// Test-fixture application for the pureboot protocol tests: prints "APP" on
// the chip's serial link (the same link the loader uses) — the proof that
// the loader's hand-over, and on the tinies the host's reset-vector
// surgery, actually launched it. Linked normally (crt, vectors at 0); on
// the tinies its reset vector is the rjmp the host re-homes.
//
// On the hardware-USART link it then listens, and an 'L' makes it jump into
// the resident loader — the application-owned loader entry a
// BOOTRST-unprogrammed mega relies on (reset always boots the application
// there), exercised by the self-update tests. The software link idles:
// reset reaches those loaders through the patched vector (or the runner
// models BOOTRST), so the application owes them nothing.
//
// PUREBOOT_HANDOVER drops the listening and jumps straight in, leaving the
// USART enabled behind it — the hand-over state a loader bit-banging on that
// USART's own pins has to survive.
//
// The fixture speaks the deployment its loader was built for: the same
// PUREBOOT_* defines configure it, and without them it assumes the stock
// deployment (the crystal/RC clock table below, the chip's natural link).
#include <libavr/libavr.hpp>

using namespace avr::literals;

namespace {

consteval avr::hertz_t clock()
{
#if defined(PUREBOOT_CLOCK_HZ)
	return avr::hertz_t{PUREBOOT_CLOCK_HZ};
#else
	auto name = std::string_view{avr::hw::db.name};
	if (name.starts_with("ATtiny13"))
		return 9.6_MHz;
	if (name.starts_with("ATtiny"))
		return 8_MHz;
	return 16_MHz;
#endif
}

#if !defined(PUREBOOT_TX)
#define PUREBOOT_TX pb1
#endif
#if !defined(PUREBOOT_USART)
#define PUREBOOT_USART 0
#endif

consteval bool use_hardware()
{
#if defined(PUREBOOT_SOFT_SERIAL)
	return false;
#else
	return avr::hw::db.has_instance("USART0") || avr::hw::db.has_instance("USART");
#endif
}

using dev = avr::device<{.clock = clock()}>;

template <avr::hertz_t C, bool Hardware = use_hardware()>
struct link {
#if defined(PUREBOOT_BAUD)
	static constexpr avr::baud_t baud{PUREBOOT_BAUD};
#else
	static constexpr avr::baud_t baud{115200};
#endif
	using tx_t = avr::uart::usart<'0' + PUREBOOT_USART, C, {.baud = baud, .max_baud_error = 2.5_pct}>;
	static void tx(char c)
	{
		tx_t::write(static_cast<std::uint8_t>(c));
	}
	// The loader sits in the top slot — 512 bytes on every chip. The jump
	// takes a word address, which is what makes the >64 KiB chips' entry
	// reachable through a 16-bit pointer at all.
	static void enter_loader()
	{
		constexpr std::uint32_t slot = 512;
		reinterpret_cast<void (*)()>(static_cast<std::uint16_t>((avr::hw::db.mem.flash_size - slot) / 2))();
	}

	[[noreturn]] static void idle()
	{
#if defined(PUREBOOT_HANDOVER)
		// Hand back at once, with this USART still enabled — the state that
		// leaves a bit-banged loader on its pins mute unless the loader
		// releases it. Unconditional because there is no command wire to
		// wait on: that loader's link is the pins, not this peripheral.
		enter_loader();
		__builtin_unreachable();
#else
		for (;;) {
			auto command = tx_t::read_blocking();
			if (command == 'L')
				enter_loader();
			// 'D' leaves every word of the SPM page buffer dirty, so that a
			// following 'L' enters the loader with the buffer it never clears.
			if (command == 'D') {
				for (std::uint16_t at = 0; at < avr::spm::page_bytes; at += 2)
					avr::spm::fill(at, 0xdead);
				tx('D');
			}
		}
#endif
	}
};

template <avr::hertz_t C>
struct link<C, false> {
#if defined(PUREBOOT_BAUD)
	static constexpr avr::baud_t baud{PUREBOOT_BAUD};
#else
	static constexpr avr::baud_t baud{57600};
#endif
	using tx_t = avr::uart::software_tx<C, avr::PUREBOOT_TX, baud>;
	static void tx(char c)
	{
		tx_t::write(static_cast<std::uint8_t>(c));
	}
	[[noreturn]] static void idle()
	{
		while (true) {
		}
	}
};

} // namespace

int main()
{
	avr::init<typename link<dev::clock>::tx_t>();
#if !defined(PUREBOOT_HANDOVER)
	link<dev::clock>::tx('A');
	link<dev::clock>::tx('P');
	link<dev::clock>::tx('P');
#endif
	// The hand-over fixture stays silent: nothing is listening on the USART it
	// brings up — the loader it hands to speaks those pins directly — so its
	// banner would be a write into a peer that does not exist.
	link<dev::clock>::idle();
}