diff --git a/src/gadgets/select.rs b/src/gadgets/select.rs index bbd36d76..9df395d8 100644 --- a/src/gadgets/select.rs +++ b/src/gadgets/select.rs @@ -10,24 +10,24 @@ impl, const D: usize> CircuitBuilder { /// Note: This does not range-check `b`. pub fn select_ext( &mut self, - b: Target, + b: ExtensionTarget, x: ExtensionTarget, y: ExtensionTarget, ) -> ExtensionTarget { - let b_ext = self.convert_to_ext(b); let gate = self.num_gates(); // Holds `by - y`. let first_out = ExtensionTarget::from_range(gate, ArithmeticExtensionGate::::wires_first_output()); - self.double_arithmetic_extension(F::ONE, F::NEG_ONE, b_ext, y, y, b_ext, x, first_out) + self.double_arithmetic_extension(F::ONE, F::NEG_ONE, b, y, y, b, x, first_out) .1 } /// See `select_ext`. pub fn select(&mut self, b: Target, x: Target, y: Target) -> Target { + let b_ext = self.convert_to_ext(b); let x_ext = self.convert_to_ext(x); let y_ext = self.convert_to_ext(y); - self.select_ext(b, x_ext, y_ext).to_target_array()[0] + self.select_ext(b_ext, x_ext, y_ext).to_target_array()[0] } } @@ -54,13 +54,13 @@ mod tests { let (x, y) = (FF::rand(), FF::rand()); let xt = builder.add_virtual_extension_target(); let yt = builder.add_virtual_extension_target(); - let truet = builder.add_virtual_target(); - let falset = builder.add_virtual_target(); + let truet = builder.add_virtual_extension_target(); + let falset = builder.add_virtual_extension_target(); pw.set_extension_target(xt, x); pw.set_extension_target(yt, y); - pw.set_target(truet, F::ONE); - pw.set_target(falset, F::ZERO); + pw.set_extension_target(truet, FF::ONE); + pw.set_extension_target(falset, FF::ZERO); let should_be_x = builder.select_ext(truet, xt, yt); let should_be_y = builder.select_ext(falset, xt, yt); diff --git a/src/gates/exponentiation.rs b/src/gates/exponentiation.rs new file mode 100644 index 00000000..2dd3b009 --- /dev/null +++ b/src/gates/exponentiation.rs @@ -0,0 +1,382 @@ +use std::marker::PhantomData; + +use crate::circuit_builder::CircuitBuilder; +use crate::field::extension_field::target::ExtensionTarget; +use crate::field::extension_field::Extendable; +use crate::field::field::Field; +use crate::gates::gate::Gate; +use crate::generator::{GeneratedValues, SimpleGenerator, WitnessGenerator}; +use crate::plonk_common::{reduce_with_powers, reduce_with_powers_recursive}; +use crate::target::Target; +use crate::vars::{EvaluationTargets, EvaluationVars, EvaluationVarsBase}; +use crate::wire::Wire; +use crate::witness::PartialWitness; + +/// A gate for raising a value to a power. +#[derive(Clone, Debug)] +pub(crate) struct ExponentiationGate, const D: usize> { + pub num_power_bits: usize, + pub _phantom: PhantomData, +} + +impl, const D: usize> ExponentiationGate { + pub fn new(num_power_bits: usize) -> Self { + Self { + num_power_bits, + _phantom: PhantomData, + } + } + + pub fn max_power_bits(num_wires: usize, num_routed_wires: usize) -> usize { + let max_for_routed_wires = num_routed_wires - 3; + let max_for_wires = (num_wires - 2) / 2; + max_for_routed_wires.min(max_for_wires) + } + + pub fn wire_base(&self) -> usize { + 0 + } + + pub fn wire_power(&self) -> usize { + 1 + } + + /// The `i`th bit of the exponent, in little-endian order. + pub fn wire_power_bit(&self, i: usize) -> usize { + debug_assert!(i < self.num_power_bits); + 2 + i + } + + pub fn wire_output(&self) -> usize { + 2 + self.num_power_bits + } + + pub fn wire_intermediate_value(&self, i: usize) -> usize { + debug_assert!(i < self.num_power_bits); + 3 + self.num_power_bits + i + } +} + +impl, const D: usize> Gate for ExponentiationGate { + fn id(&self) -> String { + format!("{:?}", self, D) + } + + fn eval_unfiltered(&self, vars: EvaluationVars) -> Vec { + let base = vars.local_wires[self.wire_base()]; + let power = vars.local_wires[self.wire_power()]; + + let power_bits: Vec<_> = (0..self.num_power_bits) + .map(|i| vars.local_wires[self.wire_power_bit(i)]) + .collect(); + let intermediate_values: Vec<_> = (0..self.num_power_bits) + .map(|i| vars.local_wires[self.wire_intermediate_value(i)]) + .collect(); + + let output = vars.local_wires[self.wire_output()]; + + let mut constraints = Vec::new(); + + let computed_power = reduce_with_powers(&power_bits, F::Extension::TWO); + constraints.push(power - computed_power); + + for i in 0..self.num_power_bits { + let prev_intermediate_value = if i == 0 { + F::Extension::ONE + } else { + intermediate_values[i - 1].square() + }; + + // power_bits is in LE order, but we accumulate in BE order. + let cur_bit = power_bits[self.num_power_bits - i - 1]; + + let not_cur_bit = F::Extension::ONE - cur_bit; + let computed_intermediate_value = + prev_intermediate_value * (cur_bit * base + not_cur_bit); + constraints.push(computed_intermediate_value - intermediate_values[i]); + } + + constraints.push(output - intermediate_values[self.num_power_bits - 1]); + + constraints + } + + fn eval_unfiltered_base(&self, vars: EvaluationVarsBase) -> Vec { + let base = vars.local_wires[self.wire_base()]; + let power = vars.local_wires[self.wire_power()]; + + let power_bits: Vec<_> = (0..self.num_power_bits) + .map(|i| vars.local_wires[self.wire_power_bit(i)]) + .collect(); + let intermediate_values: Vec<_> = (0..self.num_power_bits) + .map(|i| vars.local_wires[self.wire_intermediate_value(i)]) + .collect(); + + let output = vars.local_wires[self.wire_output()]; + + let mut constraints = Vec::new(); + + let computed_power = reduce_with_powers(&power_bits, F::TWO); + constraints.push(power - computed_power); + + for i in 0..self.num_power_bits { + let prev_intermediate_value = if i == 0 { + F::ONE + } else { + intermediate_values[i - 1].square() + }; + + // power_bits is in LE order, but we accumulate in BE order. + let cur_bit = power_bits[self.num_power_bits - i - 1]; + + let not_cur_bit = F::ONE - cur_bit; + let computed_intermediate_value = + prev_intermediate_value * (cur_bit * base + not_cur_bit); + constraints.push(computed_intermediate_value - intermediate_values[i]); + } + + constraints.push(output - intermediate_values[self.num_power_bits - 1]); + + constraints + } + + fn eval_unfiltered_recursively( + &self, + builder: &mut CircuitBuilder, + vars: EvaluationTargets, + ) -> Vec> { + let base = vars.local_wires[self.wire_base()]; + let power = vars.local_wires[self.wire_power()]; + + let power_bits: Vec<_> = (0..self.num_power_bits) + .map(|i| vars.local_wires[self.wire_power_bit(i)]) + .collect(); + let intermediate_values: Vec<_> = (0..self.num_power_bits) + .map(|i| vars.local_wires[self.wire_intermediate_value(i)]) + .collect(); + + let output = vars.local_wires[self.wire_output()]; + + let mut constraints = Vec::new(); + + let two = builder.constant(F::TWO); + let computed_power = reduce_with_powers_recursive(builder, &power_bits, two); + let power_diff = builder.sub_extension(power, computed_power); + constraints.push(power_diff); + + let one = builder.constant_extension(F::Extension::ONE); + for i in 0..self.num_power_bits { + let prev_intermediate_value = if i == 0 { + one + } else { + builder.square_extension(intermediate_values[i - 1]) + }; + + // power_bits is in LE order, but we accumulate in BE order. + let cur_bit = power_bits[self.num_power_bits - i - 1]; + let mul_by = builder.select_ext(cur_bit, base, one); + let computed_intermediate_value = + builder.mul_extension(prev_intermediate_value, mul_by); + let intermediate_value_diff = + builder.sub_extension(computed_intermediate_value, intermediate_values[i]); + constraints.push(intermediate_value_diff); + } + + let output_diff = + builder.sub_extension(output, intermediate_values[self.num_power_bits - 1]); + constraints.push(output_diff); + + constraints + } + + fn generators( + &self, + gate_index: usize, + _local_constants: &[F], + ) -> Vec>> { + let gen = ExponentiationGenerator:: { + gate_index, + gate: self.clone(), + }; + vec![Box::new(gen)] + } + + fn num_wires(&self) -> usize { + self.wire_intermediate_value(self.num_power_bits - 1) + 1 + } + + fn num_constants(&self) -> usize { + 0 + } + + fn degree(&self) -> usize { + 4 + } + + fn num_constraints(&self) -> usize { + self.num_power_bits + 2 + } +} + +#[derive(Debug)] +struct ExponentiationGenerator, const D: usize> { + gate_index: usize, + gate: ExponentiationGate, +} + +impl, const D: usize> SimpleGenerator for ExponentiationGenerator { + fn dependencies(&self) -> Vec { + let local_target = |input| Target::wire(self.gate_index, input); + + let mut deps = Vec::with_capacity(self.gate.num_power_bits + 2); + deps.push(local_target(self.gate.wire_base())); + deps.push(local_target(self.gate.wire_power())); + for i in 0..self.gate.num_power_bits { + deps.push(local_target(self.gate.wire_power_bit(i))); + } + deps + } + + fn run_once(&self, witness: &PartialWitness) -> GeneratedValues { + let local_wire = |input| Wire { + gate: self.gate_index, + input, + }; + + let get_local_wire = |input| witness.get_wire(local_wire(input)); + + let num_power_bits = self.gate.num_power_bits; + let base = get_local_wire(self.gate.wire_base()); + + let power_bits = (0..num_power_bits) + .map(|i| get_local_wire(self.gate.wire_power_bit(i))) + .collect::>(); + let mut intermediate_values = Vec::new(); + + let mut current_intermediate_value = F::ONE; + for i in 0..num_power_bits { + if power_bits[num_power_bits - i - 1] == F::ONE { + current_intermediate_value *= base; + } + intermediate_values.push(current_intermediate_value); + current_intermediate_value *= current_intermediate_value; + } + + let mut result = GeneratedValues::with_capacity(num_power_bits + 1); + for i in 0..num_power_bits { + let intermediate_value_wire = local_wire(self.gate.wire_intermediate_value(i)); + result.set_wire(intermediate_value_wire, intermediate_values[i]); + } + + let output_wire = local_wire(self.gate.wire_output()); + result.set_wire(output_wire, intermediate_values[num_power_bits - 1]); + + result + } +} + +#[cfg(test)] +mod tests { + use std::marker::PhantomData; + + use rand::{thread_rng, Rng}; + + use crate::field::crandall_field::CrandallField; + use crate::field::extension_field::quartic::QuarticCrandallField; + use crate::field::field::Field; + use crate::gates::exponentiation::ExponentiationGate; + use crate::gates::gate::Gate; + use crate::gates::gate_testing::test_low_degree; + use crate::proof::Hash; + use crate::util::log2_ceil; + use crate::vars::EvaluationVars; + + const MAX_POWER_BITS: usize = 17; + + #[test] + fn wire_indices() { + let gate = ExponentiationGate:: { + num_power_bits: 5, + _phantom: PhantomData, + }; + + assert_eq!(gate.wire_base(), 0); + assert_eq!(gate.wire_power(), 1); + assert_eq!(gate.wire_power_bit(0), 2); + assert_eq!(gate.wire_power_bit(4), 6); + assert_eq!(gate.wire_output(), 7); + assert_eq!(gate.wire_intermediate_value(0), 8); + assert_eq!(gate.wire_intermediate_value(4), 12); + } + + #[test] + fn low_degree() { + test_low_degree::(ExponentiationGate::new(5)); + } + + #[test] + fn test_gate_constraint() { + type F = CrandallField; + type FF = QuarticCrandallField; + const D: usize = 4; + + /// Returns the local wires for an exponentiation gate given the base, power, and power bit + /// values. + fn get_wires(base: F, power: u64) -> Vec { + let mut power_bits = Vec::new(); + let mut cur_power = power; + while cur_power > 0 { + power_bits.push(cur_power % 2); + cur_power /= 2; + } + + let num_power_bits = power_bits.len(); + + let power_F = F::from_canonical_u64(power); + let power_bits_F: Vec<_> = power_bits + .iter() + .map(|b| F::from_canonical_u64(*b)) + .collect(); + + let mut v = Vec::new(); + v.push(base); + v.push(power_F); + v.extend(power_bits_F.clone()); + + let mut intermediate_values = Vec::new(); + let mut current_intermediate_value = F::ONE; + for i in 0..num_power_bits { + if power_bits[num_power_bits - i - 1] == 1 { + current_intermediate_value *= base; + } + intermediate_values.push(current_intermediate_value); + current_intermediate_value *= current_intermediate_value; + } + let output_value = intermediate_values[num_power_bits - 1]; + v.push(output_value); + v.extend(intermediate_values); + + v.iter().map(|&x| x.into()).collect::>() + } + + let mut rng = rand::thread_rng(); + + let base = F::TWO; + let power = rng.gen::() % (1 << MAX_POWER_BITS); + let num_power_bits = log2_ceil(power + 1); + let gate = ExponentiationGate:: { + num_power_bits, + _phantom: PhantomData, + }; + + let vars = EvaluationVars { + local_constants: &[], + local_wires: &get_wires(base, power as u64), + public_inputs_hash: &Hash::rand(), + }; + assert!( + gate.eval_unfiltered(vars).iter().all(|x| x.is_zero()), + "Gate constraints are not satisfied." + ); + } +} diff --git a/src/gates/insertion.rs b/src/gates/insertion.rs index 17df4024..41e3e6e9 100644 --- a/src/gates/insertion.rs +++ b/src/gates/insertion.rs @@ -6,7 +6,7 @@ use crate::circuit_builder::CircuitBuilder; use crate::field::extension_field::target::ExtensionTarget; use crate::field::extension_field::{Extendable, FieldExtension}; use crate::field::field::Field; -use crate::gates::gate::{Gate, GateRef}; +use crate::gates::gate::Gate; use crate::generator::{GeneratedValues, SimpleGenerator, WitnessGenerator}; use crate::target::Target; use crate::vars::{EvaluationTargets, EvaluationVars, EvaluationVarsBase}; @@ -358,7 +358,7 @@ mod tests { type FF = QuarticCrandallField; const D: usize = 4; - /// Returns the local wires for an insertion gate for given the original vector, element to + /// Returns the local wires for an insertion gate given the original vector, element to /// insert, and index. fn get_wires(orig_vec: Vec, insertion_index: usize, element_to_insert: FF) -> Vec { let vec_size = orig_vec.len(); diff --git a/src/gates/mod.rs b/src/gates/mod.rs index 987a276d..20680141 100644 --- a/src/gates/mod.rs +++ b/src/gates/mod.rs @@ -4,6 +4,7 @@ pub mod arithmetic; pub mod base_sum; pub mod constant; +pub mod exponentiation; pub(crate) mod gate; pub mod gate_tree; pub mod gmimc;