Dynamic strategy

enum OutputFormat
{
    case markdown
    // one
    // two
    case html
    // <ul>
    //  <li>one</li>
    // </ul>
}

protocol ListStrategy
{
    init()
    func start(_ buffer: inout String)
    func end(_ buffer: inout String)
    func addListItem(buffer: inout String, item: String)
}

class MarkdownListStrategy: ListStrategy
{
    required init(){}
    func start(_ buffer: inout String) {}
    func end(_ buffer: inout String) {}
    func addListItem(buffer: inout String, item: String)
    {
        buffer.append(" * \(item)\n")
    }
    
}

class HTMLListStrategy: ListStrategy
{
    required init() {}
    func start(_ buffer: inout String) {
        buffer.append("<ul>\n")
    }
    func end(_ buffer: inout String) {
        buffer.append("</ul>\n")
    }
    func addListItem(buffer: inout String, item: String)
    {
        buffer.append(" <li> \(item) </li>\n")
    }
}


class TextProcessor : CustomStringConvertible
{
    private var buffer = ""
    
    private var listStrategy : ListStrategy
    
    init(_ outputFormat: OutputFormat)
    {
        switch outputFormat {
        case .markdown:
            listStrategy = MarkdownListStrategy()
        case .html:
            listStrategy = HTMLListStrategy()
        }
    }
    
    func setOutputFormat(_ outputFormat: OutputFormat)
    {
        switch outputFormat {
        case .markdown:
            listStrategy = MarkdownListStrategy()
        case .html:
            listStrategy = HTMLListStrategy()
        }
    }
    
    func appendList(_ items: [String])
    {
        listStrategy.start(&buffer)
        
        for item in items
        {
            listStrategy.addListItem(buffer: &buffer, item: item)
        }
        
        listStrategy.end(&buffer)
    }
    
    func clear()
    {
        buffer = ""
    }
    
    var description: String
    {
        return buffer
    }
}


func main()
{
    let tp = TextProcessor(.markdown)
    tp.appendList(["foo","bar","baz"])
    print(tp)
    
    
    tp.clear()
    tp.setOutputFormat(.html)
    tp.appendList(["foo","bar","baz"])
    print(tp)
}

Static Strategy

class TextProcessor2<LS> :
    CustomStringConvertible
where LS: ListStrategy
{
    private var buffer = ""
    private var listStrategy = LS()
    
    
    func appendList(_ items: [String])
    {
        listStrategy.start(&buffer)
        
        for item in items
        {
            listStrategy.addListItem(buffer: &buffer, item: item)
        }
        
        listStrategy.end(&buffer)
    }
    
    func clear()
    {
        buffer = ""
    }
    
    var description: String
    {
        return buffer
    }
}


func main()
{
    let tp = TextProcessor2<MarkdownListStrategy>()
    tp.appendList(["foo","bar","baz"])
    print(tp)
    
    
    let tp2 = TextProcessor2<HTMLListStrategy>()
    tp2.appendList(["foo","bar","baz"])
    print(tp2)
}