package usync

import "sync"

// Monitor guards access to a value. It must not be copied after first use.
type Monitor[T any] struct {
	value T
	mutex sync.Mutex
}

// NewMonitor creates a new Monitor with the specified value.
func NewMonitor[T any] (value T) Monitor[T] {
	return Monitor[T] {
		value: value,
	}
}

// Set sets the value of the Monitor.
func (this *Monitor[T]) Set (value T) {
	this.mutex.Lock()
	defer this.mutex.Unlock()
	this.value = value
}

// Borrow borrows the value from the Monitor, and returns a function that must
// immediately be deferred, like this:
//
//   value, done := monitor.Borrow()
//   defer done()
//
// From the time Borrow is called to the time the done function is called, it is
// safe to access the locked object from within the current goroutine.
func (this *Monitor[T]) Borrow () (T, func ()) {
	this.mutex.Lock()
	return this.value, this.mutex.Unlock
}

// BorrowReturn is like borrow, but returns a "done" function that takes in an
// updated value. The intended use of this function is like this:
//
//   value, done := monitor.BorrowReturn()
//   defer done(value)
func (this *Monitor[T]) BorrowReturn () (T, func (T)) {
	this.mutex.Lock()
	return this.value, func (value T) {
		defer this.mutex.Unlock()
		this.value = value
	}
}

// RWMonitor guards separate read/write access to a value.
type RWMonitor[T any] struct {
	value T
	mutex sync.RWMutex
}

// NewRWMonitor creates a new Monitor with the specified value. It must not be
// copied after first use.
func NewRWMonitor[T any] (value T) RWMonitor[T] {
	return RWMonitor[T] {
		value: value,
	}
}

// Set sets the value of the Monitor.
func (this *RWMonitor[T]) Set (value T) {
	this.mutex.Lock()
	defer this.mutex.Unlock()
	this.value = value
}

// Borrow borrows the value from the Monitor for write access, and returns a
// function that must immediately be deferred, like this:
//
//   value, done := monitor.Borrow()
//   defer done()
//
// From the time Borrow is called to the time the done function is called, it is
// safe to access the locked object from within the current goroutine.
func (this *RWMonitor[T]) Borrow () (T, func ()) {
	this.mutex.Lock()
	return this.value, this.mutex.Unlock
}

// BorrowReturn is like borrow, but returns a "done" function that takes in an
// updated value. The intended use of this function is like this:
//
//   value, done := monitor.BorrowReturn()
//   defer done(value)
func (this *RWMonitor[T]) BorrowReturn () (T, func (T)) {
	this.mutex.Lock()
	return this.value, func (value T) {
		defer this.mutex.Unlock()
		this.value = value
	}
}

// RBorrow is like Borrow, but returns the item for read access only. Do not
// under any circumstances modify anything returned by this method.
func (this *RWMonitor[T]) RBorrow () (T, func ()) {
	this.mutex.Lock()
	return this.value, this.mutex.Unlock
}