feat: add encode to compiler

This commit introduces `Compiler::encode` and `Compiler::decode`, two
functions that will create a compressed representation of a circuit that
can be used to generate prover and verifier keys.

The compression strategy takes advantage of the fact that circuit
representations are sparse; meaning, most of the scalars are zeroes. We
also have a higher incidence of `1` and `-1`, so any value that is
equivalent to a `i8` is compressed into a single byte, instead of the
regular `32` bytes of a bls12-381.

This will result in expressive gains in terms of storage. For instance,
a `2^12` circuit can be compressed into roughly 100Kb.

benches/plonk.rs

@@ -90,9 +90,11 @@ fn run<const DEGREE: usize>(
         Compiler::compile::<BenchCircuit<DEGREE>>(&pp, label)
             .expect("failed to compile circuit");
 
+    let circuit: BenchCircuit<DEGREE> = BenchCircuit::default();
+
     // sanity run
     let (proof, public_inputs) = prover
-        .prove(&mut rand_core::OsRng, &Default::default())
+        .prove(&mut rand_core::OsRng, &circuit)
         .expect("failed to prove");
 
     verifier
@@ -103,9 +105,7 @@ fn run<const DEGREE: usize>(
     let description = format!("Prove 2^{} = {} gates", power, DEGREE);
 
     c.bench_function(description.as_str(), |b| {
-        b.iter(|| {
-            black_box(prover.prove(&mut rand_core::OsRng, &Default::default()))
-        })
+        b.iter(|| black_box(prover.prove(&mut rand_core::OsRng, &circuit)))
     });
 
     let description = format!("Verify 2^{} = {} gates", power, DEGREE);

src/composer.rs

@@ -42,13 +42,13 @@ pub trait Composer: Sized + Index<Witness, Output = BlsScalar> {
     ///
     /// A turbo composer expects the first witness to be always present and to
     /// be zero.
-    const ZERO: Witness = Witness::new(0);
+    const ZERO: Witness = Witness::ZERO;
 
     /// `One` representation inside the constraint system.
     ///
     /// A turbo composer expects the 2nd witness to be always present and to
     /// be one.
-    const ONE: Witness = Witness::new(1);
+    const ONE: Witness = Witness::ONE;
 
     /// Identity point representation inside the constraint system
     const IDENTITY: WitnessPoint = WitnessPoint::new(Self::ZERO, Self::ONE);

src/composer/compiler.rs

@@ -4,15 +4,24 @@
 //
 // Copyright (c) DUSK NETWORK. All rights reserved.
 
+use core::{iter, mem};
+
 use dusk_bls12_381::BlsScalar;
 
 use crate::commitment_scheme::{CommitKey, OpeningKey, PublicParameters};
+use crate::constraint_system::{Constraint, Selector, Witness};
 use crate::error::Error;
 use crate::fft::{EvaluationDomain, Evaluations, Polynomial as FftPolynomial};
 use crate::proof_system::preprocess::Polynomials;
 use crate::proof_system::{widget, ProverKey};
 
-use super::{Builder, Circuit, Composer, Prover, Verifier};
+use super::{Builder, Circuit, Composer, Polynomial, Prover, Verifier};
+
+mod scalar;
+use scalar::ScalarEncoder;
+
+mod witness;
+use witness::WitnessEncoder;
 
 /// Generate the arguments to prove and verify a circuit
 pub struct Compiler;
@@ -24,48 +33,267 @@ impl Compiler {
     pub fn compile<C>(
         pp: &PublicParameters,
         label: &[u8],
-    ) -> Result<(Prover<C>, Verifier<C>), Error>
+    ) -> Result<(Prover, Verifier), Error>
     where
         C: Circuit,
     {
-        Self::compile_with_circuit(pp, label, &Default::default())
+        let max_size = Self::max_size(pp);
+        let mut builder = Builder::initialized(max_size);
+
+        C::default().circuit(&mut builder)?;
+
+        Self::compile_with_builder(pp, label, &builder)
     }
 
-    /// Create a new arguments set from a given circuit instance
+    /// Compile a circuit and encode its prover and verifier into the provided
+    /// buffer, extending its bytes.
+    ///
+    /// Returns the amount of bytes written to `buffer`.
     ///
-    /// Use the provided circuit instead of the default implementation
-    pub fn compile_with_circuit<C>(
+    /// [Self::decode] is used to return the prover/verifier.
+    pub fn encode<C, B>(
         pp: &PublicParameters,
-        label: &[u8],
-        circuit: &C,
-    ) -> Result<(Prover<C>, Verifier<C>), Error>
+        buffer: &mut B,
+    ) -> Result<usize, Error>
     where
         C: Circuit,
+        B: Extend<u8>,
     {
-        let max_size = (pp.commit_key.powers_of_g.len() - 1) >> 1;
-        let mut prover = Builder::initialized(max_size);
+        let max_size = Self::max_size(pp);
+        let mut builder = Builder::initialized(max_size);
+        let encoder = ScalarEncoder::default();
+
+        C::default().circuit(&mut builder)?;
+        let mut n = 0;
+
+        let witnesses = (builder.witnesses.len() as u64).to_le_bytes();
+        n += witnesses.len();
+        buffer.extend(witnesses);
+
+        let constraints = (builder.constraints.len() as u64).to_le_bytes();
+        n += constraints.len();
+        buffer.extend(constraints);
+
+        // TODO encode public input indexes
+
+        n += builder.constraints.iter().enumerate().fold(
+            0,
+            |n, (i, polynomial)| {
+                let has_pi = builder.public_inputs.contains_key(&i);
+                buffer.extend(iter::once(has_pi as u8));
+                let Polynomial {
+                    q_m,
+                    q_l,
+                    q_r,
+                    q_o,
+                    q_c,
+                    q_d,
+                    q_arith,
+                    q_range,
+                    q_logic,
+                    q_fixed_group_add,
+                    q_variable_group_add,
+                    w_a,
+                    w_b,
+                    w_d,
+                    w_o,
+                } = polynomial;
+                n + 1
+                    + encoder.encode(q_m, buffer)
+                    + encoder.encode(q_l, buffer)
+                    + encoder.encode(q_r, buffer)
+                    + encoder.encode(q_o, buffer)
+                    + encoder.encode(q_c, buffer)
+                    + encoder.encode(q_d, buffer)
+                    + encoder.encode(q_arith, buffer)
+                    + encoder.encode(q_range, buffer)
+                    + encoder.encode(q_logic, buffer)
+                    + encoder.encode(q_fixed_group_add, buffer)
+                    + encoder.encode(q_variable_group_add, buffer)
+                    + WitnessEncoder::encode(*w_a, buffer)
+                    + WitnessEncoder::encode(*w_b, buffer)
+                    + WitnessEncoder::encode(*w_d, buffer)
+                    + WitnessEncoder::encode(*w_o, buffer)
+            },
+        );
+
+        Ok(n)
+    }
+
+    /// Compile a circuit and encode its prover and verifier into the provided
+    /// buffer, extending its bytes.
+    ///
+    /// It will call [Self::encode] with a pre-allocated [Vec] so it will not
+    /// reallocate during runtime.
+    pub fn encode_to_vec<C>(pp: &PublicParameters) -> Result<Vec<u8>, Error>
+    where
+        C: Circuit,
+    {
+        // pre-alloc to worst case scenario
+        let max_size = Self::max_size(pp);
+        let capacity = max_size * mem::size_of::<Constraint>();
+        let mut buffer = Vec::with_capacity(capacity);
+        Self::encode::<C, _>(pp, &mut buffer)?;
+        Ok(buffer)
+    }
+
+    /// Generates a [Prover] and [Verifier] from a buffer created by
+    /// [Self::encode].
+    ///
+    /// Returns the amount of bytes read from the `buffer`.
+    pub fn decode(
+        pp: &PublicParameters,
+        label: &[u8],
+        mut buffer: &[u8],
+    ) -> Result<(usize, Prover, Verifier), Error> {
+        if buffer.len() < 17 {
+            return Err(Error::NotEnoughBytes);
+        }
+
+        let mut witnesses = [0; 8];
+        let mut constraints = [0; 8];
+
+        witnesses.copy_from_slice(&buffer[..8]);
+        constraints.copy_from_slice(&buffer[8..16]);
+
+        let witnesses = u64::from_le_bytes(witnesses);
+        let constraints = u64::from_le_bytes(constraints);
+
+        // TODO decode public input indexes
 
-        circuit.circuit(&mut prover)?;
+        let mut n = 16;
 
-        let n = (prover.constraints() + 6).next_power_of_two();
+        buffer = &buffer[n..];
+
+        #[allow(deprecated)]
+        let mut builder = Builder::uninitialized(constraints as usize);
+
+        (0..witnesses).for_each(|_| {
+            builder.append_witness(BlsScalar::zero());
+        });
+
+        (0..constraints).try_for_each::<_, Result<_, Error>>(|_| {
+            if buffer.is_empty() {
+                return Err(Error::NotEnoughBytes);
+            }
+            let has_pi = buffer[0] == true as u8;
+            n += 1;
+            buffer = &buffer[1..];
+
+            // decode the selectors
+            let q_m = Self::decode_scalar(&mut buffer, &mut n)?;
+            let q_l = Self::decode_scalar(&mut buffer, &mut n)?;
+            let q_r = Self::decode_scalar(&mut buffer, &mut n)?;
+            let q_o = Self::decode_scalar(&mut buffer, &mut n)?;
+            let q_c = Self::decode_scalar(&mut buffer, &mut n)?;
+            let q_d = Self::decode_scalar(&mut buffer, &mut n)?;
+            let q_arith = Self::decode_scalar(&mut buffer, &mut n)?;
+            let q_range = Self::decode_scalar(&mut buffer, &mut n)?;
+            let q_logic = Self::decode_scalar(&mut buffer, &mut n)?;
+            let q_fixed_group_add = Self::decode_scalar(&mut buffer, &mut n)?;
+            let q_variable_group_add =
+                Self::decode_scalar(&mut buffer, &mut n)?;
+
+            // decode the witnesses
+            let w_a = Self::decode_witness(&mut buffer, &mut n)?;
+            let w_b = Self::decode_witness(&mut buffer, &mut n)?;
+            let w_d = Self::decode_witness(&mut buffer, &mut n)?;
+            let w_o = Self::decode_witness(&mut buffer, &mut n)?;
+
+            // setup the constraint
+            let mut constraint = Constraint::default()
+                .set(Selector::Multiplication, q_m)
+                .set(Selector::Left, q_l)
+                .set(Selector::Right, q_r)
+                .set(Selector::Output, q_o)
+                .set(Selector::Constant, q_c)
+                .set(Selector::Fourth, q_d)
+                .set(Selector::Arithmetic, q_arith)
+                .set(Selector::Range, q_range)
+                .set(Selector::Logic, q_logic)
+                .set(Selector::GroupAddFixedBase, q_fixed_group_add)
+                .set(Selector::GroupAddVariableBase, q_variable_group_add)
+                .a(w_a)
+                .b(w_b)
+                .d(w_d)
+                .o(w_o);
+
+            // append a PI, if applicable
+            if has_pi {
+                constraint = constraint.public(BlsScalar::zero());
+            }
+
+            // append the constraint to the builder
+            builder.append_custom_gate(constraint);
+
+            Ok(())
+        })?;
+
+        let (prover, verifier) =
+            Self::compile_with_builder(pp, label, &builder)?;
+
+        Ok((n, prover, verifier))
+    }
+
+    /// Consumes a buffer to decode a scalar. Update the buffer to its
+    /// remainder.
+    fn decode_scalar(
+        buffer: &mut &[u8],
+        n: &mut usize,
+    ) -> Result<BlsScalar, Error> {
+        let (consumed, scalar) = ScalarEncoder::decode(*buffer);
+        if consumed == 0 {
+            return Err(Error::BlsScalarMalformed);
+        }
+        *n += consumed;
+        *buffer = &(*buffer)[consumed..];
+        Ok(scalar)
+    }
+
+    /// Consumes a buffer to decode a witness. Update the buffer to its
+    /// remainder.
+    fn decode_witness(
+        buffer: &mut &[u8],
+        n: &mut usize,
+    ) -> Result<Witness, Error> {
+        let (consumed, witness) = WitnessEncoder::decode(*buffer);
+        if consumed == 0 {
+            return Err(Error::BlsScalarMalformed);
+        }
+        *n += consumed;
+        *buffer = &(*buffer)[consumed..];
+        Ok(witness)
+    }
+
+    /// Returns the maximum constraints length for the parameters.
+    fn max_size(pp: &PublicParameters) -> usize {
+        (pp.commit_key.powers_of_g.len() - 1) >> 1
+    }
+
+    /// Create a new arguments set from a given circuit instance
+    ///
+    /// Use the default implementation of the circuit
+    fn compile_with_builder(
+        pp: &PublicParameters,
+        label: &[u8],
+        builder: &Builder,
+    ) -> Result<(Prover, Verifier), Error> {
+        let n = (builder.constraints() + 6).next_power_of_two();
 
         let (commit, opening) = pp.trim(n)?;
 
         let (prover, verifier) =
-            Self::preprocess(label, commit, opening, &prover)?;
+            Self::preprocess(label, commit, opening, &builder)?;
 
         Ok((prover, verifier))
     }
 
-    fn preprocess<C>(
+    fn preprocess(
         label: &[u8],
         commit_key: CommitKey,
         opening_key: OpeningKey,
         prover: &Builder,
-    ) -> Result<(Prover<C>, Verifier<C>), Error>
-    where
-        C: Circuit,
-    {
+    ) -> Result<(Prover, Verifier), Error> {
         let mut perm = prover.perm.clone();
 
         let constraints = prover.constraints();
@@ -386,3 +614,6 @@ impl Compiler {
         Ok((prover, verifier))
     }
 }
+
+#[test]
+fn compiler_encode_and_decode() {}