advent-of-code/day18/main.go

package main

import (
	"bufio"
	_ "embed"
	"fmt"
	"strconv"
	"strings"

	aoc "go.sour.is/advent-of-code"
	"golang.org/x/exp/maps"
)

// var log = aoc.Log

func main() { aoc.MustResult(aoc.Runner(run)) }

type result struct {
	valuePT1 int
	valuePT2 int
}

func (r result) String() string { return fmt.Sprintf("%#v", r) }

func run(scan *bufio.Scanner) (*result, error) {

	var vecsPT1 []vector
	var vecsPT2 []vector

	for scan.Scan() {
		text := scan.Text()

		if len(text) == 0 {
			continue
		}

		v, color := fromLine(text)

		vecsPT1 = append(vecsPT1, v)
		vecsPT2 = append(vecsPT2, fromColor(color))
	}
	return &result{
		valuePT1: findArea(vecsPT1),
		valuePT2: findArea(vecsPT2),
	}, nil
}

type vector struct {
	offset point
	scale  int
}

type point [2]int

func (p point) add(a point) point {
	return point{p[0] + a[0], p[1] + a[1]}
}
func (p point) scale(m int) point {
	return point{p[0] * m, p[1] * m}
}

// numPoints the number of the points inside an outline plus the number of points in the outline
func numPoints(outline []point, borderLength int) int {
	// shoelace - find the float area in a shape
	sum := 0
	for _, p := range pairwise(outline) {
		row1, col1 := p[0][0], p[0][1]
		row2, col2 := p[1][0], p[1][1]

		sum += row1*col2 - row2*col1
	}
	area := sum / 2

	// pick's theorem - find the number of points in a shape given its area
	return (aoc.ABS(area) - borderLength/2 + 1) + borderLength
}

func pairwise[T any](arr []T) [][2]T {
	var pairs [][2]T
	for i := range arr[:len(arr)-1] {
		pairs = append(pairs, [2]T{arr[i], arr[i+1]})
	}
	return pairs
}

var OFFSET = map[string]point{
	"R": {0, 1},
	"D": {1, 0},
	"L": {0, -1},
	"U": {-1, 0},
}
var OFFSET_INDEXES = maps.Values(OFFSET)

func fromLine(text string) (vector, string) {
	v := vector{}
	s, text, _ := strings.Cut(text, " ")
	v.offset = OFFSET[s]

	s, text, _ = strings.Cut(text, " ")
	v.scale = aoc.Atoi(s)

	_, text, _ = strings.Cut(text, "(#")
	s, _, _ = strings.Cut(text, ")")
	return v, s
}
 
func fromColor(c string) vector {
	scale, _ := strconv.ParseInt(c[:5], 16, 64)
	offset := OFFSET_INDEXES[c[5]-'0']

	return vector{
		offset: offset,
		scale:  int(scale),
	}
}

func findArea(vecs []vector) int {
	shoelace := []point{{0,0}}
	borderLength := 0

	for _, vec := range vecs {
		scaled_offset := vec.offset.scale(vec.scale)
		shoelace = append(shoelace, shoelace[len(shoelace)-1].add(scaled_offset))
		borderLength += vec.scale
	}

	return numPoints(shoelace, borderLength)
}