Add/text merkle_branch(leaf).

Author: evoskuil

include/bitcoin/system/chain/block.hpp

@@ -42,6 +42,7 @@ class BC_API block
 
     typedef std::shared_ptr<const block> cptr;
 
+    static std::vector<size_t> merkle_branch(size_t leaf) NOEXCEPT;
     static bool is_malleable64(const transaction_cptrs& txs) NOEXCEPT;
     static uint64_t subsidy(size_t height, uint64_t subsidy_interval,
         uint64_t initial_block_subsidy_satoshi, bool bip42) NOEXCEPT;

src/chain/block.cpp

@@ -445,6 +445,32 @@ bool block::is_invalid_merkle_root() const NOEXCEPT
     return generate_merkle_root(false) != header_->merkle_root();
 }
 
+// public/static (utility)
+std::vector<size_t> block::merkle_branch(size_t leaf) NOEXCEPT
+{
+    BC_ASSERT(leaf < power2(sub1(bits<size_t>)));
+
+    std::vector<size_t> positions{};
+    if (is_zero(leaf))
+        return positions;
+
+    // Upper bound, actual count may be less due to duplication.
+    positions.reserve(ceilinged_log2(leaf));
+
+    for (auto width = one, leaves = add1(leaf); leaves > one;)
+    {
+        const auto sibling = bit_xor(leaf, one);
+        if (sibling < leaves++)
+            positions.push_back(sibling * width);
+
+        shift_right_into(leaf);
+        shift_right_into(leaves);
+        shift_left_into(width);
+    }
+
+    return positions;
+}
+
 // Accept (contextual).
 // ----------------------------------------------------------------------------
 

test/chain/block.cpp

@@ -706,6 +706,78 @@ BOOST_AUTO_TEST_CASE(block__is_invalid_merkle_root__block100k__false)
     BOOST_REQUIRE(!instance.is_invalid_merkle_root());
 }
 
+// merkle_branch
+
+BOOST_AUTO_TEST_CASE(block__merkle_branch__leaf_zero__empty)
+{
+    BOOST_REQUIRE(block::merkle_branch(0).empty());
+}
+
+BOOST_AUTO_TEST_CASE(block__merkle_branch__one__zero)
+{
+    const std::vector<size_t> expected{ 0 };
+    BOOST_REQUIRE(block::merkle_branch(1) == expected);
+}
+
+BOOST_AUTO_TEST_CASE(block__merkle_branch__two_for_odd_length__zero)
+{
+    const std::vector<size_t> expected{ 0 };
+    BOOST_REQUIRE(block::merkle_branch(2) == expected);
+}
+
+BOOST_AUTO_TEST_CASE(block__merkle_branch__three__two_and_zero)
+{
+    const std::vector<size_t> expected{ 2, 0 };
+    BOOST_REQUIRE(block::merkle_branch(3) == expected);
+}
+
+BOOST_AUTO_TEST_CASE(block__merkle_branch__seven__six_four_and_zero)
+{
+    const std::vector<size_t> expected{ 6, 4, 0 };
+    BOOST_REQUIRE(block::merkle_branch(7) == expected);
+}
+
+BOOST_AUTO_TEST_CASE(block__merkle_branch__ten_for_odd__eight_and_zero)
+{
+    const std::vector<size_t> expected{ 8, 0 };
+    BOOST_REQUIRE(block::merkle_branch(10) == expected);
+}
+
+BOOST_AUTO_TEST_CASE(block__merkle_branch__medium_power_of_two__expected)
+{
+    const std::vector<size_t> expected{ 14u, 12u, 8u, 0u };
+    BOOST_REQUIRE(block::merkle_branch(15u) == expected);
+}
+
+BOOST_AUTO_TEST_CASE(block__merkle_branch__power_of_two_minus_one__expected)
+{
+    constexpr auto leaf = 1023u;
+    constexpr auto size = ceilinged_log2(leaf);
+    const auto positions = block::merkle_branch(leaf);
+    BOOST_CHECK_EQUAL(positions.size(), size);
+    BOOST_CHECK_EQUAL(positions.front(), 1022u);
+    BOOST_CHECK_EQUAL(positions.back(), 0u);
+}
+
+BOOST_AUTO_TEST_CASE(block__merkle_branch__odd_large_leaf_with_duplication__expected)
+{
+    constexpr auto leaf = 2047u;
+    constexpr auto size = ceilinged_log2(leaf);
+    const auto positions = block::merkle_branch(leaf);
+    BOOST_CHECK_EQUAL(positions.size(), size);
+    BOOST_CHECK_EQUAL(positions.front(), 2046u);
+    BOOST_CHECK_EQUAL(positions.back(), 0u);
+}
+
+BOOST_AUTO_TEST_CASE(block__merkle_branch__maximum_non_overflow__expected)
+{
+    constexpr auto maximum = sub1(power2(sub1(bits<size_t>)));
+    const auto positions = block::merkle_branch(maximum);
+    BOOST_CHECK_EQUAL(positions.size(), sub1(bits<size_t>));
+    BOOST_CHECK_EQUAL(positions.front(), sub1(maximum));
+    BOOST_CHECK_EQUAL(positions.back(), 0u);
+}
+
 // is_overweight
 // is_invalid_coinbase_script
 // is_hash_limit_exceeded