Data commit

Task/Convex-hull/Go/convex-hull.go

@@ -0,0 +1,63 @@
+package main
+
+import (
+	"fmt"
+	"image"
+	"sort"
+)
+
+
+// ConvexHull returns the set of points that define the
+// convex hull of p in CCW order starting from the left most.
+func (p points) ConvexHull() points {
+	// From https://en.wikibooks.org/wiki/Algorithm_Implementation/Geometry/Convex_hull/Monotone_chain
+	// with only minor deviations.
+	sort.Sort(p)
+	var h points
+
+	// Lower hull
+	for _, pt := range p {
+		for len(h) >= 2 && !ccw(h[len(h)-2], h[len(h)-1], pt) {
+			h = h[:len(h)-1]
+		}
+		h = append(h, pt)
+	}
+
+	// Upper hull
+	for i, t := len(p)-2, len(h)+1; i >= 0; i-- {
+		pt := p[i]
+		for len(h) >= t && !ccw(h[len(h)-2], h[len(h)-1], pt) {
+			h = h[:len(h)-1]
+		}
+		h = append(h, pt)
+	}
+
+	return h[:len(h)-1]
+}
+
+// ccw returns true if the three points make a counter-clockwise turn
+func ccw(a, b, c image.Point) bool {
+	return ((b.X - a.X) * (c.Y - a.Y)) > ((b.Y - a.Y) * (c.X - a.X))
+}
+
+type points []image.Point
+
+func (p points) Len() int      { return len(p) }
+func (p points) Swap(i, j int) { p[i], p[j] = p[j], p[i] }
+func (p points) Less(i, j int) bool {
+	if p[i].X == p[j].X {
+		return p[i].Y < p[i].Y
+	}
+	return p[i].X < p[j].X
+}
+
+func main() {
+	pts := points{
+		{16, 3}, {12, 17}, {0, 6}, {-4, -6}, {16, 6},
+		{16, -7}, {16, -3}, {17, -4}, {5, 19}, {19, -8},
+		{3, 16}, {12, 13}, {3, -4}, {17, 5}, {-3, 15},
+		{-3, -9}, {0, 11}, {-9, -3}, {-4, -2}, {12, 10},
+	}
+	hull := pts.ConvexHull()
+	fmt.Println("Convex Hull:", hull)
+}