feat: add build root children from proof

Author: mpolitzer

internal/merkle/builder.go

@@ -50,6 +50,29 @@ func (proof *Proof) BuildRoot() common.Hash {
 	return rootHash
 }
 
+func (proof *Proof) BuildRootChildren() (common.Hash, common.Hash) {
+	two := big.NewInt(2)
+	height := len(proof.Siblings)
+	childHash := proof.Node
+
+	for i, s := range proof.Siblings[:height-1] {
+
+		// ((pos >> i) % 2) == 0
+		if new(big.Int).Rem(new(big.Int).Rsh(proof.Pos, uint(i)), two).Cmp(zero) == 0 {
+			childHash = crypto.Keccak256Hash(childHash[:], s[:])
+		} else {
+			childHash = crypto.Keccak256Hash(s[:], childHash[:])
+		}
+	}
+
+	// ((pos >> (height-1)) % 2) == 0
+	if new(big.Int).Rem(new(big.Int).Rsh(proof.Pos, uint(height-1)), two).Cmp(zero) == 0 {
+		return childHash, proof.Siblings[height-1]
+	} else {
+		return proof.Siblings[height-1], childHash
+	}
+}
+
 func (proof *Proof) VerifyRoot(other common.Hash) bool {
 	return proof.BuildRoot() == other
 }

internal/merkle/builder_test.go

@@ -8,6 +8,7 @@ import (
 	"testing"
 
 	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/crypto"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -129,6 +130,51 @@ func TestAppendAndRepeated(t *testing.T) {
 	assert.Equal(t, tree1, tree2)
 }
 
+func TestBuildRootChildren1(t *testing.T) {
+	p := Proof{
+		Pos:  big.NewInt(1),
+		Node: common.HexToHash("0x01"),
+		Siblings: []common.Hash{
+			common.HexToHash("0x02"),
+		},
+	}
+	rootHash := p.BuildRoot()
+	lhs, rhs := p.BuildRootChildren()
+
+	assert.Equal(t, rootHash, crypto.Keccak256Hash(lhs[:], rhs[:]))
+}
+
+func TestBuildRootChildren2(t *testing.T) {
+	p := Proof{
+		Pos:  big.NewInt(1),
+		Node: common.HexToHash("0x01"),
+		Siblings: []common.Hash{
+			common.HexToHash("0x02"),
+			common.HexToHash("0x03"),
+		},
+	}
+	rootHash := p.BuildRoot()
+	lhs, rhs := p.BuildRootChildren()
+
+	assert.Equal(t, rootHash, crypto.Keccak256Hash(lhs[:], rhs[:]))
+}
+
+func TestBuildRootChildren3(t *testing.T) {
+	p := Proof{
+		Pos:  big.NewInt(1),
+		Node: common.HexToHash("0x01"),
+		Siblings: []common.Hash{
+			common.HexToHash("0x02"),
+			common.HexToHash("0x03"),
+			common.HexToHash("0x04"),
+		},
+	}
+	rootHash := p.BuildRoot()
+	lhs, rhs := p.BuildRootChildren()
+
+	assert.Equal(t, rootHash, crypto.Keccak256Hash(lhs[:], rhs[:]))
+}
+
 // repanicked
 //func TestBuildNotPow2(t *testing.T) {
 //	defer recover()