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lookup_range_check: Replace short_lookup_bitshift with selector. 

Previously, the short_lookup_bitshift fixed column was a non-binary
selector that both provided a constant value and toggled a gate.

Now, the constant value is copied in from the global constants API,
and the toggle is handled by a q_lookup_bitshift selector.
This commit is contained in:
therealyingtong 2021-07-24 00:31:01 +08:00
parent 283b45169a
commit 76c73531c8
1 changed files with 19 additions and 18 deletions
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37
src/circuit/gadget/utilities/lookup_range_check.rs
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@ -17,7 +17,7 @@ use super::*;
pub struct LookupRangeCheckConfig<F: FieldExt + PrimeFieldBits, const K: usize> { pub struct LookupRangeCheckConfig<F: FieldExt + PrimeFieldBits, const K: usize> {
pub q_lookup: Selector, pub q_lookup: Selector,
pub q_lookup_short: Selector, pub q_lookup_short: Selector,
pub short_lookup_bitshift: Column<Fixed>, pub q_lookup_bitshift: Selector,
pub running_sum: Column<Advice>, pub running_sum: Column<Advice>,
table_idx: Column<Fixed>, table_idx: Column<Fixed>,
_marker: PhantomData<F>, _marker: PhantomData<F>,
@ -44,11 +44,11 @@ impl<F: FieldExt + PrimeFieldBits, const K: usize> LookupRangeCheckConfig<F, K>
let q_lookup = meta.selector(); let q_lookup = meta.selector();
let q_lookup_short = meta.selector(); let q_lookup_short = meta.selector();
let short_lookup_bitshift = meta.fixed_column(); let q_lookup_bitshift = meta.selector();
let config = LookupRangeCheckConfig { let config = LookupRangeCheckConfig {
q_lookup, q_lookup,
q_lookup_short, q_lookup_short,
short_lookup_bitshift, q_lookup_bitshift,
running_sum, running_sum,
table_idx, table_idx,
_marker: PhantomData, _marker: PhantomData,
@ -78,15 +78,16 @@ impl<F: FieldExt + PrimeFieldBits, const K: usize> LookupRangeCheckConfig<F, K>
// For short lookups, check that the word has been shifted by the correct number of bits. // For short lookups, check that the word has been shifted by the correct number of bits.
meta.create_gate("Short lookup bitshift", |meta| { meta.create_gate("Short lookup bitshift", |meta| {
let inv_two_pow_s = meta.query_fixed(config.short_lookup_bitshift, Rotation::cur()); let q_lookup_bitshift = meta.query_selector(config.q_lookup_bitshift);
let word = meta.query_advice(config.running_sum, Rotation::prev()); let word = meta.query_advice(config.running_sum, Rotation::prev());
let shifted_word = meta.query_advice(config.running_sum, Rotation::cur()); let shifted_word = meta.query_advice(config.running_sum, Rotation::cur());
let inv_two_pow_s = meta.query_advice(config.running_sum, Rotation::next());
let two_pow_k = F::from_u64(1 << K); let two_pow_k = F::from_u64(1 << K);
// shifted_word = word * 2^{K-s} // shifted_word = word * 2^{K-s}
// = word * 2^K * inv_two_pow_s // = word * 2^K * inv_two_pow_s
vec![inv_two_pow_s.clone() * (word * two_pow_k * inv_two_pow_s - shifted_word)] vec![q_lookup_bitshift * (word * two_pow_k * inv_two_pow_s - shifted_word)]
}); });
config config
@ -317,17 +318,11 @@ impl<F: FieldExt + PrimeFieldBits, const K: usize> LookupRangeCheckConfig<F, K>
// Enable lookup for `element`, to constrain it to 10 bits. // Enable lookup for `element`, to constrain it to 10 bits.
self.q_lookup_short.enable(region, 0)?; self.q_lookup_short.enable(region, 0)?;
// Assign 2^{-num_bits} in a fixed column. // Enable lookup for shifted element, to constrain it to 10 bits.
{ self.q_lookup_short.enable(region, 1)?;
// 2^{-num_bits}
let inv_two_pow_s = F::from_u64(1 << num_bits).invert().unwrap(); // Check element has been shifted by the correct number of bits.
region.assign_fixed( self.q_lookup_bitshift.enable(region, 1)?;
|| format!("2^(-{})", num_bits),
self.short_lookup_bitshift,
1,
|| Ok(inv_two_pow_s),
)?;
}
// Assign shifted `element * 2^{K - num_bits}` // Assign shifted `element * 2^{K - num_bits}`
let shifted = element.value().map(|element| { let shifted = element.value().map(|element| {
@ -342,8 +337,14 @@ impl<F: FieldExt + PrimeFieldBits, const K: usize> LookupRangeCheckConfig<F, K>
|| shifted.ok_or(Error::SynthesisError), || shifted.ok_or(Error::SynthesisError),
)?; )?;
// Enable lookup for shifted element, to constrain it to 10 bits. // Assign 2^{-num_bits} from a fixed column.
self.q_lookup_short.enable(region, 1)?; let inv_two_pow_s = F::from_u64(1 << num_bits).invert().unwrap();
region.assign_advice_from_constant(
|| format!("2^(-{})", num_bits),
self.running_sum,
2,
inv_two_pow_s,
)?;
Ok(()) Ok(())
} }