kernel: expose witness stack for btck_TransactionInput

diff --git a/src/kernel/bitcoinkernel.cpp b/src/kernel/bitcoinkernel.cpp
index 0a315ff3..92700d16 100644
--- a/src/kernel/bitcoinkernel.cpp
+++ b/src/kernel/bitcoinkernel.cpp
@@ -498,6 +498,7 @@ struct btck_TransactionSpentOutputs : Handle<btck_TransactionSpentOutputs, CTxUn
 struct btck_Coin : Handle<btck_Coin, Coin> {};
 struct btck_BlockHash : Handle<btck_BlockHash, uint256> {};
 struct btck_TransactionInput : Handle<btck_TransactionInput, CTxIn> {};
+struct btck_WitnessStack : Handle<btck_WitnessStack, CScriptWitness> {};
 struct btck_TransactionOutPoint: Handle<btck_TransactionOutPoint, COutPoint> {};
 struct btck_Txid: Handle<btck_Txid, Txid> {};
 struct btck_PrecomputedTransactionData : Handle<btck_PrecomputedTransactionData, PrecomputedTransactionData> {};
@@ -709,11 +710,38 @@ uint32_t btck_transaction_input_get_sequence(const btck_TransactionInput* input)
     return btck_TransactionInput::get(input).nSequence;
 }
 
+const btck_WitnessStack* btck_transaction_input_get_witness_stack(const btck_TransactionInput* input)
+{
+    return btck_WitnessStack::ref(&btck_TransactionInput::get(input).scriptWitness);
+}
+
 void btck_transaction_input_destroy(btck_TransactionInput* input)
 {
     delete input;
 }
 
+size_t btck_witness_stack_count_items(const btck_WitnessStack* witness_stack)
+{
+    return btck_WitnessStack::get(witness_stack).stack.size();
+}
+
+int btck_witness_stack_get_item_at(const btck_WitnessStack* witness_stack, size_t index, btck_WriteBytes writer, void* user_data)
+{
+    const auto& stack{btck_WitnessStack::get(witness_stack).stack};
+    assert(index < stack.size());
+    return writer(stack[index].data(), stack[index].size(), user_data);
+}
+
+btck_WitnessStack* btck_witness_stack_copy(const btck_WitnessStack* witness_stack)
+{
+    return btck_WitnessStack::copy(witness_stack);
+}
+
+void btck_witness_stack_destroy(btck_WitnessStack* witness_stack)
+{
+    delete witness_stack;
+}
+
 btck_TransactionOutPoint* btck_transaction_out_point_copy(const btck_TransactionOutPoint* out_point)
 {
     return btck_TransactionOutPoint::copy(out_point);
diff --git a/src/kernel/bitcoinkernel.h b/src/kernel/bitcoinkernel.h
index 6ce65a04..09b1caf0 100644
--- a/src/kernel/bitcoinkernel.h
+++ b/src/kernel/bitcoinkernel.h
@@ -298,10 +298,17 @@ typedef struct btck_BlockHash btck_BlockHash;
 /**
  * Opaque data structure for holding a transaction input.
  *
- * Holds information on the @ref btck_TransactionOutPoint held within.
+ * Holds information on the @ref btck_TransactionOutPoint, @ref btck_WitnessStack and script_sig held within.
  */
 typedef struct btck_TransactionInput btck_TransactionInput;
 
+/**
+ * Opaque data structure for holding a witness stack.
+ *
+ * Holds a sequence of witness stack items.
+ */
+typedef struct btck_WitnessStack btck_WitnessStack;
+
 /**
  * Opaque data structure for holding a transaction out point.
  *
@@ -1680,6 +1687,16 @@ BITCOINKERNEL_API const btck_TransactionOutPoint* btck_transaction_input_get_out
 BITCOINKERNEL_API uint32_t btck_transaction_input_get_sequence(
     const btck_TransactionInput* transaction_input) BITCOINKERNEL_ARG_NONNULL(1);
 
+/**
+ * @brief Get the witness stack of a transaction input. The returned witness
+ * stack is not owned and depends on the lifetime of the transaction input.
+ *
+ * @param[in] transaction_input Non-null.
+ * @return                      The witness stack.
+ */
+BITCOINKERNEL_API const btck_WitnessStack* btck_transaction_input_get_witness_stack(
+    const btck_TransactionInput* transaction_input) BITCOINKERNEL_ARG_NONNULL(1);
+
 /**
  * Destroy the transaction input.
  */
@@ -1687,6 +1704,53 @@ BITCOINKERNEL_API void btck_transaction_input_destroy(btck_TransactionInput* tra
 
 ///@}
 
+/** @name Witness Stack
+ * Functions for working with witness stacks.
+ */
+///@{
+
+/**
+ * @brief Return the number of items in a witness stack.
+ *
+ * @param[in] witness_stack Non-null.
+ * @return                  The number of witness stack items.
+ */
+BITCOINKERNEL_API size_t btck_witness_stack_count_items(
+    const btck_WitnessStack* witness_stack) BITCOINKERNEL_ARG_NONNULL(1);
+
+/**
+ * @brief Serialize a witness stack item at a given index through the passed in
+ * callback.
+ *
+ * @param[in] witness_stack Non-null.
+ * @param[in] index         Index of the item in the witness stack.
+ * @param[in] writer        Non-null, function pointer for writing bytes.
+ * @param[in] user_data     Nullable, passed back through the writer callback.
+ * @return                  The return value of the writer.
+ * @pre                    index < btck_witness_stack_count_items(witness_stack)
+ */
+BITCOINKERNEL_API int BITCOINKERNEL_WARN_UNUSED_RESULT btck_witness_stack_get_item_at(
+    const btck_WitnessStack* witness_stack,
+    size_t index,
+    btck_WriteBytes writer,
+    void* user_data) BITCOINKERNEL_ARG_NONNULL(1, 3);
+
+/**
+ * @brief Copy a witness stack.
+ *
+ * @param[in] witness_stack Non-null.
+ * @return                  The copied witness stack.
+ */
+BITCOINKERNEL_API btck_WitnessStack* BITCOINKERNEL_WARN_UNUSED_RESULT btck_witness_stack_copy(
+    const btck_WitnessStack* witness_stack) BITCOINKERNEL_ARG_NONNULL(1);
+
+/**
+ * Destroy the witness stack.
+ */
+BITCOINKERNEL_API void btck_witness_stack_destroy(btck_WitnessStack* witness_stack);
+
+///@}
+
 /** @name Transaction Out Point
  * Functions for working with transaction out points.
  */
diff --git a/src/kernel/bitcoinkernel_wrapper.h b/src/kernel/bitcoinkernel_wrapper.h
index ff7899d1..f84a06ed 100644
--- a/src/kernel/bitcoinkernel_wrapper.h
+++ b/src/kernel/bitcoinkernel_wrapper.h
@@ -564,6 +564,49 @@ public:
         : Handle(view) {}
 };
 
+template <typename Derived>
+class WitnessStackApi
+{
+private:
+    auto impl() const
+    {
+        return static_cast<const Derived*>(this)->get();
+    }
+
+    friend Derived;
+    WitnessStackApi() = default;
+
+public:
+    size_t CountItems() const
+    {
+        return btck_witness_stack_count_items(impl());
+    }
+
+    std::vector<std::byte> GetItem(size_t index) const
+    {
+        struct Item { const btck_WitnessStack* stack; size_t index; };
+        Item item{impl(), index};
+        return write_bytes(&item, +[](const Item* c, btck_WriteBytes w, void* ud) {
+            return btck_witness_stack_get_item_at(c->stack, c->index, w, ud);
+        });
+    }
+
+    MAKE_RANGE_METHOD(Items, Derived, &WitnessStackApi<Derived>::CountItems, &WitnessStackApi<Derived>::GetItem, *static_cast<const Derived*>(this))
+};
+
+class WitnessStackView : public View<btck_WitnessStack>, public WitnessStackApi<WitnessStackView>
+{
+public:
+    explicit WitnessStackView(const btck_WitnessStack* ptr) : View{ptr} {}
+};
+
+class WitnessStack : public Handle<btck_WitnessStack, btck_witness_stack_copy, btck_witness_stack_destroy>, public WitnessStackApi<WitnessStack>
+{
+public:
+    WitnessStack(const WitnessStackView& view)
+        : Handle(view) {}
+};
+
 template <typename Derived>
 class TransactionInputApi
 {
@@ -586,6 +629,12 @@ public:
     {
         return btck_transaction_input_get_sequence(impl());
     }
+
+    WitnessStackView GetWitnessStack() const
+    {
+        return WitnessStackView{btck_transaction_input_get_witness_stack(impl())};
+    }
+
 };
 
 class TransactionInputView : public View<btck_TransactionInput>, public TransactionInputApi<TransactionInputView>
diff --git a/src/test/kernel/test_kernel.cpp b/src/test/kernel/test_kernel.cpp
index e6d215dc..8dc63fd6 100644
--- a/src/test/kernel/test_kernel.cpp
+++ b/src/test/kernel/test_kernel.cpp
@@ -513,6 +513,28 @@ BOOST_AUTO_TEST_CASE(btck_transaction_input)
     OutPoint point_0 = input_0.OutPoint();
     OutPoint point_1 = input_1.OutPoint();
     CheckHandle(point_0, point_1);
+
+    WitnessStackView ws_0 = input_0.GetWitnessStack();
+    BOOST_CHECK_EQUAL(ws_0.CountItems(), 0);
+    BOOST_CHECK(ws_0.Items().empty());
+
+    // P2WSH input: OP_0, sig, sig, redeem_script (0, 71, 71, 105 bytes); no scriptSig.
+    Transaction segwit_tx{hex_string_to_byte_vec("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")};
+    TransactionInputView segwit_input = segwit_tx.GetInput(0);
+    WitnessStackView ws = segwit_input.GetWitnessStack();
+    BOOST_CHECK_EQUAL(ws.CountItems(), 4);
+    BOOST_CHECK(ws.GetItem(0).empty());
+    BOOST_CHECK(ws.GetItem(1) == hex_string_to_byte_vec("30440220565d170eed95ff95027a69b313758450ba84a01224e1f7f130dda46e94d13f8602207bdd20e307f062594022f12ed5017bbf4a055a06aea91c10110a0e3bb23117fc01"));
+    BOOST_CHECK(ws.GetItem(2) == hex_string_to_byte_vec("30440220647d2dc5b15f60bc37dc42618a370b2a1490293f9e5c8464f53ec4fe1dfe067302203598773895b4b16d37485cbe21b337f4e4b650739880098c592553add7dd435501"));
+    BOOST_CHECK(ws.GetItem(3) == hex_string_to_byte_vec("52210375e00eb72e29da82b89367947f29ef34afb75e8654f6ea368e0acdfd92976b7c2103a1b26313f430c4b15bb1fdce663207659d8cac749a0e53d70eff01874496feff2103c96d495bfdd5ba4145e3e046fee45e84a8a48ad05bd8dbb395c011a32cf9f88053ae"));
+    auto items = ws.Items();
+    BOOST_CHECK_EQUAL(items.size(), 4);
+    for (size_t i = 0; i < items.size(); ++i) {
+        BOOST_CHECK(items[i] == ws.GetItem(i));
+    }
+    WitnessStack owned_ws_0{ws_0};
+    WitnessStack owned_ws{ws};
+    CheckHandle(owned_ws_0, owned_ws);
 }
 
 BOOST_AUTO_TEST_CASE(btck_precomputed_txdata) {