pub const SingleThreadedRwLock = struct {
    state: enum { unlocked, locked_exclusive, locked_shared } = .unlocked,
    shared_count: usize = 0,

    /// Attempts to obtain exclusive lock ownership.
    /// Returns `true` if the lock is obtained, `false` otherwise.
    pub fn tryLock(rwl: *SingleThreadedRwLock) bool {
        switch (rwl.state) {
            .unlocked => {
                assert(rwl.shared_count == 0);
                rwl.state = .locked_exclusive;
                return true;
            },
            .locked_exclusive, .locked_shared => return false,
        }
    }

    /// Blocks until exclusive lock ownership is acquired.
    pub fn lock(rwl: *SingleThreadedRwLock) void {
        assert(rwl.state == .unlocked); // deadlock detected
        assert(rwl.shared_count == 0); // corrupted state detected
        rwl.state = .locked_exclusive;
    }

    /// Releases a held exclusive lock.
    /// Asserts the lock is held exclusively.
    pub fn unlock(rwl: *SingleThreadedRwLock) void {
        assert(rwl.state == .locked_exclusive);
        assert(rwl.shared_count == 0); // corrupted state detected
        rwl.state = .unlocked;
    }

    /// Attempts to obtain shared lock ownership.
    /// Returns `true` if the lock is obtained, `false` otherwise.
    pub fn tryLockShared(rwl: *SingleThreadedRwLock) bool {
        switch (rwl.state) {
            .unlocked => {
                rwl.state = .locked_shared;
                assert(rwl.shared_count == 0);
                rwl.shared_count = 1;
                return true;
            },
            .locked_shared => {
                rwl.shared_count += 1;
                return true;
            },
            .locked_exclusive => return false,
        }
    }

    /// Blocks until shared lock ownership is acquired.
    pub fn lockShared(rwl: *SingleThreadedRwLock) void {
        switch (rwl.state) {
            .unlocked => {
                rwl.state = .locked_shared;
                assert(rwl.shared_count == 0);
                rwl.shared_count = 1;
            },
            .locked_shared => {
                rwl.shared_count += 1;
            },
            .locked_exclusive => unreachable, // deadlock detected
        }
    }

    /// Releases a held shared lock.
    pub fn unlockShared(rwl: *SingleThreadedRwLock) void {
        switch (rwl.state) {
            .unlocked => unreachable, // too many calls to `unlockShared`
            .locked_exclusive => unreachable, // exclusively held lock
            .locked_shared => {
                rwl.shared_count -= 1;
                if (rwl.shared_count == 0) {
                    rwl.state = .unlocked;
                }
            },
        }
    }
}