aoc2024/day-18/internal/two/graph.go

package two

import (
	"errors"
	"math"
	"slices"
	"strings"

	"github.com/onyx-and-iris/aoc2024/day-18/internal/queue"
)

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

func newGraph(width, height, numCorruptions int, corruptedCoords [][]int) *graph {
	var data []string
	var sb strings.Builder
	for range height {
		for range width {
			sb.WriteRune('.')
		}
		data = append(data, sb.String())
		sb.Reset()
	}

	for _, coords := range corruptedCoords[:numCorruptions] {
		data[coords[1]] = replaceAtIndex(data[coords[1]], '#', coords[0])
	}

	return &graph{point{0, 0}, point{len(data[0]) - 1, len(data) - 1}, data}
}

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

func (g *graph) isOutOfBounds(p point) bool {
	return p.x < 0 || p.y < 0 || p.y >= len(g.data) || p.x >= len(g.data[p.y])
}

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

func (g *graph) addCorruption(coords []int) {
	g.data[coords[1]] = replaceAtIndex(g.data[coords[1]], '#', coords[0])
}

func (g *graph) dijkstra(start, end point) ([]point, error) {
	queue := queue.New[point]()
	queue.Enqueue(start)
	visited := make(map[point]struct{})
	costs := make(map[point]int)
	prev := make(map[point]point)

	for !queue.IsEmpty() {
		current := queue.Dequeue()

		// we found a shortest path
		if current == end {
			return g.generatePath(start, end, prev), nil
		}

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

		for _, n := range neighbours(current) {
			if g.isOutOfBounds(n) {
				continue
			}

			if g.valueAt(n) == '#' {
				continue
			}

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

			new_cost := costs[current] + 1
			if new_cost < costs[n] {
				costs[n] = new_cost
				prev[n] = current
				queue.Enqueue(n)
			}
		}
	}

	return nil, errors.New("unable to find a shortest path")
}

func (g *graph) generatePath(start, end point, prev map[point]point) []point {
	path := []point{end}
	node := prev[end]
	for node != start {
		path = append(path, prev[node])
		node = prev[node]
	}
	return path
}

func (g *graph) debug(path []point) string {
	temp := slices.Clone(g.data)
	for _, p := range path {
		if g.valueAt(p) == '#' {
			continue
		}
		temp[p.y] = replaceAtIndex(temp[p.y], 'O', p.x)
	}
	return strings.Join(temp, "\n")
}