aoc2023/day-17/one.go

package main

import (
	"container/heap"

	log "github.com/sirupsen/logrus"
)

type option func(*dijkstra)

func WithMinDistance(distance int) option {
	return func(d *dijkstra) {
		d.minDistance = distance
	}
}

func WithMaxDistance(distance int) option {
	return func(d *dijkstra) {
		d.maxDistance = distance
	}
}

type dijkstra struct {
	graph       [][]int
	minDistance int
	maxDistance int
}

func newDijkstra(graph [][]int, opts ...option) *dijkstra {
	d := &dijkstra{graph: graph}

	for _, opt := range opts {
		opt(d)
	}

	return d
}

func (d dijkstra) initialize(start coords) *pqueue {
	pq := newPriorityQueue()
	heap.Init(pq)
	// we don't encounter heat loss for start point unless we enter this block again
	heap.Push(pq, newNode(0, 0, 0, 0, start.X, start.Y))
	return pq
}

// run performs the lowest cost dijkstra algorithm with a min heap
func (d dijkstra) run(start, end coords) int {
	pq := d.initialize(start)

	visited := map[string]bool{}

	for pq.Len() > 0 {
		cost, node := func() (int, *node) {
			x := heap.Pop(pq).(*node)
			return x.cost, x
		}()

		// we reached final location, return its lowest cost
		if node.X == end.X && node.Y == end.Y && node.distance >= d.minDistance {
			log.Debug("returning lowest cost with min distance >= ", d.minDistance)
			return node.cost
		}

		if _, ok := visited[node.String()]; ok {
			continue
		}
		visited[node.String()] = true

		var neighbours = [][]int{{0, -1}, {0, 1}, {-1, 0}, {1, 0}} // N, S, W, E
		for _, n := range neighbours {
			nextX := node.X + n[0]
			nextY := node.Y + n[1]

			if nextY < 0 || nextY >= len(d.graph) || nextX < 0 || nextX >= len(d.graph[nextY]) {
				continue
			}

			if node.directionX == -n[0] && node.directionY == -n[1] { // are we going backwards?
				continue
			}

			var distance = 1
			if node.directionX == n[0] || node.directionY == n[1] { // same direction
				distance = node.distance + 1
			} else {
				if node.distance < d.minDistance {
					continue
				}
			}

			if distance > d.maxDistance {
				continue
			}

			new_cost := cost + d.graph[nextY][nextX]
			heap.Push(pq, newNode(new_cost, distance, n[0], n[1], nextX, nextY))
		}
	}

	return 0
}

// one returns the lowest cost path from start to end
func one(lines []string) int {
	graph := buildGraph(lines)

	start := newCoords(0, 0)
	end := newCoords(len(graph[0])-1, len(graph)-1)
	dijkstra := newDijkstra(graph, WithMaxDistance(3))
	cost := dijkstra.run(start, end)

	return cost
}