import SwiftUI

struct SplitView<L: View, R: View>: View {
    let direction: Direction
    let left: L
    let right: R
    
    private let splitterVisibleSize: CGFloat = 5
    private let splitterInvisibleSize: CGFloat = 5
    
    @State var split: CGFloat = 0.5
    
    var body: some View {
        GeometryReader { geo in
            let leftRect = self.leftRect(for: geo.size)
            let rightRect = self.rightRect(for: geo.size, leftRect: leftRect)
            let splitterPoint = self.splitterPoint(for: geo.size, leftRect: leftRect)
            
            ZStack(alignment: .topLeading) {
                left
                    .frame(width: leftRect.size.width, height: leftRect.size.height)
                    .offset(x: leftRect.origin.x, y: leftRect.origin.y)
                right
                    .frame(width: rightRect.size.width, height: rightRect.size.height)
                    .offset(x: rightRect.origin.x, y: rightRect.origin.y)
                Splitter(direction: direction, visibleSize: splitterVisibleSize, invisibleSize: splitterInvisibleSize)
                    .position(splitterPoint)
            }
        }
    }
    
    func leftRect(for size: CGSize) -> CGRect {
        // Initially the rect is the full size
        var result = CGRect(x: 0, y: 0, width: size.width, height: size.height)
        switch (direction) {
        case .horizontal:
            result.size.width = result.size.width * split
            result.size.width -= splitterVisibleSize / 2
            
        case .vertical:
            result.size.height = result.size.height * split
            result.size.height -= splitterVisibleSize / 2
        }
        
        return result
    }
    
    func rightRect(for size: CGSize, leftRect: CGRect) -> CGRect {
        // Initially the rect is the full size
        var result = CGRect(x: 0, y: 0, width: size.width, height: size.height)
        switch (direction) {
        case .horizontal:
            // For horizontal layouts we offset the starting X by the left rect
            // and make the width fit the remaining space.
            result.origin.x += leftRect.size.width
            result.origin.x += splitterVisibleSize / 2
            result.size.width -= result.origin.x
            
        case .vertical:
            assert(false)
        }
        
        return result
    }
    
    func splitterPoint(for size: CGSize, leftRect: CGRect) -> CGPoint {
        switch (direction) {
        case .horizontal:
            return CGPoint(x: leftRect.size.width, y: size.height / 2)
            
        case .vertical:
            return CGPoint(x: size.width / 2, y: leftRect.size.height)
        }
    }
    
    init(_ direction: Direction, @ViewBuilder left: (() -> L), @ViewBuilder right: (() -> R)) {
        self.direction = direction
        self.left = left()
        self.right = right()
    }
}

extension SplitView {
    enum Direction {
        case horizontal, vertical
    }
}