInt]
29    let mut coeffs_a: Vec<BigInt> = (0..a.n()).map(|i| BigInt::from(i)).collect();
30    ring_rns
31        .at_level(a.level())
32        .to_bigint_inplace(&a, 1, &mut coeffs_a);
33
34    // Performs c = intt(ntt(a) / q_level)
35    if NTT {
36        ring_rns.ntt_inplace::<false>(&mut a);
37    }
38
39    ring_rns.div_floor_by_last_modulus::<NTT>(&a, &mut b, &mut c);
40
41    if NTT {
42        ring_rns.at_level(c.level()).intt_inplace::<false>(&mut c);
43    }
44
45    // Exports c to coeffs_c
46    let mut coeffs_c = vec![BigInt::from(0); c.n()];
47    ring_rns
48        .at_level(c.level())
49        .to_bigint_inplace(&c, 1, &mut coeffs_c);
50
51    // Performs floor division on a
52    let scalar_big = BigInt::from(ring_rns.0[ring_rns.level()].modulus.q);
53    coeffs_a.iter_mut().for_each(|a| {
54        // Emulates floor division in [0, q-1] and maps to [-(q-1)/2, (q-1)/2-1]
55        *a /= &scalar_big;
56        if a.sign() == Sign::Minus {
57            *a -= 1;
58        }
59    });
60
61    assert!(coeffs_a == coeffs_c);
62}