Point hit tester for lines, quadratic bezier curves, and cubic bezier curves in Swift.

CurveHitTester.swift

import UIKit

public class CurveHitTester {
    
    typealias CurvePointMapper = (t: CGFloat, u: CGFloat) -> (x: CGFloat, y: CGFloat)
    
    public static func hitTestLine(point: CGPoint, p1: CGPoint, p2: CGPoint, segments: Int, lineWidth: CGFloat) -> Bool {
        return hitTestPoints(point, segments: segments, lineWidth: lineWidth) { (t, u) in
            let x = u * p1.x + t * p2.x
            let y = u * p1.y + t * p2.y
            return (x, y)
        }
    }
    
    public static func hitTestQuad(point: CGPoint, p1: CGPoint, c1: CGPoint, p2: CGPoint, segments: Int, lineWidth: CGFloat) -> Bool {
        return hitTestPoints(point, segments: segments, lineWidth: lineWidth) { (t, u) in
            let x = u * u * p1.x + 2.0 * u * t * c1.x + t * t * p2.x
            let y = u * u * p1.y + 2.0 * u * t * c1.y + t * t * p2.y
            return (x, y)
        }
    }
    
    public static func hitTestCubic(point: CGPoint, p1: CGPoint, c1: CGPoint, c2: CGPoint, p2: CGPoint, segments: Int, lineWidth: CGFloat) -> Bool {
        return hitTestPoints(point, segments: segments, lineWidth: lineWidth) { (t, u) in
            let x = u * u * u * p1.x + 3.0 * u * u * t * c1.x + 3.0 * u * t * t * c2.x + t * t * t * p2.x
            let y = u * u * u * p1.y + 3.0 * u * u * t * c1.y + 3.0 * u * t * t * c2.y + t * t * t * p2.y
            return (x, y)
        }
    }
    
    // 曲線上の点との当たり判定
    // シンプルにタッチ座標周囲の円が点を含むかで判定
    private static func hitTestPoints(point: CGPoint, segments: Int, lineWidth: CGFloat, toCurvePoint: CurvePointMapper) -> Bool {
        guard segments > 0 else {
            return false
        }
        let ll = lineWidth * lineWidth / 4.0
        
        func isNearPoint(x: CGFloat, y: CGFloat) -> Bool {
            let dx = point.x - x
            let dy = point.y - y
            return (dx*dx + dy*dy <= ll)
        }
        
        return [Int](0...segments)
            .map { i in
                let t = CGFloat(i) / CGFloat(segments)
                return (t: t, u: 1 - t)
            }
            .map(toCurvePoint)
            .filter(isNearPoint)
            .count > 0
    }
    
}