diff --git a/M6502/HarteTest_6502/TestRunner.cpp b/M6502/HarteTest_6502/TestRunner.cpp
index a120842..5c6c155 100644
--- a/M6502/HarteTest_6502/TestRunner.cpp
+++ b/M6502/HarteTest_6502/TestRunner.cpp
@@ -7,7 +7,7 @@ bool TestRunner::m_undocumented_opcodes_initialised = false;
 TestRunner::TestRunner(const test_t test)
 : m_test(test) {}
 
-EightBit::MemoryMapping TestRunner::mapping(uint16_t address) noexcept {
+EightBit::MemoryMapping TestRunner::mapping(const uint16_t address) noexcept {
     return { RAM(), 0x0000, 0xffff, EightBit::MemoryMapping::AccessLevel::ReadWrite };
 }
 
@@ -26,23 +26,23 @@ void TestRunner::lowerPOWER() {
     EightBit::Bus::lowerPOWER();
 }
 
-void TestRunner::addActualCycle(uint16_t address, uint8_t value, std::string action) {
+void TestRunner::addActualCycle(const uint16_t address, const uint8_t value, const std::string action) {
     m_actualCycles.push_back({ address, value, action });
 }
 
-void TestRunner::addActualCycle(EightBit::register16_t address, uint8_t value, std::string action) {
+void TestRunner::addActualCycle(const EightBit::register16_t address, const uint8_t value, const std::string action) {
     addActualCycle(address.word, value, action);
 }
 
-void TestRunner::addActualReadCycle(EightBit::register16_t address, uint8_t value) {
+void TestRunner::addActualReadCycle(const EightBit::register16_t address, const uint8_t value) {
     addActualCycle(address, value, "read");
 }
 
-void TestRunner::addActualWriteCycle(EightBit::register16_t address, uint8_t value) {
+void TestRunner::addActualWriteCycle(const EightBit::register16_t address, const uint8_t value) {
     addActualCycle(address, value, "write");
 }
 
-void TestRunner::dumpCycle(uint16_t address, uint8_t value, std::string action) {
+void TestRunner::dumpCycle(const uint16_t address, const uint8_t value, const std::string action) {
     os()
         << std::setfill('0') << std::hex
         << "Address: " << std::setw(4) << (int)address
@@ -51,7 +51,7 @@ void TestRunner::dumpCycle(uint16_t address, uint8_t value, std::string action)
     pushCurrentMessage();
 }
 
-void TestRunner::dumpCycles(std::string which, const actual_cycles_t& events) {
+void TestRunner::dumpCycles(const std::string which, const actual_cycles_t& events) {
     m_messages.push_back(which);
     dumpCycles(events);
 }
@@ -65,17 +65,17 @@ void TestRunner::dumpCycle(const actual_cycle_t& cycle) {
     dumpCycle(std::get<0>(cycle), std::get<1>(cycle), std::get<2>(cycle));
 }
 
-void TestRunner::dumpCycles(std::string which, cycles_t events) {
+void TestRunner::dumpCycles(const std::string which, const cycles_t events) {
     m_messages.push_back(which);
     dumpCycles(events);
 }
 
-void TestRunner::dumpCycles(cycles_t cycles) {
-    for (auto cycle: cycles)
+void TestRunner::dumpCycles(const cycles_t cycles) {
+    for (const auto cycle: cycles)
         dumpCycle(cycle_t(cycle));
 }
 
-void TestRunner::dumpCycle(cycle_t cycle) {
+void TestRunner::dumpCycle(const cycle_t cycle) {
     dumpCycle(cycle.address(), cycle.value(), cycle.action());
 }
 
@@ -94,7 +94,7 @@ void TestRunner::initialise() {
     os() << std::hex << std::uppercase;
 }
 
-void TestRunner::raise(std::string what, uint16_t expected, uint16_t actual) {
+void TestRunner::raise(const std::string what, const uint16_t expected, const uint16_t actual) {
     os()
         << std::setw(2) << std::setfill(' ')
         << what
@@ -104,7 +104,7 @@ void TestRunner::raise(std::string what, uint16_t expected, uint16_t actual) {
     pushCurrentMessage();
 }
 
-void TestRunner::raise(std::string what, uint8_t expected, uint8_t actual) {
+void TestRunner::raise(const std::string what, const uint8_t expected, const uint8_t actual) {
     os()
         << std::setw(2) << std::setfill(' ')
         << what
@@ -116,7 +116,7 @@ void TestRunner::raise(std::string what, uint8_t expected, uint8_t actual) {
     pushCurrentMessage();
 }
 
-void TestRunner::raise(std::string what, std::string expected, std::string actual) {
+void TestRunner::raise(const std::string what, const std::string expected, const std::string actual) {
     os()
         << std::setw(0) << std::setfill(' ')
         << what
@@ -125,7 +125,7 @@ void TestRunner::raise(std::string what, std::string expected, std::string actua
     pushCurrentMessage();
 }
 
-bool TestRunner::check(std::string what, uint16_t address, uint8_t expected, uint8_t actual) {
+bool TestRunner::check(const std::string what, const uint16_t address, const uint8_t expected, const uint8_t actual) {
     const auto success = actual == expected;
     if (!success) {
         os() << what << ": " << std::setw(4) << std::setfill('0') << (int)address;
@@ -144,7 +144,7 @@ void TestRunner::initialiseState() {
     CPU().X() = initial.x();
     CPU().Y() = initial.y();
     CPU().P() = initial.p();
-    for (auto entry : initial.ram()) {
+    for (const auto entry : initial.ram()) {
         const byte_t byte(entry);
         RAM().poke(byte.address(), byte.value());
     }
@@ -152,14 +152,14 @@ void TestRunner::initialiseState() {
 
 bool TestRunner::checkState() {
 
-    const auto& expected_cycles = test().cycles();
-    const auto& actual_cycles = m_actualCycles;
+    const auto expected_cycles = test().cycles();
+    const auto actual_cycles = m_actualCycles;
     m_cycle_count_mismatch = expected_cycles.size() != actual_cycles.size();
     if (m_cycle_count_mismatch)
         return false;
 
     size_t actual_idx = 0;
-    for (auto expected_cycle : expected_cycles) {
+    for (const auto expected_cycle : expected_cycles) {
         const auto expected = cycle_t(expected_cycle);
         const auto actual = actual_cycles.at(actual_idx++);   // actual could be less than expected
         check("Cycle address", expected.address(), std::get<0>(actual));
@@ -194,8 +194,7 @@ void TestRunner::pushCurrentMessage() {
 void TestRunner::disassemble(uint16_t address) {
     try {
         os() << m_disassembler.disassemble(address);
-    }
-    catch (const std::domain_error& error) {
+    } catch (const std::domain_error& error) {
         os() << "Disassembly problem: " << error.what();
     }
     pushCurrentMessage();
diff --git a/M6502/HarteTest_6502/array_t.cpp b/M6502/HarteTest_6502/array_t.cpp
index cebc1f9..2a5f72c 100644
--- a/M6502/HarteTest_6502/array_t.cpp
+++ b/M6502/HarteTest_6502/array_t.cpp
@@ -1,5 +1,5 @@
 #include "stdafx.h"
 #include "array_t.h"
 
-array_t::array_t(simdjson::dom::array input) noexcept
+array_t::array_t(const simdjson::dom::array input) noexcept
 : m_raw(input) {}
diff --git a/M6502/HarteTest_6502/array_t.h b/M6502/HarteTest_6502/array_t.h
index 8d8b69e..959f2fe 100644
--- a/M6502/HarteTest_6502/array_t.h
+++ b/M6502/HarteTest_6502/array_t.h
@@ -11,7 +11,7 @@ private:
 protected:
 	array_t(simdjson::dom::array input) noexcept;
 
-	auto raw() const noexcept { return m_raw; }
+	[[nodiscard]] auto raw() const noexcept { return m_raw; }
 
 	[[nodiscard]] auto at(size_t idx) const noexcept { return raw().at(idx); }
 	[[nodiscard]] auto integer_at(size_t idx) const noexcept { return (int64_t)at(idx); }
@@ -22,5 +22,5 @@ public:
 	[[nodiscard]] auto begin() const noexcept { return m_raw.begin(); }
 	[[nodiscard]] auto end() const noexcept { return m_raw.end(); }
 	[[nodiscard]] auto size() const noexcept { return m_raw.size(); }
-	[[nodiscard]] auto operator[](size_t idx) const noexcept { return m_raw.at(idx); }
+	[[nodiscard]] auto operator[](size_t idx) const noexcept { return at(idx); }
 };
diff --git a/M6502/HarteTest_6502/byte_t.cpp b/M6502/HarteTest_6502/byte_t.cpp
index 525bda0..49f1263 100644
--- a/M6502/HarteTest_6502/byte_t.cpp
+++ b/M6502/HarteTest_6502/byte_t.cpp
@@ -1,5 +1,5 @@
 #include "stdafx.h"
 #include "byte_t.h"
 
-byte_t::byte_t(simdjson::dom::array input) noexcept
+byte_t::byte_t(const simdjson::dom::array input) noexcept
 : array_t(input) {}
diff --git a/M6502/HarteTest_6502/cycle_t.cpp b/M6502/HarteTest_6502/cycle_t.cpp
index f819550..5bdf91b 100644
--- a/M6502/HarteTest_6502/cycle_t.cpp
+++ b/M6502/HarteTest_6502/cycle_t.cpp
@@ -1,5 +1,5 @@
 #include "stdafx.h"
 #include "cycle_t.h"
 
-cycle_t::cycle_t(simdjson::dom::array input) noexcept
+cycle_t::cycle_t(const simdjson::dom::array input) noexcept
 : byte_t(input) {}
diff --git a/M6502/HarteTest_6502/cycles_t.cpp b/M6502/HarteTest_6502/cycles_t.cpp
index 85f309d..442155c 100644
--- a/M6502/HarteTest_6502/cycles_t.cpp
+++ b/M6502/HarteTest_6502/cycles_t.cpp
@@ -1,5 +1,5 @@
 #include "stdafx.h"
 #include "cycles_t.h"
 
-cycles_t::cycles_t(simdjson::dom::array input) noexcept
+cycles_t::cycles_t(const simdjson::dom::array input) noexcept
 : array_t(input) {}
diff --git a/M6502/HarteTest_6502/element_t.h b/M6502/HarteTest_6502/element_t.h
index 8c2a279..7d9abd1 100644
--- a/M6502/HarteTest_6502/element_t.h
+++ b/M6502/HarteTest_6502/element_t.h
@@ -15,6 +15,7 @@ protected:
     auto raw() const noexcept { return m_raw; }
 
     [[nodiscard]] auto at(std::string key) const noexcept { return raw()[key]; }
+    [[nodiscard]] auto operator[](std::string key) const noexcept { return at(key); }
     [[nodiscard]] auto array_at(std::string key) const noexcept { return at(key).get_array(); }
     [[nodiscard]] auto integer_at(std::string key) const noexcept { return (int64_t)at(key); }
 };
diff --git a/M6502/HarteTest_6502/opcode_test_suite_t.cpp b/M6502/HarteTest_6502/opcode_test_suite_t.cpp
index 01f8111..7ddf676 100644
--- a/M6502/HarteTest_6502/opcode_test_suite_t.cpp
+++ b/M6502/HarteTest_6502/opcode_test_suite_t.cpp
@@ -8,7 +8,7 @@
 
 simdjson::dom::parser opcode_test_suite_t::m_parser;
 
-opcode_test_suite_t::opcode_test_suite_t(std::string path)
+opcode_test_suite_t::opcode_test_suite_t(const std::string path)
 : m_path(path) {}
 
 void opcode_test_suite_t::load() {
diff --git a/M6502/HarteTest_6502/ram_t.cpp b/M6502/HarteTest_6502/ram_t.cpp
index b2d214c..09e9bab 100644
--- a/M6502/HarteTest_6502/ram_t.cpp
+++ b/M6502/HarteTest_6502/ram_t.cpp
@@ -1,5 +1,5 @@
 #include "stdafx.h"
 #include "ram_t.h"
 
-ram_t::ram_t(simdjson::dom::array input) noexcept
+ram_t::ram_t(const simdjson::dom::array input) noexcept
 : array_t(input) {}
diff --git a/M6502/HarteTest_6502/tests.cpp b/M6502/HarteTest_6502/tests.cpp
index 20475c2..a6bdc08 100644
--- a/M6502/HarteTest_6502/tests.cpp
+++ b/M6502/HarteTest_6502/tests.cpp
@@ -26,14 +26,13 @@ int main() {
         opcode_test_suite_t opcode(path.string());
         opcode.load();
 
-        for (auto opcode_test_element : opcode) {
+        for (const auto opcode_test_element : opcode) {
 
             const auto opcode_test = test_t(opcode_test_element);
             TestRunner runner(opcode_test);
             runner.check();
 
-            auto invalid = runner.invalid();
-            if (invalid) {
+            if (runner.invalid()) {
                 ++invalid_opcode_count;
                 if (runner.unimplemented())
                     ++unimplemented_opcode_count;