adventofcode/2023/day10/main.go

package main

import (
	"fmt"

	h "git.bullercodeworks.com/brian/adventofcode/helpers"
)

func main() {
	inp := h.StdinToStringSlice()
	m, l := part1(inp)
	fmt.Println()
	part2(m, l)
}

const (
	NS = '|'
	EW = '-'
	NE = 'L'
	NW = 'J'
	SW = '7'
	SE = 'F'

	START = 'S'
)

var pipes = []byte{NS, EW, NE, NW, SW, SE}

func part1(input []string) (h.CoordByteMap, []h.Coordinate) {
	m := h.StringSliceToCoordByteMap(input)
	_, loop := findLoop(&m)
	fmt.Println("# Part 1")
	fmt.Println(len(loop) / 2)
	return m, loop
}

func part2(m h.CoordByteMap, loop []h.Coordinate) {
	var insideCoords []h.Coordinate
	var inside bool
	var enclosed int
	for y := m.TLY; y <= m.BRY; y++ {
		inside = false
		var rowPathStart byte
		for x := m.TLX; x <= m.BRX; x++ {
			wrk := h.Coordinate{X: x, Y: y}
			wrkV := m.Get(wrk)
			if h.CoordListContains(wrk, loop) {
				// This coordinate is part of the loop, so it can change the 'inside' flag
				// A corner _only_ changes the flag if the 'exiting' corner continues in the same direction
				// i.e.: L--J doesn't change the flag, L--7 does change the flag
				switch wrkV {
				case NE, SE:
					rowPathStart = wrkV
				case NW, SW:
					if continues(rowPathStart, wrkV) {
						inside = !inside
					}
				}
				if wrkV == NS {
					// a | always changes the flag
					inside = !inside
				}
			} else {
				if inside {
					enclosed++
					insideCoords = append(insideCoords, wrk)
				}
			}
		}
	}
	fmt.Println("# Part 2")
	fmt.Println(enclosed)
	for _, insides := range insideCoords {
		m.Put(insides, 'I')
	}
}

// continues checks if two _corner_ pieces continue the flow in the same direction
func continues(p1, p2 byte) bool {
	switch p1 {
	case NE:
		return p2 == SW
	case NW:
		return p2 == SE
	case SW:
		return p2 == NE
	case SE:
		return p2 == NW
	}
	return false
}

func connectsNorth(v byte) bool {
	return v == START || v == NS || v == NE || v == NW
}
func connectsSouth(v byte) bool {
	return v == START || v == NS || v == SE || v == SW
}
func connectsEast(v byte) bool {
	return v == START || v == EW || v == NE || v == SE
}
func connectsWest(v byte) bool {
	return v == START || v == EW || v == NW || v == SW
}

func connects(m *h.CoordByteMap, p1, p2 h.Coordinate) bool {
	p1v, p2v := m.Get(p1), m.Get(p2)
	switch {
	case p1.North().Equals(p2):
		return connectsNorth(p1v) && connectsSouth(p2v)
	case p1.East().Equals(p2):
		return connectsEast(p1v) && connectsWest(p2v)
	case p1.South().Equals(p2):
		return connectsSouth(p1v) && connectsNorth(p2v)
	case p1.West().Equals(p2):
		return connectsWest(p1v) && connectsEast(p2v)
	}
	return false
}

func countConnections(m *h.CoordByteMap, p h.Coordinate) int {
	var res int
	for _, tst := range []h.Coordinate{p.North(), p.East(), p.South(), p.West()} {
		if connects(m, p, tst) {
			res++
		}
	}
	return res
}

// Find loop finds all coordinates on the loop and returns:
// 1. The starting coordinate
// 2. All coordinates on the loop
// It also replaces the 'S' for the starting coordinate with the appropriate
// pipe byte
func findLoop(m *h.CoordByteMap) (h.Coordinate, []h.Coordinate) {
	start, _ := m.FindFirst(START)
	next := start
	path := []h.Coordinate{start}

	// Figure out the starting pipe byte
	for _, v := range pipes {
		m.Put(start, v)
		if countConnections(m, start) == 2 {
			break
		}
	}

	for {
		for _, tst := range []h.Coordinate{
			next.North(),
			next.East(),
			next.South(),
			next.West(),
		} {
			if !connects(m, next, tst) || (!tst.Equals(start) && h.CoordListContains(tst, path)) {
				continue
			}
			next = tst
		}
		if next.Equals(start) {
			// We're done
			return start, path
		} else {
			path = append(path, next)
		}
	}
}