IndexPath+OutlineViewInterop.swift

import Cocoa

private func rowIndexForLayer(at rowIndex: Int, in layers: [Layer]) -> Int {
    return layers.count - 1 - rowIndex
}

private func outlineViewRowIndex(for path: IndexPath, parent: LayerContainer) -> Int {
    let layerIndex = path.last!
    return rowIndexForLayer(at: layerIndex, in: parent.layers)
}

func rowIndexesForMoving(_ layer: Layer, from: IndexPath, below: IndexPath,
                         sourceContainer: LayerContainer, destinationContainer: LayerContainer)
    -> (fromRow: Int, fromParent: LayerContainer?, toRow: Int, toParent: LayerContainer?)?
{
    guard from.next != below else { return nil }
    guard from != below else { return nil }
    
    let updatedFrom = from.parent == below.parent ? from.havingInserted(below) : from
    var fromRow = outlineViewRowIndex(for: updatedFrom,
                                      parent: sourceContainer)
    
    let updatedBelow = from.parent == below.parent ? below.havingRemoved(from) : below
    let toRow = outlineViewRowIndex(for: updatedBelow,
                                    parent: destinationContainer)
    
    if sourceContainer === destinationContainer && toRow > fromRow {
       fromRow += 1
    }
    
    if sourceContainer !== destinationContainer {
        fromRow += 1
    }
    
    guard fromRow != toRow || sourceContainer !== destinationContainer else { return nil }
    let fromParent = from.count > 1 ? sourceContainer : nil
    let toParent = below.count > 1 ? destinationContainer : nil
    return (fromRow, fromParent, toRow, toParent)
}

extension NSOutlineView {
    /// Convenience API for the similar `NSOutlineView.moveItem(at:inParent:to:inParent:)`, but in Canvas
    /// coordinates.
    ///
    /// Canvas coordinates refer to the fact that the first layer (index 0) in canvas is the background one (ie. below other layers),
    /// although it is displayed as the last one in the outline view (ie. the bottom layer is the background).
    ///
    /// - Parameters:
    ///   - layer: The layer being moved.
    ///   - canvas: The canvas to which the layer belongs.
    ///   - sourcePath: The original path to the layer, in Canvas coordinates.
    ///   - destinationPath: The destination path at which the layer is inserted, in Canvas coordinates.
    func move(_ layer: Layer, in canvas: Canvas, at sourcePath: IndexPath, below destinationPath: IndexPath,
              sourceContainer: LayerContainer, destinationContainer: LayerContainer) {
        
        if let (fromRow, fromParent, toRow, toParent)
            = rowIndexesForMoving(layer, from: sourcePath, below: destinationPath,
                                  sourceContainer: sourceContainer,
                                  destinationContainer: destinationContainer) {
            moveItem(at: fromRow, inParent: fromParent, to: toRow, inParent: toParent)
        }
    }
}