package dijkstra

import (
	"container/heap"
	"math"
	"reflect"
	"testing"
)

// Edge represents a weighted directed edge to another node.
type Edge struct {
	To     int
	Weight int
}

// Dijkstra returns the shortest distances from start to every reachable node.
// Unreachable nodes keep distance math.MaxInt.
func Dijkstra(graph [][]Edge, start int) []int {
	dist := make([]int, len(graph))
	for i := range dist {
		dist[i] = math.MaxInt
	}

	dist[start] = 0
	pq := priorityQueue{{node: start, dist: 0}}
	heap.Init(&pq)

	for pq.Len() > 0 {
		cur := heap.Pop(&pq).(item)
		if cur.dist != dist[cur.node] {
			continue
		}

		for _, edge := range graph[cur.node] {
			nextDist := cur.dist + edge.Weight
			if nextDist < dist[edge.To] {
				dist[edge.To] = nextDist
				heap.Push(&pq, item{node: edge.To, dist: nextDist})
			}
		}
	}

	return dist
}

type item struct {
	node int
	dist int
}

type priorityQueue []item

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

func (pq priorityQueue) Less(i, j int) bool {
	return pq[i].dist < pq[j].dist
}

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

func (pq *priorityQueue) Push(x any) {
	*pq = append(*pq, x.(item))
}

func (pq *priorityQueue) Pop() any {
	old := *pq
	n := len(old)
	x := old[n-1]
	*pq = old[:n-1]
	return x
}

func TestDijkstra(t *testing.T) {
	graph := [][]Edge{
		{{To: 1, Weight: 4}, {To: 2, Weight: 1}},
		{{To: 3, Weight: 1}},
		{{To: 1, Weight: 2}, {To: 3, Weight: 5}},
		{},
	}

	got := Dijkstra(graph, 0)
	want := []int{0, 3, 1, 4}

	if !reflect.DeepEqual(got, want) {
		t.Fatalf("Dijkstra() = %v, want %v", got, want)
	}
}

func TestDijkstraUnreachable(t *testing.T) {
	graph := [][]Edge{
		{{To: 1, Weight: 7}},
		{},
		{},
	}

	got := Dijkstra(graph, 0)
	want := []int{0, 7, math.MaxInt}

	if !reflect.DeepEqual(got, want) {
		t.Fatalf("Dijkstra() = %v, want %v", got, want)
	}
}