feat: solve part two

internal/2025/DayNine/code.go

@@ -3,6 +3,7 @@ package daynine
 import (
 	"advent-of-code/internal/registry"
 	"os"
+	"slices"
 	"strconv"
 	"strings"
 )
@@ -38,19 +39,142 @@ func abs(x int) int {
 	return x
 }
 
+func minMax(a, b int) (int, int) {
+	if a > b {
+		return b, a
+	}
+	return a, b
+}
+
+func compressCoordinates(points []Point) ([]int, []int, map[int]int, map[int]int) {
+	xs := make(map[int]bool)
+	ys := make(map[int]bool)
+	for _, point := range points {
+		xs[point.x] = true
+		ys[point.y] = true
+	}
+	sortedX := make([]int, 0, len(xs))
+	sortedY := make([]int, 0, len(ys))
+	for x := range xs {
+		sortedX = append(sortedX, x)
+	}
+	for y := range ys {
+		sortedY = append(sortedY, y)
+	}
+	slices.Sort(sortedX)
+	slices.Sort(sortedY)
+
+	mapX := make(map[int]int, len(sortedX))
+	mapY := make(map[int]int, len(sortedY))
+	for idx, x := range sortedX {
+		mapX[x] = 2 * idx
+	}
+	for idx, y := range sortedY {
+		mapY[y] = 2 * idx
+	}
+	return sortedX, sortedY, mapX, mapY
+}
+
+func drawBoundary(grid [][]bool, points []Point, mapX, mapY map[int]int) {
+	for idx := range points {
+		current, next := points[idx], points[(idx+1)%len(points)]
+		cx1, cy1 := mapX[current.x], mapY[current.y]
+		cx2, cy2 := mapX[next.x], mapY[next.y]
+
+		if cx1 == cx2 {
+			minY, maxY := minMax(cy1, cy2)
+			for y := minY; y <= maxY; y++ {
+				grid[cx1][y] = true
+			}
+		} else {
+			minX, maxX := minMax(cx1, cx2)
+			for x := minX; x <= maxX; x++ {
+				grid[x][cy1] = true
+			}
+		}
+	}
+}
+
+func scanLineFill(grid [][]bool, points []Point, mapX, mapY map[int]int, _, height int) {
+	type edge struct {
+		x, yMin, yMax int
+	}
+	edges := make([]edge, 0, len(points))
+	for idx := range points {
+		current, next := points[idx], points[(idx+1)%len(points)]
+		if current.x == next.x {
+			minY, maxY := minMax(mapY[current.y], mapY[next.y])
+			edges = append(edges, edge{mapX[current.x], minY, maxY})
+		}
+	}
+
+	for y := range height {
+		crossings := make([]int, 0, len(edges))
+		for _, e := range edges {
+			if e.yMin <= y && y < e.yMax {
+				crossings = append(crossings, e.x)
+			}
+		}
+		slices.Sort(crossings)
+		for i := 0; i < len(crossings)-1; i += 2 {
+			for x := crossings[i]; x <= crossings[i+1]; x++ {
+				grid[x][y] = true
+			}
+		}
+	}
+}
+
+func forceEmptyGaps(grid [][]bool, sortedX, sortedY []int, width, height int) {
+	for idx := 0; idx < len(sortedX)-1; idx++ {
+		if sortedX[idx+1] == sortedX[idx]+1 {
+			cx := 2*idx + 1
+			for y := range height {
+				grid[cx][y] = true
+			}
+		}
+	}
+	for idx := 0; idx < len(sortedY)-1; idx++ {
+		if sortedY[idx+1] == sortedY[idx]+1 {
+			cy := 2*idx + 1
+			for x := range width {
+				grid[x][cy] = true
+			}
+		}
+	}
+}
+
+func buildPrefixSum(grid [][]bool, width, height int) [][]int {
+	sum := make([][]int, width+1)
+	for idx := range sum {
+		sum[idx] = make([]int, height+1)
+	}
+	for i := range width {
+		for j := range height {
+			value := 0
+			if grid[i][j] {
+				value = 1
+			}
+			sum[i+1][j+1] = value + sum[i][j+1] + sum[i+1][j] - sum[i][j]
+		}
+	}
+	return sum
+}
+
+func getSum(sum [][]int, x1, y1, x2, y2 int) int {
+	return sum[x2+1][y2+1] - sum[x1][y2+1] - sum[x2+1][y1] + sum[x1][y1]
+}
+
 func PartOne(data []string) int {
 	points := parsePoints(data)
 	maxArea := 0
 
 	for i := range points {
-		for j := 1; j < len(points); j++ {
-			distanceX := abs(points[i].x - points[j].x)
-			distanceY := abs(points[i].y - points[j].y)
+		for j := i + 1; j < len(points); j++ {
+			dx := abs(points[i].x - points[j].x)
+			dy := abs(points[i].y - points[j].y)
 
-			if distanceX > 0 && distanceY > 0 {
-				if area := (distanceX + 1) * (distanceY + 1); area > maxArea {
-					maxArea = area
-				}
+			if area := (dx + 1) * (dy + 1); area > maxArea {
+				maxArea = area
 			}
 		}
 	}
@@ -58,5 +182,53 @@ func PartOne(data []string) int {
 }
 
 func PartTwo(data []string) int {
-	return 0
+	points := parsePoints(data)
+	sortedX, sortedY, mapX, mapY := compressCoordinates(points)
+
+	width, height := 2*len(sortedX), 2*len(sortedY)
+	grid := make([][]bool, width)
+	for idx := range grid {
+		grid[idx] = make([]bool, height)
+	}
+
+	drawBoundary(grid, points, mapX, mapY)
+	scanLineFill(grid, points, mapX, mapY, width, height)
+	forceEmptyGaps(grid, sortedX, sortedY, width, height)
+
+	sum := buildPrefixSum(grid, width, height)
+
+	compressedPoints := make([]Point, len(points))
+	for idx, point := range points {
+		compressedPoints[idx] = Point{mapX[point.x], mapY[point.y]}
+	}
+
+	maxArea := 0
+	for i := range points {
+		for j := i + 1; j < len(points); j++ {
+			p1, p2 := points[i], points[j]
+			dx := abs(p1.x - p2.x)
+			dy := abs(p1.y - p2.y)
+
+			if dx == 0 || dy == 0 {
+				continue
+			}
+
+			area := (dx + 1) * (dy + 1)
+			if area <= maxArea {
+				continue
+			}
+
+			cx1, cy1 := compressedPoints[i].x, compressedPoints[i].y
+			cx2, cy2 := compressedPoints[j].x, compressedPoints[j].y
+			minCX, maxCX := minMax(cx1, cx2)
+			minCY, maxCY := minMax(cy1, cy2)
+
+			totalCells := (maxCX - minCX + 1) * (maxCY - minCY + 1)
+			if getSum(sum, minCX, minCY, maxCX, maxCY) == totalCells {
+				maxArea = area
+			}
+		}
+	}
+
+	return maxArea
 }