trie: use bitarray for path encoding

trie/bintrie/binary_node.go

@@ -25,7 +25,7 @@ import (
 )
 
 type (
-	NodeFlushFn    func([]byte, BinaryNode)
+	NodeFlushFn    func(*BitArray, BinaryNode)
 	NodeResolverFn func([]byte, common.Hash) ([]byte, error)
 )
 
@@ -54,7 +54,7 @@ type BinaryNode interface {
 	Hash() common.Hash
 	GetValuesAtStem([]byte, NodeResolverFn) ([][]byte, error)
 	InsertValuesAtStem([]byte, [][]byte, NodeResolverFn, int) (BinaryNode, error)
-	CollectNodes([]byte, NodeFlushFn) error
+	CollectNodes(*BitArray, NodeFlushFn) error
 
 	toDot(parent, path string) string
 	GetHeight() int

trie/bintrie/binary_node_test.go

@@ -199,28 +199,28 @@ func TestKeyToPath(t *testing.T) {
 			name:     "depth 0",
 			depth:    0,
 			key:      []byte{0x80}, // 10000000 in binary
-			expected: []byte{1},
+			expected: []byte{1},    // first 1 bit = 1
 			wantErr:  false,
 		},
 		{
 			name:     "depth 7",
 			depth:    7,
 			key:      []byte{0xFF}, // 11111111 in binary
-			expected: []byte{1, 1, 1, 1, 1, 1, 1, 1},
+			expected: []byte{0xFF}, // first 8 bits = 0xFF
 			wantErr:  false,
 		},
 		{
 			name:     "depth crossing byte boundary",
 			depth:    10,
 			key:      []byte{0xFF, 0x00}, // 11111111 00000000 in binary
-			expected: []byte{1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0},
+			expected: []byte{0x07, 0xF8}, // first 11 bits = 11111111000 = 0x7F8
 			wantErr:  false,
 		},
 		{
 			name:     "max valid depth",
 			depth:    StemSize * 8,
 			key:      make([]byte, HashSize),
-			expected: make([]byte, StemSize*8+1),
+			expected: make([]byte, StemSize), // 248 bits of zeros (capped to key length)
 			wantErr:  false,
 		},
 		{