xt/fibheap.go

package main

import "math/bits"

type fibTree[T any] struct {
	value  *T
	parent *fibTree[T]
	child  []*fibTree[T]
	mark   bool
}

func (t *fibTree[T]) Value() *T { return t.value }
func (t *fibTree[T]) addAtEnd(n *fibTree[T]) {
	n.parent = t
	t.child = append(t.child, n)
}

type fibHeap[T any] struct {
	trees []*fibTree[T]
	least *fibTree[T]
	count uint
	less  func(a, b *T) bool
}

func FibHeap[T any](less func(a, b *T) bool) *fibHeap[T] {
	return &fibHeap[T]{less: less}
}

func (h *fibHeap[T]) GetMin() *T {
	return h.least.value
}

func (h *fibHeap[T]) IsEmpty() bool { return h.least == nil }

func (h *fibHeap[T]) Insert(v *T) {
	ntree := &fibTree[T]{value: v}
	h.trees = append(h.trees, ntree)
	if h.least == nil || h.less(v, h.least.value) {
		h.least = ntree
	}
	h.count++
}

func (h *fibHeap[T]) ExtractMin() *T {
	smallest := h.least
	if smallest != nil {
		// Remove smallest from root trees.
		for i := range h.trees {
			pos := h.trees[i]
			if pos == smallest {
				h.trees[i] = h.trees[len(h.trees)-1]
				h.trees = h.trees[:len(h.trees)-1]
				break
			}
		}

		// Add children to root
		h.trees = append(h.trees, smallest.child...)
		smallest.child = smallest.child[:0]

		h.least = nil
		if len(h.trees) > 0 {
			h.consolidate()
		}

		h.count--
		return smallest.value
	}
	return nil
}

func (h *fibHeap[T]) consolidate() {
	aux := make([]*fibTree[T], bits.Len(h.count)+1)
	for _, x := range h.trees {
		order := len(x.child)

		// consolidate the larger roots under smaller roots of same order until we have at most one tree per order.
		for aux[order] != nil {
			y := aux[order]
			if h.less(y.value, x.value) {
				x, y = y, x
			}
			x.addAtEnd(y)
			aux[order] = nil
			order++
		}
		aux[order] = x
	}

	h.trees = h.trees[:0]
	// move ordered trees to root and find least node.
	for _, k := range aux {
		if k != nil {
			k.parent = nil
			h.trees = append(h.trees, k)
			if h.least == nil || h.less(k.value, h.least.value) {
				h.least = k
			}
		}
	}
}

func (h *fibHeap[T]) Merge(a *fibHeap[T]) {
	h.trees = append(h.trees, a.trees...)
	h.count += a.count
	if h.least == nil || a.least != nil && h.less(a.least.value, h.least.value) {
		h.least = a.least
	}
}

func (h *fibHeap[T]) find(fn func(*T) bool) *fibTree[T] {
	var st []*fibTree[T]
	st = append(st, h.trees...)
	var tr *fibTree[T]

	for len(st) > 0 {
		tr, st = st[0], st[1:]
		ro := *tr.value
		if fn(&ro) {
			break
		}
		st = append(st, tr.child...)
	}

	return tr
}

func (h *fibHeap[T]) Find(fn func(*T) bool) *T {
	if needle := h.find(fn); needle != nil {
		return needle.value
	}

	return nil
}

func (h *fibHeap[T]) DecreaseKey(find func(*T) bool, decrease func(*T)) {
	needle := h.find(find)
	if needle == nil {
		return
	}
	decrease(needle.value)

	if h.less(needle.value, h.least.value) {
		h.least = needle
	}

	if parent := needle.parent; parent != nil {
		if h.less(needle.value, parent.value) {
			h.cut(needle)
			h.cascadingCut(parent)
		}
	}
}

func (h *fibHeap[T]) cut(x *fibTree[T]) {
	parent := x.parent
	for i := range parent.child {
		pos := parent.child[i]
		if pos == x {
			parent.child[i] = parent.child[len(parent.child)-1]
			parent.child = parent.child[:len(parent.child)-1]
			break
		}
	}

	x.parent = nil
	x.mark = false
	h.trees = append(h.trees, x)

	if h.less(x.value, h.least.value) {
		h.least = x
	}
}

func (h *fibHeap[T]) cascadingCut(y *fibTree[T]) {
	if y.parent != nil {
		if !y.mark {
			y.mark = true
			return
		}

		h.cut(y)
		h.cascadingCut(y.parent)
	}
}