package structr

import (
	"reflect"
	"sync"
	"unsafe"
)

// QueueConfig defines config vars
// for initializing a struct queue.
type QueueConfig[StructType any] struct {

	// Indices defines indices to create
	// in the Queue for the receiving
	// generic struct parameter type.
	Indices []IndexConfig

	// Pop is called when queue values
	// are popped, during calls to any
	// of the Pop___() series of fns.
	Pop func(StructType)
}

// Queue provides a structure model queue with
// automated indexing and popping by any init
// defined lookups of field combinations.
type Queue[StructType any] struct {

	// indices used in storing passed struct
	// types by user defined sets of fields.
	indices []Index

	// main underlying
	// struct item queue.
	queue list

	// hook functions.
	copy func(StructType) StructType
	pop  func(StructType)

	// protective mutex, guards:
	// - Queue{}.queue
	// - Index{}.data
	// - Queue{} hook fns
	mutex sync.Mutex
}

// Init initializes the queue with given configuration
// including struct fields to index, and necessary fns.
func (q *Queue[T]) Init(config QueueConfig[T]) {
	t := reflect.TypeOf((*T)(nil)).Elem()

	if len(config.Indices) == 0 {
		panic("no indices provided")
	}

	// Safely copy over
	// provided config.
	q.mutex.Lock()
	q.indices = make([]Index, len(config.Indices))
	for i, cfg := range config.Indices {
		q.indices[i].ptr = unsafe.Pointer(q)
		q.indices[i].init(t, cfg, 0)
	}
	q.pop = config.Pop
	q.mutex.Unlock()
}

// Index selects index with given name from queue, else panics.
func (q *Queue[T]) Index(name string) *Index {
	for i := range q.indices {
		if q.indices[i].name == name {
			return &q.indices[i]
		}
	}
	panic("unknown index: " + name)
}

// PopFront pops the current value at front of the queue.
func (q *Queue[T]) PopFront() (T, bool) {
	t := q.PopFrontN(1)
	if len(t) == 0 {
		var t T
		return t, false
	}
	return t[0], true
}

// PopBack pops the current value at back of the queue.
func (q *Queue[T]) PopBack() (T, bool) {
	t := q.PopBackN(1)
	if len(t) == 0 {
		var t T
		return t, false
	}
	return t[0], true
}

// PopFrontN attempts to pop n values from front of the queue.
func (q *Queue[T]) PopFrontN(n int) []T {
	return q.pop_n(n, func() *list_elem {
		return q.queue.head
	})
}

// PopBackN attempts to pop n values from back of the queue.
func (q *Queue[T]) PopBackN(n int) []T {
	return q.pop_n(n, func() *list_elem {
		return q.queue.tail
	})
}

// Pop attempts to pop values from queue indexed under any of keys.
func (q *Queue[T]) Pop(index *Index, keys ...Key) []T {
	if index == nil {
		panic("no index given")
	} else if index.ptr != unsafe.Pointer(q) {
		panic("invalid index for queue")
	}

	// Acquire lock.
	q.mutex.Lock()

	// Preallocate expected ret slice.
	values := make([]T, 0, len(keys))

	for i := range keys {
		// Delete all items under key from index, collecting
		// value items and dropping them from all their indices.
		index.delete(keys[i], func(item *indexed_item) {

			// Append deleted to values.
			value := item.data.(T)
			values = append(values, value)

			// Delete queued.
			q.delete(item)
		})
	}

	// Get func ptrs.
	pop := q.pop

	// Done with lock.
	q.mutex.Unlock()

	if pop != nil {
		// Pass all popped values
		// to given user hook (if set).
		for _, value := range values {
			pop(value)
		}
	}

	return values
}

// PushFront pushes values to front of queue.
func (q *Queue[T]) PushFront(values ...T) {
	q.mutex.Lock()
	for i := range values {
		item := q.index(values[i])
		q.queue.push_front(&item.elem)
	}
	q.mutex.Unlock()
}

// PushBack pushes values to back of queue.
func (q *Queue[T]) PushBack(values ...T) {
	q.mutex.Lock()
	for i := range values {
		item := q.index(values[i])
		q.queue.push_back(&item.elem)
	}
	q.mutex.Unlock()
}

// MoveFront attempts to move values indexed under any of keys to the front of the queue.
func (q *Queue[T]) MoveFront(index *Index, keys ...Key) {
	q.mutex.Lock()
	for i := range keys {
		index.get(keys[i], func(item *indexed_item) {
			q.queue.move_front(&item.elem)
		})
	}
	q.mutex.Unlock()
}

// MoveBack attempts to move values indexed under any of keys to the back of the queue.
func (q *Queue[T]) MoveBack(index *Index, keys ...Key) {
	q.mutex.Lock()
	for i := range keys {
		index.get(keys[i], func(item *indexed_item) {
			q.queue.move_back(&item.elem)
		})
	}
	q.mutex.Unlock()
}

// Len returns the current length of queue.
func (q *Queue[T]) Len() int {
	q.mutex.Lock()
	l := q.queue.len
	q.mutex.Unlock()
	return l
}

func (q *Queue[T]) pop_n(n int, next func() *list_elem) []T {
	if next == nil {
		panic("nil fn")
	}

	// Acquire lock.
	q.mutex.Lock()

	// Preallocate ret slice.
	values := make([]T, 0, n)

	// Iterate over 'n' items.
	for i := 0; i < n; i++ {

		// Get next elem.
		next := next()
		if next == nil {

			// reached
			// end.
			break
		}

		// Cast the indexed item from elem.
		item := (*indexed_item)(next.data)

		// Append deleted to values.
		value := item.data.(T)
		values = append(values, value)

		// Delete queued.
		q.delete(item)
	}

	// Get func ptrs.
	pop := q.pop

	// Done with lock.
	q.mutex.Unlock()

	if pop != nil {
		// Pass all popped values
		// to given user hook (if set).
		for _, value := range values {
			pop(value)
		}
	}

	return values
}

func (q *Queue[T]) index(value T) *indexed_item {
	item := new_indexed_item()

	// Set item value.
	item.data = value

	// Get ptr to value data.
	ptr := unsafe.Pointer(&value)

	// Acquire key buf.
	buf := new_buffer()

	for i := range q.indices {
		// Get current index ptr.
		idx := &(q.indices[i])

		// Extract fields comprising index key.
		parts := extract_fields(ptr, idx.fields)
		if parts == nil {
			continue
		}

		// Calculate index key.
		key := idx.key(buf, parts)
		if key.Zero() {
			continue
		}

		// Append item to index.
		idx.append(key, item)
	}

	// Done with buf.
	free_buffer(buf)

	return item
}

func (q *Queue[T]) delete(item *indexed_item) {
	for len(item.indexed) != 0 {
		// Pop last indexed entry from list.
		entry := item.indexed[len(item.indexed)-1]
		item.indexed = item.indexed[:len(item.indexed)-1]

		// Drop index_entry from index.
		entry.index.delete_entry(entry)
	}

	// Drop entry from queue list.
	q.queue.remove(&item.elem)

	// Free now-unused item.
	free_indexed_item(item)
}