Gift Wrapping Algorithm
Gift wrapping is the act of enclosing a gift in some sort of material. An option to gift wrapping is use a gift box or bag. Although the Hall brothers Rollie and Joyce Hall, founders of Hallmark card, make not invent gift wrapping, their innovations led to the development of modern gift wrapping. They helped to popularize the idea of decorative gift wrapping in the 20th century, and according to Joyce Hall," the decorative gift-wrapping business was birth the day Rollie put those French envelope linings on top of that showcase."The purpose of wrapping paper is first documented in ancient China, where paper was invented in 2nd century BC.
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package com.algorithmexamples.geometry.convexhull
import com.algorithmexamples.datastructures.Stack
import com.algorithmexamples.geometry.Point
class GiftWrapping: ConvexHullAlgorithm {
override fun convexHull(points: Array<Point>): Collection<Point> {
if (points.size < 3) throw IllegalArgumentException("there must be at least 3 points")
val hull = Stack<Point>()
// Find the leftmost point
var l = 0
points.indices
.asSequence()
.filter { points[it].x < points[l].x }
.forEach { l = it }
// Start from leftmost point, keep moving counterclockwise
// until reach the start point again. This loop runs O(h)
// times where h is number of points in result or output.
var p = l
var q: Int
do {
// Add current point to result
hull.push(points[p])
// Search for a point 'q' such that orientation(p, x,
// q) is counterclockwise for all points 'x'. The idea
// is to keep track of last visited most counterclock-
// wise point in q. If any point 'i' is more counterclock-
// wise than q, then update q.
q = ( p+ 1) % points.size
points.indices
.asSequence()
.filter { Point.orientation(points[p], points[it], points[q]) < 0 }
.forEach { q = it }
// Now q is the most counterclockwise with respect to p
// Set p as q for next iteration, so that q is added to
// result 'hull'
p = q
} while (p != l) // While we don't come to first point
return hull
}
}
