/* libcr_ipc/rwmutex.c - Simple read/write mutexes for libcr
 *
 * Copyright (C) 2024-2025  Luke T. Shumaker <lukeshu@lukeshu.com>
 * SPDX-License-Identifier: AGPL-3.0-or-later
 */

#include <libcr/coroutine.h> /* for cid_t, cr_* */
#include <libmisc/assert.h>

#define IMPLEMENTATION_FOR_LIBCR_IPC_RWMUTEX_H YES
#include <libcr_ipc/rwmutex.h>

struct cr_rwmutex_waiter {
	bool                     is_reader;
	cid_t                    cid;
};
SLIST_DECLARE_NODE(_cr_rwmutex_waiter_list, struct cr_rwmutex_waiter);

void cr_rwmutex_lock(cr_rwmutex_t *mu) {
	assert(mu);
	cr_assert_in_coroutine();

	struct _cr_rwmutex_waiter_list_node self = { .val = {
		.is_reader = false,
		.cid       = cr_getcid(),
	}};
	slist_push_to_rear(&mu->waiters, &self);
	if (mu->waiters.front != &self || mu->locked)
		cr_pause_and_yield();
	assert(mu->waiters.front == &self);

	/* We now hold the lock (and are mu->waiters.front).  */
	slist_pop_from_front(&mu->waiters);
	assert(mu->nreaders == 0);
	mu->locked = true;
	mu->unpausing = false;
}

void cr_rwmutex_rlock(cr_rwmutex_t *mu) {
	assert(mu);
	cr_assert_in_coroutine();

	struct _cr_rwmutex_waiter_list_node self = { .val = {
		.is_reader = true,
		.cid       = cr_getcid(),
	}};
	slist_push_to_rear(&mu->waiters, &self);
	if (mu->waiters.front != &self || (mu->locked && mu->nreaders == 0))
		cr_pause_and_yield();
	assert(mu->waiters.front == &self);

	/* We now hold the lock (and are mu->waiters.front).  */
	slist_pop_from_front(&mu->waiters);
	mu->nreaders++;
	mu->locked = true;
	struct _cr_rwmutex_waiter_list_node *waiter = mu->waiters.front;
	if (waiter && waiter->val.is_reader) {
		assert(mu->unpausing);
		cr_unpause(waiter->val.cid);
	} else {
		mu->unpausing = false;
	}
}

void cr_rwmutex_unlock(cr_rwmutex_t *mu) {
	assert(mu);
	cr_assert_in_coroutine();

	assert(mu->locked);
	assert(mu->nreaders == 0);
	assert(!mu->unpausing);
	if (mu->waiters.front) {
		struct _cr_rwmutex_waiter_list_node *waiter = mu->waiters.front;
		mu->unpausing = true;
		cr_unpause(waiter->val.cid);
	} else {
		mu->locked = false;
	}
}

void cr_rwmutex_runlock(cr_rwmutex_t *mu) {
	assert(mu);
	cr_assert_in_coroutine();

	assert(mu->locked);
	assert(mu->nreaders > 0);
	mu->nreaders--;
	if (mu->nreaders == 0 && !mu->unpausing) {
		if (mu->waiters.front) {
			struct _cr_rwmutex_waiter_list_node *waiter = mu->waiters.front;
			assert(!waiter->val.is_reader);
			mu->unpausing = true;
			cr_unpause(waiter->val.cid);
		} else {
			mu->locked = false;
		}
	}
}