kleingrep/regex/priorityQueue.go

package regex

import "container/heap"

// Implement a priority queue using container/heap

const (
	min_priority int = iota
	zerostate_priority
	alternation_priority
	kleene_priority
	char_priority
	max_priority
)

func getPriority(state *nfaState) int {
	if state.isKleene {
		return zerostate_priority
	} else if state.isAlternation {
		return alternation_priority
	} else {
		if state.isEmpty {
			return zerostate_priority
		} else {
			return char_priority
		}
	}
}

type priorQueueItem struct {
	state    *nfaState
	priority int
	index    int
}

func newPriorQueueItem(state *nfaState) *priorQueueItem {
	return &priorQueueItem{
		state:    state,
		index:    -1,
		priority: getPriority(state),
	}
}

type priorityQueue []*priorQueueItem

func (pq priorityQueue) Len() int {
	return len(pq)
}

func (pq priorityQueue) Less(i, j int) bool {
	if pq[i].priority == pq[j].priority {
		return pq[i].index < pq[j].index
	}
	return pq[i].priority > pq[j].priority // We want max-heap, so we use greater-than
}

func (pq priorityQueue) Swap(i, j int) {
	pq[i], pq[j] = pq[j], pq[i]
	pq[i].index = i
	pq[j].index = j
}

func (pq *priorityQueue) Push(x any) {
	length := len(*pq)
	item := x.(*priorQueueItem)
	item.index = length
	*pq = append(*pq, item)
}

func (pq *priorityQueue) Pop() any {
	old := *pq
	n := len(old)
	item := old[n-1]
	old[n-1] = nil
	item.index = -1
	*pq = old[0 : n-1]
	return item
}
func (pq *priorityQueue) peek() any {
	queue := *pq
	n := len(queue)
	return queue[n-1]
}

func (pq *priorityQueue) update(item *priorQueueItem, value *nfaState, priority int) {
	item.state = value
	item.priority = priority
	heap.Fix(pq, item.index)
}