TweetDuck/Data/InjectedHTML.cs

using System;

namespace TweetDuck.Data{
    sealed class InjectedHTML{
        public enum Position{
            Before, After
        }

        private readonly Position position;
        private readonly string search;
        private readonly string html;

        public InjectedHTML(Position position, string search, string html){
            this.position = position;
            this.search = search;
            this.html = html;
        }

        public string InjectInto(string targetHTML){
            int index = targetHTML.IndexOf(search, StringComparison.Ordinal);

            if (index == -1){
                return targetHTML;
            }

            int cutIndex;

            switch(position){
                case Position.Before: cutIndex = index; break;
                case Position.After: cutIndex = index+search.Length; break;
                default: return targetHTML;
            }

            return targetHTML.Insert(cutIndex, html);
        }
    }
}