KillRing.swift

import Foundation

/// KillRing with macOS text text system semantics.
///
/// Use when implementing `yank:` and `yankAndSelect:` actions.
/// 
/// Learn more at http://hogbaysoftware.com/posts/mac-text-editing-mark-kill-yank/
public class KillRing {
    
    public static let shared = KillRing()
    
    public enum Action {
        case delete
        case deleteToMark
        case deleteToBeginningOfLine
        case deleteToEndOfLine
        case deleteToBeginningOfParagraph
        case deleteToEndOfParagraph
    }
    
    private var index: Int = 0
    private var buffer: [String]
    private var yanking: Bool = false
    private var lastAction: Action?
    
    init() {
        let killRingSize = UserDefaults.standard.integer(forKey: "NSTextKillRingSize")
        buffer = Array(repeating: "", count: max(killRingSize, 1))
    }
    
    /// Call when selection changes in editor
    public func selectionChanged() {
        lastAction = nil
    }

    /// Call when text changes in editor
    public func textChanged() {
        yanking = false
        lastAction = nil
    }

    /// Call when text editor performs any of the defined Actions.
    /// The editor should perform all selection and buffer mutations within the `killBlock`.
    public func kill(action: Action, killBlock: () -> String) {
        let savedLastAction = lastAction
        let string = killBlock()
        
        if action != savedLastAction {
            lastAction = action
            
            if index == buffer.count - 1 {
                index = 0
            } else {
                index += 1
            }

            buffer[index] = ""
        } else {
            self.lastAction = savedLastAction
        }

        switch action {
        case .delete:
            buffer[index] = string
        case .deleteToBeginningOfLine, .deleteToBeginningOfParagraph:
            buffer[index] = string + buffer[index]
        case .deleteToEndOfLine, .deleteToEndOfParagraph, .deleteToMark:
            buffer[index] = buffer[index] + string
        }
    }

    /// Call in response to `yank:` action.
    /// The editor should perform all selection and buffer mutations within the `yankBlock`.
    public func yank(yankBlock: (String) -> ()) {
        yankBlock(buffer[index])
        yanking = true
    }

    /// Call in response to `yankAndSelect:` action.
    /// The editor should perform all selection and buffer mutations within the `yankAndSelectBlock`.
    public func yankAndSelect(yankAndSelectBlock: (String) -> ()) {
        if yanking {
            if index == 0 {
                index = buffer.count - 1
            } else {
                index -= 1
            }
        }
        yankAndSelectBlock(buffer[index])
        yanking = true
    }

}