feat(proof.go): adding JSON marshal/unmarshal (#214)

Adds json marshaling/unmarshaling to `nmt.Proof`.

As found elsewhere in the go stdlib, `UnmarshalJSON` must be defined on
the pointer receiver although `MarshalJSON` is a value receiver.

This is needed because the fields are private. Does this need to be the
case? If we make them public, JSON marshaling will work by default.

proof.go

@@ -2,6 +2,7 @@ package nmt
 
 import (
 	"bytes"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"hash"
@@ -47,6 +48,39 @@ type Proof struct {
 	isMaxNamespaceIDIgnored bool
 }
 
+type jsonProof struct {
+	Start                   int      `json:"start"`
+	End                     int      `json:"end"`
+	Nodes                   [][]byte `json:"nodes"`
+	LeafHash                []byte   `json:"leaf_hash"`
+	IsMaxNamespaceIDIgnored bool     `json:"is_max_namespace_id_ignored"`
+}
+
+func (proof Proof) MarshalJSON() ([]byte, error) {
+	jsonProofObj := jsonProof{
+		Start:                   proof.start,
+		End:                     proof.end,
+		Nodes:                   proof.nodes,
+		LeafHash:                proof.leafHash,
+		IsMaxNamespaceIDIgnored: proof.isMaxNamespaceIDIgnored,
+	}
+	return json.Marshal(jsonProofObj)
+}
+
+func (proof *Proof) UnmarshalJSON(data []byte) error {
+	var jsonProofObj jsonProof
+	err := json.Unmarshal(data, &jsonProofObj)
+	if err != nil {
+		return err
+	}
+	proof.start = jsonProofObj.Start
+	proof.end = jsonProofObj.End
+	proof.nodes = jsonProofObj.Nodes
+	proof.leafHash = jsonProofObj.LeafHash
+	proof.isMaxNamespaceIDIgnored = jsonProofObj.IsMaxNamespaceIDIgnored
+	return nil
+}
+
 // Start index of this proof.
 func (proof Proof) Start() int {
 	return proof.start

proof_test.go

@@ -13,6 +13,29 @@ import (
 	"github.com/celestiaorg/nmt/namespace"
 )
 
+func TestJsonMarshal_Proof(t *testing.T) {
+	// create a tree with 4 leaves
+	nIDSize := 1
+	tree := exampleNMT(nIDSize, true, 1, 2, 3, 4)
+
+	// build a proof for an NID that is within the namespace range of the tree
+	nID := []byte{1}
+	proof, err := tree.ProveNamespace(nID)
+	require.NoError(t, err)
+
+	// marshal the proof to JSON
+	jsonProof, err := proof.MarshalJSON()
+	require.NoError(t, err)
+
+	// unmarshal the proof from JSON
+	var unmarshalledProof Proof
+	err = unmarshalledProof.UnmarshalJSON(jsonProof)
+	require.NoError(t, err)
+
+	// verify that the unmarshalled proof is equal to the original proof
+	assert.Equal(t, proof, unmarshalledProof)
+}
+
 // TestVerifyNamespace_EmptyProof tests the correct behaviour of VerifyNamespace for valid and invalid empty proofs.
 func TestVerifyNamespace_EmptyProof(t *testing.T) {
 	// create a tree with 4 leaves