Calculate Y-axis distance between a point and a line segment.

distanceAboveLineSegment.swift

import CoreGraphics

extension CGPoint {

    /// Calculate Y-axis distance between a point and a line segment.
    ///
    /// - parameter endpointA: One endpoint of the line segment.
    /// - parameter endpointB: The other endpoint of the line segment.
    ///
    /// - returns: Distance, or `nil` if this point's x-coordinate is not between those of the endpoints.
    func distanceAboveLineSegment(endpointA: CGPoint,
                                  endpointB: CGPoint) -> CGFloat?
    {
        // Fail if endpoints are on a vertical line.
        if endpointA.x == endpointB.x {
            return nil
        }

        // Swap endpoints if necessary so that endpointLeft is the leftmost point.
        var endpointLeft = endpointA
        var endpointRight = endpointB
        if endpointLeft.x > endpointRight.x {
            swap(&endpointLeft, &endpointRight)
        }

        let x = self.x

        // If not over the line segment, fail.
        if x < endpointLeft.x || endpointRight.x < x {
            return nil
        }

        // Find the y-coordinate where a vertical line crosses the line segment.
        let slope = (endpointRight.y - endpointLeft.y) / (endpointRight.x - endpointLeft.x)
        let dx = x - endpointLeft.x
        let dy = slope * dx
        let yIntercept = endpointLeft.y + dy

        // Return distance.
        return self.y - yIntercept
    }
}