// Package mq implements a concurrent, dual-priority message queue.
//
// [Q] is similar to a buffered channel, except that senders can assign one of
// two priority levels to each item, "high" or "low." Receivers will always
// get a high-priority item ahead of any low-priority ones.
//
// For example:
//
//	q := mq.Make[string](8)
//	mq.SendLow("world")
//	mq.SendHigh("hello")
//	word1, _ := mq.Recv()
//	word2, _ := mq.Recv()
//	fmt.Println(word1, word2)
//	q.Close()
//	// Output: hello world
//
// # Implementation
//
// Each queue has two circular buffers, one for each priority level.
// Currently, the capacities for these are fixed and equal. If one buffer is
// full, attempts to send further items with its priority level will block
// ([Q.Send]) or fail ([Q.TrySend]).
//
// Compared the pq package, the limitation on priority levels increases
// performance, as its circular buffers are much less expensive than the heap
// operations of a traditional priority queue.
package mq

import (
	"sync"

	"gogs.humancabbage.net/sam/priorityq"
	"gogs.humancabbage.net/sam/priorityq/circ"
)

// Q is a concurrent, dual-priority message queue.
type Q[T any] struct {
	*state[T]
}

// Make a new queue.
func Make[T any](cap int) Q[T] {
	high := circ.Make[T](cap)
	low := circ.Make[T](cap)
	s := &state[T]{
		high: high,
		low:  low,
	}
	s.canRecv = sync.Cond{L: &s.mu}
	s.canSendHigh = sync.Cond{L: &s.mu}
	s.canSendLow = sync.Cond{L: &s.mu}
	return Q[T]{s}
}

type state[T any] struct {
	mu          sync.Mutex
	high        circ.B[T]
	low         circ.B[T]
	canSendHigh sync.Cond
	canSendLow  sync.Cond
	canRecv     sync.Cond
	closed      bool
}

// Close marks the queue as closed.
//
// Attempting to close an already-closed queue results in a panic.
func (s *state[T]) Close() {
	s.mu.Lock()
	if s.closed {
		panic("close of closed queue")
	}
	s.closed = true
	s.mu.Unlock()
	s.canRecv.Broadcast()
}

// Recv gets an item, blocking when empty until one is available.
//
// The returned bool will be true if the queue still has items or is open.
// It will be false if the queue is empty and closed.
func (s *state[T]) Recv() (T, bool) {
	s.mu.Lock()
	defer s.mu.Unlock()
	for {
		for !s.closed && !s.high.CanPop() && !s.low.CanPop() {
			s.canRecv.Wait()
		}
		if s.closed && !s.high.CanPop() && !s.low.CanPop() {
			var empty T
			return empty, false
		}
		if s.high.CanPop() {
			value := s.high.PopFront()
			s.canSendHigh.Broadcast()
			return value, true
		}
		if s.low.CanPop() {
			value := s.low.PopFront()
			s.canSendLow.Broadcast()
			return value, true
		}
	}
}

// Send is an alias for SendLow.
func (s *state[T]) Send(value T) {
	s.SendLow(value)
}

// SendHigh adds an item with high priority, blocking if full.
func (s *state[T]) SendHigh(value T) {
	s.send(value, &s.high, &s.canSendHigh)
}

// SendLow adds an item with low buffer, blocking if full.
func (s *state[T]) SendLow(value T) {
	s.send(value, &s.low, &s.canSendLow)
}

// TryRecv attempts to get an item from the queue, without blocking.
//
// The error indicates whether the attempt succeeded, the queue is empty, or
// the queue is closed.
func (s *state[T]) TryRecv() (value T, err error) {
	s.mu.Lock()
	defer s.mu.Unlock()
	if s.high.CanPop() {
		value = s.high.PopFront()
		s.canSendHigh.Broadcast()
		return
	}
	if s.low.CanPop() {
		value = s.low.PopFront()
		s.canSendLow.Broadcast()
		return
	}
	if s.closed {
		err = priorityq.ErrClosed
	} else {
		err = priorityq.ErrEmpty
	}
	return
}

// TrySend is an alias for TrySendLow.
func (s *state[T]) TrySend(value T) error {
	return s.trySend(value, &s.low)
}

// TrySendHigh attempts to add an item with high priority, without blocking.
//
// Returns an error from the root priorityq package, or nil if successful.
func (s *state[T]) TrySendHigh(value T) error {
	return s.trySend(value, &s.high)
}

// TrySendLow attempts to add an item with low priority, without blocking.
//
// Returns an error from the root priorityq package, or nil if successful.
func (s *state[T]) TrySendLow(value T) error {
	return s.trySend(value, &s.low)
}

func (s *state[T]) send(value T, buf *circ.B[T], cond *sync.Cond) {
	s.mu.Lock()
	defer s.mu.Unlock()
	for {
		for !s.closed && !buf.CanPush() {
			cond.Wait()
		}
		if s.closed {
			panic("send on closed queue")
		}
		if buf.CanPush() {
			buf.PushBack(value)
			s.canRecv.Broadcast()
			return
		}
	}
}

func (s *state[T]) trySend(value T, buf *circ.B[T]) error {
	s.mu.Lock()
	defer s.mu.Unlock()
	if s.closed {
		return priorityq.ErrClosed
	}
	if !buf.CanPush() {
		return priorityq.ErrFull
	}
	buf.PushBack(value)
	s.canRecv.Broadcast()
	return nil
}