Get shortest unique substring - code review on the webpage: https://codereview.stackexchange.com/a/176779/123986

getShortestUniqueSubstring_codereview.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace substring_practice
{
    class Program
    {
        static void Main(string[] args)
        {
            var testResult = GetShortestUniqueSubstring(new char[] { 'a', 'b', 'c' }, "aefbcgaxy");
            Debug.Assert(testResult.CompareTo("bcga") == 0);
        }

        /// <summary>
        /// Oct. 25, 2017
        /// code is reviewed on this webpage:
        /// https://codereview.stackexchange.com/questions/176769/find-the-smallest-substring-that-contains-some-given-subset-of-characters
        /// </summary>
        /// <param name="search"></param>
        /// <param name="source"></param>
        /// <returns></returns>
        public static string GetShortestUniqueSubstring(char[] search, string source)
        {
            if (search == null || search.Length == 0)
            {
                throw new ArgumentException("Search collection is null or empty.", "Search argument"); // code review 1
            }

            // assume that search string is not empty
            if (string.IsNullOrEmpty(source))
            {
                throw new ArgumentException("Source string in null or empty", "Source argument"); // code review 2
            }

            // put unique chars in search string to the dictionary
            var map = search.ToDictionary(c => c, c => 1);  // code review 3

            var needChars = search.Length; // 'xyz' - 3, var match = needChars == 0 
            // iterate the string and find match, and also keep track of minimum 
            var left = 0;
            var length = source.Length;

            var smallestLength = length + 1;
            var smallestSubstring = "";

            for (int index = 0; index < length; index++)
            {
                var visit = source[index];

                //TryGetValue method to avoid double search of the value by key. 
                int count;   // code review
                var inMap = map.TryGetValue(visit, out count); // code review 4
                var needOne = inMap && count > 0;

                if (inMap)
                {
                    map[visit]--;
                }

                if (needOne)
                {
                    needChars--;
                }

                var findMatch = needChars == 0;
                if (!findMatch)
                {
                    continue;
                }

                // move left point forward - while loop                
                while (left <= index && (!map.ContainsKey(source[left]) || (map[source[left]] < 0))) // code review 5
                {
                    var removeChar = source[left];

                    // update the variable needChars                     
                    if (map.ContainsKey(source[left]))
                    {
                        map[removeChar]++;
                    }

                    left++;
                }

                var currentLength = index - left + 1;
                var findSmallerOne = currentLength < smallestLength;
                if (findSmallerOne)
                {
                    smallestLength = currentLength;
                    smallestSubstring = source.Substring(left, currentLength);

                    needChars++;
                    map[source[left]]++;
                    left = left + 1;
                }
            }

            // edge case
            return smallestLength == length + 1   // code review 6
                ? string.Empty
                : smallestSubstring;            
        }
    }
}