Fix massif index from mmr index (#21)

re: AB#9491

Co-authored-by: jgough <[email protected]>

massifs/masssifreader.go

@@ -182,28 +182,10 @@ func MassifIndexFromLeafIndex(massifHeight uint8, leafIndex uint64) uint64 {
 //	given the mmr index of the leaf.
 //
 // NOTE: if the mmrIndex is not a leaf node, then error is returned.
-func MassifIndexFromMMRIndex(massifHeight uint8, mmrIndex uint64) (uint64, error) {
+func MassifIndexFromMMRIndex(massifHeight uint8, mmrIndex uint64) uint64 {
 
-	// First check if the given mmrIndex is a leaf node.
-	//
-	// NOTE: leaf nodes are always on height 0.
-	height := mmr.IndexHeight(mmrIndex)
-	if height != 0 {
-		return 0, ErrNotleaf
-	}
-
-	// HeightSize returns the maximum number of nodes for a given height of MMR. Where the leaf nodes
-	//  start on height 1.
-	mmrSize := mmr.HeightSize(uint64(massifHeight))
-
-	// now find the massif.
-	//
-	// for context, see: https://github.com/datatrails/epic-8120-scalable-proof-mechanisms/blob/main/mmr/forestrie-mmrblobs.md#blob-size
-	//
-	// Note: massif indexes start at 0.
-	// Note: mmr indexes starts at 0.
-	massifIndex := mmrIndex / mmrSize
+	leafIndex := mmr.LeafIndex(mmrIndex)
 
-	return massifIndex, nil
+	return MassifIndexFromLeafIndex(massifHeight, leafIndex)
 
 }

massifs/masssifreader_test.go

@@ -104,11 +104,16 @@ func TestMassifIndexFromLeafIndex(t *testing.T) {
 // 1. a height of 3 and a mmr index of 4, returns a massif index of 0
 // 2. a height of 3 and a mmr index of 15, returns a massif index of 2
 // 3. a height of 3 and a mmr index of 10, returns a massif index of 1
-// 4. a height of 5 and a mmr index of 33, returns a massif index of 1
-// 5. a height of 5 and a mmr index of 70, returns a massif index of 1
-// 6. a height of 3 and a mmr index of 12, returns not a leaf err
-// 7. a height of 5 and a mmr index of 72, returns not a leaf err
-// 8. a height of 14 and a mmr index of 16382, returns a massif index of 1
+// 4. a height of 3 and a mmr index of 11, returns a massif index of 1
+// 5. a height of 3 and a mmr index of 14, returns a massif index of 1
+// 6. a height of 5 and a mmr index of 33, returns a massif index of 1
+// 7. a height of 5 and a mmr index of 70, returns a massif index of 1
+// 8. a height of 3 and a mmr index of 12, returns a massif index of 1
+// 9. a height of 5 and a mmr index of 72, returns a massif index of 2
+// 10. a height of 14 and a mmr index of 16382, returns a massif index of 0
+// 11. a height of 14 and a mmr index of 16383, returns a massif index of 1
+// 12. a height of 14 and a mmr index of 999418, retruns a massif index of 60
+// 13. a height of 14 and a mmr index of 999419, retruns a massif index of 61
 func TestMassifIndexFromMMRIndex(t *testing.T) {
 	type args struct {
 		massifHeight uint8
@@ -144,6 +149,22 @@ func TestMassifIndexFromMMRIndex(t *testing.T) {
 			},
 			expected: 1,
 		},
+		{
+			name: "height 3, mmr index 11",
+			args: args{
+				massifHeight: 3,
+				mmrIndex:     11,
+			},
+			expected: 1,
+		},
+		{
+			name: "height 3, mmr index 14",
+			args: args{
+				massifHeight: 3,
+				mmrIndex:     14,
+			},
+			expected: 1,
+		},
 		{
 			name: "height 5, mmr index 32",
 			args: args{
@@ -174,32 +195,53 @@ func TestMassifIndexFromMMRIndex(t *testing.T) {
 				massifHeight: 3,
 				mmrIndex:     12,
 			},
-			expected: 0,
-			err:      ErrNotleaf,
+			expected: 1,
 		},
 		{
 			name: "height 5, mmr index 72",
 			args: args{
 				massifHeight: 5,
 				mmrIndex:     72,
 			},
-			expected: 0,
-			err:      ErrNotleaf,
+			expected: 2,
 		},
 		{
 			name: "height 14, mmr index 16382",
+			args: args{
+				massifHeight: 14,
+				mmrIndex:     16382,
+			},
+			expected: 0,
+		},
+		{
+			name: "height 14, mmr index 16383",
 			args: args{
 				massifHeight: 14,
 				mmrIndex:     16383,
 			},
 			expected: 1,
 		},
+		{
+			name: "height 14, mmr index 999418",
+			args: args{
+				massifHeight: 14,
+				mmrIndex:     999418,
+			},
+			expected: 60,
+		},
+		{
+			name: "height 14, mmr index 999419",
+			args: args{
+				massifHeight: 14,
+				mmrIndex:     999419,
+			},
+			expected: 61,
+		},
 	}
 	for _, test := range tests {
 		t.Run(test.name, func(t *testing.T) {
-			actual, err := MassifIndexFromMMRIndex(test.args.massifHeight, test.args.mmrIndex)
+			actual := MassifIndexFromMMRIndex(test.args.massifHeight, test.args.mmrIndex)
 
-			assert.Equal(t, test.err, err)
 			assert.Equal(t, test.expected, actual)
 		})
 	}