Add Script::Tree note about prehashed merkle root, update tests

Author: bbrtj

lib/Bitcoin/Crypto/Script/Tree.pm

@@ -300,6 +300,11 @@ instead:
 		hash => bytestring with prehashed leaf,
 	}
 
+Same logic can be applied if only the merkle root of the tree is known - simply
+introduce a prehashed structure as the only element of the tree. This can be
+useful in places where a script tree object is required for its root hash, but
+you don't have the full tree - for example in PSBTs.
+
 =head1 INTERFACE
 
 =head2 Attributes

t/Script/Tree.t

@@ -2,53 +2,53 @@ use Test2::V0;
 use Bitcoin::Crypto qw(btc_script_tree);
 use Bitcoin::Crypto::Util qw(to_format);
 
-# this example is a modified test case from BIP341
-my $tree = btc_script_tree->new(
-	tree => [
-		{
-			id => 0,
-			leaf_version => 192,
-			script => [hex => '2071981521ad9fc9036687364118fb6ccd2035b96a423c59c5430e98310a11abe2ac']
-		},
-		[
-			{
-				id => 1,
-				leaf_version => 192,
-				script => [hex => '20d5094d2dbe9b76e2c245a2b89b6006888952e2faa6a149ae318d69e520617748ac']
-			},
-			{
-				id => 2,
-				leaf_version => 192,
-				script => [hex => '20c440b462ad48c7a77f94cd4532d8f2119dcebbd7c9764557e62726419b08ad4cac']
-			}
-		]
-	]
-);
+subtest 'should calculate tree root' => sub {
 
-my $tree_prehashed = btc_script_tree->new(
-	tree => [
-		{hash => [hex => 'f154e8e8e17c31d3462d7132589ed29353c6fafdb884c5a6e04ea938834f0d9d']},
-		[
+	# this example is a modified test case from BIP341
+	my $tree = btc_script_tree->new(
+		tree => [
 			{
+				id => 0,
 				leaf_version => 192,
-				script => [hex => '20d5094d2dbe9b76e2c245a2b89b6006888952e2faa6a149ae318d69e520617748ac']
+				script => [hex => '2071981521ad9fc9036687364118fb6ccd2035b96a423c59c5430e98310a11abe2ac']
 			},
-			{hash => [hex => 'd7485025fceb78b9ed667db36ed8b8dc7b1f0b307ac167fa516fe4352b9f4ef7']},
+			[
+				{
+					id => 1,
+					leaf_version => 192,
+					script => [hex => '20d5094d2dbe9b76e2c245a2b89b6006888952e2faa6a149ae318d69e520617748ac']
+				},
+				{
+					id => 2,
+					leaf_version => 192,
+					script => [hex => '20c440b462ad48c7a77f94cd4532d8f2119dcebbd7c9764557e62726419b08ad4cac']
+				}
+			]
 		]
-	]
-);
+	);
 
-my $tree_from_path = btc_script_tree->from_path(
-	{
-		leaf_version => 192,
-		script => [hex => '20d5094d2dbe9b76e2c245a2b89b6006888952e2faa6a149ae318d69e520617748ac']
-	}, [
-		[hex => 'd7485025fceb78b9ed667db36ed8b8dc7b1f0b307ac167fa516fe4352b9f4ef7'],
-		[hex => 'f154e8e8e17c31d3462d7132589ed29353c6fafdb884c5a6e04ea938834f0d9d'],
-	]
-);
+	my $tree_prehashed = btc_script_tree->new(
+		tree => [
+			{hash => [hex => 'f154e8e8e17c31d3462d7132589ed29353c6fafdb884c5a6e04ea938834f0d9d']},
+			[
+				{
+					leaf_version => 192,
+					script => [hex => '20d5094d2dbe9b76e2c245a2b89b6006888952e2faa6a149ae318d69e520617748ac']
+				},
+				{hash => [hex => 'd7485025fceb78b9ed667db36ed8b8dc7b1f0b307ac167fa516fe4352b9f4ef7']},
+			]
+		]
+	);
 
-subtest 'testing merkle_root' => sub {
+	my $tree_from_path = btc_script_tree->from_path(
+		{
+			leaf_version => 192,
+			script => [hex => '20d5094d2dbe9b76e2c245a2b89b6006888952e2faa6a149ae318d69e520617748ac']
+		}, [
+			[hex => 'd7485025fceb78b9ed667db36ed8b8dc7b1f0b307ac167fa516fe4352b9f4ef7'],
+			[hex => 'f154e8e8e17c31d3462d7132589ed29353c6fafdb884c5a6e04ea938834f0d9d'],
+		]
+	);
 	is(
 		to_format [hex => $tree->get_merkle_root],
 		'2f6b2c5397b6d68ca18e09a3f05161668ffe93a988582d55c6f07bd5b3329def',
@@ -68,10 +68,20 @@ subtest 'testing merkle_root' => sub {
 	);
 };
 
-subtest 'testing from_path with empty path' => sub {
-	my $tree;
+subtest 'tree merkle root should be equal to the hash of the single tree element' => sub {
+	my $root = '2f6b2c5397b6d68ca18e09a3f05161668ffe93a988582d55c6f07bd5b3329def';
+	my $tree_root_only = btc_script_tree->new(tree => [{hash => [hex => $root]}]);
+
+	is(
+		to_format [hex => $tree_root_only->get_merkle_root],
+		$root,
+		'precalculated merkle root ok'
+	);
+};
+
+subtest 'should create a tree using from_path with empty path' => sub {
 	ok lives {
-		$tree = btc_script_tree->from_path(
+		btc_script_tree->from_path(
 			{
 				leaf_version => 192,
 				script => [hex => '20d5094d2dbe9b76e2c245a2b89b6006888952e2faa6a149ae318d69e520617748ac']