//go:build !solution

package rwmutex

import "sync/atomic"

// A RWMutex is a reader/writer mutual exclusion lock.
// The lock can be held by an arbitrary number of readers or a single writer.
// The zero value for a RWMutex is an unlocked mutex.
//
// If a goroutine holds a RWMutex for reading and another goroutine might
// call Lock, no goroutine should expect to be able to acquire a read lock
// until the initial read lock is released. In particular, this prohibits
// recursive read locking. This is to ensure that the lock eventually becomes
// available; a blocked Lock call excludes new readers from acquiring the
// lock.
type RWMutex struct {
    readerSem   chan struct{}
    writerSem   chan struct{}
    readerCount atomic.Int32
}

// New creates *RWMutex.
func New() *RWMutex {
    rSem := make(chan struct{}, 1)
    wrSem := make(chan struct{}, 1)
    return &RWMutex{rSem, wrSem, atomic.Int32{}}
}

// RLock locks rw for reading.
//
// It should not be used for recursive read locking; a blocked Lock
// call excludes new readers from acquiring the lock. See the
// documentation on the RWMutex type.
func (rw *RWMutex) RLock() { // мы по сути под мьютексом это делаем
    rw.writerSem <- struct{}{} // error
    rw.readerCount.Add(1)

    select { // обозначили, что есть читатели
    case rw.readerSem <- struct{}{}:
    default:
    }

    <-rw.writerSem
}

// RUnlock undoes a single RLock call;
// it does not affect other simultaneous readers.
// It is a run-time error if rw is not locked for reading
// on entry to RUnlock.
func (rw *RWMutex) RUnlock() {
    select {
    case rw.writerSem <- struct{}{}: // Если RLock владеет, то он быстро отдаст; если Lock, то долго стоим
        if readersRemain := rw.readerCount.Add(-1); readersRemain == 0 {
            <-rw.readerSem
        }
        <-rw.writerSem
    default:
        if readersRemain := rw.readerCount.Add(-1); readersRemain == 0 {
            <-rw.readerSem
        }
    }
}

// Lock locks rw for writing.
// If the lock is already locked for reading or writing,
// Lock blocks until the lock is available.
func (rw *RWMutex) Lock() {
    rw.writerSem <- struct{}{}

    rw.readerSem <- struct{}{}
    <-rw.readerSem
}

// Unlock unlocks rw for writing. It is a run-time error if rw is
// not locked for writing on entry to Unlock.
//
// As with Mutexes, a locked RWMutex is not associated with a particular
// goroutine. One goroutine may RLock (Lock) a RWMutex and then
// arrange for another goroutine to RUnlock (Unlock) it.
func (rw *RWMutex) Unlock() {
    <-rw.writerSem
}

// TODO: 2 ридера сделали RLock, далее врайтер сделал Lock, после этого если придёт ридер, то он исполнится после врайтера
// Когда в очереди есть писатель, то читатели блочатся

// В RUnlock мы иногда могли выпихнуть <-rw.writersSem, когда там ждал писатель

// 1) RLock
// 2) Lock
// 3) 100 x RLock
// 4) RUnlock