tests: add unit tests for CBlockIndex::GetAncestor and LastCommonAncestor

diff --git a/src/test/CMakeLists.txt b/src/test/CMakeLists.txt
index aacefb3f..b4c1ec69 100644
--- a/src/test/CMakeLists.txt
+++ b/src/test/CMakeLists.txt
@@ -26,6 +26,7 @@ add_executable(test_bitcoin
   bloom_tests.cpp
   bswap_tests.cpp
   caches_tests.cpp
+  chain_tests.cpp
   chainstate_write_tests.cpp
   checkqueue_tests.cpp
   cluster_linearize_tests.cpp
diff --git a/src/test/chain_tests.cpp b/src/test/chain_tests.cpp
new file mode 100644
index 00000000..a782e880
--- /dev/null
+++ b/src/test/chain_tests.cpp
@@ -0,0 +1,85 @@
+// Copyright (c) The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <boost/test/unit_test.hpp>
+
+#include <chain.h>
+#include <test/util/setup_common.h>
+
+#include <memory>
+
+BOOST_FIXTURE_TEST_SUITE(chain_tests, BasicTestingSetup)
+
+namespace {
+
+const CBlockIndex* NaiveGetAncestor(const CBlockIndex* a, int height)
+{
+    while (a->nHeight > height) {
+        a = a->pprev;
+    }
+    BOOST_REQUIRE_EQUAL(a->nHeight, height);
+    return a;
+}
+
+const CBlockIndex* NaiveLastCommonAncestor(const CBlockIndex* a, const CBlockIndex* b)
+{
+    while (a->nHeight > b->nHeight) {
+        a = a->pprev;
+    }
+    while (b->nHeight > a->nHeight) {
+        b = b->pprev;
+    }
+    while (a != b) {
+        BOOST_REQUIRE_EQUAL(a->nHeight, b->nHeight);
+        a = a->pprev;
+        b = b->pprev;
+    }
+    BOOST_REQUIRE_EQUAL(a, b);
+    return a;
+}
+
+} // namespace
+
+BOOST_AUTO_TEST_CASE(chain_test)
+{
+    FastRandomContext ctx;
+    std::vector<std::unique_ptr<CBlockIndex>> block_index;
+    // Run 10 iterations of the whole test.
+    for (int i = 0; i < 10; ++i) {
+        block_index.clear();
+        // Create genesis block.
+        auto genesis = std::make_unique<CBlockIndex>();
+        genesis->nHeight = 0;
+        block_index.push_back(std::move(genesis));
+        // Create 10000 more blocks.
+        for (int b = 0; b < 10000; ++b) {
+            auto new_index = std::make_unique<CBlockIndex>();
+            // 95% of blocks build on top of the last block; the others fork off randomly.
+            if (ctx.randrange(20) != 0) {
+                new_index->pprev = block_index.back().get();
+            } else {
+                new_index->pprev = block_index[ctx.randrange(block_index.size())].get();
+            }
+            new_index->nHeight = new_index->pprev->nHeight + 1;
+            new_index->BuildSkip();
+            block_index.push_back(std::move(new_index));
+        }
+        // Run 10000 random GetAncestor queries.
+        for (int q = 0; q < 10000; ++q) {
+            const CBlockIndex* block = block_index[ctx.randrange(block_index.size())].get();
+            unsigned height = ctx.randrange<unsigned>(block->nHeight + 1);
+            const CBlockIndex* result = block->GetAncestor(height);
+            BOOST_CHECK(result == NaiveGetAncestor(block, height));
+        }
+        // Run 10000 random LastCommonAncestor queries.
+        for (int q = 0; q < 10000; ++q) {
+            const CBlockIndex* block1 = block_index[ctx.randrange(block_index.size())].get();
+            const CBlockIndex* block2 = block_index[ctx.randrange(block_index.size())].get();
+            const CBlockIndex* result = LastCommonAncestor(block1, block2);
+            BOOST_CHECK(result == NaiveLastCommonAncestor(block1, block2));
+        }
+    }
+}
+
+BOOST_AUTO_TEST_SUITE_END()