Day 6, part 2. A bit brute-force, but it gets the job done.

2018/day06-2.go

@@ -0,0 +1,16 @@
+package main
+
+import (
+	"fmt"
+
+	"internal/coords"
+	"internal/util"
+)
+
+func main() {
+	data := util.ReadInput()
+	points := coords.ParsePoints(data)
+
+	area := coords.ComputeNearnessRegion(points, 10000)
+	fmt.Println(area)
+}

2018/internal/coords/coords.go

@@ -11,6 +11,7 @@ type Point struct {
 	RectArea int
 }
 
+// Turn input data into useable structured point objects.
 func ParsePoints(data []string) []*Point {
 	points := []*Point{}
 
@@ -33,16 +34,7 @@ func ParsePoints(data []string) []*Point {
 // unbounded areas) to -1
 func ComputeRectilinearAreas(data []*Point) {
 	// first we find the bounds
-	maxX := 0
-	maxY := 0
-	for _, point := range data {
-		if point.X > maxX {
-			maxX = point.X
-		}
-		if point.Y > maxY {
-			maxY = point.Y
-		}
-	}
+	maxX, maxY := findUpperBounds(data)
 
 	// now for each integer point in our range, we compute the distance to each
 	// point. We save the closest point and its distance, then:
@@ -50,7 +42,7 @@ func ComputeRectilinearAreas(data []*Point) {
 	// 2. Otherwise, we increment RectArea for the closest point.
 	for x := -maxX; x <= maxX*2; x++ {
 		for y := -maxY; y <= maxY*2; y++ {
-			closest := FindClosest(data, &Point{X: x, Y: y})
+			closest := findClosest(data, &Point{X: x, Y: y})
 			if closest == nil {
 				continue
 			}
@@ -61,16 +53,47 @@ func ComputeRectilinearAreas(data []*Point) {
 
 	// now we find any unbounded points and set their areas to the sentinel
 	// value
-	DetectUnboundedPoints(data)
+	detectUnboundedPoints(data)
 }
 
-func FindClosest(data []*Point, point *Point) *Point {
+// ComputeNearnessRegion takes a list of points, returns the number of points
+// whose rectilinear distance to *all* the listed points sums to less than the
+// target value.
+func ComputeNearnessRegion(points []*Point, target int) int {
+	maxX, maxY := findUpperBounds(points)
+
+	// this is a 'brute-force' solution that just picks a suitably wide
+	// area to search.
+	area := 0
+	for x := -target; x < maxX+target; x++ {
+		for y := -target; y < maxY+target; y++ {
+			if sumDistances(points, &Point{X: x, Y: y}) < target {
+				area++
+			}
+		}
+	}
+
+	return area
+}
+
+// sumDistances calculates the distance from each point in `points` to `target`, and
+// returns the sum of all of those distances
+func sumDistances(points []*Point, target *Point) int {
+	area := 0
+	for _, point := range points {
+		area += Distance(point, target)
+	}
+	return area
+}
+
+// Find the nearest point in `data` to `point`, using rectilinear distance.
+func findClosest(data []*Point, point *Point) *Point {
 	closest := data[0]
-	distance := computeDistance(data[0], point)
+	distance := Distance(data[0], point)
 
 	for i := 1; i < len(data); i++ {
 		candidate := data[i]
-		newDistance := computeDistance(candidate, point)
+		newDistance := Distance(candidate, point)
 
 		if newDistance < distance {
 			closest = candidate
@@ -83,13 +106,13 @@ func FindClosest(data []*Point, point *Point) *Point {
 	return closest
 }
 
-func DetectUnboundedPoints(data []*Point) {
+func detectUnboundedPoints(data []*Point) {
 	for _, candidate := range data {
 		// look in each direction
-		up := FindClosest(data, &Point{X: candidate.X, Y: candidate.Y + 1000000})
-		down := FindClosest(data, &Point{X: candidate.X, Y: candidate.Y - 1000000})
-		left := FindClosest(data, &Point{X: candidate.X + 1000000, Y: candidate.Y})
-		right := FindClosest(data, &Point{X: candidate.X - 1000000, Y: candidate.Y})
+		up := findClosest(data, &Point{X: candidate.X, Y: candidate.Y + 1000000})
+		down := findClosest(data, &Point{X: candidate.X, Y: candidate.Y - 1000000})
+		left := findClosest(data, &Point{X: candidate.X + 1000000, Y: candidate.Y})
+		right := findClosest(data, &Point{X: candidate.X - 1000000, Y: candidate.Y})
 
 		// if any of those points are closest to our point, we're unbounded
 		if up == candidate || down == candidate ||
@@ -99,7 +122,22 @@ func DetectUnboundedPoints(data []*Point) {
 	}
 }
 
-func computeDistance(p1 *Point, p2 *Point) int {
+func findUpperBounds(points []*Point) (int, int) {
+	maxX := 0
+	maxY := 0
+	for _, point := range points {
+		if point.X > maxX {
+			maxX = point.X
+		}
+		if point.Y > maxY {
+			maxY = point.Y
+		}
+	}
+
+	return maxX, maxY
+}
+
+func Distance(p1 *Point, p2 *Point) int {
 	return abs(p2.Y-p1.Y) + abs(p2.X-p1.X)
 }