aoc2024/day-12/internal/two/solve.go

package two

import (
	"bytes"

	log "github.com/sirupsen/logrus"
)

func Solve(buf []byte) (int, error) {
	r := bytes.NewReader(buf)
	graph, err := parseLines(r)
	if err != nil {
		return 0, err
	}

	var totalCost int
	totalAreaVisited := make(map[point]struct{})
	for i := 0; i < len(graph.data); i++ {
		for j := 0; j < len(graph.data[i]); j++ {
			start := newPoint(j, i)
			if graph.isOutOfBounds(start) {
				continue
			}

			_, ok := totalAreaVisited[start]
			if ok {
				continue
			}
			totalAreaVisited[start] = struct{}{}

			path := exploreAreaSequentially(start, graph)
			numSides := analyzeSides(graph.valueAt(firstPointFromMap(path.visited)), graph, path)

			totalCost += len(path.visited) * numSides
			for point := range path.visited {
				totalAreaVisited[point] = struct{}{}
			}
		}
	}

	return totalCost, nil
}

func analyzeSides(kind rune, graph *graph, path path) int {
	log.Debugf("graph for values %s\n%s\n", string(kind), graph.debug(path.visited))

	var corners int
	for current := range path.visited {
		ns := neighbours(current)

		if isCorner(graph, current, ns[N], ns[E]) {
			corners++
		}

		if isCorner(graph, current, ns[S], ns[E]) {
			corners++
		}

		if isCorner(graph, current, ns[N], ns[W]) {
			corners++
		}

		if isCorner(graph, current, ns[S], ns[W]) {
			corners++
		}

		corners += numInnerCorner(graph, current, ns)
	}

	log.Debugf("this path has %d corners", corners)

	return corners
}

func isCorner(graph *graph, current, p, q point) bool {
	if graph.isOutOfBounds(p) && graph.isOutOfBounds(q) {
		return true
	}

	if !graph.isOutOfBounds(p) {
		if graph.isOutOfBounds(q) && !graph.sameKind(p, current) {
			return true
		}
	}

	if !graph.isOutOfBounds(q) {
		if graph.isOutOfBounds(p) && !graph.sameKind(q, current) {
			return true
		}
	}

	if !graph.isOutOfBounds(p) && !graph.isOutOfBounds(q) {
		if !graph.sameKind(p, current) && !graph.sameKind(q, current) {
			return true
		}
	}

	return false
}

func numInnerCorner(graph *graph, current point, ns [4]point) int {
	var corners int

	if !graph.isOutOfBounds(ns[N]) && !graph.isOutOfBounds(ns[E]) {
		if graph.sameKind(ns[N], current) && graph.sameKind(ns[E], current) &&
			!graph.sameKind(point{current.x + 1, current.y - 1}, current) {
			corners++
		}
	}

	if !graph.isOutOfBounds(ns[N]) && !graph.isOutOfBounds(ns[W]) {
		if graph.sameKind(ns[N], current) && graph.sameKind(ns[W], current) &&
			!graph.sameKind(point{current.x - 1, current.y - 1}, current) {
			corners++
		}
	}

	if !graph.isOutOfBounds(ns[S]) && !graph.isOutOfBounds(ns[E]) {
		if graph.sameKind(ns[S], current) && graph.sameKind(ns[E], current) &&
			!graph.sameKind(point{current.x + 1, current.y + 1}, current) {
			corners++
		}
	}

	if !graph.isOutOfBounds(ns[S]) && !graph.isOutOfBounds(ns[W]) {
		if graph.sameKind(ns[S], current) && graph.sameKind(ns[W], current) &&
			!graph.sameKind(point{current.x - 1, current.y + 1}, current) {
			corners++
		}
	}

	return corners
}