// Package npq implements a concurrent, n-priority message queue.
//
// [Q] is similar to a buffered channel, except that senders can assign one of
// some fixed number priority levels to each item. Receivers will always get
// the item with the highest priority.
//
// For example:
//
//	q := npq.Make[int, string](8, 1, 2)
//	q.Send(1, "world")
//	q.Send(2, "hello")
//	word1, _ := q.Recv()
//	word2, _ := q.Recv()
//	fmt.Println(word1, word2)
//	q.Close()
//	// Output: hello world
//
// # Implementation
//
// Each [Q] has an array of 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 to [mq.Q] , which has only two priority levels, [Q] performs
// similarly in the Send and Recv benchmarks. In the ConcSendRecv benchmark,
// though, this package is around 75% slower. Even more peculiar, however, in
// HighContention, it is 7% faster.
//
// Compared the pq package, this package is faster in the HighContention
// benchmark until around 145-150 priority levels; in ConcSendRecv, until
// around 200 levels; and in Recv, until around 750 levels.
//
// It's important to remember the memory requirement differences. A [pq.Q]
// with capacity N can store N items; a [Q] with capacity N and P priority
// levels can store N*P items.
package npq

import (
	"fmt"
	"sync"

	"gogs.humancabbage.net/sam/priorityq"
	"gogs.humancabbage.net/sam/priorityq/mq"
	"gogs.humancabbage.net/sam/priorityq/pq"
	"gogs.humancabbage.net/sam/priorityq/queue"
	"golang.org/x/exp/constraints"
)

// so that godoc (kinda) works
var _ *pq.Q[int, int]
var _ *mq.Q[int]

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

// Make a new queue.
func Make[P constraints.Integer, T any](cap int, min P, max P) Q[P, T] {
	numLevels := max - min + 1
	levels := make([]queue.Q[T], numLevels)
	for i := 0; i < int(numLevels); i++ {
		levels[i] = queue.Make[T](cap)
	}
	s := &state[P, T]{
		levels: levels,
		min:    min,
		max:    max,
	}
	canSend := make([]sync.Cond, numLevels)
	for i := 0; i < len(canSend); i++ {
		canSend[i].L = &s.mu
	}
	s.canRecv = sync.Cond{L: &s.mu}
	s.canSend = canSend
	return Q[P, T]{s}
}

type state[P constraints.Integer, T any] struct {
	mu      sync.Mutex
	levels  []queue.Q[T]
	canSend []sync.Cond
	canRecv sync.Cond
	min     P
	max     P
	closed  bool
}

// Close marks the queue as closed.
//
// Attempting to close an already-closed queue results in a panic.
func (s *state[P, T]) Close() {
	s.mu.Lock()
	if s.closed {
		s.mu.Unlock()
		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[P, T]) Recv() (T, bool) {
	s.mu.Lock()
	defer s.mu.Unlock()
	for {
		var available int = -1
		findAvailable := func() {
			for levelIdx := len(s.levels) - 1; levelIdx > -1; levelIdx-- {
				level := &s.levels[levelIdx]
				if level.CanPop() {
					available = levelIdx
					break
				}
			}
		}
		findAvailable()
		for !s.closed && available == -1 {
			s.canRecv.Wait()
			findAvailable()
		}
		if s.closed && available == -1 {
			var empty T
			return empty, false
		}
		if available != -1 {
			level := &s.levels[available]
			value := level.PopFront()
			s.canSend[available].Broadcast()
			return value, true
		}
	}
}

// Send adds an item with some priority, blocking if full.
func (s *state[P, T]) Send(priority P, value T) {
	s.validatePriority(priority)
	s.mu.Lock()
	defer s.mu.Unlock()

	levelIdx := priority - s.min
	level := &s.levels[levelIdx]
	for {
		for !s.closed && !level.CanPush() {
			s.canSend[levelIdx].Wait()
		}
		if s.closed {
			panic("send on closed queue")
		}
		if level.CanPush() {
			level.PushBack(value)
			s.canRecv.Broadcast()
			return
		}
	}
}

// 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[P, T]) TryRecv() (value T, err error) {
	s.mu.Lock()
	defer s.mu.Unlock()

	for levelIdx := len(s.levels) - 1; levelIdx > -1; levelIdx-- {
		level := &s.levels[levelIdx]
		if level.CanPop() {
			value = level.PopFront()
			s.canSend[levelIdx].Broadcast()
			return
		}
	}
	if s.closed {
		err = priorityq.ErrClosed
	} else {
		err = priorityq.ErrEmpty
	}

	return
}

// TrySend attempts to add an item with some priority, without blocking.
//
// Returns an error from the root priorityq package, or nil if successful.
func (s *state[P, T]) TrySend(priority P, value T) error {
	s.validatePriority(priority)
	s.mu.Lock()
	defer s.mu.Unlock()
	if s.closed {
		return priorityq.ErrClosed
	}
	levelIdx := priority - s.min
	level := &s.levels[levelIdx]
	if !level.CanPush() {
		return priorityq.ErrFull
	}
	level.PushBack(value)
	s.canRecv.Broadcast()

	return nil
}

func (s *state[P, T]) validatePriority(priority P) {
	if priority < s.min || priority > s.max {
		panic(fmt.Errorf("priority %d out of range (%d, %d)",
			priority, s.min, s.max))
	}
}