Kernel: Add functions for working with outpoints

diff --git a/src/kernel/bitcoinkernel.cpp b/src/kernel/bitcoinkernel.cpp
index 531df3c8..8bba3cf1 100644
--- a/src/kernel/bitcoinkernel.cpp
+++ b/src/kernel/bitcoinkernel.cpp
@@ -491,6 +491,9 @@ struct btck_BlockSpentOutputs : Handle<btck_BlockSpentOutputs, std::shared_ptr<C
 struct btck_TransactionSpentOutputs : Handle<btck_TransactionSpentOutputs, CTxUndo> {};
 struct btck_Coin : Handle<btck_Coin, Coin> {};
 struct btck_BlockHash : Handle<btck_BlockHash, uint256> {};
+struct btck_TransactionInput : Handle<btck_TransactionInput, CTxIn> {};
+struct btck_TransactionOutPoint: Handle<btck_TransactionOutPoint, COutPoint> {};
+struct btck_Txid: Handle<btck_Txid, Txid> {};
 
 btck_Transaction* btck_transaction_create(const void* raw_transaction, size_t raw_transaction_len)
 {
@@ -519,6 +522,17 @@ size_t btck_transaction_count_inputs(const btck_Transaction* transaction)
     return btck_Transaction::get(transaction)->vin.size();
 }
 
+const btck_TransactionInput* btck_transaction_get_input_at(const btck_Transaction* transaction, size_t input_index)
+{
+    assert(input_index < btck_Transaction::get(transaction)->vin.size());
+    return btck_TransactionInput::ref(&btck_Transaction::get(transaction)->vin[input_index]);
+}
+
+const btck_Txid* btck_transaction_get_txid(const btck_Transaction* transaction)
+{
+    return btck_Txid::ref(&btck_Transaction::get(transaction)->GetHash());
+}
+
 btck_Transaction* btck_transaction_copy(const btck_Transaction* transaction)
 {
     return btck_Transaction::copy(transaction);
@@ -637,6 +651,60 @@ int btck_script_pubkey_verify(const btck_ScriptPubkey* script_pubkey,
     return result ? 1 : 0;
 }
 
+btck_TransactionInput* btck_transaction_input_copy(const btck_TransactionInput* input)
+{
+    return btck_TransactionInput::copy(input);
+}
+
+const btck_TransactionOutPoint* btck_transaction_input_get_out_point(const btck_TransactionInput* input)
+{
+    return btck_TransactionOutPoint::ref(&btck_TransactionInput::get(input).prevout);
+}
+
+void btck_transaction_input_destroy(btck_TransactionInput* input)
+{
+    delete input;
+}
+
+btck_TransactionOutPoint* btck_transaction_out_point_copy(const btck_TransactionOutPoint* out_point)
+{
+    return btck_TransactionOutPoint::copy(out_point);
+}
+
+uint32_t btck_transaction_out_point_get_index(const btck_TransactionOutPoint* out_point)
+{
+    return btck_TransactionOutPoint::get(out_point).n;
+}
+
+const btck_Txid* btck_transaction_out_point_get_txid(const btck_TransactionOutPoint* out_point)
+{
+    return btck_Txid::ref(&btck_TransactionOutPoint::get(out_point).hash);
+}
+
+void btck_transaction_out_point_destroy(btck_TransactionOutPoint* out_point)
+{
+    delete out_point;
+}
+
+btck_Txid* btck_txid_copy(const btck_Txid* txid)
+{
+    return btck_Txid::copy(txid);
+}
+
+void btck_txid_to_bytes(const btck_Txid* txid, unsigned char output[32])
+{
+    std::memcpy(output, btck_Txid::get(txid).begin(), 32);
+}
+
+int btck_txid_equals(const btck_Txid* txid1, const btck_Txid* txid2)
+{
+    return btck_Txid::get(txid1) == btck_Txid::get(txid2);
+}
+
+void btck_txid_destroy(btck_Txid* txid)
+{
+    delete txid;
+}
 
 void btck_logging_set_options(const btck_LoggingOptions options)
 {
diff --git a/src/kernel/bitcoinkernel.h b/src/kernel/bitcoinkernel.h
index e9283e3d..14cd8145 100644
--- a/src/kernel/bitcoinkernel.h
+++ b/src/kernel/bitcoinkernel.h
@@ -249,6 +249,22 @@ typedef struct btck_Coin btck_Coin;
  */
 typedef struct btck_BlockHash btck_BlockHash;
 
+/**
+ * Opaque data structure for holding a transaction input.
+ *
+ * Holds information on the @ref btck_TransactionOutPoint held within.
+ */
+typedef struct btck_TransactionInput btck_TransactionInput;
+
+/**
+ * Opaque data structure for holding a transaction out point.
+ *
+ * Holds the txid and output index it is pointing to.
+ */
+typedef struct btck_TransactionOutPoint btck_TransactionOutPoint;
+
+typedef struct btck_Txid btck_Txid;
+
 /** Current sync state passed to tip changed callbacks. */
 typedef uint8_t btck_SynchronizationState;
 #define btck_SynchronizationState_INIT_REINDEX ((btck_SynchronizationState)(0))
@@ -498,6 +514,18 @@ BITCOINKERNEL_API size_t BITCOINKERNEL_WARN_UNUSED_RESULT btck_transaction_count
 BITCOINKERNEL_API const btck_TransactionOutput* BITCOINKERNEL_WARN_UNUSED_RESULT btck_transaction_get_output_at(
     const btck_Transaction* transaction, size_t output_index) BITCOINKERNEL_ARG_NONNULL(1);
 
+/**
+ * @brief Get the transaction input at the provided index. The returned
+ * transaction input is not owned and depends on the lifetime of the
+ * transaction.
+ *
+ * @param[in] transaction Non-null.
+ * @param[in] input_index The index of the transaction input to be retrieved.
+ * @return                 The transaction input
+ */
+BITCOINKERNEL_API const btck_TransactionInput* BITCOINKERNEL_WARN_UNUSED_RESULT btck_transaction_get_input_at(
+    const btck_Transaction* transaction, size_t input_index) BITCOINKERNEL_ARG_NONNULL(1);
+
 /**
  * @brief Get the number of inputs of a transaction.
  *
@@ -507,6 +535,16 @@ BITCOINKERNEL_API const btck_TransactionOutput* BITCOINKERNEL_WARN_UNUSED_RESULT
 BITCOINKERNEL_API size_t BITCOINKERNEL_WARN_UNUSED_RESULT btck_transaction_count_inputs(
     const btck_Transaction* transaction) BITCOINKERNEL_ARG_NONNULL(1);
 
+/**
+ * @brief Get the txid of a transaction. The returned txid is not owned and
+ * depends on the lifetime of the transaction.
+ *
+ * @param[in] transaction Non-null.
+ * @return                The txid.
+ */
+BITCOINKERNEL_API const btck_Txid* BITCOINKERNEL_WARN_UNUSED_RESULT btck_transaction_get_txid(
+    const btck_Transaction* transaction) BITCOINKERNEL_ARG_NONNULL(1);
+
 /**
  * Destroy the transaction.
  */
@@ -1313,6 +1351,119 @@ BITCOINKERNEL_API void btck_transaction_spent_outputs_destroy(btck_TransactionSp
 
 ///@}
 
+/** @name Transaction Input
+ * Functions for working with transaction inputs.
+ */
+///@{
+
+/**
+ * @brief Copy a transaction input.
+ *
+ * @param[in] transaction_input Non-null.
+ * @return                      The copied transaction input.
+ */
+BITCOINKERNEL_API btck_TransactionInput* BITCOINKERNEL_WARN_UNUSED_RESULT btck_transaction_input_copy(
+    const btck_TransactionInput* transaction_input) BITCOINKERNEL_ARG_NONNULL(1);
+
+/**
+ * @brief Get the transaction out point. The returned transaction out point is
+ * not owned and depends on the lifetime of the transaction.
+ *
+ * @param[in] transaction_input Non-null.
+ * @return                      The transaction out point.
+ */
+BITCOINKERNEL_API const btck_TransactionOutPoint* BITCOINKERNEL_WARN_UNUSED_RESULT btck_transaction_input_get_out_point(
+    const btck_TransactionInput* transaction_input) BITCOINKERNEL_ARG_NONNULL(1);
+
+/**
+ * Destroy the transaction input.
+ */
+BITCOINKERNEL_API void btck_transaction_input_destroy(btck_TransactionInput* transaction_input);
+
+///@}
+
+/** @name Transaction Out Point
+ * Functions for working with transaction out points.
+ */
+///@{
+
+/**
+ * @brief Copy a transaction out point.
+ *
+ * @param[in] transaction_out_point Non-null.
+ * @return                          The copied transaction out point.
+ */
+BITCOINKERNEL_API btck_TransactionOutPoint* BITCOINKERNEL_WARN_UNUSED_RESULT btck_transaction_out_point_copy(
+    const btck_TransactionOutPoint* transaction_out_point) BITCOINKERNEL_ARG_NONNULL(1);
+
+/**
+ * @brief Get the output position from the transaction out point.
+ *
+ * @param[in] transaction_out_point Non-null.
+ * @return                          The output index.
+ */
+BITCOINKERNEL_API uint32_t BITCOINKERNEL_WARN_UNUSED_RESULT btck_transaction_out_point_get_index(
+    const btck_TransactionOutPoint* transaction_out_point) BITCOINKERNEL_ARG_NONNULL(1);
+
+/**
+ * @brief Get the txid from the transaction out point. The returned txid is
+ * not owned and depends on the lifetime of the transaction out point.
+ *
+ * @param[in] transaction_out_point Non-null.
+ * @return                          The txid.
+ */
+BITCOINKERNEL_API const btck_Txid* BITCOINKERNEL_WARN_UNUSED_RESULT btck_transaction_out_point_get_txid(
+    const btck_TransactionOutPoint* transaction_out_point) BITCOINKERNEL_ARG_NONNULL(1);
+
+/**
+ * Destroy the transaction out point.
+ */
+BITCOINKERNEL_API void btck_transaction_out_point_destroy(btck_TransactionOutPoint* transaction_out_point);
+
+///@}
+
+/** @name Txid
+ * Functions for working with txids.
+ */
+///@{
+
+/**
+ * @brief Copy a txid.
+ *
+ * @param[in] txid Non-null.
+ * @return         The copied txid.
+ */
+BITCOINKERNEL_API btck_Txid* BITCOINKERNEL_WARN_UNUSED_RESULT btck_txid_copy(
+    const btck_Txid* txid) BITCOINKERNEL_ARG_NONNULL(1);
+
+/**
+ * @brief Check if two txids are equal.
+ *
+ * @param[in] txid1 Non-null.
+ * @param[in] txid2 Non-null.
+ * @return          0 if the txid is not equal.
+ */
+BITCOINKERNEL_API int BITCOINKERNEL_WARN_UNUSED_RESULT btck_txid_equals(
+    const btck_Txid* txid1, const btck_Txid* txid2) BITCOINKERNEL_ARG_NONNULL(1, 2);
+
+/**
+ * @brief Serializes the txid to bytes.
+ *
+ * @param[in] txid    Non-null.
+ * @param[out] output The serialized txid.
+ */
+BITCOINKERNEL_API void btck_txid_to_bytes(
+    const btck_Txid* txid, unsigned char output[32]) BITCOINKERNEL_ARG_NONNULL(1, 2);
+
+/**
+ * Destroy the txid.
+ */
+BITCOINKERNEL_API void btck_txid_destroy(btck_Txid* txid);
+
+///@}
+
+///@}
+
 /** @name Coin
  * Functions for working with coins.
  */
@@ -1328,7 +1479,8 @@ BITCOINKERNEL_API btck_Coin* BITCOINKERNEL_WARN_UNUSED_RESULT btck_coin_copy(
     const btck_Coin* coin) BITCOINKERNEL_ARG_NONNULL(1);
 
 /**
- * @brief Returns the height of the block that contains the coin's prevout.
+ * @brief Returns the block height where the transaction that
+ * created this coin was included in.
  *
  * @param[in] coin Non-null.
  * @return         The block height of the coin.
diff --git a/src/kernel/bitcoinkernel_wrapper.h b/src/kernel/bitcoinkernel_wrapper.h
index d06878fa..06a4ccfa 100644
--- a/src/kernel/bitcoinkernel_wrapper.h
+++ b/src/kernel/bitcoinkernel_wrapper.h
@@ -451,6 +451,119 @@ public:
         : Handle(view) {}
 };
 
+template <typename Derived>
+class TxidApi
+{
+private:
+    auto impl() const
+    {
+        return static_cast<const Derived*>(this)->get();
+    }
+
+    friend Derived;
+    TxidApi() = default;
+
+public:
+    bool operator==(const TxidApi& other) const
+    {
+        return btck_txid_equals(impl(), other.impl()) != 0;
+    }
+
+    bool operator!=(const TxidApi& other) const
+    {
+        return btck_txid_equals(impl(), other.impl()) == 0;
+    }
+
+    std::array<std::byte, 32> ToBytes() const
+    {
+        std::array<std::byte, 32> hash;
+        btck_txid_to_bytes(impl(), reinterpret_cast<unsigned char*>(hash.data()));
+        return hash;
+    }
+};
+
+class TxidView : public View<btck_Txid>, public TxidApi<TxidView>
+{
+public:
+    explicit TxidView(const btck_Txid* ptr) : View{ptr} {}
+};
+
+class Txid : public Handle<btck_Txid, btck_txid_copy, btck_txid_destroy>, public TxidApi<Txid>
+{
+public:
+    Txid(const TxidView& view)
+        : Handle(view) {}
+};
+
+template <typename Derived>
+class OutPointApi
+{
+private:
+    auto impl() const
+    {
+        return static_cast<const Derived*>(this)->get();
+    }
+
+    friend Derived;
+    OutPointApi() = default;
+
+public:
+    uint32_t index() const
+    {
+        return btck_transaction_out_point_get_index(impl());
+    }
+
+    TxidView Txid() const
+    {
+        return TxidView{btck_transaction_out_point_get_txid(impl())};
+    }
+};
+
+class OutPointView : public View<btck_TransactionOutPoint>, public OutPointApi<OutPointView>
+{
+public:
+    explicit OutPointView(const btck_TransactionOutPoint* ptr) : View{ptr} {}
+};
+
+class OutPoint : public Handle<btck_TransactionOutPoint, btck_transaction_out_point_copy, btck_transaction_out_point_destroy>, public OutPointApi<OutPoint>
+{
+public:
+    OutPoint(const OutPointView& view)
+        : Handle(view) {}
+};
+
+template <typename Derived>
+class TransactionInputApi
+{
+private:
+    auto impl() const
+    {
+        return static_cast<const Derived*>(this)->get();
+    }
+
+    friend Derived;
+    TransactionInputApi() = default;
+
+public:
+    OutPointView OutPoint() const
+    {
+        return OutPointView{btck_transaction_input_get_out_point(impl())};
+    }
+};
+
+class TransactionInputView : public View<btck_TransactionInput>, public TransactionInputApi<TransactionInputView>
+{
+public:
+    explicit TransactionInputView(const btck_TransactionInput* ptr) : View{ptr} {}
+};
+
+class TransactionInput : public Handle<btck_TransactionInput, btck_transaction_input_copy, btck_transaction_input_destroy>, public TransactionInputApi<TransactionInput>
+{
+public:
+    TransactionInput(const TransactionInputView& view)
+        : Handle(view) {}
+};
+
 template <typename Derived>
 class TransactionApi
 {
@@ -476,8 +589,20 @@ public:
         return TransactionOutputView{btck_transaction_get_output_at(impl(), index)};
     }
 
+    TransactionInputView GetInput(size_t index) const
+    {
+        return TransactionInputView{btck_transaction_get_input_at(impl(), index)};
+    }
+
+    TxidView Txid() const
+    {
+        return TxidView{btck_transaction_get_txid(impl())};
+    }
+
     MAKE_RANGE_METHOD(Outputs, Derived, &TransactionApi<Derived>::CountOutputs, &TransactionApi<Derived>::GetOutput, *static_cast<const Derived*>(this))
 
+    MAKE_RANGE_METHOD(Inputs, Derived, &TransactionApi<Derived>::CountInputs, &TransactionApi<Derived>::GetInput, *static_cast<const Derived*>(this))
+
     std::vector<std::byte> ToBytes() const
     {
         return write_bytes(impl(), btck_transaction_to_bytes);
diff --git a/src/test/kernel/test_kernel.cpp b/src/test/kernel/test_kernel.cpp
index fca76f4f..d9875ee1 100644
--- a/src/test/kernel/test_kernel.cpp
+++ b/src/test/kernel/test_kernel.cpp
@@ -14,6 +14,7 @@
 #include <cstdint>
 #include <cstdlib>
 #include <filesystem>
+#include <format>
 #include <iostream>
 #include <memory>
 #include <optional>
@@ -61,6 +62,19 @@ std::vector<std::byte> hex_string_to_byte_vec(std::string_view hex)
     return bytes;
 }
 
+std::string byte_span_to_hex_string_reversed(std::span<const std::byte> bytes)
+{
+    std::ostringstream oss;
+
+    // Iterate in reverse order
+    for (auto it = bytes.rbegin(); it != bytes.rend(); ++it) {
+        oss << std::hex << std::setw(2) << std::setfill('0')
+            << static_cast<unsigned int>(static_cast<uint8_t>(*it));
+    }
+
+    return oss.str();
+}
+
 constexpr auto VERIFY_ALL_PRE_SEGWIT{ScriptVerificationFlags::P2SH | ScriptVerificationFlags::DERSIG |
                                      ScriptVerificationFlags::NULLDUMMY | ScriptVerificationFlags::CHECKLOCKTIMEVERIFY |
                                      ScriptVerificationFlags::CHECKSEQUENCEVERIFY};
@@ -465,6 +479,18 @@ BOOST_AUTO_TEST_CASE(btck_transaction_output)
     CheckHandle(output, output2);
 }
 
+BOOST_AUTO_TEST_CASE(btck_transaction_input)
+{
+    Transaction tx{hex_string_to_byte_vec("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")};
+    TransactionInput input_0 = tx.GetInput(0);
+    TransactionInput input_1 = tx.GetInput(1);
+    CheckHandle(input_0, input_1);
+    CheckRange(tx.Inputs(), tx.CountInputs());
+    OutPoint point_0 = input_0.OutPoint();
+    OutPoint point_1 = input_1.OutPoint();
+    CheckHandle(point_0, point_1);
+}
+
 BOOST_AUTO_TEST_CASE(btck_script_verify_tests)
 {
     // Legacy transaction aca326a724eda9a461c10a876534ecd5ae7b27f10f26c3862fb996f80ea2d45d
@@ -702,6 +728,7 @@ void chainman_mainnet_validation_test(TestDirectory& test_directory)
     auto raw_block = hex_string_to_byte_vec("010000006fe28c0ab6f1b372c1a6a246ae63f74f931e8365e15a089c68d6190000000000982051fd1e4ba744bbbe680e1fee14677ba1a3c3540bf7b1cdb606e857233e0e61bc6649ffff001d01e362990101000000010000000000000000000000000000000000000000000000000000000000000000ffffffff0704ffff001d0104ffffffff0100f2052a0100000043410496b538e853519c726a2c91e61ec11600ae1390813a627c66fb8be7947be63c52da7589379515d4e0a604f8141781e62294721166bf621e73a82cbf2342c858eeac00000000");
     Block block{raw_block};
     TransactionView tx{block.GetTransaction(block.CountTransactions() - 1)};
+    BOOST_CHECK_EQUAL(byte_span_to_hex_string_reversed(tx.Txid().ToBytes()), "0e3e2357e806b6cdb1f70b54c3a3a17b6714ee1f0e68bebb44a74b1efd512098");
     BOOST_CHECK_EQUAL(tx.CountInputs(), 1);
     Transaction tx2 = tx;
     BOOST_CHECK_EQUAL(tx2.CountInputs(), 1);
@@ -832,6 +859,50 @@ BOOST_AUTO_TEST_CASE(btck_chainman_regtest_tests)
     auto read_block_2 = chainman->ReadBlock(tip_2).value();
     check_equal(read_block_2.ToBytes(), hex_string_to_byte_vec(REGTEST_BLOCK_DATA[REGTEST_BLOCK_DATA.size() - 2]));
 
+    Txid txid = read_block.Transactions()[0].Txid();
+    Txid txid_2 = read_block_2.Transactions()[0].Txid();
+    BOOST_CHECK(txid != txid_2);
+    BOOST_CHECK(txid == txid);
+    CheckHandle(txid, txid_2);
+
+    auto find_transaction = [&chainman](const TxidView& target_txid) -> std::optional<Transaction> {
+        auto chain = chainman->GetChain();
+        for (const auto block_tree_entry : chain.Entries()) {
+            auto block{chainman->ReadBlock(block_tree_entry)};
+            for (const TransactionView transaction : block->Transactions()) {
+                if (transaction.Txid() == target_txid) {
+                    return Transaction{transaction};
+                }
+            }
+        }
+        return std::nullopt;
+    };
+
+    for (const auto block_tree_entry : chain.Entries()) {
+        auto block{chainman->ReadBlock(block_tree_entry)};
+        for (const auto transaction : block->Transactions()) {
+            std::vector<TransactionInput> inputs;
+            std::vector<TransactionOutput> spent_outputs;
+            for (const auto input : transaction.Inputs()) {
+                OutPointView point = input.OutPoint();
+                if (point.index() == std::numeric_limits<uint32_t>::max()) {
+                    continue;
+                }
+                inputs.emplace_back(input);
+                BOOST_CHECK(point.Txid() != transaction.Txid());
+                std::optional<Transaction> tx = find_transaction(point.Txid());
+                BOOST_CHECK(tx.has_value());
+                BOOST_CHECK(point.Txid() == tx->Txid());
+                spent_outputs.emplace_back(tx->GetOutput(point.index()));
+            }
+            BOOST_CHECK(inputs.size() == spent_outputs.size());
+            ScriptVerifyStatus status = ScriptVerifyStatus::OK;
+            for (size_t i{0}; i < inputs.size(); ++i) {
+                BOOST_CHECK(spent_outputs[i].GetScriptPubkey().Verify(spent_outputs[i].Amount(), transaction, spent_outputs, i, ScriptVerificationFlags::ALL, status));
+            }
+        }
+    }
+
     // Read spent outputs for current tip and its previous block
     BlockSpentOutputs block_spent_outputs{chainman->ReadBlockSpentOutputs(tip)};
     BlockSpentOutputs block_spent_outputs_prev{chainman->ReadBlockSpentOutputs(*tip.GetPrevious())};