aoc2023/day-18/two.go

package main

import (
	"math"
	"strings"
)

func fromHex(imager *imager, lines []string) {
	for _, line := range lines {
		direction, count := func() (string, int) {
			var dirs = []string{"R", "D", "L", "U"}
			m := getParams(r, line)
			code := strings.TrimLeft(m["colour"], "#")
			return dirs[mustConv(string(code[len(code)-1]))], mustConvHex(code[:len(code)-1])
		}()
		imager.add(direction, count)
	}
}

// two returns the area of the polygon described by imager.space
// it uses the hex codes to define points
func two(lines []string) int {
	imager := newImager()
	buildImage(fromHex, imager, lines)

	area := 0
	for i := 0; i < len(imager.space); i++ {
		next := imager.space[(i+1)%len(imager.space)]
		area += imager.space[i].X*next.Y - imager.space[i].Y*next.X
	}

	// add perimeter to area within perimeter
	area = len(imager.space) + (int(math.Abs(float64(area))) / 2)

	return area - len(imager.space)/2 + 1
}