aoc2024/day-16/internal/one/graph.go

package one

import (
	hp "container/heap"
	"errors"
	"math"
	"slices"
	"strings"

	log "github.com/sirupsen/logrus"
)

const turnCost int = 1000

type graph struct {
	start node
	end   node
	data  []string
}

func newGraph() *graph {
	return &graph{}
}

func (g *graph) String() string {
	return strings.Join(g.data, "\n")
}

func (g *graph) valueAt(n node) rune {
	return rune(g.data[n.y][n.x])
}

func (g *graph) dijkstra() (int, error) {
	heap := newHeap()
	hp.Push(heap, move{g.start, 0})
	visited := make(map[node]struct{})
	costs := make(map[node]int)
	prev := make(map[node]node)

	for heap.Len() > 0 {
		current := hp.Pop(heap).(move)

		if current.node.coords == g.end.coords {
			log.Debugf("\n%s\n", g.debug(prev, costs))
			return costs[g.end], nil
		}

		if _, ok := visited[current.node]; ok {
			continue
		}
		visited[current.node] = struct{}{}

		for _, n := range neighbours(current.node) {
			if g.valueAt(n) == '#' {
				continue
			}

			next_cost := current.cost + 1
			if n.direction != current.node.direction {
				next_cost += turnCost
			}

			_, ok := costs[n]
			if !ok {
				costs[n] = math.MaxInt
			}

			if next_cost < costs[n] {
				costs[n] = next_cost
				prev[n] = current.node
				hp.Push(heap, move{n, next_cost})
			}
		}
	}

	return 0, errors.New("unable to get shortest path cost")
}

func (g *graph) debug(prev map[node]node, costs map[node]int) string {
	path := []node{g.end}

	node := prev[g.end]
	for node != g.start {
		path = append(path, prev[node])
		node = prev[node]
	}

	temp := slices.Clone(g.data)
	for _, node := range path {
		if g.valueAt(node) == 'S' || g.valueAt(node) == 'E' {
			continue
		}
		temp[node.y] = replaceAtIndex(temp[node.y], []rune{'^', '>', 'v', '<'}[node.direction], node.x)
	}

	log.Debugf("len of shortest path: %d", len(path))
	log.Debugf("cost of shortest path: %d", costs[g.end])

	return strings.Join(temp, "\n")
}