Add HashFunction interface (w/ Poseidon & Sha256)

hash.go

@@ -0,0 +1,87 @@
+package arbo
+
+import (
+	"crypto/sha256"
+	"math/big"
+
+	"github.com/iden3/go-iden3-crypto/poseidon"
+)
+
+var (
+	// TypeHashSha256 represents the label for the HashFunction of Sha256
+	TypeHashSha256 = []byte("sha256")
+	// TypeHashPoseidon represents the label for the HashFunction of
+	// Poseidon
+	TypeHashPoseidon = []byte("poseidon")
+
+	// HashFunctionSha256 contains the HashSha256 struct which implements
+	// the HashFunction interface
+	HashFunctionSha256 HashSha256
+	// HashFunctionPoseidon contains the HashPoseidon struct which implements
+	// the HashFunction interface
+	HashFunctionPoseidon HashPoseidon
+)
+
+// Once Generics are at Go, this will be updated (August 2021
+// https://blog.golang.org/generics-next-step)
+
+// HashFunction defines the interface that is expected for a hash function to be
+// used in a generic way in the Tree.
+type HashFunction interface {
+	Type() []byte
+	Len() int
+	Hash(...[]byte) ([]byte, error)
+	// CheckInputs checks if the inputs are valid without computing the hash
+	// CheckInputs(...[]byte) error
+}
+
+// HashSha256 implements the HashFunction interface for the Sha256 hash
+type HashSha256 struct{}
+
+// Type returns the type of HashFunction for the HashSha256
+func (f HashSha256) Type() []byte {
+	return TypeHashSha256
+}
+
+// Len returns the length of the Hash output
+func (f HashSha256) Len() int {
+	return 32 //nolint:gomnd
+}
+
+// Hash implements the hash method for the HashFunction HashSha256
+func (f HashSha256) Hash(b ...[]byte) ([]byte, error) {
+	var toHash []byte
+	for i := 0; i < len(b); i++ {
+		toHash = append(toHash, b[i]...)
+	}
+	h := sha256.Sum256(toHash)
+	return h[:], nil
+}
+
+// HashPoseidon implements the HashFunction interface for the Poseidon hash
+type HashPoseidon struct{}
+
+// Type returns the type of HashFunction for the HashPoseidon
+func (f HashPoseidon) Type() []byte {
+	return TypeHashPoseidon
+}
+
+// Len returns the length of the Hash output
+func (f HashPoseidon) Len() int {
+	return 32 //nolint:gomnd
+}
+
+// Hash implements the hash method for the HashFunction HashPoseidon
+func (f HashPoseidon) Hash(b ...[]byte) ([]byte, error) {
+	var toHash []*big.Int
+	for i := 0; i < len(b); i++ {
+		bi := BytesToBigInt(b[i])
+		toHash = append(toHash, bi)
+	}
+	h, err := poseidon.Hash(toHash)
+	if err != nil {
+		return nil, err
+	}
+	hB := BigIntToBytes(h)
+	return hB, nil
+}