adventofcode2023/day11.go

package main

import (
	"bufio"
	"bytes"
	"fmt"
	"io"
	"unicode"
)

type galaxy struct {
	val byte // either '#' or '.'
	x   int
	y   int
}

func galaxyDistance(g1, g2 galaxy, blankRows, blankColumns []int) int {
	maxX, minX := maxmin(g1.x, g2.x)

	// g2.y will always be greater than g1.y
	diffy := g2.y - g1.y
	diffx := maxX - minX

	for _, col := range blankColumns {
		if maxX > col && minX < col {
			diffy++
		}
	}

	for _, row := range blankRows {
		if g2.y > row && g1.y < row {
			diffx++
		}
	}

	return diffy + diffx
}

type day11 struct{}

func (d day11) solve1(r io.Reader) string {
	scanner := bufio.NewScanner(r)
	space := make([][]galaxy, 0, 50)
	galaxies := make([]galaxy, 0, 50)

	blankRows := make([]int, 0, 10)
	blankColumns := make([]int, 0, 10)

	for y := 0; scanner.Scan(); y++ {
		layer := make([]galaxy, 0, 50)
		bytes := bytes.TrimFunc(scanner.Bytes(), unicode.IsSpace)

		seenGalaxy := false
		for x, c := range bytes {
			galaxy := galaxy{x: x, y: y, val: c}
			layer = append(layer, galaxy)

			// only galaxies get tracked in
			// the galaxy slice
			if c == '#' {
				galaxies = append(galaxies, galaxy)
				seenGalaxy = true
			}

		}
		space = append(space, layer)

		if !seenGalaxy {
			blankRows = append(blankRows, y)
		}
	}

	for i := 0; i < len(space[0]); i++ {
		seenGalaxy := false
		for j := range space {
			if space[j][i].val == '#' {
				seenGalaxy = true
				break
			}
		}

		if !seenGalaxy {
			blankColumns = append(blankColumns, i)
		}
	}

	sum := 0
	for i := 0; i < len(galaxies)-1; i++ {
		for j := i + 1; j < len(galaxies); j++ {
			g1 := galaxies[i]
			g2 := galaxies[j]
			sum += galaxyDistance(g1, g2, blankRows, blankColumns)
		}
	}

	return fmt.Sprintf("%d", sum)
}

func (d day11) solve2(r io.Reader) string {
	return "incomplete"
}