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Adds Collection<GeoJSON.Position>.convexHull() (#7)
Adds convexHull() Also: - SPM update to get closed polygon rings - Refactoring
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,40 @@ | ||
| // | ||
| // Turf+ConvexHull.swift | ||
| // | ||
| // | ||
| // Created by Adrian Schönig on 14/6/2022. | ||
| // | ||
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| import Foundation | ||
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| import GeoJSONKit | ||
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| extension Collection where Element == GeoJSON.Position { | ||
| /// Calculates the convex hull of a given sequence of positions. | ||
| /// | ||
| /// - Complexity: O(*n* log *n*), where *n* is the count of `points`. | ||
| /// | ||
| /// - Returns: The convex hull of this sequence as a polygon | ||
| public func convexHull() -> GeoJSON.Polygon { | ||
| let positions = AndrewsMonotoneChain.convexHull(self).map { $0.removingAltitude } | ||
| return .init(exterior: .init(positions: positions)) | ||
| } | ||
| } | ||
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| extension GeoJSON { | ||
| /// Calculates the convex hull of all the elements of this GeoJSON | ||
| /// | ||
| /// - Returns: The convex hull of this GeoJSON as a polygon | ||
| public func convexHull() -> GeoJSON.Polygon { | ||
| return positions.convexHull() | ||
| } | ||
| } | ||
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| extension GeoJSON.Position { | ||
| fileprivate var removingAltitude: GeoJSON.Position { | ||
| guard altitude != nil else { return self } | ||
| var updated = self | ||
| updated.altitude = nil | ||
| return updated | ||
| } | ||
| } |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,52 @@ | ||
| // | ||
| // GeoJSON+Helpers.swift | ||
| // | ||
| // | ||
| // Created by Adrian Schönig on 14/6/2022. | ||
| // | ||
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| import GeoJSONKit | ||
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| extension GeoJSON.GeometryObject { | ||
| public var geometries: [GeoJSON.Geometry] { | ||
| switch self { | ||
| case .single(let geo): return [geo] | ||
| case .multi(let geos): return geos | ||
| case .collection(let geoObjects): return geoObjects.flatMap(\.geometries) | ||
| } | ||
| } | ||
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| public var positions: [GeoJSON.Position] { | ||
| geometries.flatMap(\.positions) | ||
| } | ||
| } | ||
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| extension GeoJSON.Geometry { | ||
| public var positions: [GeoJSON.Position] { | ||
| switch self { | ||
| case .point(let position): return [position] | ||
| case .lineString(let line): return line.positions | ||
| case .polygon(let polygon): | ||
| // Ignore the interior positions as the purpose of this is getting | ||
| // bounding boxes, convex hull or alike | ||
| return polygon.exterior.positions | ||
| } | ||
| } | ||
| } | ||
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| extension GeoJSON { | ||
| public var geometries: [GeoJSON.Geometry] { | ||
| switch object { | ||
| case .feature(let feature): | ||
| return feature.geometry.geometries | ||
| case .featureCollection(let features): | ||
| return features.flatMap(\.geometry.geometries) | ||
| case .geometry(let geometry): | ||
| return geometry.geometries | ||
| } | ||
| } | ||
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| public var positions: [GeoJSON.Position] { | ||
| geometries.flatMap(\.positions) | ||
| } | ||
| } |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,83 @@ | ||
| // | ||
| // MonotoneChain.swift | ||
| // | ||
| // | ||
| // Created by Adrian Schönig on 14/6/2022. | ||
| // | ||
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| import Foundation | ||
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| import GeoJSONKit | ||
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| // Adopted from https://en.wikibooks.org/wiki/Algorithm_Implementation/Geometry/Convex_hull/Monotone_chain | ||
| struct AndrewsMonotoneChain { | ||
| private static func cross(_ o: GeoJSON.Position, _ a: GeoJSON.Position, _ b: GeoJSON.Position) -> Double { | ||
| let lhs = (a.x - o.x) * (b.y - o.y) | ||
| let rhs = (a.y - o.y) * (b.x - o.x) | ||
| return lhs - rhs | ||
| } | ||
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| /// Calculate and return the convex hull of a given sequence of points. | ||
| /// | ||
| /// - Remark: Implements Andrew’s monotone chain convex hull algorithm. | ||
| /// | ||
| /// - Complexity: O(*n* log *n*), where *n* is the count of `points`. | ||
| /// | ||
| /// - Parameter points: A sequence of `GeoJSON.Position` elements. | ||
| /// | ||
| /// - Returns: An array containing the convex hull of `points`, ordered | ||
| /// lexicographically from the smallest coordinates to the largest, | ||
| /// turning counterclockwise. | ||
| /// | ||
| static func convexHull<Source>(_ points: Source) -> [GeoJSON.Position] | ||
| where Source : Collection, | ||
| Source.Element == GeoJSON.Position | ||
| { | ||
| // Exit early if there aren’t enough points to work with. | ||
| guard points.count > 1 else { return Array(points) } | ||
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| // Create storage for the lower and upper hulls. | ||
| var lower = [GeoJSON.Position]() | ||
| var upper = [GeoJSON.Position]() | ||
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| // Sort points in lexicographical order. | ||
| let points = points.sorted { a, b in | ||
| a.x < b.x || a.x == b.x && a.y < b.y | ||
| } | ||
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| // Construct the lower hull. | ||
| for point in points { | ||
| while lower.count >= 2 { | ||
| let a = lower[lower.count - 2] | ||
| let b = lower[lower.count - 1] | ||
| if cross(a, b, point) > 0 { break } | ||
| lower.removeLast() | ||
| } | ||
| lower.append(point) | ||
| } | ||
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| // Construct the upper hull. | ||
| for point in points.lazy.reversed() { | ||
| while upper.count >= 2 { | ||
| let a = upper[upper.count - 2] | ||
| let b = upper[upper.count - 1] | ||
| if cross(a, b, point) > 0 { break } | ||
| upper.removeLast() | ||
| } | ||
| upper.append(point) | ||
| } | ||
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| // Remove each array’s last point, as it’s the same as the first point | ||
| // in the opposite array, respectively. | ||
| lower.removeLast() | ||
| upper.removeLast() | ||
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| // Join the arrays to form the convex hull. | ||
| return lower + upper | ||
| } | ||
| } | ||
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| fileprivate extension GeoJSON.Position { | ||
| var x: Double { longitude } | ||
| var y: Double { latitude } | ||
| } |
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