Implement conversions

Cargo.toml

@@ -53,6 +53,8 @@ ethers-core = { git = "https://github.com/gakonst/ethers-rs", default-features =
 
 [dev-dependencies]
 pretty_assertions = "1.0"
+rand_chacha = "0.3.1"
+rand_core = "0.6.3"
 serde_json = "1.0.79"
 tempfile = "3.0"
 tiny-keccak = "2.0.2"

src/lib.rs

@@ -16,13 +16,12 @@ pub mod mimc_hash;
 #[cfg(feature = "mimc")]
 pub mod mimc_tree;
 
-use ark_bn254::{FrParameters, Parameters};
+use ark_bn254::{Fr, FrParameters, Parameters};
 use ark_ec::bn::Bn;
-use ark_ff::Fp256;
 
 pub use crate::posseidon_hash::posseidon_hash;
 
-pub type Field = Fp256<FrParameters>;
+pub type Field = Fr;
 pub type Groth16Proof = ark_groth16::Proof<Bn<Parameters>>;
 pub type EthereumGroth16Proof = ark_circom::ethereum::Proof;
 

src/poseidon_tree.rs

@@ -3,7 +3,7 @@ use crate::{
     merkle_tree::{self, Hasher, MerkleTree},
     posseidon_hash, Field,
 };
-use ark_ff::PrimeField;
+use ark_ff::{PrimeField, ToBytes};
 use serde::{Deserialize, Serialize};
 
 #[allow(dead_code)]
@@ -33,7 +33,12 @@ impl From<Hash> for Field {
 #[allow(clippy::fallible_impl_from)] // TODO
 impl From<Field> for Hash {
     fn from(n: Field) -> Self {
-        todo!()
+        let mut bytes = [0_u8; 32];
+        n.into_repr()
+            .write(&mut bytes[..])
+            .expect("write should succeed");
+        bytes.reverse(); // Convert to big endian
+        Self(bytes)
     }
 }
 
@@ -48,7 +53,22 @@ impl Hasher for PoseidonHash {
 #[cfg(test)]
 pub mod test {
     use super::*;
+    use ark_ff::UniformRand;
     use hex_literal::hex;
+    use rand_chacha::ChaChaRng;
+    use rand_core::SeedableRng;
+
+    #[test]
+    fn test_ark_hash_ark_roundtrip() {
+        use ark_ff::One;
+        let mut rng = ChaChaRng::seed_from_u64(123);
+        for i in 0..1000 {
+            let n = Field::rand(&mut rng);
+            let n = Field::one();
+            let m = Hash::from(n).into();
+            assert_eq!(n, m);
+        }
+    }
 
     #[test]
     fn test_tree_4() {

src/posseidon_hash.rs

@@ -3,7 +3,8 @@ use crate::{
     merkle_tree::{self, Hasher, MerkleTree},
     Field,
 };
-use ff::{PrimeField, PrimeFieldRepr};
+use ark_ff::{BigInteger256, PrimeField as _};
+use ff::{PrimeField as _, PrimeFieldRepr as _};
 use once_cell::sync::Lazy;
 use poseidon_rs::{Fr, FrRepr, Poseidon};
 use serde::{Deserialize, Serialize};
@@ -11,11 +12,11 @@ use serde::{Deserialize, Serialize};
 static POSEIDON: Lazy<Poseidon> = Lazy::new(Poseidon::new);
 
 fn ark_to_posseidon(n: Field) -> Fr {
-    todo!()
+    Fr::from_repr(FrRepr(n.into_repr().0)).expect("n is a valid field element")
 }
 
 fn posseidon_to_ark(n: Fr) -> Field {
-    todo!()
+    Field::from_repr(BigInteger256(n.into_repr().0)).expect("n is a valid field element")
 }
 
 pub fn posseidon_hash(input: &[Field]) -> Field {
@@ -33,14 +34,26 @@ pub fn posseidon_hash(input: &[Field]) -> Field {
 
 #[cfg(test)]
 mod test {
-    use ff::{Field, PrimeField, PrimeFieldRepr};
+    use super::{ark_to_posseidon, posseidon_to_ark};
+    use crate::Field;
+    use ark_ff::{Field as _, UniformRand};
+    use ff::{Field as _, PrimeField, PrimeFieldRepr};
     use poseidon_rs::Fr;
+    use rand_chacha::ChaChaRng;
+    use rand_core::SeedableRng;
 
     #[test]
     fn test_modulus_identical() {
-        let mut modulus = [0_u8; 32];
-        let writer = Fr::char().write_be(&mut modulus[..]).unwrap();
+        assert_eq!(Fr::char().0, Field::characteristic());
+    }
 
-        todo!()
+    #[test]
+    fn test_ark_pos_ark_roundtrip() {
+        let mut rng = ChaChaRng::seed_from_u64(123);
+        for i in 0..1000 {
+            let n = Field::rand(&mut rng);
+            let m = posseidon_to_ark(ark_to_posseidon(n));
+            assert_eq!(n, m)
+        }
     }
 }