phantom-zone/src/decomposer.rs
src/decomposer.rsRaw
1use itertools::{izip, Itertools};
2use num_traits::{
3 AsPrimitive, FromPrimitive, Num, One, PrimInt, ToPrimitive, WrappingAdd, WrappingSub, Zero,
4};
5use std::{
6 fmt::{Debug, Display},
7 marker::PhantomData,
8 ops::Rem,
9};
10
11use crate::backend::{ArithmeticOps, ModularOpsU64};
12
13fn gadget_vector<T: PrimInt>(logq: usize, logb: usize, d: usize) -> Vec<T> {
14 assert!(logq >= (logb * d));
15 let ignored_bits = logq - (logb * d);
16
17 (0..d)
18 .into_iter()
19 .map(|i| T::one() << (logb * i + ignored_bits))
20 .collect_vec()
21}
22
23pub trait RlweDecomposer {
24 type Element;
25 type D: Decomposer<Element = Self::Element>;
26
27 /// Decomposer for RLWE Part A
28 fn a(&self) -> &Self::D;
29 /// Decomposer for RLWE Part B
30 fn b(&self) -> &Self::D;
31}
32
33impl<D> RlweDecomposer for (D, D)
34where
35 D: Decomposer,
36{
37 type D = D;
38 type Element = D::Element;
39 fn a(&self) -> &Self::D {
40 &self.0
41 }
42 fn b(&self) -> &Self::D {
43 &self.1
44 }
45}
46
47pub trait Decomposer {
48 type Element;
49 type Iter: Iterator<Item = Self::Element>;
50 fn new(q: Self::Element, logb: usize, d: usize) -> Self;
51
52 fn decompose_to_vec(&self, v: &Self::Element) -> Vec<Self::Element>;
53 fn decompose_iter(&self, v: &Self::Element) -> Self::Iter;
54 fn decomposition_count(&self) -> usize;
55}
56
57pub struct DefaultDecomposer<T> {
58 /// Ciphertext modulus
59 q: T,
60 /// Log of ciphertext modulus
61 logq: usize,
62 /// Log of base B
63 logb: usize,
64 /// base B
65 b: T,
66 /// (B - 1). To simulate (% B) as &(B-1), that is extract least significant
67 /// logb bits
68 b_mask: T,
69 /// B/2
70 bby2: T,
71 /// Decomposition count
72 d: usize,
73 /// No. of bits to ignore in rounding
74 ignore_bits: usize,
75}
76
77pub trait NumInfo {
78 const BITS: u32;
79}
80
81impl NumInfo for u64 {
82 const BITS: u32 = u64::BITS;
83}
84impl NumInfo for u32 {
85 const BITS: u32 = u32::BITS;
86}
87impl NumInfo for u128 {
88 const BITS: u32 = u128::BITS;
89}
90
91impl<T: PrimInt + NumInfo + Debug> DefaultDecomposer<T> {
92 fn recompose<Op>(&self, limbs: &[T], modq_op: &Op) -> T
93 where
94 Op: ArithmeticOps<Element = T>,
95 {
96 let mut value = T::zero();
97 let gadget_vector = self.gadget_vector();
98 assert!(limbs.len() == gadget_vector.len());
99 izip!(limbs.iter(), gadget_vector.iter())
100 .for_each(|(d_el, beta)| value = modq_op.add(&value, &modq_op.mul(d_el, beta)));
101
102 value
103 }
104
105 pub(crate) fn gadget_vector(&self) -> Vec<T> {
106 return gadget_vector(self.logq, self.logb, self.d);
107 }
108}
109
110impl<
111 T: PrimInt
112 + ToPrimitive
113 + FromPrimitive
114 + WrappingSub
115 + WrappingAdd
116 + NumInfo
117 + From<bool>
118 + Display
119 + Debug,
120 > Decomposer for DefaultDecomposer<T>
121{
122 type Element = T;
123 type Iter = DecomposerIter<T>;
124
125 fn new(q: T, logb: usize, d: usize) -> DefaultDecomposer<T> {
126 // if q is power of 2, then `BITS - leading_zeros` outputs logq + 1.
127 let logq = if q & (q - T::one()) == T::zero() {
128 (T::BITS - q.leading_zeros() - 1) as usize
129 } else {
130 (T::BITS - q.leading_zeros()) as usize
131 };
132
133 assert!(
134 logq >= (logb * d),
135 "Decomposer wants logq >= logb*d but got logq={logq}, logb={logb}, d={d}"
136 );
137
138 let ignore_bits = logq - (logb * d);
139
140 DefaultDecomposer {
141 q,
142 logq,
143 logb,
144 b: T::one() << logb,
145 b_mask: (T::one() << logb) - T::one(),
146 bby2: T::one() << (logb - 1),
147 d,
148 ignore_bits,
149 }
150 }
151
152 // TODO(Jay): Outline the caveat
153 fn decompose_to_vec(&self, value: &T) -> Vec<T> {
154 let q = self.q;
155 let logb = self.logb;
156 let b = T::one() << logb;
157 let full_mask = b - T::one();
158 let bby2 = b >> 1;
159
160 let mut value = *value;
161 if value >= (q >> 1) {
162 value = !(q - value) + T::one()
163 }
164 value = round_value(value, self.ignore_bits);
165 let mut out = Vec::with_capacity(self.d);
166 for _ in 0..(self.d) {
167 let k_i = value & full_mask;
168
169 value = (value - k_i) >> logb;
170
171 if k_i > bby2 || (k_i == bby2 && ((value & T::one()) == T::one())) {
172 out.push(q - (b - k_i));
173 value = value + T::one();
174 } else {
175 out.push(k_i);
176 }
177 }
178
179 return out;
180 }
181
182 fn decomposition_count(&self) -> usize {
183 self.d
184 }
185
186 fn decompose_iter(&self, value: &T) -> DecomposerIter<T> {
187 let mut value = *value;
188 if value >= (self.q >> 1) {
189 value = !(self.q - value) + T::one()
190 }
191 value = round_value(value, self.ignore_bits);
192
193 DecomposerIter {
194 value,
195 q: self.q,
196 logq: self.logq,
197 logb: self.logb,
198 b: self.b,
199 bby2: self.bby2,
200 b_mask: self.b_mask,
201 steps_left: self.d,
202 }
203 }
204}
205
206impl<T: PrimInt> DefaultDecomposer<T> {}
207
208pub struct DecomposerIter<T> {
209 /// Value to decompose
210 value: T,
211 steps_left: usize,
212 /// (1 << logb) - 1 (for % (1<<logb); i.e. to extract least signiciant logb
213 /// bits)
214 b_mask: T,
215 logb: usize,
216 // b/2 = 1 << (logb-1)
217 bby2: T,
218 /// Ciphertext modulus
219 q: T,
220 /// Log of ciphertext modulus
221 logq: usize,
222 /// b = 1 << logb
223 b: T,
224}
225
226impl<T: PrimInt + From<bool> + WrappingSub + Display> Iterator for DecomposerIter<T> {
227 type Item = T;
228
229 fn next(&mut self) -> Option<Self::Item> {
230 if self.steps_left != 0 {
231 self.steps_left -= 1;
232 let k_i = self.value & self.b_mask;
233
234 self.value = (self.value - k_i) >> self.logb;
235
236 // if k_i > self.bby2 || (k_i == self.bby2 && ((self.value &
237 // T::one()) == T::one())) { self.value = self.value
238 // + T::one(); Some(self.q + k_i - self.b)
239 // } else {
240 // Some(k_i)
241 // }
242
243 // Following is without branching impl of the commented version above. It
244 // happens to speed up bootstrapping for `SMALL_MP_BOOL_PARAMS` (& other
245 // parameters as well but I haven't tested) by roughly 15ms.
246 // Suprisingly the improvement does not show up when I benchmark
247 // `decomposer_iter` in isolation. Putting this remark here as a
248 // future task to investiage (TODO).
249 let carry_bool =
250 k_i > self.bby2 || (k_i == self.bby2 && ((self.value & T::one()) == T::one()));
251 let carry = <T as From<bool>>::from(carry_bool);
252 let neg_carry = (T::zero().wrapping_sub(&carry));
253 self.value = self.value + carry;
254 Some((neg_carry & self.q) + k_i - (carry << self.logb))
255
256 // Some(
257 // (self.q & ((carry << self.logq) - (T::one() & carry))) + k_i
258 // - (carry << self.logb), )
259
260 // Some(k_i)
261 } else {
262 None
263 }
264 }
265}
266
267fn round_value<T: PrimInt + WrappingAdd>(value: T, ignore_bits: usize) -> T {
268 if ignore_bits == 0 {
269 return value;
270 }
271
272 let ignored_msb = (value & ((T::one() << ignore_bits) - T::one())) >> (ignore_bits - 1);
273 (value >> ignore_bits).wrapping_add(&ignored_msb)
274}
275
276#[cfg(test)]
277mod tests {
278
279 use itertools::{izip, Itertools};
280 use rand::{thread_rng, Rng};
281
282 use crate::{
283 backend::{ModInit, ModularOpsU64, Modulus},
284 decomposer::round_value,
285 utils::{generate_prime, tests::Stats, TryConvertFrom1},
286 };
287
288 use super::{Decomposer, DefaultDecomposer};
289
290 #[test]
291 fn decomposition_works() {
292 let ring_size = 1 << 11;
293
294 let mut rng = thread_rng();
295
296 for logq in [37, 55] {
297 let logb = 11;
298 let d = 3;
299 let mut stats = vec![Stats::new(); d];
300
301 for i in [true, false] {
302 let q = if i {
303 generate_prime(logq, 2 * ring_size, 1u64 << logq).unwrap()
304 } else {
305 1u64 << logq
306 };
307 let decomposer = DefaultDecomposer::new(q, logb, d);
308 let modq_op = ModularOpsU64::new(q);
309 for _ in 0..1000000 {
310 let value = rng.gen_range(0..q);
311 let limbs = decomposer.decompose_to_vec(&value);
312 let limbs_from_iter = decomposer.decompose_iter(&value).collect_vec();
313 assert_eq!(limbs, limbs_from_iter);
314 let value_back = round_value(
315 decomposer.recompose(&limbs, &modq_op),
316 decomposer.ignore_bits,
317 );
318 let rounded_value = round_value(value, decomposer.ignore_bits);
319 assert!((rounded_value as i64 - value_back as i64).abs() <= 1,);
320
321 izip!(stats.iter_mut(), limbs.iter()).for_each(|(s, l)| {
322 s.add_more(&vec![q.map_element_to_i64(l)]);
323 });
324 }
325 }
326
327 stats.iter().enumerate().for_each(|(index, s)| {
328 println!(
329 "Limb {index} - Mean: {}, Std: {}",
330 s.mean(),
331 s.std_dev().abs().log2()
332 );
333 });
334 }
335 }
336}