chore: use shoelace and picks theorem

day18/main.go

@@ -3,9 +3,7 @@ package main
 import (
 	"bufio"
 	_ "embed"
-	"encoding/hex"
 	"fmt"
-	"os"
 	"strconv"
 	"strings"
 
@@ -26,8 +24,8 @@ func (r result) String() string { return fmt.Sprintf("%#v", r) }
 
 func run(scan *bufio.Scanner) (*result, error) {
 
-	var vecs []vec
+	var vecsPT1 []vector
-	var vecs2 []vec
+	var vecsPT2 []vector
 
 	for scan.Scan() {
 		text := scan.Text()
@@ -36,199 +34,96 @@ func run(scan *bufio.Scanner) (*result, error) {
 			continue
 		}
 
-		var s string
+		v, color := fromLine(text)
-		v := vec{}
+
-		s, text, _ = strings.Cut(text, " ")
+		vecsPT1 = append(vecsPT1, v)
-		v.Direction = direction(s[0])
+		vecsPT2 = append(vecsPT2, fromColor(color))
-		s, text, _ = strings.Cut(text, " ")
-		v.Steps = aoc.Atoi(s)
-		_, text, _ = strings.Cut(text, "#")
-		s, _, _ = strings.Cut(text, ")")
-		b, _ := hex.DecodeString(s)
-		copy(v.Color[:], b)
-		vecs = append(vecs, v)
-		vecs2 = append(vecs2, fromColor(s))
 	}
 	return &result{
-		valuePT1: findArea(vecs),
+		valuePT1: findArea(vecsPT1),
-		valuePT2: skip("AOC_DAY18P2", func() int { return findArea(vecs2) }),
+		valuePT2: findArea(vecsPT2),
 	}, nil
 }
 
-func findArea(vecs []vec) int {
+type vector struct {
-	var points []point
+	offset point
-
+	scale  int
-	var x, y int
-	var minX, minY int
-	last := direction('S')
-	for _, v := range vecs {
-		// fmt.Println("pt ", i, v)
-		for i := 0; i < v.Steps; i++ {
-			switch v.Direction {
-			case 'U':
-				y++
-			case 'D':
-				y--
-			case 'R':
-				x++
-			case 'L':
-				x--
-			}
-			// fmt.Println("pt ", i, y, x)
-
-			minX = min(minX, x)
-			minY = min(minY, y)
-			if len(points) > 0 {
-				points[len(points)-1].d = v.Direction
-			}
-			points = append(points, point{d: v.Direction, w: opposite(v.Direction), x: x, y: y, color: v.Color})
-			last = v.Direction
-		}
-	}
-	_ = last
-	// points[0].w = last
-	points[len(points)-1].d = points[0].w
-
-	adjX := aoc.ABS(min(0, minX))
-	adjY := aoc.ABS(min(0, minY))
-	fmt.Println("minX", minX, "minY", minY)
-	fmt.Println("adjX", adjX, "adjY", adjY)
-
-	trace := make(map[int]map[int]point)
-
-	minX, minY = 0, 0
-	maxX, maxY := 0, 0
-
-	for i, p := range points {
-
-		p.x += adjX
-		p.y += adjY
-		// fmt.Println("raw", i, p.x, p.y, string(p.w), string(p.d))
-
-		maxX = max(maxX, p.x)
-		maxY = max(maxY, p.y)
-
-		points[i] = p
-
-		if row, ok := trace[p.y]; true {
-			if !ok {
-				row = make(map[int]point)
-			}
-			row[p.x] = p
-			trace[p.y] = row
-		}
-	}
-	fmt.Println("maxX", maxX, "maxY", maxY)
-
-	area := 0
-
-	for y := maxY; y >= 0; y-- {
-		row, ok := trace[y]
-		if !ok {
-			continue
 }
 
-		var last direction
+type point [2]int
-		p := row[0]
-		last = p.d
-		inLoop := false
 
-		for x := 0; x <= maxX; x++ {
+func (p point) add(a point) point {
-			if p, ok := row[x]; ok {
+	return point{p[0] + a[0], p[1] + a[1]}
-				// fmt.Println("vec", string(p.w), string(p.d))
-				switch string([]rune{rune(p.w), rune(p.d)}) {
-				case "LD", "DL", "UD", "DU", "RD", "UU", "DR":
-					inLoop = !inLoop
 }
-				if last != p.d {
+func (p point) scale(m int) point {
-					last = p.d
+	return point{p[0] * m, p[1] * m}
 }
 
-				// fmt.Print(string(p.w)+string(p.d))
+// 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]
 
-				// On loop
+		sum += row1*col2 - row2*col1
-				area++
-
-				continue // 203338
 	}
-			last = direction('0')
+	area := sum / 2
 
-			if inLoop {
+	// pick's theorem - find the number of points in a shape given its area
-				area++
+	return (aoc.ABS(area) - borderLength/2 + 1) + borderLength
-				// fmt.Print("XX")
-				// } else {
-				// 	fmt.Print("..")
 }
 
+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]})
 	}
-		// fmt.Println("")
+	return pairs
-		fmt.Println(y, area)
-	}
-	return area
 }
 
-type direction rune
+var OFFSET = map[string]point{
-
+	"R": {0, 1},
-const (
+	"D": {1, 0},
-	U direction = 'U'
+	"L": {0, -1},
-	D direction = 'D'
+	"U": {-1, 0},
-	L direction = 'L'
-	R direction = 'R'
-)
-
-func opposite(d direction) direction {
-	switch d {
-	case U:
-		return D
-	case D:
-		return U
-	case L:
-		return R
-	case R:
-		return L
 }
-	return '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
 }
  
-type vec struct {
+func fromColor(c string) vector {
-	Direction direction
+	scale, _ := strconv.ParseInt(c[:5], 16, 64)
-	Steps     int
+	offset := OFFSET_INDEXES[c[5]-'0']
-	Color     [3]byte
-}
 
-type point struct {
+	return vector{
-	w     direction
+		offset: offset,
-	d     direction
+		scale:  int(scale),
-	x, y  int
-	color [3]byte
-}
-
-func fromColor(c string) vec {
-	steps, _ := strconv.ParseInt(c[:5], 16, 64)
-
-	d := '_'
-	switch c[5] {
-	case '0':
-		d = 'R'
-	case '1':
-		d = 'D'
-	case '2':
-		d = 'L'
-	case '3':
-		d = 'U'
-	}
-
-	return vec{
-		Direction: direction(d),
-		Steps:     int(steps),
 	}
 }
 
-func skip[T any](env string, fn func() T) T {
+func findArea(vecs []vector) int {
-	var zero T
+	shoelace := []point{{0,0}}
-	if e, err := strconv.ParseBool(os.Getenv(env)); err == nil && e {
+	borderLength := 0
-		return zero
+
+	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 fn()
+	return numPoints(shoelace, borderLength)
 }
+

day18/main_test.go

@@ -3,7 +3,6 @@ package main
 import (
 	"bufio"
 	"bytes"
-	"os"
 	"testing"
 
 	_ "embed"
@@ -33,13 +32,11 @@ func TestSolution(t *testing.T) {
 	is := is.New(t)
 	scan := bufio.NewScanner(bytes.NewReader(input))
 
-	os.Setenv("AOC_DAY18P2", "1")
-
 	result, err := run(scan)
 	is.NoErr(err)
 
 	t.Log(result)
 	is.True(result.valuePT1 < 68834) // first attempt too high.
 	is.Equal(result.valuePT1, 46334)
-	is.Equal(result.valuePT2, 0)
+	is.Equal(result.valuePT2, 102000662718092)
 }