// Package osthread allows functions to be called on the main operating system thread.
//
// Usage:
//
//	func main() {
//		osthread.Start(Entrypoint) // Initialize osthread and calls Entrypoint. Blocks until Entrypoint returns.
//	}
//
//	func Entrypoint() {
//		osthread.Call(FuncA) // Call FuncA on the main operating system thread, block until it returns
//		// ...
//		osthread.Go(FuncB) // Schedule FuncB to be called on the main operating system thread
//		// ...
//	}
package osthread

import "runtime"

// Start allows arbitrary functions to be run on the main operating system thread.
//
// Start must be called from the program's main function. Once called, it blocks until entrypoint returns.
func Start(entrypoint func()) {
	done := make(chan any)

	// Run entrypoint in a separate goroutine.
	go func() {
		defer func() {
			done <- nil
		}()

		entrypoint()
	}()

	// Call functions in our queue until entrypoint returns.
	// These functions are called on the main operating system thread.
	for {
		select {
		case fn := <-queue:
			fn()
		case <-done:
			return
		}
	}
}

// Go schedules a function to be run on the main operating system thread, returning immediately.
func Go(fn func()) {
	queue <- fn
}

// Call schedules a function to be run on the main operating system thread, blocking until it returns.
func Call(fn func()) {
	done := make(chan any)
	queue <- func() {
		defer func() {
			done <- nil
		}()

		fn()
	}

	<-done
}

var queue chan func()

func init() {
	runtime.LockOSThread()
	queue = make(chan func(), runtime.GOMAXPROCS(0))
}